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Maximum Lift Flap Setting

Some pilots express concern about using flaps for takeoff from a backcountry strip. Should flaps be used or not, and if used, how much flap should be used?

The majority of experienced mountain pilots agree ... flaps should be used for takeoff.

How much flaps? The POH or Owner's Manual may give a recommendation, in which case you are obligated to use their wisdom. But, if there is no blessing listed, the following procedure will provide the maximum lift from any particular airfoil section.



To determine the best flap setting begin by making full control deflection of the ailerons, the control wheel or stick is moved full left in this picture. This represents the maximum lift for this airplane's airfoil design.

Remember, lift and drag are directly proportional. Increase lift and you increase drag. Here the manufacturer determined the maximum lift for the aileron deflection is obtained at the particular angle formed with full control deflection. Through testing they found that more aileron deflection would create drag causing a decrease in performance. And obviously any less deflection would result in decreased roll capability.

Maximum aileron deflection is a compromise providing the greatest performance for the particular airfoil section (design).

Next, match the flap deflection to the aileron deflection. This provides the maximum amount of lift versus drag for the airfoil considering the effect of drag. This works for normally aspirated engines.

With the Cessna-type airplanes that incorporate fowler flaps that move out and down, it is necessary to parallel the flap deflection to match the aileron deflection since they are not side-by-side. This will result, for example, in about 12-degrees flap extension in the Cessna 150-170 series airplanes.

If your airplane happens to be a turbo-super charged wild duck, or some such derivative, the flaps will probably be set at 50 percent because you are not as concerned with the balance of engine power and lift. These airplanes have the performance available to take off and accelerate with half flaps extended.





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