Introduction to Aerospace Engineering with a Flight Test Perspective Chapter 1

First Flights

Author: Eze-Odikwa Tochukwu Jed

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“Wilbur, having used his turn in the unsuccessful attempt on the 14th, the right to the first trial now belonged to me. After running the motor a few minutes to heat it up, I released the wire that held the machine to the track, and the machine started forward in the wind. Wilbur ran at the side of the machine, holding the wing to balance it on the track. Unlike the start on the 14th, made in a calm, the machine, facing a 27-mile wind, started very slowly. Wilbur was able to stay with it till it lifted from the track after a forty-foot run. One of the Lifesaving men snapped the camera for us, taking a picture just as the machine had reached the end of the track and had risen to a height of about two feet. The slow forward speed of the machine over the ground is clearly shown in the picture by Wilbur’s attitude. He stayed along beside the machine without any effort.

The course of the flight up and down was exceedingly erratic, partly due to the irregularity of the air, and partly to lack of experience in handling this machine. The control of the front rudder was difficult on account of its being balanced too near the center. This gave it a tendency to turn itself when started; so that it turned too far on one side and then too far on the other. As a result the machine would rise suddenly to about ten feet, and then as suddenly dart for the ground. A sudden dart when a little over a hundred feet from the end of the track, or a little over 120 ft from the point at which it rose into the air, ended the flight. As the velocity of the wind was over 35 ft per second and the speed of the machine over the ground against this wind ten feet per second, the speed of the machine relative to the air was over 45 ft per second, and the length of the flight was equivalent to a flight of 540 feet made in calm air. This flight lasted only 12 seconds, but it was nevertheless the first in the history of the world in which a machine carrying a man had raised itself by its own power into the air in full flight, had sailed forward without reduction of speed and had finally landed at a point as high as that from which it started.”

—— Orville Wright writing about the first successful flight of a heavier-than-air flying machine from Kill Devil Hills, North Carolina, on 17 December, 1903.

1.1 Introduction

The history of aerospace engineering is full of firsts, such as the first balloon flight, the first airplane flight, the first helicopter flight, the first artificial satellite flight, the first manned spacecraft flight, and many others. In this first chapter, these many firsts are discussed in the context of the aerospace engineering involved in making these historic events happen. The first flight of a new vehicle design is a significant achievement and milestone. It is usually the culmination of years of hard work by many people, including engineers, technicians, managers, pilots, and other support personnel. First flights often represent firsts in the application of new aerospace engineering concepts or theories that are being validated by the actual flight.

As an aerospace engineer, you have the opportunity to contribute to the first flight of a new aircraft, a new spacecraft, or a new technology. Aerospace engineers are involved in all facets of the design, analysis, research, development, and testing of aerospace vehicles. This encompasses many different aerospace engineering discipline specialties, including aerodynamics, propulsion, performance, stability, control, structures, systems, and others. Several of these fundamental disciplines of aerospace engineering are introduced in this text. The aerospace engineer tests the vehicle, on the ground and in flight, to verify that it can perform as predicted and to improve its operating characteristics. Flight testing is usually the final test to be performed on the complete vehicle or system.

In many areas of engineering and technology, there is sometimes a perception that there is “nothing left to be done”, or that “there is nothing left to be invented”. The impressive successes of our aerospace past may appear, to some, to dim the prospects for future innovations. Aerospace engineers have indeed designed, built, and flown some of the most innovative, complex, and amazing machines known to humanity. However, there is still ample room for creativity and innovation in the design of aerospace vehicles, and opportunities for technological breakthroughs to make the skies and stars far more accessible. By the end of this textbook, you will have greatly increased your knowledge of aerospace engineering, but you will also be humbled by how much more there is to be discovered.

1.1.1 Organization of the Book

Aerospace engineering encompasses the fields of aeronautical and astronautical engineering. As a broad generalization, the aeronautical field tends to deal with vehicles that fly through the sensible atmosphere, that is, aircraft. Astronautics deals with vehicles that operate in the airless space environment, that is, spacecraft. Aerospace engineering is, in many ways, a merging of these two fields, and includes aircraft, spacecraft, and other vehicles that operate in both the air and space environments. In the coming sections, we get more precise with the definitions of the various types of aerospace vehicles, such as aircraft and spacecraft.

The material in the text is organized in an academic building-block fashion as shown in Figure 1.1. In Chapter 1, we start by defining and discussing some of the many different types of aircraft and spacecraft. Many first flights of these different types of aerospace vehicles are described, providing insights and perspectives into the development and evolution of aerospace engineering. The terms aircraft and spacecraft are clearly defined, along with definitions of the various parts, components, and assemblies that make up various examples of these types of vehicles. The reader also makes a literary “first flight” in a modern, supersonic jet airplane, which introduces many of the areas to be discussed in the coming chapters.

In Chapter 2, several introductory concepts in aerospace engineering and flight test are discussed. This chapter gives the reader some of the basic concepts and terminology, in aerospace engineering and flight test, from which to learn the material in the subsequent chapters. Some basic mathematical ideas, definitions, and concepts are reviewed, which starts to fill our engineering toolbox with the basic tools required to analyze and design aerospace vehicles. Basic aerospace engineering concepts, relating to the flight of aerospace vehicles, are introduced, including aircraft axis systems, free-body diagrams, the regimes of flight, and the flight envelope. Basic flight test concepts are introduced, including the different types of flight test, the flight test process, the players involved, and the use of flight test techniques.

The fundamental disciplines of aerodynamics and propulsion are discussed in Chapters 3 and 4, respectively. The study of aerodynamics, in Chapter 3, provides the theories and tools required to analyze the flow of air over aerospace vehicles, the flow that produces aerodynamic forces such as lift and drag. We discover how and why these aerodynamic forces are created, and how this affects the design of aerodynamic surfaces such as airfoils and wings. In studying propulsion in Chapter 4, we learn about the devices that generate the thrust force to propel aerospace vehicles both in the atmosphere and in space. We develop a deeper understanding of how thrust is produced, regardless of the type of machinery that is used.

The study of performance, in Chapter 5, builds upon an understanding of aerodynamics and propulsion, as shown graphically in Figure 1.1. Performance deals with the linear motion of the vehicle caused by the aerodynamic forces (lift and drag) and propulsive force (thrust) acting upon it. Performance seeks to determine how fast, how high, how far, and how long a vehicle can fly. In Chapter 6, the study of stability and control also builds upon the fundamental disciplines of aerodynamics and propulsion. Stability and control deals with the angular motion of the vehicle caused by the aerodynamic and propulsive moments acting on it. We investigate the vehicle’s stability when disturbed from its equilibrium condition and seek to understand the impacts of various vehicle configurations and geometries. We also look at the means by which the vehicle can be controlled throughout its flight regime.

Many examples of ground and flight testing are integrated throughout the text, in sections entitled Ground Test Techniques and Flight Test Techniques. The flight test techniques are described in a unique manner, by placing the reader “in the cockpit” of different aircraft as the test pilot or flight test engineer. The reader obtains an intimate knowledge of the engineering concepts, test techniques, and in-flight data collection by “flying” the flight test techniques. A collateral benefit of this approach is that the reader is familiarized with several different types of real aircraft.

1.1.2 FTT: Your Familiarization Flight

This is the first of many flight test techniques (FTTs) that are “flown” in the text. The FTT is a precise and standardized method, used to efficiently collect data during flight test, research, and evaluation of aerospace vehicles. The FTT process is discussed in more detail in a later section of this chapter.

This first FTT introduces you to aerospace engineering in an exciting way, by taking a flight in a supersonic jet aircraft. A flight test engineer (FTE) often flies a familiarization flight in an aircraft prior to performing test flights, especially if this is an aircraft that is new to the FTE. As the name implies, this flight serves to familiarize the FTE with the aircraft and the flight environment. The areas of familiarization usually include the aircraft’s performance, flying qualities, cockpit environment, avionics, or other special test equipment and instrumentation. The present FTT provides a general description of a familiarization flight, but the primary objective is to introduce you to a wide range of aerospace engineering and test concepts that are explored in later chapters. Your familiarization flight will raise many technical questions about aerospace engineering and flight test, and this provides motivation to seek answers in the chapters to come.

For your familiarization flight, you will be flying the McDonnell Douglas (now Boeing) F/A-18B Hornet supersonic jet aircraft, shown in Figure 1.2. The F/A-18B is a two-seat, twin-engine, supersonic fighter jet aircraft, designed for launching from and landing on an aircraft carrier. Almost all aerospace vehicles are designated with letters and numbers, which we will decipher in a later section. A three-view drawing of the F/A-18A is shown in Figure 1.3. You will get very familiar with these types of drawings of aerospace vehicles, where typically side, top, and front views of the vehicle are depicted. Selected specifications of the F-18 Hornet are given in Table 1.1. The chapters to come will help you understand all of the technical details in these specifications, such as what defines a “low bypass turbofan jet engine with an afterburner” or why wing area, maximum weights, or load factor limits are important.

Table 1.1 Selected specifications of the McDonnell Douglas F/A-18B Hornet.

