Aerodynamics - 3

Aerodynamic Forces

When two solid objects interact in a mechanical process, forces are transmitted, or applied, at the point of contact. But when a solid object interacts with a fluid, things are more difficult to describe because the fluid can change its shape. For a solid body immersed in a fluid, the "point of contact" is every point on the surface of the body. The fluid can flow around the body and maintain physical contact at all points. The transmission, or application, of mechanical forces between a solid body and a fluid occurs at every point on the surface of the body. And the transmission occurs through the fluid pressure.
 
Definitions of Lift and Drag
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Since the fluid is in motion, we can define a flow direction along the motion. The component of the net force perpendicular (or normal) to the flow direction is called the lift; the component of the net force along the flow direction is called the drag. These are definitions. In reality, there is a single, net, integrated force caused by the pressure variations along a body. This aerodynamic force acts through the average location of the pressure variation which is called the center of pressure 

The Lift Formula 

Lift=1/2*D*V2 *S*CL

Here D=density of air (High density air means lots of air particles passing over leads to increase in lift, Low density air means less of air particles passing over leads to decrease in lift. This is why aircraft have to run down the run way quickly to get lift).

V= Velocity of aircraft (Travelling fast means lots of air particles passing over higher lift, travelling slow means less air particles in contact thus less lift).

S = Area of wing (Small wing-less lift, longer wing greater lift cause more particles in contact, more lift. If the area of wing doubled lift & drag also doubled).

CL-Coefficient of lift (Depends upon the shape of wing & angle of attack).

We can calculate drag on a similar way

Drag=1/2*D*V2*S*CD

Here CD is called coefficient of drag which is also determined by AoA & wing shape. It is more difficult to calculate than CL because it has more dependencies than Lift (Apart from AoA & shape there are numerous other sources that increases drag Eg. Air Friction).

CL & CD is determined experimentally.
Lift & drag vary directly as its square of the relative wind..
If the velocity doubled lift & drag multiplied by 4.
Lift and drag proportional to density of the air passing over the airfoil.
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When an aircraft flying horizontally at constant velocity the forces acting up on it are in equilibrium. The retarding force (drag) equals the propelling force (thrust). The lift must equal the total weight of the aircraft so the lift & drag equations serves to determine the how much weight airfoil will lift under certain conditions & how much thrust force power is required to overcome the total drag of the air plane. The air pressure at various points on air foil is also measured (through wind tunnel test) in order to determine the airfoil efficiency & suitability for a given purpose.

Angle Of Attack  (AoA)

Before continuing with AOA we must understand the concepts like wing chord & relative air flow (RAF).
Wing chord – A line drawn from the leading edge to the trailing edge of the air foil is called wing chord.
Relative air flow – Direction of the air flow relative to the wing i.e. the air unaffected by the wing (horizontal in most cases)
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AoA is the the angle between chord of the wing & relative air flow/wind. In general when AoA increases lift increases & drag also increase but not as lift. Lift increases with the increase in AoA up to a point after that the lift decrease abruptly & the drag become the major component this is called stalling angle/critical AoA sometimes called burble point .

AoA is based on the design of the aircraft. The angle of attack of an airfoil directly controls the distribution of pressure below and above it. When a wing is at a low but positive angle of attack, most of the lift is due to the wing's negative pressure (upper surface) and downwash. (Negative pressure is any pressure less than atmospheric, and positive pressure is pressure greater than atmospheric.) At any angle of attack, other than the angle at zero lift, all the forces acting on the wing as a result of the pressure distribution surrounding it may be summed up and represented as one force - the center of pressure.
 
In any AoA lift & drag directly proportional to area of wing. If area of wing doubled lift and drag also doubled. Lift & drag proportional to relative wind & velocity squared.
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AILERONS are using to increase and decrease AoA. When we need to increase the AoA the pilot moves the ailerons (Our next part is about aileron) back words (aileron is fitted in the trailing edge of the wing) this means when the aileron moves the trailing edge also moves while the leading edge remains same this changes the chord of the wing but the relative air flow stays same. Thus the AoA also changes if we want to reduce the AoA pilot does the opposite. So we can change AoA using ailerons and control lift.

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