airplane

Aerodynamics

An aircraft in straight-and-level unaccelerated flight has four forces acting on it. (In turning, diving, or climbing flight, additional forces come into play.) These forces are lift, an upward-acting force; drag, a retarding force of the resistance to lift and to the friction of the aircraft moving through the air; weight, the downward effect that gravity has on the aircraft; and thrust, the forward-acting force provided by the propulsion system (or, in the case of unpowered aircraft, by using gravity to translate altitude into speed). Drag and weight are elements inherent in any object, including an aircraft. Lift and thrust are artificially created elements devised to enable an aircraft to fly.

Understanding lift first requires an understanding of an airfoil, which is a structure designed to obtain reaction upon its surface from the air through which it moves. Early airfoils typically had little more than a slightly curved upper surface and a flat undersurface. Over the years, airfoils have been adapted to meet changing needs. By the 1920s, airfoils typically had a rounded upper surface, with the greatest height being reached in the first third of the chord (width). In time, both upper and lower surfaces were curved to a greater or lesser degree, and the thickest part of the airfoil gradually moved backward. As airspeeds grew, there was a requirement for a very smooth passage of air over the surface, which was achieved in the laminar-flow airfoil, where the camber was farther back than contemporary practice dictated. Supersonic aircraft required even more drastic changes in airfoil shapes, some losing the roundness formerly associated with a wing and having a double-wedge shape.

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By moving forward in the air, the wing’s airfoil obtains a reaction useful for flight from the air passing over its surface. (In flight the airfoil of the wing normally produces the greatest amount of lift, but propellers, tail surfaces, and the fuselage also function as airfoils and generate varying amounts of lift.) In the 18th century the Swiss mathematician Daniel Bernoulli discovered that, if the velocity of air is increased over a certain point of an airfoil, the pressure of the air is decreased. Air flowing over the curved top surface of the wing’s airfoil moves faster than the air flowing on the bottom surface, decreasing the pressure on top. The higher pressure from below pushes (lifts) the wing up to the lower pressure area. Simultaneously the air flowing along the underside of the wing is deflected downward, providing a Newtonian equal and opposite reaction and contributing to the total lift.

The lift an airfoil generates is also affected by its “angle of attack”—i.e., its angle relative to the wind. Both lift and angle of attack can be immediately, if crudely, demonstrated, by holding one’s hand out the window of a moving automobile. When the hand is turned flat to the wind, much resistance is felt and little “lift” is generated, for there is a turbulent region behind the hand. The ratio of lift to drag is low. When the hand is held parallel to the wind, there is far less drag and a moderate amount of lift is generated, the turbulence smooths out, and there is a better ratio of lift to drag. However, if the hand is turned slightly so that its forward edge is raised to a higher angle of attack, the generation of lift will increase. This favourable increase in the lift-to-drag ratio will create a tendency for the hand to “fly” up and over. The greater the speed, the greater the lift and drag will be. Thus, total lift is related to the shape of the airfoil, the angle of attack, and the speed with which the wing passes through the air.

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Weight is a force that acts opposite to lift. Designers thus attempt to make the aircraft as light as possible. Because all aircraft designs have a tendency to increase in weight during the development process, modern aerospace engineering staffs have specialists in the field controlling weight from the beginning of the design. In addition, pilots must control the total weight that an aircraft is permitted to carry (in passengers, fuel, and freight) both in amount and in location. The distribution of weight (i.e., the control of the centre of gravity of the aircraft) is as important aerodynamically as the amount of weight being carried.

Thrust, the forward-acting force, is opposed to drag as lift is opposed to weight. Thrust is obtained by accelerating a mass of ambient air to a velocity greater than the speed of the aircraft; the equal and opposite reaction is for the aircraft to move forward. In reciprocating or turboprop-powered aircraft, thrust derives from the propulsive force caused by the rotation of the propeller, with residual thrust provided by the exhaust. In a jet engine, thrust derives from the propulsive force of the rotating blades of a turbine compressing air, which is then expanded by the combustion of introduced fuel and exhausted from the engine. In a rocket-powered aircraft, the thrust is derived from the equal and opposite reaction to the burning of the rocket propellant. In a sailplane, height attained by mechanical, orographic, or thermal techniques is translated into speed by means of gravity.