Item	Specification
Primary function	All-weather, supersonic fighter/attack jet aircraft
Manufacturer	McDonnell Douglas Aircraft, St Louis, Missouri
First flight	18 November 1978
Crew	1 pilot+1 instructor pilot or flight test engineer
Power plant	2 × F404-GE-400 afterburning turbofan engine
Thrust, MIL (ea. engine)	10,700 lb (47,600 N), military power
Thrust, MAX (ea. engine)	17,700 lb (78,700 N), maximum afterburner
Empty weight	∼25,000 lb (11,300 kg)
Maximum takeoff weight	51,900 lb (23,500 kg)
Length	56 ft (17.1 m)
Height	15 ft 4 in (4.67 m)
Wingspan	37 ft 6 in (11.4 m)
Wing area	400 ft2 (37.2 m2)
Airfoil, wing root	NACA 65A005 modified
Airfoil, wingtip	NACA 65A003.5 modified
Maximum speed	1190 mph (1915 km/h), Mach 1.7+
Service ceiling	>50,000 ft (>15,240 m)
Load factor limits	+7.5 g, −3.0 g

Before you can go flying in an F-18, you need to be properly dressed. You don an olive-green flight suit, black flight boots, and an anti-G suit, an outer garment that fits snuggly over the lower half of your body. Inflatable bladders, sewn into the anti-G suit, inflate with pressurized air to prevent blood from pooling in your lower extremities, keeping the blood in your head, so that you do not lose consciousness when the aircraft is maneuvering at high load factors or g’s. You slip your arms into a parachute harness that buckles around your chest and both legs. You are wearing the harness for the parachute, but not the actual parachute, as you will buckle this harness into your ejection seat, which contains your emergency parachute in the headrest.

With your flight helmet, oxygen mask, and kneeboard, a small clipboard-type writing surface, in your helmet bag, you walk out to the airport ramp, where the jet is parked. As you walk up to the aircraft, you note its general configuration. The aircraft has a slender fuselage with a low-mounted, thin wing, aft-mounted horizontal tail, twin vertical tails, and tricycle landing gear, comprising two main wheels, extending from either side of the fuselage, and a fuselage nose wheel. You observe that the landing gear looks quite sturdy, designed for harsh aircraft carrier landings. The jet is powered by twin engines, with semicircular air inlets on each side of the fuselage and side-by-side exhaust nozzles at the aft end of the fuselage. The two aviators sit in a tandem configuration, beneath a long “bubble” canopy that is hinged behind the aft cockpit. Your test pilot will be seated in the front cockpit and you will be in the aft cockpit.

You approach the aircraft from its left side, next to the cockpit, as shown in Figure 1.4. Before you climb into the cockpit, you perform a walk-around of the jet to learn a little more about it. Underneath the left wing, near the fuselage, you look into the left engine inlet, which is a semicircular opening. This inlet feeds air to the turbofan jet engine. Later, we will learn about why the inlet is shaped in this way and how the air mass flow, which is ingested through the inlet, is related to the production of thrust. Looking underneath the fuselage, you see a large cylindrical fuel tank with pointed ends, hung underneath the centerline of the fuselage. Of course, you know that the fuel quantity carried aboard the aircraft dictates how far and how long the aircraft can fly. We will see that the range and endurance is a function of more than just the fuel quantity; it is also a function of key parameters related to the aerodynamics and propulsion of the vehicle. We will also learn about how to obtain range and endurance through flight testing.

You move towards the leading edge of the left wing. You observe that the wing is thin, with a somewhat sharp leading edge, and that the wing leading edge is swept backward. There is a large hinged flap surface at the inboard wing trailing edge. We will explore the aerodynamics of three-dimensional wings and their two-dimensional cross-sectional shapes, known as airfoils. We will learn why airfoils and wings are shaped differently for flight at different speeds, including why wings are swept back. We will discuss how hinged flaps increase the lift of a wing. Fundamentally, we will discuss how a wing produces aerodynamic lift, and will discuss the many ways of quantifying the lift and drag of an aircraft, through analysis, ground test, or flight test.

Now you are at the rear of the aircraft, looking at the two engine nozzles, as shown in Figure 1.5. The nozzles have interlocking metal petals that can expand and contract to change the nozzle exit area. We will examine how the flow properties change with area in subsonic and supersonic nozzle flows. We will learn how to calculate the velocity, pressure, and temperature of the gas flowing through the nozzle. You look down the afterburner of the jet engine, which appears to be an almost empty duct. We will discuss the various components of the jet engine, including the afterburner, and will explain their functions. The jet engine is an amazing engineering achievement. We will explore its beginnings, and the engineers who invented it. We also learn about how engines are tested in the ground and flight environments.

Coming around the right, aft end of the airplane, as shown in Figure 1.6, you look at the horizontal and vertical tail surfaces. We will learn why these surfaces are critical to the stability and control of the aircraft. We will see that the locations and sizes of these surfaces are important parameters in defining the aircraft’s stability in flight, and will also learn about the control forces associated with deflection of these surfaces in an air stream. We will discuss several different flight test techniques used to quantify an aircraft’s stability. Near the nose of the airplane, you notice several L-shaped tubes mounted on the lower side of the fuselage. We will learn about these Pitot tubes, which are used to measure the F-18’s airspeed, and will investigate how they work in subsonic and supersonic flight. We will also see that flight testing is required to calibrate these probes to obtain accurate airspeed information. You come to the aircraft nose, which has a pointed shape. We will explore the aerodynamics of two and three-dimensional bodies, such as this nose shape. We will also touch on the interesting phenomena that occur when these types of pointed shapes are at high angles of attack.

Returning to the left side of the fuselage, with your walk-around complete, you meet up with your test pilot. It is time to get into the airplane and go flying. The pilot climbs the ladder into the front cockpit and you follow, making your way into the back cockpit. You buckle your lap belt, plug in your G-suit hose, and connect the ejection seat shoulder belts to your harness. Next, you don your flight helmet, connect your oxygen mask hose, plug in your communications cable, and slip on your flying gloves. You strap your kneeboard on your right thigh so that you will be able to take some notes during your flight. Now that you are strapped in and have connected all of your gear, you have a chance to relax and look around. There is a center control stick, two rudder pedals at your feet, and two throttle levers by your left side. The instrument panel in front of you has three square display screens, surrounded by buttons, and an array of other circular, analog instruments (Figure 1.7).

You hear the test pilot in your helmet earphones, asking if you can hear him and if you are ready for engine start. You reply affirmatively, and a few seconds later, you hear the whir of the engines coming alive. After engine start, the canopy lowers, and the pilot performs various checks, including checks of the flight control system. After these checks, the pilot taxis the jet to the end of the runway. The pilot performs the pre-takeoff checks, then tells you to arm your ejection seat, and asks if you are ready for takeoff. You say you are ready to go. The pilot contacts the control tower and requests a takeoff clearance with an unrestricted climb. The tower grants both requests, and the pilot taxis the jet to the centerline of the runway.

The pilot pushes the throttles forward into full afterburner and the jet accelerates forward. In what seems like a very short distance, the F-18 is airborne. We will learn how to calculate the takeoff distance and define the parameters that affect this calculation. We will also learn how to measure the takeoff distance in flight test. The pilot keeps the jet low to the ground, continuing to gain airspeed, and then pulls the jet up to what seems like a near vertical climb. You feel pushed down heavily into your seat. Looking at the g-meter, you see that you are pulling about 4 g’s, making you feel four times heavier than your normal weight. We will see how the load factor affects the turn radius of this type of pull-up maneuver and we will calculate the radius of this vertical turn. We will discuss climb performance and define how to calculate the rate and angle of climb. We will also investigate climb performance from an energy perspective, where we account for kinetic and potential energies of the vehicle. We will make energy plots that define the performance capabilities of the aircraft, and we will discuss the flight test techniques used to quantify climb performance.

Looking at the altitude indication, you see that the numbers are increasing rapidly. At an altitude of about 14,000 ft (4270 m), the pilot rolls and pulls out of the vertical climb, so that you are upside down, and then rolls the aircraft upright to wings-level flight. Reducing the engine power, the pilot stabilizes the aircraft at a constant airspeed and altitude to let you catch your breath for a moment. Looking at the cockpit instruments, you see that you are at an airspeed of about 220 knots (253 mph, 407 km/h) and a Mach number of 0.6. We will discover that there are many different kinds of airspeeds and look at the reason for these different definitions. We will learn about Mach number, how it is defined, what it physically means, and why it is so important in high-speed aerodynamics. We will see that in this steady-state flight condition, there are four forces acting on the aircraft, which are in balance, and we will learn that this steady-state trim condition is an important starting point for most of the flight test techniques.

The pilot climbs the F-18 higher, leveling off at an altitude of 30,000 ft (9140 m). The airspeed indicates 350 knots (403 mph, 643.7 km/h), the outside air temperature (OAT) is a frigid −48^∘F (412^∘R, 228.7 K), and the Mach number reads about 0.6. We will learn about how the atmosphere changes, from sea level to high altitudes, and how this affects the calculations of aircraft performance. We will develop models of the atmosphere that will be used in our analyses. The pilot advances the throttles into full afterburner, and the F-18 accelerates in level flight. You watch the Mach indicator, waiting for it to indicate that you have broken the sound barrier and are flying at supersonic speed. We will discuss what is meant by the “sound barrier” and how it was “broken” for the first time. Looking out at the wing, you see something that looks like blurry light-and-dark lines or bands, dancing on the wing surface. You glance at the airspeed indication and it shows 530 knots (609.9 mph, 981.6 km/h). These are shock waves forming on the wing as the jet reaches transonic speeds. We will explain why these form on the wing and at what flight speeds. We will discuss the implications of these shock waves on the aerodynamics of the aircraft. We will learn that there are techniques to visualize these flow structures in flight.

The jet continues the level acceleration, and the Mach meter is indicating about Mach 0.96, when it jumps to Mach 1.1. We will explain the aerodynamic cause of this jump in the Mach indication. You have been on the ground and heard the sonic boom of a jet flying overhead at supersonic speed, yet you heard no sonic boom as your F-18 went supersonic. We will learn about sonic booms and some of the research that has been conducted to understand them. The F-18 continues to accelerate, reaching about Mach 1.3 in level flight. You are now flying at supersonic speed, traveling a distance of about one mile every four seconds. We will learn about high-speed supersonic flow and how it is fundamentally different from low-speed, subsonic flow. We will delve into discussions about even higher speed hypersonic flow, where the Mach number is greater than about five, and the flow physics is distinctly different.

