Takeoff and Landing Flap Settings

An excellent question, and once again the best way to answer it is to go back to our trusty friend the lift equation:

Please read the previous question linked above for a full explanation of this equation, but of interest to us in this case are the variables

Since we've already answered questions on slats and flaps in the past, you should be aware that these devices are mounted on the wings and deflect downwards to generate additional lift. We see this effect in the lift equation through the lift coefficient--as the angle of flap deflection increases, the value of C L increases and the lift increases (assuming all other variables are constant). You are also correct in pointing out that the amount of lift a plane needs at any given time is directly related to its weight at that time. So it is understandable that you might expect the flaps to be set at a greater angle of deflection when the plane is heavier and fully loaded with fuel at takeoff than when most of that fuel is gone and the plane is much lighter at landing.

However, the above discussion assumes that all the other variables are constant at takeoff and landing, and this is not the case. The key parameter we also need to consider is the aircraft's speed. Most aircraft takeoff at a speed that is about 20% to 30% faster than the landing speed. Since the lift varies linearly with the lift coefficient but with the square of the velocity, the velocity has a much greater impact on the lift equation. As a result, we need more flap deflection at landing than at takeoff to compensate for the reduced speed even though the aircraft is lighter. In fact, the primary purpose for adding flaps to aircraft wings in the first place was to reduce landing speeds and give pilots more margin for error in this most critical stage of flight.
- answer by Jeff Scott, 9 December 2001

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