In aviation, the topic of aerodynamics is inevitable. Aerodynamics involves studying forces that affect an object that is moving through air or an object exposed to moving air. The desired result is that the plane moves forward with some degree of navigation using minimum fuel or power. As an aeroplane is in motion, it is subjected to forces from air, gravity and the engine thrust. These forces acting on the plane’s surface in flight are mainly as a result of the division of pressure created by the flowing air on the plane’s surfaces. The distribution of pressure is as a factor of speed and density of air flowing on the plane. The density of air is inversely proportional to temperature and altitude, and the pressure itself increases with the air density. On the other hand “Pressure increases exponentially with increasing airspeed” (Spark, 1998, p. 201). With this in mind, the climate and location of air adversely affect the type of flight.
A normal plane is designed with asymmetric wings to have air flow faster on the top side and slower at the bottom side. This causes the pressure on the lower side to be higher than the upper side; this causes an up lift force on the wings. This is the basic principle which keeps planes and choppers in flight. Also, the upward force generated counters the gravity force to enable planes to ‘float’ in air. On the wings, there are wing flaps which are essentially used to change the plane’s direction; both horizontally and vertically. Also, the wing flaps are used to slow down the plane.
When air flows smoothly on the surface, it is regarded as laminar flow. On the other hand, if the flow is accompanied by swirling of the wind, the flow will be known as turbulent. The shape of the wing affects the type of flow on the surfaces. Streamlined wings usually experience laminar flow. This is because the distance between the free flowing air and the wing is minimal or zero. If this separation is big, stalling will occur and would make the plane inefficient. Stalling arises when the air flow is not smooth but rather turbulent. Since the lift depends on the pressure difference between the upper and lower parts of the wing, it is very crucial to maintain the pressure under the wing higher than on top. During turbulent flow, the pressure difference is interrupted such that the plane starts to lose the upward suspension force. This effect is called stalling and may result to the plane losing its flying capabilities. The parts of airplane that results to stalling due to drag include, the landing gear, wings together with their flaps, planes bare body and the engines. Also, the types of surfaces may be a source of stalling. Smooth surfaces offer less stalling as compared to rough surfaces (Spark, 1998). This means that the plane’s surfaces should be kept smooth by cleaning and polishing.
Despite of that, stalling the plane is necessary when it comes to braking; especially the moment before landing. This type of drug is called induced drag since it is intentionally caused by the pilot. The common way which pilots increase stalling (especially during landing) is by increasing the angle of attack on the wings. The angle of attack is the change of the orientation of the wing flaps (up or down); which is done to manoeuvre the plane. When the angle of attack is increased, the flow changes from laminar to turbulent and in turn, the plan will slow down. As the pilot increases the angle of attack, the more he increases the plan’s drag. The drag increases as a result of “circular air motion occurring at the tip of the wing” (Furgerson, 1995, p. 398). However, during flight, the pilot has to change direction at one point or the other. The only way to do this is to increase the angle of attack. To have minimum drag, he has to minimally increase the angle of attack to have low drag forces. This will eventually cause a huge turning circle but wastage of power will be reduced. Small changes of the angle of attack do not have any effect on the laminar flow (Furgerson, 1995). Nonetheless, there is a limit in which this angle can go while maintaining the laminar flow (15º). All these factors take effect when the plane is travelling at subsonic speeds (speeds below the speed of sound).
When an airplane is moving, there is usually a thin layer of air on the plane surface which flows slower than the rest of the air. This slender layer is called the boundary layer and it directly influences the friction of air on the plane. The friction developed relies on the “rate of change of velocity throughout the boundary layer” (Grishm, 1999, p. 139). Ideally, the boundary layer should have laminar flow but that is not the case. As the width of the wing increases, the flow changes from laminar to turbulent. As the air travelling over the lengthy wing, turbulence occurs as a result of abrupt pressure changes.
The changing speed of air on and below the wing, leads to speeds higher than the speed of the plane on the upper surface. When the plane’s velocity is nearing the velocity of the sound, the air flowing on top of the wing will be and eventually surpass the acoustic velocity; this will lead to periods of supersonic speeds. What usually happens at this point, a shock wave tries to restore the plane’s speed to just under the speed of sound. This shock wave happens when the air ‘refuses to be cut’ since the plane is moving at a speed which does not allow the formation of a boundary layer. As a result, the air flow is no longer laminar and this will tend to stall the plane (Hammond, 1996).
Any additional speed of the plane results to yet another shock wave referred to as a bow wave; it moves from the further edge of the wing to the front tip of the wing. This bow wave is as a result of “weak pressure waves propagating forward from the wing’s leading edge at the local speed of sound”. At supersonic speeds, these waves are encountered with the wind speed in such way that they are not able to go in front of the wing. At subsonic speeds, the air is ‘cautioned’ on the oncoming wing and has time to alter itself to smoothly flow on and under the wing. However, at supersonic speeds, the air has no prior warning of the oncoming wing and has to alter itself at once. As a result, the air flow will be turbulent and cause the plane to stall instantly (Hammond, 1996).
Planes which are designed to fly at supersonic speeds usually have their wings sharpened at the edges. This means that “the bow wave will move in and become attached to the leading edge as the speed increases” (Chi, 1998, p. 443). The result will be a smooth laminar flow with minimal stalling. At these speeds, the angle of attack range has to be reduced to even lower levels (7º). Surpassing the minimum angle will result to a catastrophic stalling; it will be as if the plane hit the wall. High performance planes are thus designed with sharper edges and choose to change the direction of the thrust on the engine rather than changing the angle of attack when it comes to changing direction (Chi, 1998).
References
Chi, B. (1998). Basic Aerodynamics. Singapore: Colset.
Furgerson, C. (1995). Drag In Small Airplanes. Chicago: Universal Publishers.
Grishm, A. (1999). Air Loads. New York: Whitford.
Hammond, R. (1996). Wing Design. London: Butler and Tanner.
Spark, A. (1998). Design For Air Combat. New York: Jane’s Publishing.