The pilot pulls the throttles back, and the F-18 decelerates to subsonic speed. The pilot asks if you are ready to do some maneuvering. Of course, you say that you are more than ready. First, the pilot does some high-g, level turns, so that you can acclimatize to higher g-loadings. You successively fly level turns at 2g, then 4g, then 6g. With each successive turn, the high load factors push you down further into your seat. We will learn about level turn performance capabilities, the important parameters involved, and the associated flight test techniques. We will also learn about the flight envelope of the aircraft, as related to the airspeed and load factors that are within the aircraft’s capabilities.

Now, the pilot asks if you want to fly, so you grab hold of the control stick and get a “feel” for the F-18. We will discuss why the handling qualities of an aircraft are important and how they are evaluated. The pilot tells you to do some rolls, so you push the stick full over to the left and the jet rolls around the horizon in a blur. We will discuss the aircraft stability in all three of its axes, and determine what characteristics determine whether its motion is stable or unstable about these axes. The pilot tells you to pull back the throttles to slow the jet down so that you can do some low-speed flight. You pull the control stick back, raising the nose of the aircraft, and increasing the angle-of-attack. You continue slowly pulling the nose up and, at an angle-of-attack of about 20^∘, the aircraft starts to gently rock from side-to-side. Pulling back a little more and the wing rock increases and the nose wanders a bit from side to side. At about 25^∘ angle-of-attack, you cannot pull the stick back any further. Looking at the altimeter, you see that you are descending at a high rate. The pilot tells you to recover by returning the control stick to its center or neutral position. After you do this, the nose attitude decreases rapidly and the aircraft is back flying in level flight. We will learn about the aerodynamics associated with high angle-of-attack flight and stall. We will also discuss the aerodynamics and issues involved with aircraft spins. The various ground and flight test techniques to learn about stalls and spins will be covered.

It is time to head back to the airport. The pilot takes the flight controls, rolls the F-18 inverted, and then pulls down in a maneuver called a split-S. As the aircraft is coming through the vertical, you have a great view of the ground below, as shown in Figure 1.8. While this is a fun maneuver to lose altitude quickly, we will see that it can also be used to obtain aerodynamic data about the aircraft. The pilot enters the landing pattern, lowers the landing gear, and slows for the landing approach. Similar to takeoff performance, we will discuss the important parameters, associated with landing performance, and determine how to calculate the landing distance. We will see how the type of runway surface and other factors affects this distance. The F-18 touches down and rolls to a stop on the runway. You have had a successful familiarization flight during which we have identified many areas to be discussed and explored in the chapters ahead.

1.2 Aircraft

In the broadest sense, the term “aircraft” refers to all types of vehicles that fly within our Earth’s sensible atmosphere. The Federal Aviation Administration (FAA), in its Federal Aviation Regulations [6], defines an aircraft as “a device that is used or intended to be used for flight in the air”. Aircraft support their weight with the force derived from either static or dynamic sources. For example, a lighter-than-air balloon supports its weight with static buoyancy, while a heavier-than-air airplane generates aerodynamic lift, which balances its weight, due to the dynamic reaction of air flowing over its wings.

1.2.1 Classification of Aircraft

There are many different types of aircraft, and a wide variety of ways that one could classify these different types. We could classify the different types of aircraft based on their geometric configuration, the type of propulsion, the mission or function, or other factors. Perhaps, a reasonable first distinction that we can make is between aircraft that are lighter-than-air and those that are heavier-than-air. A classification of aircraft, based on this starting point, is shown in Figure 1.9.

Figure 1.9 Classification of different types of aircraft.

Lighter-than-air aircraft include airships and balloons. We can further subdivide heavier-than-aircraft into powered and unpowered aircraft, that is, aircraft with and without one or more propulsive devices or engines. Unpowered, heavier-than-air aircraft include gliders or sailplanes. Powered, heavier-than-air aircraft can be subdivided into airplanes, rotorcraft, and ornithopters, where the distinction between these different types of aircraft is based on their type of lift production. Airplanes have a fixed wing, which produces lift due to the air flowing over it. Rotorcraft encompass all heavier-than-air aircraft that generate lift from rotating wings or spinning rotor blades. Rotorcraft can be further divided into autogyro and helicopter. The autogyro has unpowered, free-spinning rotor blades, which require forward motion for lift production, whereas the helicopter has powered rotors that can produce lift even without forward speed. Ornithopters use flapping wings to generate both lift and thrust, similar to a bird. Many early would-be inventors of the first heavier-than-air airplane attempted to fly this type of flapping wing machine, but without success. We generally follow the classifications given in Figure 1.9 to describe aircraft in the following sections. We start our discussion of aircraft with the fixed-wing airplane.

1.2.2 The Airplane

Since most of us have grown up in a time where airplanes are commonplace, it is difficult to imagine that we do not know what an airplane is “supposed to look like”. However, if we were living in the late 1800s, prior to the first successful flight of a heavier-than-air machine, we would probably be influenced by nature, and think that airplane flight should mimic bird flight. Some of the early aviation enthusiasts took this to the extreme, attempting to construct flyable ornithopters. Other early aviation pioneers made careful observations of bird flight, trying to understand nature’s secrets about flight. There are now many variants on what an airplane looks like, but there are several common fundamental engineering aspects to heavier-than-air flight that have made it successful. We will see examples of this in the design and successful flight of the first airplane.

1.2.2.1 The First Airplane

At the beginning of this chapter, the iconic photograph of the first controlled, sustained flight of a heavier-than-air, powered airplane was presented. This first flight was the culmination of years of hard work by two brothers from Dayton, Ohio: Orville (1871–1948) and Wilbur (1867–1912) Wright. The brothers followed a logical and systematic approach in the design, construction, and flight test of their powered airplane. They critically reviewed much of the existing technical information and data relevant to aeronautical theory and aircraft design. In several important areas, the Wright brothers determined that the state-of-the-art information and data was not adequate or was incorrect, so they performed their own, independent analyses and tests to obtain what they needed. An example of this is the designs of the airfoil shapes for their wings and propellers, which were based on data that they collected using a wind tunnel of their own design. They also developed their own aircraft internal combustion engine, with the help of expert machinist, Charlie Taylor. The Wright brothers’ determination to ensure that their airplane design was based on sound technical data was fundamental to their success.

The Wright brothers were also methodical and systematic in their approach to flying and flight testing. Between 1900 and 1903, they performed extensive flight testing with gliders of their own design. Starting first with unmanned, kite-like gliders (Figure 1.10), they systematically progressed to manned glider flights (Figure 1.11). The Wright brothers designed, built, and flew their first manned glider at Kitty Hawk, North Carolina, in 1900 with disappointing results. They test flew another glider design in 1901, but this second manned glider also flew poorly. It was not until their third glider design in 1903 that the Wright brothers were satisfied with how the glider flew. These glider design iterations systematically improved the performance and flying qualities of their unpowered airplanes and these lessons learned were incorporated into their 1903 powered airplane design.

The glider flying had another very important purpose, in addition to collecting flight data to improve their designs. By flying these many glider flights, the Wright brothers were learning how to fly. They gained extensive piloting experience in how to control their aircraft in the new three-dimensional world of flying. They understood that not only must a successful heavier-than-air vehicle lift its own weight, but it must also be controllable. They designed their aircraft to be controllable by the pilot in all three axes, with independent control effectors in pitch, roll, and yaw. Their airplane design had an elevator for pitch control, a rudder for yaw control, and for roll control, they used a scheme of warping or twisting of the wings.

The Wright brothers spent a considerable amount of time observing the flight of birds, and in particular the flights of buzzards. Their observations of bird flight gave them valuable insights into how to control a flying vehicle. They observed that as the birds soared and turned, the shape of their wings changed. Realizing that this wing twisting or warping was critical to the roll control of the maneuvering birds, the Wright brothers incorporated the wing warping concept into their airplane designs, and finally into the design of the first successful heavier-than-air airplane. It is interesting to read the Wright brothers’ description of their invention of a heavier-than-air flying machine in their original patent, as shown below. Note, that they make particular mention of the stability and control aspects of their airplane.

Be it known that we, Orville Wright and Wilbur Wright, citizens of the United States, residing in the city of Dayton, county of Montgomery, and State of Ohio, have invented certain new and useful Improvements in Flying-Machines, of which the following is a specification. Our invention relates to that class of flying machines in which the weight is sustained by the reactions resulting when one or more aero planes are moved through the air edge-wise at a small angle of incidence, either by the application of mechanical power or by the utilization of the force of gravity. The objects of our invention are to provide means for maintaining or restoring the equilibrium or lateral balance of the apparatus, to provide means for guiding the machine both vertically and horizontally, and to provide a structure combining lightness, strength, convenience of construction, and certain other advantages which will hereinafter appear.

——- US patent 821,393, “Flying-Machine” Application filed March 23, 1903 Patent granted May 22, 1906

The Wright brothers’ successful, first powered airplane, the Flyer I, was a canard configuration biplane, with a forward-mounted, all-moving horizontal, biplane elevator and an aft-mounted, vertical, twin rudder. (The all-moving nature of the elevator is clearly seen in the photograph of the Flyer I’s first flight, shown at the beginning of the chapter.). The airplane structure was a spruce and ash wooden framework, covered with finely woven muslin cotton fabric. The wing bracing wires were 15-gauge bicycle spoke wire. The airplane had a single, four-cylinder, gasoline-fueled piston engine, capable of producing about 12 horsepower (8.9 kW). Less than half a gallon of gasoline fuel was carried onboard the airplane. There was no engine throttle, the pilot could only open or close the fuel line that supplied the engine. The engine drove two contra-rotating, pusher propellers through a chain-drive transmission system. The propellers rotated at an average speed of about 350 revolutions per minute (rpm). The 170 lb (77 kg) engine was mounted on the right wing. To counterbalance the engine weight, the pilot was placed on the left wing. Since the typical pilot weight of about 145 lb (66 kg) was less than the engine weight, the right wing was about 4′′ (10 cm) longer than the left.

