G. Casey Cassidy
Aerodynamics is the branch of dynamics that deals with the motion of air and other gaseous fluids, and with the forces acting on bodies in motion relative to such fluids. For the purposes of this unit and in hopes of maintaining simplicity and clarity, aerodynamics will focus itself on air as it relates to the wings and body of an airplane, or more simply, on what makes airplanes fly.
2.1
Flight
Because airplanes are heavier than air, they have to rely on their engines to give them power and their wings to provide lift, an upward force that overcomes the plane’s weight. Without them both, airplanes would not be able to stay in the air. Four forces affect the flying of an airplane: lift, weight, thrust, and drag. Flight depends upon these forces being balanced. If lift is greater than weight, the plane climbs higher and higher. If thrust is greater than drag, the plane’s forward speed will continue to increase.
The process of powered flight requires the application of forces to accomplish it. In order to sustain an object at a given elevation above the surface of the earth, a constant lifting force must be provided to counteract the effect of gravity (3). Something must supply this lifting force.
In the case of the fixed-wing aircraft, the basic idea is to get the aircraft into the air by creating forward motion to supply lift. Since this forward motion is running the plane into the air, the lift can be provided by a fixed structure (4). Essentially the forward propulsion has nothing to do with lift, but is necessary to get the wings moving against the air at a rate sufficient to provide moving against the air at a rate sufficient to provide lift.
In the design of an airplane for forward flight, total drag must be kept to a minimum if the power requirements are to be kept reasonably low (5). Any drag, whatever the reason, will require the use of power to overcome it. The process of streamlining is designed to reduce drag to a point as low as possible.
In forward flight the lift is provided by the reaction of the wings upon the air as the plane is driven forward to engage it. Both lift and drag are to be viewed in terms of dynamic pressures caused by air flowing against the aircraft(6).
As a general rule it is considered that about 3/4 of the total lift is generated by the action of the upper surface of the airfoil. The efficiency of an airfoil can be described as a ratio of lift to drag (7).
2.2
Lift
Wings have a special shape called an airfoil. This shape gives strength, a smooth airflow, and better lift. The top half of the wing is curved more than its underside, and thus air flowing over it moves faster than air beneath it. As a result, the air exerts less pressure on the wing’s upper surface and higher pressure below produces an upward lift.
It had been proposed many years ago that a “vortex acting on the wing is the cause of lift” (7). This vortex or circular air motion has since been studied extensively in the field of fluid dynamics. Professor Peter P. Wegener in his article, “The Science of Flight”, in the Journal,
American Scientist
, explains this phenomenon thusly, “We begin with a circulatory flow with concentric streamlines around a rotating cylinder. To this flow is added a second flow from left to right. A flow pattern is created with asymmetrical streamlines above and below the cylinder. The flow speed above is higher than that below’‘(8). These effects upon the cylinder are in reality what happens to the airfoil; pressure above is lower, pressure below is greater and lift is created. This is known as the Magnus effect(9). A wing or airfoil is forced by a streamlined contour. The line that is equidistant between upper and lower surfaces is the “mean camber line”. The “chord” or “chord line” of the section is determined by joining the two end points of the mean camber line by a straight line. The “angle of attack” of the section is the angle between the undisturbed freestream direction of the flow and the direction of the chord line(10). The amount of lift is determined by the angle of attack, the angle at which the airfoil is inclined to the air. Because of this definition, cambered wings can be expected to provide lift at zero angle of attack(11).
2.3
Weight
The takeoff weight of an airplane varies by type. It is the plane itself, the passengers, cargo, and the fuel. New materials (plastics and “mixed” metals) save weight and fuel. Planes taking off from areas with high, thin altitudes and hot air temperatures must carry less weight(l2).
2.4
Drag
Planes are smooth-surfaced and are shaped to reduce drag (the resistance of the air to the plane flying through it). The shape of both the wings and the body or fuselage affects drag, and the most efficient planes are those with the best ratio of lift to drag(l3). Today computers are used to find the best possible shapes. Reduced drag means less fuel is required.
