Lift Flashcards
With respect to the CL curve, describe the effect of
a) Increased camber.
Increasing the camber produces more CL and changes the angle at which stall occurs (increasing the stall angle).
With respect to the CL curve, describe the effect of b) Surface roughness.
CL Max is very sensitive to the roughness of the forward part of upper surface, from the leading edge to about 20 - 30% of chord. Roughness can cause wings to stall at lower angle of attack due to FLOW BREAKAWAY.
Identify the factors affecting lift (low-subsonic speed airflow).
Freestream air density, freestream velocity, Size and shape of the wing, condition of the surface, angle of attack.
Describe the meaning of the term coefficient of lift.
CL is the amount of lift created by the aerofoils of an aircraft.
Define aspect ratio (AR) and describe the effect of AR on CL.
Aspect ratio is the ratio of the wingspan to the chord of a wing.
Its effect on the Coefficient of lift is that a decrease in AR will decrease CL and increase the geometric stalling angle.
Explain the meaning of a high CLmax.
Maximum value of coeffecient of lift. Attained just before the stalling angle of attack.
Given a typical CL versus AOA curve for a GP-type aerofoil, identify:
(a) the zero lift angle;
This is the point at which the angle of attack is correlated with a CL of 0. Usually around -3o for a standard general purpose aerofoil.
Given a typical CL versus AOA curve for a GP-type aerofoil, identify:
(b) the angle for maximum CL (CLmax).
The angle for CLmax is shown at the peak of the curve. Note that it is also the stalling angle of attack.
Explain the effect of induced downwash on AoA.
Downwash will decrease effective AoA. To then maintain level flight the effective AoA will need to be increased, (geometric will also be higher than before). This increase also generates more drag, referred to as induced drag.
Describe three-dimensional flow over a wing and explain how wingtip and trailing edge vortices are formed.
3D flow is caused by the high pressure on the bottom of the wing migrating towards the wingtip to reach the low pressure on the top of the wing.
Wing-tip vortices are generated when the airflow spills over the wingtip from high to low pressure.
Trailing edge vortices are generated when air flow doesn’t make it the wingtip, but rather meeting at the trailing edge. These can flow towards and add to the wingtip vortices. See fig 4-7.
Vortices are least when a high airspeed and low AOA are being maintained. A low aspect ratio will produce smaller wingtip vortices than a high aspect ratio.