Unusual by today’s standards, the pilot lay prone on his stomach, with his hips in a padded wooden cradle, facing towards the front-mounted elevator. The wing-warping roll control and rudder-deflection yaw control were interconnected, such that sliding of the hip cradle sideways caused the wings to warp and the rudders to deflect. A wooden lever in the pilot’s left hand controlled the aircraft pitch by changing both the angle of the elevator and the camber or shape of the elevator airfoil section. If the pilot pulled back on the lever, the elevator angle and camber were increased, thereby increasing its lift. If the pilot pushed the lever forward, the elevator angle and camber were decreased, resulting in less elevator lift. (Airfoil camber is discussed in Chapter 3.)

Table 1.2 Selected specifications of the 1903 Wright Flyer I

Item	Specification
Primary function	First heavier-than-air flying machine
Manufacturer	Orville and Wilbur Wright, Dayton, Ohio
First flight	17 December 1903
Crew	One pilot
Power plant	In-line, 4-cylinder, water-cooled piston engine
Engine power	12 hp (8.9 kW) at 1020 rpm
Fuel capacity	0.2 gal (0.65 l) of gasoline
Propellers	Two 2-bladed, 8 ft (2.4 m) diameter
Empty weight	605 lb (274 kg)
Gross weight	750 lb (341 kg)
Length	21 ft 1 in (6.43 m)
Wingspan	40 ft 4 in (12.3 m)
Wing area	510 ft2 (47.4 m2) (upper and lower wings)
Wing loading	1.47 lb/ft2 (7.18 kgf /m2)
Maximum speed	30 mph (48.3 km/h)
Stall speed	22 mph (35 km/h)
Ceiling	30 ft (9.0 m)

The Flyer I used a 60 ft (18.3 m) launch rail for takeoff. The aircraft was restrained, sitting on the rail, until the pilot was ready for takeoff. He then released the restraining rope and the aircraft started its takeoff roll along the rail, riding on two modified bicycle wheel hubs. The aircraft had wooden skids for landing on the sandy ground. The Flyer I had a maximum airspeed of about 30 mph (48 km/h) and a maximum altitude of about 30 ft (9.0 m). Selected specifications of the Wright Flyer I are given in Table 1.2.

After winning a coin toss, Wilbur Wright attempted the first flight of the Flyer I on 14 December 1903. The launch rail was placed on an incline, giving the aircraft a downhill, gravity-assisted takeoff roll. Taking off in a light wind, Wilbur pulled the Flyer I off the launch rail, but almost immediately stalled the aircraft, causing it to return to earth in about three seconds. This “powered hop”, with a gravity-assisted takeoff, could not be considered a first, controlled flight of a heavier-than-air airplane. The aircraft sustained some minor damage, which took three days to repair.

Table 1.3 Wright brothers’ flights of 14 and 17 December 1903.

Flight No.	Date	Flight Time	Ground Distance	Pilot
1	14 Dec	3 sec	112 ft (34.1 m)	Wilbur
2	17 Dec	12 sec	120 ft (36.6 m)	Orville
3	17 Dec	13 sec	175 ft (53.3 m)	Wilbur
4	17 Dec	15 sec	200 ft (61.0 m)	Orville
2	17 Dec	59 sec	852 ft (260 m)	Wilbur

On 17 December 1903, it was Orville’s turn to attempt the first flight. Since the winds were blowing at more than 20 mph (32.2 km/h), the launch rail was placed on level ground and pointed into the wind. At 10:35 am, Orville Wright made the first controlled, powered flight in a heavier-than-air airplane, with the flight lasting about 12 seconds, landing 120 ft (37 m) from the point of take-off. The Wright brothers made four flights that day, with the final flight lasting almost a full minute. A summary of the initial flights of the Flyer I on 14 and 17 December 1903 is given in Table 1.3. After the successful flights of 17 December, the Wright brothers sent a telegram to their father, telling him about their accomplishment (Figure 1.12). Soon after the fourth landing, a gust of wind picked up the Flyer I and it tumbled end-over-end across the rough and sandy terrain. The Flyer I was destroyed and never flew again. Quite fittingly, a part of the Flyer I would soar again, when a piece of its wing fabric and a piece of wood from one of its propellers were carried inside a spacesuit pocket of Neil Armstrong when he stepped onto the surface of the moon on 20 July 1969.

1.2.2.2 Parts of an Airplane

In this section, the major parts of a fixed-wing airplane are described. There are many different aircraft configurations, as discussed in the next section. For our present purpose, we reference a somewhat standard aircraft configuration, with a single fuselage, a single wing attached to the fuselage, podded engines mounted underneath the wings, and horizontal and vertical tail surfaces mounted to the fuselage, aft of the wing, as shown in Figure 1.13. This configuration is in wide use today for commercial, military, and general aviation applications. The following discussion is generally applicable to other aircraft configurations, discussed in the next section. The major components of an airplane are the fuselage, main wing, empennage, engines, and landing gear. The fuselage contains the cockpit, passenger, and cargo compartments. The main wing extends from either side of the fuselage and often has integral fuel tanks within it. The empennage is the tail area of the airplane, comprising the horizontal and vertical stabilizers and the associated moving control surfaces: the elevators and rudders, respectively. If the airplane is a powered airplane, there is one or more wing or fuselage-mounted engines. The power plant may be a reciprocating-engine–propeller combination or a jet engine. The engines may be podded, with the engine pods or nacelles mounted above or below the wings or on the sides of the fuselage. The engines may be buried in the fuselage, with an inlet or intake opening towards the front of the fuselage and exhaust openings at the aft end. The landing gear is composed of wheels with tires attached to struts, extending from the fuselage, wings, or engine pods. Often, the landing gear configuration consists of two main gear assemblies under the wings and a nose gear at the front of the fuselage, although other configurations are possible.

The elevators and the rudder on the empennage, and the ailerons on the wings comprise the primary flight control system. Each of these control system surfaces provides an incremental aerodynamic force that creates a moment to rotate the aircraft about its center of gravity (CG) in the desired direction. As shown in Figure 1.14, these control surfaces enable rotation of the airplane in three dimensions, where the elevator, ailerons, and rudder provide pitch, roll, and yaw rotations, respectively. Elevators are flap-like devices located at the trailing edges of the horizontal stabilizers. Some aircraft, typically military fighter aircraft, have all-moving horizontal stabilizers, called stabilators or stabs, instead of a combination of stabilizers and elevators.

The ailerons on the left and right wings deflect in opposite directions; that is, when the right aileron deflects upward, the left aileron deflects downward and vice versa. The downward deflected aileron results in additional lift on one side of the wing, while the upward deflected aileron results in decreased lift on the other side of the wing, creating the rolling moment. The additional lift produced by the downward deflected aileron also results in additional drag. This additional drag produces a yawing moment in a direction opposite or adverse to the desired direction of roll, and therefore is called adverse yaw. To counter this adverse yaw, the rudder is deflected to produce an opposing yawing moment, resulting in what is termed a coordinated turn.

High-speed aircraft also have secondary or auxiliary flight controls, which include devices on the wings called flaps, slats, and spoilers. Flaps are high-lift devices, located at the inboard wing trailing edge sections. When deflected or lowered, the flaps provide increased lift at lower airspeeds, enabling steeper landing approach glide paths without an increase in the approach airspeed. Slats, which are extended from the wing leading edge, are also high-lift devices that increase the wing lift at low speeds. There are several different types of wing flaps and slats, of varying mechanical complexity and aerodynamic effectiveness, which are discussed in Chapter 3. Spoilers, which extend upward from the wing upper surface, reduce or “spoil” the lift, to assist the airplane in slowing down and descending. They are also deployed after landing, to “dump” the wing lift and transfer the airplane’s weight from the wings to the landing gear, which improves braking. Spoilers can also be used as a means of airplane roll control, when deployed differentially (extending from one wing and not the other).

1.2.2.3 Airplane Configurations

Airplanes come in all shapes and sizes. Usually, the configuration of an airplane is driven, or at least strongly influenced, by its mission requirements. For example, a commercial airliner has a large fuselage cabin area due to the requirement to transport passengers. A military fighter jet may have a highly swept wing to allow it to fly supersonically (we will see why this is so in Chapter 3.). A utility airplane that must be able to take off and land on snow might have skis for landing gear. These are a few examples of the types of aircraft configurations that may be driven by the mission requirements. It may be possible to satisfy the mission requirements with a variety of design solutions, limited only by the imagination and creativity of the airplane designer, and influenced by advancements in technology. A listing of possible airplane configurations for different components is given in Table 1.4. This listing is not meant to be exhaustive, but rather to illustrate the many possibilities in airplane designs. We briefly discuss several of these configuration options, citing real airplane design examples along the way, to better appreciate the possibilities.

Table 1.4 Sampling of possible airplane configurations.