(figure available in print form)
2.5
Thrust
The plane is moved forward through the air by thrust. In a jet plane, air escaping from the exhausts propels it forward. A larger or heavier plane requires more powerful engines. So does a plane with poor drag or lift.
2.6
Control of The Plane
A plane with only a fuselage, wings, and engines would be very unstable(l4). A tailplane, which is a small “wing”, and a vertical tail fin provide stability. Adjustable surfaces on both wings and tail control the direction of flight.
(figure available in print form)
2.7
Pitching
If the nose of the plane is displaced upwards by air currents, the tail will move down(15).
The pilot makes a correction by pushing the control column forward. The elevators are moved downward, increasing the upward force on the tail. This lowers the nose and returns the plane to level flight, preventing the to-and-fro rocking called “pitch”. If the elevators remain lowered, the plane will descend.
(figure available in print form)
2.8
Yawing
If the plane is yawing, the nose goes one way first, then the other way(16). The vertical tail fin helps prevent this and keeps the plane flying straight. A moveable rudder on the tail gives directional control. Turning the rudder to the left increases the force on that side and pushes the plane’s nose left.
(figure available in print form)
2.9
Rolling
The plane will roll if the wing tips are displaced up or down(17). It will also slide sideways toward the lower wing unless corrected by the rudder. Wings are designed to slope upward from the body of the plane to improve stability, but the ailerons at the ends of the wings on their rear (tailing) edge give pilots control. To bank and turn the plane to the left, the left aileron is raised. The right aileron is lowered, increasing the lift on the right wing. Then the left wing tilts downward and the plane turns.
(figure available in print form)
2.10
Flying The Plane
Once cleared for takeoff a plane turns into the wind which assists in providing lift for takeoff. You need a certain air speed for takeoff, this is the speed on the runway for still air, or the differential speed with respect to wind. As the speed for takeoff is increased the pilot pulls back the control column, to raise the nose. As the plane’s angle to the ground increases, so does its lift which is more powerful than the plane’s weight(18). On landing the plane, the pilot flies to a point in line with the runway threshold, the throttles are closed and forward speed is reduced. Lift is maintained by lifting the nose, thereby keeping lift equal to the weight of the plane and assisted by the use of flaps. Speed continues to fall as the plane is lined up for landing until the plane touches down, transferring its weight from air to ground.
2.11
Flaps and Spoilers
The elevators, rudder, and ailerons give the pilot control over the plane’s direction. Flaps and spoilers are designed to help with lift on takeoff and landing. The flaps are moveable extensions of the wing; they increase the wing area to provide extra lift when it is needed. Spoilers mounted on top of the wing assist the ailerons. When a spoiler is raised, the increased drag means that the plane descends.
In normal flight the flaps are retracted into the wing and the spoilers are kept lowered.
(figure available in print form)
*Looking down on section of wing. #3,4,7, and 8 are the flaps.
%5 and 6 are spoilers.
(figure available in print form)
3.1 Propellers
Propellers are not flat but are curved, like aerofoils. They behave in the same way, with the blades striking the air at a low angle of attack and developing thrust in the way a wing develops lift(19).
Advance design propellers can change their angle of attack to produce the maximum degree of thrust. Variable pitch propellers can be set fine for full power and low speeds (takeoff) or coarse for high forward speeds with reduced engine revolutions (20).
Propellers are common on small light piston-engine planes. When driven by a jet engine they become a turbo prop, developing much more thrust at takeoff.
3.2
Jet Engines
In the jet engine, air is drawn into the intake and compressed, with a resulting rise in pressure. Fuel is added and burned in a combustion chamber. This produces a high-speed hot gas. It flows through a turbine, which uses just enough of the energy from the gas to power the compressor, while the rest of the energy provides thrust, as it escapes at speed through the exhaust nozzle.
(figure available in print form)