Control surfaces produce lift or drag. Lift and drag increase and decrease with increasing and decreasing speed. Therefore, only small deflections are necessary when flying with high airspeeds and larger deflections at lower speeds.

__The lift equation states:__

**LIFT = ½ρV ^{2}SCL**

- CL = coefficient of lift. This represents angle of attack of the airfoil and the shape of the airfoil. This described in terms of thickness/chord ratio and a camber and can be altered in flight, for example, by lowering the flaps.
- ρ = The density of the air. For any given altitude during subsonic flight, the density may be considered to be constant.
- V
^{2}= The velocity of the aircraft, squared. - S = The wing area.

The lift formula can also be rearranged and written as: LIFT = q × S × CL (q = dynamic pressure, i.e. ½ρ × V^{2}).

In the lift equation all factors except density and speed (TAS) are constant. At higher altitude ρ is reduced; therefore TAS (V) must be increased to maintain the same lift (= weight).

__Drag equation states that:__

**DRAG = ½ρV ^{2}SCD**

- ρ = the mass density of the fluid (rho),
- V = the velocity of the object relative to the air,
- S = the reference area,
- CD = the drag coefficient - a dimensionless constant.

The drag formula can be rearranged and written as:

DRAG = CD_{ }_{× }½ρ × V^{2 }× S

From the drag equation it is clear that drag is directly proportional to the frontal area: if it increases by a factor of e.g. 3 the drag will be increased by the same factor.

Your Notes (not visible to others)

This question has appeared on the real examination, you can find the related countries below.