Area	Possible airplane configurations
Fuselage type	Single fuselage, twin fuselage, twin boom
Number of wings	Monoplane, biplane, triplane
Wing location	Low-wing, mid-wing, high-wing
Wing type	Straight, aft-swept, forward-swept
Horizontal tail	Aft-mounted, forward-mounted (canard), tailless
Vertical tail	Single or twin vertical fin
Propulsion	Reciprocating piston, gas turbine (jet), rocket
Number of engines	Single or multi-engine
Engine(s) location	Above or below wing, fuselage side-mounted, internal
Landing gear type	Wheel, skid, float, ski
Landing gear	Tricycle, tail wheel, bicycle

It is quite common for airplanes to have a single fuselage, whereas twin fuselage airplane designs are somewhat rare. A twin fuselage aircraft may offer some advantages for some applications. The twin fuselage airplane may have reduced design and development time and costs, if an existing single-fuselage airplane can be used as a baseline. This was the case for the North American F-82 Twin Mustang, developed near the end of World War II (see photo in Figure 1.15, drawing in Figure 1.16). Based on the single-fuselage XP-51 Mustang (see Figure 3.72), the F-82 was designed as a very long range fighter escort aircraft, with a nominal range of over 2000 miles (3200 km). The F-82 twin fuselages were from the single-fuselage P-51, which was stretched by 57′′ (1.45 m), allowing for the installation of additional fuel tanks. Both cockpits were retained from the single-fuselage airplanes, so that a pilot in either cockpit could fly the airplane, which was advantageous for very long duration flights. The F-82 saw combat during the Korean War, being the first fighter to shoot down a North Korean aircraft. The F-82 Twin Mustang still holds the record for the longest, non-stop flight by a propeller-driven fighter airplane, when it flew from Hawaii to New York, a distance of 5051 miles (8128 km), in 14 hours 32 minutes on 27 February 1947.

The twin fuselage configuration has found an application for airplanes that carry a large, centerline payload, such as the Virgin Galactic White Knight Two, which carries the Spaceship Two (see photo in Figure 1.17, drawing in Figure 1.18). The twin-fuselage White Knight Two is the first stage of a two-stage space launch system, with the Spaceship Two being the second stage. Only the right fuselage of the White Knight Two is configured to carry pilots and passengers, but, conceivably, the left fuselage could be designed to do so also. All three fuselages, the two White Knight Two and single Spaceship Two fuselages, are similar in design. This is an interesting design philosophy, whereby the White Knight Two is configured to be flown like the Spaceship Two, with a similar cockpit arrangement, equipment, and pilot sight picture. This allows for training and proficiency flying in the White Knight Two airplane which simulates, at least, the glide, approach, and landing phases of the Spaceship Two.

Similar to the twin fuselage configuration, an airplane may have twin longitudinal booms that extend from the main wing to the tail. The twin boom configuration may be advantageous for power plant integration or for ease of access to aft fuselage cargo doors. The twin booms also provide additional volume for carrying fuel or equipment. The Cessna 337 Skymaster is an example of a twin-boom, twin-engine airplane that has been used as a general aviation and military utility aircraft (see photo in Figure 1.19, drawing in Figure 1.20). The twin booms allow both engines to be mounted on the fuselage centerline, with one in a puller or tractor configuration (forward-mounted engine) and the other in a pusher configuration (aft-mounted engine). An advantage of having both engines along the airplane centerline, versus mounted on either side of the fuselage, is that lateral-directional control is not degraded in the event of an engine failure, i.e. there is no yawing tendency with the power loss of one engine.

An airplane with tailwheel landing gear, also sometimes called conventional landing gear, is the Extra 300 airplane (see photo in Figure 1.21, drawing in Figure 1.22). The Extra 300 is a two-place, single engine, high-performance, aerobatic, general aviation airplane with an all-composite, carbon fiber main wing. The tailwheel configuration is needed to provide ground clearance for the large-diameter propeller at the front of the airplane. The wing is attached to the middle of the fuselage, hence, it is termed a mid-wing configuration. The North American Twin Mustang is a low-wing monoplane and the Cessna Skymaster is a high-wing monoplane, where the main wing is attached to the bottom and top of the fuselage, respectively.

An example of a forward-swept wing configuration is the Grumman X-29 experimental, supersonic research aircraft (see photo in Figure 1.23, drawing in Figure 1.24). The X-29 investigated forward-swept wing maneuverability and other advanced technologies. Two X-29 aircraft were built, with test flights conducted by NASA and the US Air Force. The single-seat X-29 had a forward-swept main wing and trapezoidal-shaped canard surfaces forward of the wing. Forward-swept wings are susceptible to divergent aero elastic twisting, so the X-29 wing was fabricated with advanced composite materials, which could provide the required structural stiffness with low weight. The forward-swept wing X-29 was inherently unstable, requiring a state-of-the-art “fly-by-wire” flight control system, where the aircraft was constantly flown and stabilized by computers. A single General Electric F404 turbofan jet engine powered the X-29, enabling a top speed of Mach 1.8 at 33,000 ft (10,000 m). The first flight of the X-29 was on 14 December 1984. The two X-29 aircraft completed 422 research test flights over a period from 1984 to 1991.

Most of the airplane configurations that we have discussed so far are single-wing or monoplane configurations. An example of an airplane with two main wings, a biplane, is the Russian Antonov An-2 Colt (see photo in Figure 1.25, drawing in Figure 1.26). The two wings need not have the same dimensions. In fact, a biplane’s wings can differ in size, airfoil shape, wing sweep, or other characteristics. The An-2 is a large, rugged, single-engine aircraft designed to perform a variety of utility asks such as cargo hauling, crop dusting, water bombing (for fighting forest fires), parachute drop, glider towing, or military troop or civilian passenger transport. Designed by the Antonov Design Bureau, Kiev, Ukraine in 1946, the An-2 was produced for the next 45 years. Because of its sturdy construction, relatively simple systems, low speed capabilities, and large payload capacity, the An-2 has become a popular “bush” plane for flying people and cargo in and out of remote, unimproved areas. Known as a short takeoff and landing, or STOL, airplane, the An-2 can takeoff in less than about 600 ft (180 m) and, due to its extremely low stall speed of less than 30 mph (48 km/h), it needs only about 700 ft (210 m) to land. The An-2 shown in Figure 1.25 has conventional landing gear, but with skis for operation on snow-covered terrain replacing the tires.

All of the airplane configurations that we have discussed so far have distinct fuselage, wing, and tail components. The flying wing is a tailless airplane configuration, where the fuselage and wing are blended together. The flying wing concept is not new. Flying wing prototype aircraft were built and flown as early as the 1940s. Several flying wing designs were also built and flown in the early 20th century. The Northrop B-2 Spirit “stealth bomber” is a modern example of a flying wing airplane (see photo in Figure 1.27, drawing in Figure 1.28). Its two jet engines are “buried” in the blended wing-fuselage to mask their heat signature, enhancing its stealth capability. While there are significant aerodynamic advantages, especially in terms of reduced drag, for a tailless flying wing configuration, the stability and control issues require some special considerations. The advent of “fly-by-wire” flight control technology has made these design issues much easier to manage. We discuss the interesting stability and control considerations of flying wings further in Chapter 6.

1.2.3 Rotorcraft: the Helicopter

Thus far, we have discussed only fixed-wing aircraft. We now discuss rotary-wing aircraft or rotorcraft, where the lift-producing surfaces are rotating. The rotating wings, more properly called rotor blades, are attached to the rotor hub at the top of a rotor mast above the aircraft. The rotor blades, hub, and mast collectively are simply called the rotor. Rotorcraft include helicopters and autogyros. Helicopters are heavier-than-air flying machines that can take off and land vertically, translate in any direction, including backwards, and remain stationary in the air or hover. They have engine-driven rotor blades that produce both lift and thrust. The lift produced does not depend on the forward speed, so the helicopter can take off and land with zero forward velocity. The rotor can still produce lift if the engine is not running, as long as there is forward speed to keep the rotor blades spinning or auto-rotating. In this manner, the helicopter can glide like a fixed-wing airplane.

Two models of helicopters are shown in Figure 1.29, the Sikorsky UH-60 Black Hawk helicopter and the Bell OH-58 Kiowa light helicopter. The UH-60 is a twin-engine, single-rotor, 4-bladed military helicopter, designed for utility and transport operations. It carries a crew of two, and up to 11 passengers. The rotor of the UH-60 has a diameter of 53 ft 8 in (16.36 m). The UH-60 has a cruise speed of about 170 mph (294 km/h) and can climb to a maximum altitude of about 20,000 ft (6100 m). A three-view drawing of the UH-60 is shown in Figure 1.30. The Bell OH-58 Kiowa is a single-rotor, 2-bladed, military helicopter, designed for light utility and transport. The Kiowa is the military version of the popular Model 206A Jet Ranger civilian helicopter. The OH-58 carries a crew of one or two pilots with the civilian version capable of carrying up to four passengers. The rotor diameter of the OH-58 is 35 ft (10.7 m). The OH-58 has a cruise speed of 127 mph (204 km/h) and a maximum ceiling of about 15,000 ft (4600 m).

Autogyros, also known as gyrocopters or gyroplanes, have unpowered, free-spinning rotor blades that require forward motion to produce lift. Thrust is provided by an independent engine–propeller combination mounted in the fuselage as in a fixed-wing airplane. A short, fixed wing attached to the fuselage may also generate lift. The autogyro has many attributes of a helicopter, but since it requires forward motion to generate lift, it cannot take off and land vertically, fly backwards, or hover in still air. For completeness, we make the distinction between helicopters and autogyros, but our discussion focuses on the helicopter, as it is the predominant rotorcraft in use today.

1.2.3.1 The First Rotorcraft

Early inspiration for the design of rotorcraft may have come from nature. There are several examples of rotating winged seeds in nature that glide through the air as a means of dispersion. These flying seeds or samaras (a fruit with a wing) have been the interest of past aeronautical enthusiasts or inventors and current aeronautical engineers. In 1808, aeronautical pioneer, Sir George Cayley (1773–1857) wrote about the sycamore seed, as follows.

