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7 Answers

Flap Usage

Asked by: 4017 views Aerodynamics, Aircraft Systems

Usually most POH/Flight Manuals recommend to use a a specific flap setting for takeoff (usually short or soft field).

 

Before you read my question below, I know that flaps add drag but also increase lift (or in other words:  a tradeoff of more lift with a measurable drag penalty) and also the higher the density altitude the more overall performance decreases.

However, I overheard that if density altitude is quite high that the use of flaps for takeoff actually is not recommended because it degrades performance. What is correlation between high density altitude and flaps as it relates to a decrease in performance?

Thanks for the feedback.

 

 

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7 Answers



  1. Ray Salmon on Aug 30, 2014

    I would refer you to the guidance in your POH. The performance numbers in there are for a certain configuration and are given at various altitudes. The manufacturer expects the aircraft to perform to those numbers in that configuration. Once you deviate from the manufacturers’ guidance, those numbers become unreliable and you become a test pilot.

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  2. Aaron on Aug 30, 2014

    Think about this one: increased drag decreases acceleration, and thus increases takeoff roll. Thus, lower flap settings help get you off the ground a little sooner by decreasing stall speed, but without inducing as much drag as higher flaps settings. As they teach on the ATP written test: the first 50% of flaps increases lift by MORE than 50% of total lift from flap usage, and the last 50% of flaps increases drag by MORE than 50% of total drag flap usage.

    However, another consideration is clim performance. Climb is really where flap usage can be detrimental. Because of the increased drag you mentioned, climb performance is degraded further with flaps down. Thus you can re-think of your “tradeoff” as such: The lift increase from flaps is good for getting you off the ground, but the drag is bad for reaching effective climb speeds and climbing out at a high enough gradient. So it really depends on what you’re looking for. If you’ve got a short field with few obstacles, then flaps down is okay on takeoff, because it gets you off the ground quicker, and climbing out isn’t as much of a concern. However, at mountainous airports where climb considerations are more important, use of flaps decreases climb performance which may be drastic enough to the point where climb performance requirements are not met.

    Density altitude factors into the equation as you would expect: higher density altitudes both decrease acceleration on takeoff AND decrease performance in climb and shallow out your climb gradient. So it really is sort of exacerbating any problems you may have in your flap selections. You really just have to decide which is more important: ground roll, or obstacle clearance.

    Here’s a good reference: http://www.skybrary.aero/index.php/AP4ATCO_-_Factors_Affecting_Aircraft_Performance_During_Takeoff_and_Climb

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  3. Aaron on Aug 30, 2014

    Also, a big +1 to what Ray said, practically speaking the manufacturer (and the FAA) has poured over performance and operating techniques to give you the best guidance possible, so that is really the first and last thing to consult in operating your aircraft.

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  4. Sam Dawson on Aug 31, 2014

    What I will write will be controversial and will probably lead to some thumbs down, but so be it.

    Some POHs are awful and will give you no information on short field technique. Beechcraft Bonanzas, for example, had a short field technique that called for flaps 10-20 degrees, but then took the procedure out of the handbook until 1984 for some reason. Then, in 1984 a flaps 15 procedure suddenly appeared. In such cases where performance information is lacking the owner association can be a source of supplemental information. In the case of the Beechcraft, the American Bonanza Society has procedures and profiles through the BPPP (Beechcraft Pilot Proficiency Program), that can supplement the gaps in the POH.

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  5. Dan Chitty on Sep 04, 2014

    Thank you all for the great feedback. Much appreciated.

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  6. Brian on Sep 17, 2014

    Lift isn’t increased, it equals weight. Flaps do only three things: add drag, change the camber of the wing, and lower stall speed. Stall speed is lowered by the alteration of lift’s coefficient, but do not mistake that for an increase in lift.

    Dan, I’ve never read in any reliable text that flap usage on takeoff should be changed at a higher density altitude. Flaps allow you to get off the ground sooner due to the decrease in stall speed and therefore let you climb away from the ground sooner. Density changes will only effect the distance this takes, the rest of the variables remain constant assuming you don’t change them (I.E. changing flap settings).

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  7. Dan Chitty on Sep 18, 2014

    Thank you Brian for the additional feedback.

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