Acting in continual opposition to thrust is drag, which has two elements. Parasitic drag is that caused by form resistance (due to shape), skin friction, interference, and all other elements that are not contributing to lift; induced drag is that created as a result of the generation of lift.

Parasitic drag rises as airspeed increases. For most flights it is desirable to have all drag reduced to a minimum, and for this reason considerable attention is given to streamlining the form of the aircraft by eliminating as much drag-inducing structure as possible (e.g., enclosing the cockpit with a canopy, retracting the landing gear, using flush riveting, and painting and polishing surfaces). Some less obvious elements of drag include the relative disposition and area of fuselage and wing, engine, and empennage surfaces; the intersection of wings and tail surfaces; the unintentional leakage of air through the structure; the use of excess air for cooling; and the use of individual shapes that cause local airflow separation.

Induced drag is caused by that element of the air deflected downward which is not vertical to the flight path but is tilted slightly rearward from it. As the angle of attack increases, so does drag; at a critical point, the angle of attack can become so great that the airflow is broken over the upper surface of the wing, and lift is lost while drag increases. This critical condition is termed the stall.

Lift, drag, and stall are all variously affected by the shape of the wing planform. An elliptical wing like that used on the Supermarine Spitfire fighter of World War II, for example, while ideal aerodynamically in a subsonic aircraft, has a more undesirable stall pattern than a simple rectangular wing.

The aerodynamics of supersonic flight are complex. Air is compressible, and, as speeds and altitudes increase, the speed of the air flowing over the aircraft begins to exceed the speed of the aircraft through the air. The speed at which this compressibility affects an aircraft is expressed as a ratio of the speed of the aircraft to the speed of sound, called the Mach number, in honour of the Austrian physicist Ernst Mach. The critical Mach number for an aircraft has been defined as that at which on some point of the aircraft the airflow has reached the speed of sound.

At Mach numbers in excess of the critical Mach number (that is, speeds at which the airflow exceeds the speed of sound at local points on the airframe), there are significant changes in forces, pressures, and moments acting on the wing and fuselage caused by the formation of shock waves. One of the most important effects is a very large increase in drag as well as a reduction in lift. Initially designers sought to reach higher critical Mach numbers by designing aircraft with very thin airfoil sections for the wing and horizontal surfaces and by ensuring that the fineness ratio (length to diameter) of the fuselage was as high as possible. Wing thickness ratios (the thickness of the wing divided by its width) were about 14 to 18 percent on typical aircraft of the 1940–45 period; in later jets the ratio was reduced to less than 5 percent. These techniques delayed the local airflow reaching Mach 1.0, permitting slightly higher critical Mach numbers for the aircraft. Independent studies in Germany and the United States showed that reaching the critical Mach could be delayed further by sweeping the wings back. Wing sweep was extremely important to the development of the German World War II Messerschmitt Me 262, the first operational jet fighter, and to postwar fighters such as the North American F-86 Sabre and the Soviet MiG-15. These fighters operated at high subsonic speeds, but the competitive pressures of development required aircraft that could operate at transonic and supersonic speeds. The power of jet engines with afterburners made these speeds technically possible, but designers were still handicapped by the huge rise in drag in the transonic area. The solution involved adding volume to the fuselage ahead of and behind the wing and reducing it near the wing and tail, to create a cross-sectional area that more nearly approximated the ideal area to limit transonic drag. Early applications of this rule resulted in a “wasp-waist” appearance, such as that of the Convair F-102. In later jets application of this rule is not as apparent in the aircraft’s planform.