I was much struck with the beautiful contrivance of the chat of the sycamore tree. It is an oval seed furnished with one thin wing, which one would at first imagine would not impede its fall but only guide the seed downward, like the feathers of an arrow. But it is so formed and balanced that it no sooner is blown from the tree than it instantly creates a rotative motion preserving the seed for the center, and the …wing keeps it nearly horizontal, meeting the air in a very small angle like the bird’s wing.

The aerodynamics of this natural rotating wing has been studied extensively, including through the application of modern computational techniques, attempting to unlock the secrets of another example of nature’s optimization of flight.

The notion of a flying, rotating wing dates back to an ancient Chinese rotating toy, which was essentially a feather acting as a propeller, attached to the end of a stick. By applying rotation to the stick between the palms of one’s hands and releasing the toy, the rotating feather propeller would generate lift, making the toy fly for a short time. In 1483, Leonardo da Vinci conceived of a human-carrying rotorcraft he called an “aerial screw”. The da Vinci aerial screw concept did not address several critical issues in the design of a practical rotorcraft, which would not be solved for many centuries.

One of these issues was the development of a propulsion system, to rotate the blades with sufficient power and yet light enough in weight to lift the rotorcraft into the air. This propulsion issue was shared by the designers of heavier-than-air fixed-wing airplanes and would not be solved until the early 20th century, with the advent of the internal combustion engine. Another issue, unique to rotary-wing aircraft, had to do with the reaction torque developed by a rotating wing. The torque imparted by the engine to the rotor blade shaft also results in a reaction torque, which tends to want to rotate the vehicle in the opposite direction of the blade motion. This reaction-torque must be countered by some means, so that the vehicle does not rotate when the rotor blades are spinning. Other issues to be solved included the high vibration environment due to the large, spinning rotor, which can lead to mechanical failure and structural metal fatigue. Many of these issues are still being actively worked on today to improve helicopter designs.

During the early 20th century, there were many attempts at building and flying a vehicle capable of vertical flight. On 13 November 1907, about four years after the Wright brothers’ first successful flight of a heavier-than-air fixed-wing airplane, a French bicycle maker, Paul Cornu (1881–1944), flew a helicopter of his own design to a vertical height of 1 ft (30 cm) and hovered for 20 seconds, making this the first free flight of a heavier-than-air rotorcraft. Cornu’s helicopter had two 20 ft (6.1 m) diameter rotors with large low aspect ratio blades mounted on spinning spoked wheels, as shown in Figure 1.31. The opposite rotation of the rotors, located at opposite ends of the vehicle, served to counter the reaction-torque. A 24 horsepower, gasoline-fueled internal combustion engine, powered the rotors. In the several flights of the Cornu helicopter, it achieved a maximum vertical height of only about 6 ft (1.8 m), never rising above the region of aerodynamic ground effect, where there is increased lift and decreased drag. (Aerodynamic ground effect is discussed in Chapter 3.).

1.2.3.2 The Helicopter

The major components of a typical, modern helicopter with a single main rotor and anti-torque tail rotor are shown in Figure 1.34. Most, if not all, of the major components are attached to or contained within the structural airframe, including the cockpit, passenger or cargo cabin, engine, fuel tanks, transmission, and landing gear. The landing gear may be skids, fixed or retractable wheels, or amphibious floats. The power plant may be an internal combustion engine or a turbo shaft engine. There may be a single engine or dual engines for additional power and redundancy. The main rotor, comprising the blades, hub, and mast, and the tail rotor are connected to the engine through the transmission, where gearboxes reduce the engine’s rotational speed, allowing them to rotate at the required lower speed.

Helicopter main rotor systems are usually of a single or dual rotor configuration. As we have discussed, the single rotor configuration requires an anti-torque mechanism, such as a tail rotor. In a dual rotor system, the rotors spin in opposite directions, which cancels the rotor torque. The Boeing CH-47 Chinook, shown in Figure 1.35, is an example of a twin engine, heavy-lift helicopter with dual tandem rotors.

The main rotor blades are attached to the top of the rotor mast at the rotor hub. A rotor system, whether single or dual, is classified as a fully articulated, semi-rigid, or rigid rotor system, based on the method of attachment of the blades to the hub and the way that the blades move relative to the rotor plane of rotation. Of the three rotor systems, a fully articulated rotor system has the most degrees of freedom for blade movement. With this system, each rotor blade can move independently in three directions relative to the plane of rotation: up or down, called blade flap, and fore or aft, called blade lead or lag, respectively, and in rotation about the blade span wise axis, that is, a rotation that changes the blade pitch angle, called blade feathering. The blades are attached to the hub using three independent mechanical hinges, appropriately called the flapping hinge, the lead/lag hinge, and the feathering hinge. Fully articulated rotor systems are used on helicopters with more than two main rotor blades.

Blade flapping and lead/lag motion is needed to balance the unequal lift being produced across the rotor disk. In forward flight, the rotation of the blades results in an increase in lift for the rotor blade that is advancing into the relative wind and a decrease in lift for the retreating blade. Blade feathering controls the amount of lift that is produced by changing the blade pitch or angle-of-attack. Increasing or decreasing the blade pitch increases or decreases the lift, respectively.

With the semi-rigid rotor system, the rotor blades have two degrees of motion relative to the rotor plane of motion, flapping and feathering. The rotor blades are rigidly attached to the rotor hub, but the hub attachment to the mast is such that it can have a see-saw or teetering motion relative to the plane of rotation. This teetering motion allows the rotor blades to flap, but since the blades are rigidly attached to the hub, the blades on either side of the hub flap as a unit. This means that for a typical two-blade semi-rigid rotor system, when the blade on one side goes down, the blade on the opposite side goes up. Blade feathering is the same as in the fully articulated system, using a feathering hinge. Semi-rigid rotor systems are usually found on helicopters with two main rotor blades.

In the rigid rotor system, the rotor blades are rigidly attached to the hub and the hub is rigidly attached to the mast, such that the blades have a single degree of motion relative to the rotor plane of motion, that of feathering. Mechanically, the rigid system is much simpler than the other systems, since there are no flapping and lead/lag hinges and mechanisms. Any aerodynamically induced flapping and lead/lag motions of the blades must be absorbed by the blades and hub, making the structural design of these components more complex. A rigid rotor system may also have higher vibration characteristics than the other types of systems.

Returning to the single main rotor configuration, let us investigate other types of anti-torque devices. In addition to the conventional tail rotor, other types of anti-torque devices may be used, such as a Fenestron or NOTAR® system. The Fenestron design, also called a fantail, is essentially a tail rotor with multiple blades shrouded within a circular duct. While a conventional tail rotor may have two to five rotor blades, a fantail may have as many as 8–13 blades. The fantail blades are also shorter in length or span, and spin at a higher rotational speed than conventional tail rotor blades. The shrouded fantail acts like a ducted fan, which is more aerodynamically efficient than an exposed tail rotor. Vibration and noise are also reduced with the fantail. The shrouding has some safety advantages, protecting the rotor from striking foreign objects in flight, such as trees or power lines, and reducing risk to personnel on the ground. A disadvantage of the fantail is the added weight due to the structure around the rotor.

NOTAR® is an acronym for NO TAil Rotor. The NOTAR® system is based on a combination of an aerodynamic phenomenon, known as the Coanda effect, and direct jet thrust. A fan, located at the forward end of the tail boom, produces a low pressure, high volume flow of ambient air that is expelled through two longitudinal slots on the right side of the tail boom. These horizontal air jets create a low pressure area that causes the downwash flow from the main rotor to curve around the circular cross-section of the boom. This circulation control around the boom, created by the air jets, is known as the Coanda effect. The accelerated flow around the right side of the tail boom results in an aerodynamic lift force in a direction that counteracts the main rotor torque. In hovering flight, this circulation control system provides up to 60% of the required anti-torque. Additional anti-torque is provided by a rotating, direct jet thruster that is fed by the fan air in the boom. Vertical stabilizers provide additional directional control in forward flight. Advantages of the NOTAR® system include the elimination of tail rotor mechanisms and transmissions, and the safety benefit of not having a tail rotor with regards to tail strike. Several desirable features of rotary-wing and fixed-wing aircraft are brought together in the tilt-rotor aircraft, such as the Boeing V-22 Osprey (Figure 1.36). The tilt-rotor aircraft combines the rotorcraft capabilities of vertical takeoff, hover, and landing with the benefits of a fixed-wing aircraft, such as improved speed, range, and fuel efficiency, as compared with a pure rotorcraft. The tilt-rotor has two counter-rotating main rotors or propellers that are mounted on engine nacelles at the ends of a short wing. The nacelles can be rotated in flight between horizontal and vertical positions. With the nacelles in their vertical position, the tilt-rotor can operate like a helicopter with two counter-rotating main rotors. With the nacelles in the horizontal position, the tilt-rotor flies like a fixed-wing, twin-engine airplane with two large propellers. As can be seen in Figure 1.36, the 38 ft (11.6 m) diameter, rotating blades are a compromise between a helicopter and an airplane. With a cruising speed of about 240 knots (444 km/h), a maximum altitude of about 25,000 ft (7600 m), and a capability to takeoff vertically at a weight of about 53,000 pounds (24,040 kg), the V-22 tilt-rotor combines the benefits of rotary and fixed-wing aircraft.

1.2.4 Lighter-Than-Air Aircraft: Balloon and Airship

As the name implies, lighter-than-air vehicles are aircraft that utilize gases that are less dense than atmospheric air. The gas may be less dense because it is heated, as in a hot air balloon, or because it has an inherently lower density than air, such as helium or hydrogen in a gas balloon or airship. Lighter-than-air aircraft obtain their lift primarily from buoyancy, rather than from aerodynamic lift. We discuss two types of lighter-than-air aircraft, the balloon and the airship. The distinction between a balloon and an airship has to do with the ability to propel and steer the vehicle. A balloon does not have a propulsion system, while the airship has a means of propulsion, and is steerable. Buoyancy is based on Archimedes’ principle, which states that an object, submerged in a fluid, is acted upon by a buoyant force with a magnitude equal to the weight of the fluid displaced by the object, and in a direction that is opposite to the weight of the object. The fluid can be a liquid or a gas, so Archimedes’ principle is applicable to a ship or submarine in the ocean or a balloon or airship in the air. Assuming that the fluid is air, the buoyancy force, F_b, can be written as

F_{b}=W_{a}=m_{a}g=p{g}V{g} (1.1)

where W_a, m_a, and 𝜌_a are the weight, mass, and density, respectively, of the air displaced by the object of volume, V, and g is the acceleration due to gravity. Now imagine that we have a hollow object, with a volume V, which we can fill with a substance of density, 𝜌_g. The weight of the substance in the object is given by

W_{g}=p{g}V{g} (1.2)

(For simplicity, we ignore the weight of the hollow object that contains the substance of density 𝜌_g.)

If we fill the object with a substance that has the same density as air, 𝜌_g = 𝜌_a, the weight of the object equals the buoyancy force, and the object remains stationary in the air, as shown in Figure 1.37a. If we fill the object with a substance that has a density greater than air, 𝜌_g > 𝜌_a, the object’s weight is greater than the buoyancy force and the object sinks. If we fill the object with a substance that has a density less than air, 𝜌_g < 𝜌_a, the object’s weight is less than the buoyancy force and the object rises. As common sense would dictate, if we fill the hollow object with lead, it sinks, and if we fill it with helium or hydrogen, the object rises. We could also fill the object with air, at a higher temperature than the external, ambient air, so that the air inside the object is at a lower density. This then is the fundamental physics behind the buoyancy of the balloon and airship.

1.2.4.1 The First Balloon

Balloons are perhaps the earliest form of manned flying vehicles. The first recorded, manned flight of a hot air balloon occurred on November 21, 1783 in Paris, France. The balloon, with aeronauts Jean Francis Pilatre de Rozier and the Marquis d’Arlandes onboard (a person who operates or travels in a balloon or airship is called an aeronaut), flew for about 25 minutes over the city of Paris, rising to an altitude of about 3000 ft (914 m) and covering a distance of about 5 miles (8.1 km). This was the first free flight by mankind in an aerial vehicle. The balloon was built by Joseph and Etienne Montgolfier of France, who were to play a major role in the future development of balloon flight. A firebox, suspended underneath an opening at the bottom of the balloon, held a fire that filled the balloon with hot air. The balloon aeronauts stood on a platform, encircling the bottom of the balloon, from which they could add fuel to and tend the fire in the firebox.

With a background in paper manufacturing, the Montgolfier brothers were supposedly inspired by seeing scraps of paper, in their paper mill, being lifted aloft by smoke from a fire. Based on these observations, they believed that the smoke was a new, undiscovered gas that was less dense than air, which they dubbed “Montgolfier gas”. They believed that a thicker smoke contained more of this “Montgolfier gas”, so they sometimes burned unusual materials, such as rotted meat and shoes, to produce as thick a smoke as possible for their balloons. They did not realize that the smoke was simply heated air and was therefore less dense than unheated air. The brothers used a trial-and-error method, rather than one based on an understanding of the physics, in developing their balloons.

An aspect of the Montgolfier brothers’ balloon development, which was insightful and may have contributed to their success, was their incremental design and flight test approach. They started with the flight of a smaller scale, 10 m (32.8 ft) diameter, unmanned hot air balloon that was tethered to the ground, and built up to flights of larger, manned balloons. The Montgolfier brothers’ flight test approach was also admirable from a risk reduction standpoint. Prior to risking a balloon flight with people onboard, they flew a balloon carrying three farm animals, a sheep (aptly named Montauciel, French for “climb to the heavens”), a duck, and a rooster, to assess the effects of balloon flight on living creatures. There was logic to their selection of these three particular animals. The sheep was thought to have a physiology that was similar to a human being, thus it was selected to assess the physiological effects of altitude. Since the duck was capable of flight at the balloon altitudes, it was used to assess any non-physiological effects of balloon flight. The rooster was a non-flight capable bird, so it was used to assess altitude effects in comparison with the duck. On 9 September 1783, the sheep, duck, and rooster made history as the first living creatures to fly in a balloon. Their balloon flight lasted about 8 minutes, ascending to an altitude of about 1500 ft (460 m) and landing safely about 2 miles (3.2 km) from their launch point.

The next incremental step was the flight of a 75 ft (22.9 m) tall, 55 ft (16.8 m) diameter, tethered balloon with a man onboard. On October 15, 1783, Etienne Montgolfier was the first person to ascend in a tethered balloon followed, later that day, by Jean Francis Pilatre de Rozier, who rode the tethered balloon to a height of about 80 ft (24.4 m), the length of the tethered line attached to the balloon. Just a little over a month later, the first free flight of a balloon was completed by de Rozier and the Marquis d’Arlandes in a Montgolfier balloon.

Ten days after the first manned hot air balloon flight, aeronauts Jacques Alexander Charles and Nicholas Louis Robert flew the first manned flight of a gas balloon on 1 December 1783, also in Paris, France. Charles and Robert ascended to an altitude of about 1800 ft (550 m), covered a distance of about 25 miles (40.2 km), and were airborne for about 2 hours. The rubber-coated, silk balloon was filled with flammable hydrogen gas, an attractive choice from a buoyancy standpoint, but a poor choice from a flight safety perspective. Hydrogen gas would be used in balloons (and airships) well into the 20th century – with many instances of catastrophic events due to its high flammability – until being replaced by helium gas. In fact, de Rozier, of hot air balloon fame, died when his hydrogen gas balloon exploded while attempting to cross the English Channel in 1785. De Rozier’s balloon was a hybrid gas-and-hot air balloon, essentially a hot air balloon with an internal hydrogen gas chamber. Sadly, in addition to being one of the first persons to fly, de Rozier was also the first air crash fatality. The first manned balloon flight in the USA was in a hydrogen gas balloon, piloted by the Frenchman Jean-Pierre Blanchard on 9 January 1793. Blanchard’s balloon lifted off from Philadelphia, Pennsylvania, climbed to an altitude of about 5800 ft (1770 m) and landed in New Jersey.

In addition to opening up a new era in flight, balloons also found military applications. Tethered balloons were used as military observation platforms by the French in the late 18th century and by the armies during the US Civil War. Balloons were also used for artillery spotting in World War I.

1.2.4.2 The Balloon

The two types of balloons that we have been discussing, the hot air balloon and the gas balloon, are different based on the source of the lighter-than-air substance that provides the buoyancy. As its name implies, the hot air balloon is filled with air at a higher temperature, hence, a lower density, than the external, ambient air. The gas balloon is filled with an unheated gas, with a lower density than air, such as hydrogen, helium, or ammonia.

Balloons do not have a means of propulsion, so they literally drift with the wind. By adjusting the balloon’s buoyancy, the balloon pilot can cause the balloon to rise or sink, moving the balloon vertically into different wind currents and thereby having some, albeit limited, control of horizontal motion. Both types of balloons have a fabric envelope that is filled with the lifting gas, a basket or payload suspended underneath the envelope, and a means of adjusting the buoyancy in flight. The basket is used to carry people, while the payload could be any type of equipment or instrumentation that is carried aloft.

The major components of a conventional, modern hot air balloon are shown in Figure 1.38. The envelope of the modern hot air balloon is constructed of lightweight, synthetic fabric panels that are sewn together in banana peel shaped vertical rows, called gores. The fabric is structurally reinforced with horizontal and vertical load tapes. In a conventional hot air balloon, the envelope has a teardrop shape, but it can have a variety of other shapes. Hot air can be vented from the envelope, either through a deflation port located at the top of the envelope or through other vents in the side of the envelope. Venting of hot air is one means of buoyancy control for the balloon pilot. The envelope side vents can also be used to turn the balloon about its vertical axis, providing some control of the basket position relative to the direction of motion, which may be useful to the pilot in landing the balloon. The burners, mounted beneath the envelope, are used to heat the air inside the envelope. Unlike the first manned balloon flight, which used damp straw, old rags, and rotting meat as fuel for their firebox, modern balloons use liquid propane, which is stored in tanks inside the basket. The opening at the bottom of the envelope, called the skirt or scoop, is coated with a fire resistant material to prevent the burner flames from igniting the envelope. By controlling the firing of the burner, the balloon pilot can control the temperature of the hot air in the envelope and hence the buoyancy of the balloon. The basket or gondola is suspended beneath the envelope using stainless steel or Kevlar composite cables. The basket is commonly made of wicker, metal, or fabric, covering a metal frame. Flight instruments and avionics, such as an altimeter, variometer or rate-of-climb indicator, radio, and transponder, are mounted in the basket. In the example problem below, we gain an appreciation for the size of a hot air balloon required to carry a reasonable weight, which includes the weight of the envelope, heating system, basket, aeronauts, and hot air inside the envelope.

The early hot air balloons had the obvious disadvantages of literally carrying a fire aloft and needing to carry the heavy load of firewood or other combustibles to fuel the fire. In fact, the first hot air balloon flight by de Rozier and d’Arlandes was cut short due to their concern that the balloon was starting to catch fire. Once balloon designers figured out how to adequately seal balloons to prevent the leakage of the buoyant gas, the gas balloon soon became preferred over the hot air balloon. However, the burners of modern-day hot air balloons are much more efficient and safer, making hot air balloons the current preference for sport ballooning.

The major components of a typical gas balloon are shown in Figure 1.39. Similar to a hot air balloon, the gas balloon has an envelope that is inflated with the buoyant gas. The gas balloon envelope is typically spherical in shape and made of a thin, gas-tight synthetic material. Typical lifting gases include helium, hydrogen, and ammonia. A net surrounds the gas envelope and is connected via ropes to the load ring, from which the gondola or basket is suspended. The net serves to spread the load of the payload evenly over the surface of the envelope. A valve is located at the top of the envelope, which can be opened by the pilot, allowing gas to escape, to control the rate of ascent. In addition to this vent valve, the ascent and descent of gas balloons are controlled by throwing ballast bags, filled with sand or water, overboard. A tube at the bottom of the envelope, called the appendix, is used to fill the balloon and serves as an outlet to relieve the buildup of gas pressure inside the envelope due to temperature increases. There is also a rip panel on the envelope, which can be opened to rapidly deflate the balloon on the ground, in a high wind condition, or in an emergency situation. The basket or gondola is similar to that used for hot air balloons.

Gas balloons are used for sport ballooning, but less so than hot air balloons, due to their increased complexity and the high cost of the lifting gas. The maximum altitude capability of gas balloons is much greater than hot air balloons. Gas balloons can ascend to near-space altitudes of over 120,000 ft (37 km), above 99.5% of the earth’s atmosphere. For this reason, high-altitude gas balloons are used extensively for scientific research.

Scientific gas balloons are used for a myriad of research and observation purposes, including studies of the weather, the upper atmosphere, and deep space. The envelope volume of the gas balloon expands significantly as it ascends and the external, ambient air pressure decreases. When fully expanded, these specialized gas balloons can be as large as 400 ft (120 m) in height and 460 ft (140 m) in diameter, with a volume of 40 million ft³ (1.1 million m³). The gas envelope skin of these massive balloons is made of a thin polyethylene film, with a thickness of only 0.8 mil (one mil is one thousandth of an inch) or 20 microns. With a maximum payload capability of about 8000 pounds (3629 kg), a scientific balloon can reach an altitude of 120,000 ft (37 km). They are also used as a means of lifting a test object, such as a parachute or vehicle, to an altitude where it can be released to study aerodynamics, flight dynamics, or other characteristics.

Unlike the gas balloons that expand as they ascend, the super pressure gas balloon is designed to maintain a constant volume at all altitudes. The gas envelope of a super pressure balloon is constructed of a high-strength polyester film that can bear the high loads as the gas pressure changes. Super pressure balloons can stay aloft for months, making ideal long endurance, high altitude scientific platforms.

The hybrid balloon combines features of the hot air and gas balloons. The hybrid balloon generates its buoyancy from a combination of heated gas from a burner and the carriage of an unheated, lighter-than-air gas such as helium or hydrogen. De Rozier attempted to cross the English Channel in a hybrid hot air–hydrogen gas balloon. Since de Rozier’s time, hybrid balloons have been used for several long distance flights, including a solo, around-the-world flight by Steve Fossett in 2002. Fossett’s circumnavigation in a hot air–helium hybrid balloon took over 14 days.

1.2.4.3 The Airship

An airship is distinguished from a balloon by both its ability to propel itself through the air, typically using internal combustion engines driving propellers, and also its ability to be steered. Early airships were called dirigible balloons, after the French dirigible, meaning “capable of being directed or steerable”. Developed in the early 20th century, airships were the first powered aircraft that had flight controls for steering.

Similar to a balloon, an airship obtains its lift from a buoyant gas that is contained within a gas envelope. The buoyant gas used in airships is the same as that used in gas balloons, with the inert helium being used in most modern airships. Early airships were filled with hydrogen gas, with the same disastrous results as experienced with hydrogen-filled balloons. Typically, an airship’s envelope has an axisymmetric, streamlined shape that contains the buoyant gas in separate gas bags or cells. Airships can be classified as rigid or non-rigid, based on the construction of the envelope, as shown in Figure 1.40.

The rigid airship has an envelope comprising a structural frame with a fabric outer covering. In early airships, the structural frame was made of wood, covered by cotton cloth fabric, similar to the construction of early airplane airframes. Modern airships use a metal, typically aluminum, framework and synthetic materials for the covering. The rigid structure maintains the shape of the airship and carries the structural loads of the vehicle. The gas bags or cells are mounted inside the rigid envelope. While the gas bags or cells are filled with a pressurized, buoyant gas, the rigid envelope is typically a non-pressurized structure.

A non-rigid airship, also called a blimp, is somewhat similar to a gas balloon in that the internal pressure of the buoyant gas maintains the shape of the envelope. Gas bags inside the envelope are filled with the buoyant gas, while other gas bags called ballonets are filled with air at sea level pressure. The combination of these gas bags is used to maintain the shape of the non-rigid airship hull. To compensate for the change in size of the buoyant gas bags, due to changes in pressure with altitude, air is forced into or out of the ballonets. Air is pumped into the ballonets using auxiliary blowers and is released from the ballonets using vents.

A third type of airship, the semi-rigid airship, is a combination of the rigid and non-rigid designs. Its envelope shape is maintained by the internal gas bags, but there is also a supporting structure, such as a “backbone” keel, much like a ship.

All of these airship types usually have a gondola beneath the envelope structure, where the air[1]crew, passengers, and cargo are carried. On larger, rigid airships, passenger and cargo compartments can be located inside the rigid envelope structure. This is not possible for non-rigid airships. Larger airships typically may have multiple power cars or engine cars – separate nacelles where the internal combustion engine and propeller are installed. These engine cars may be mounted from the gondola structure or from other locations on the envelope. The propeller mounting may be of the pusher (facing aft) or tractor (facing forward) configuration. The multiple engines allow for the application of asymmetric thrust, which is used to help steer the airship. Movable, horizontal and vertical, control surfaces, located at the tail of the airship, are also used for steering and to control the attitude of the airship. To descend, the airship can vent gas and to climb, it can drop ballast. Longitudinal trimming of the airship can be accomplished through weight shifting, by pumping water or gas fore and aft inside the vehicle.

Takeoff, landing, and general ground handling of an airship requires unique facilities and a large ground crew. An airship can take off or ascend much like a balloon, but it can also use its engines to assist in the lift-off. The landing is made by slowly descending towards a ground crew, dropping ropes for them to grab, and being anchored to the ground. A mooring mast may also be used, where the nose or bow of the airship is attached or moored to the mast. As might be imagined, a large number of people are required in the ground crew for ground handling of the airship. The process is more difficult in gusty or high wind conditions.

Perhaps the most famous rigid airships were those built in the early 20th century by the German Zeppelin Company. One of the most famous Zeppelin passenger airships was the LZ-129 Hindenburg (LZ stood for Luftschiff Zeppelin, German for “Airship Zeppelin” and “129” is the airship designation number). The Hindenburg was 803.8 ft (245.0 m) in length and 135.1 ft (41.2 m) in diameter. The giant airship contained over 7 million ft³ (200,000 m³) of hydrogen gas. Powered by four Daimler-Benz 16-cylinder diesel engines, the Hindenburg had a cruise speed of 76 mph (125 km/h) and a typical cruise altitude of only 650 ft (198 m). The Hindenburg had a flight crew of 39 men, an additional dozen chefs and stewards, and a doctor on board. Luxury accommodation for 72 passengers included private cabins, promenade observation areas, a dining room, a lounge with a grand baby piano, and a smoking room, which was kept at a higher than ambient pressure to prevent any leaking hydrogen gas from entering. For the passengers, flying in a Zeppelin passenger airship was much like taking a voyage on a luxury cruise ship. These large passenger airships were the first commercial airliners, able to cross long distances, including routinely flying across the Atlantic Ocean.

Even though these Zeppelins were filled with highly flammable hydrogen gas, they had a very good safety record and flew all over the world carrying passengers and cargo for almost a decade. However, on 6 May 1937, the Hindenburg caught fire while attempting to dock at the Naval Air Station in Lakehurst, New Jersey, reducing the huge airship to a skeleton and ashes in less than a minute. Although never definitively proven, the leading theory for the cause of the fire was the ignition of leaking hydrogen gas by a static electric spark. This high profile disaster, along with a series of other airship accidents, contributed to the decline and ultimate demise of the airship as a viable means of commercial air travel. Replacing the flammable hydrogen gas with helium made the airship safer for flight, but the advancements in fixed-wing airplanes soon made the airship obsolete.

Airships are still used today, but mostly for applications where flying “low and slow” is desired, such as aerial advertising, tourism, remote sensing, and aerial observation. There has been renewed interest in using airships as long duration, very high altitude, scientific and commercial platforms, similar to high altitude balloons. An advantage of the airship is its ability to maintain a constant location over a point on the earth, similar to a stationary satellite. Scientific applications of airships include astronomical or weather observations. Commercial uses include acting as telecommunications platforms.

An example of a modern airship is the Zeppelin NT, shown in Figure 1.41. The Zeppelin NT is a semi-rigid, helium-filled airship with a gas volume of 290,450 ft³ (8255 m³). With a length of 246 ft (75 m) and a diameter of 46 ft (14.2 m), the Zeppelin NT has a gross weight of about 23,500 lb (10,700 kg). It carries a crew of 2 and 12 passengers at speeds up to 77 mph (125 km/h) and altitudes up to about 8500 ft (2600 m). The airship is powered by four 200 hp (149 kW) Lycoming IO-360, air-cooled, piston engines.

Another related aircraft, the hybrid airship, combines elements of the lighter-than-air airship and the heavier-than-air, fixed-wing airplane. The hybrid airship obtains its lift from a combination of aerostatic (buoyant) lift and aerodynamic lift. By virtue of its more aerodynamic shape and higher cruise airspeeds, as compared with a conventional airship, the aerodynamic lift of a hybrid airship can approach 50% of the total lift.