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Best Glide??

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Well-known member
Dec 15, 2001
It is a commonly held belief that the first 10 degrees of flaps on most training aircraft generally increase lift more than drag. If this is true, why don't procedures dictate using 10 degrees of flaps to increase glide performance?
It all based on how much power is required for a certain airspeed. When you are in a clean configuration at L/Dmax you are maximizing the ratio between power required for a specific airspeed (best glide). When you add flaps, it does increase lift for a given airspeed but it also increases your power required due to the increased drag. The change in drag is greater than the increase in lift therefore decreasing the ratio between the two. Remember flaps were developed for increased angles of descent at slower airspeeds not for takeoffs and increased performance. When you deflect anything into the airstream, its surface area, dynamic pressure, and deflection determine the increase in drag and moment change. Flaps do add a lot of drag.

This is my rationale, as always, somebody correct me if I'm wrong.
Adding flaps doesn't necessarily increase drag; it depends on the airplane and the type of flaps. In a split flap, any addition of flap will increase drag. In a fowler flap, initial flap application will increase lift by increasing camber, downwash, and induced angle of attack. A slight increase in induced drag will occur as the natural byproduct of lift, as the aerodynamic chord increases the induced angle of attack.

Generally speaking, the addition of flap causes the center of lift to move forward on the wing. In many cases, the download on the horizontal stab may increase, or be required to increase, for stability. This has a negative effect on aircraft performance, and is akin to adding weight to the aircraft in direct proportion to the increased download required on the tail. The airplane gets heavier (in effect), it doesn't glide as far.

Most airfoils are optimized at a given speed for the glide; altering the airfoil by deploying flaps alters the characteristics of the airfoil, and subsequently the glide performance (among other things).

Considering an increase in interference drag around the inboard and outboard end of the flaps, assymetrical span loading on the wing and airfoil, changes in downwash, etc, one can see that for many designs, the addition of flap would not be recommended, and will decrease glide performance.

With power available, the addition of small increments of flap (8-12 degrees, typically), will provide additional lift at the expense of forward speed restrictions. In a power off glide, addition of flap will create additional lift, but at the expense of gliding ability. One must specify the type of glide; best distance, or minimum sink. In some cases, minimum sink may be optimized by the addition of flap. However, in all cases, one must strictly abide by the manufacturers recommendation.

Remember that ever design is different. So are the recommendations for each design. Each design has been tested and developed with real-time testing for every phase of the published performance. In order to duplicate the performance, or hope to be able to duplicate the performance, one must as closely as possible approximate the conditions under which that performance was verified. You do this by following the directions.

While the initial flap settings do provide a higher ratio of lift than drag, generally speaking, the flap settings do increase induced drag. Often initial flap settings are compared against flap settings of higher deflections (generally greater than 20 degrees); as flap increases beyond the median 20 degree point, both parasitic and induced drag tend to increase far beyond the benifits gained by the lift produced. However, this doesn't change the fact that drag is produced with any flap deflection, even if only minimal.

Some aircraft benifit greatly from the addition of flap, even in the glide. One airplane I flew experienced a 50 knot decrease in stall speed by application of flap, and an increase in glide of substantial amount. (Which was good, because without the flap, it had the glide ratio of a brick).

Again, you must consider each aircraft on an individual basis per the make and model, and often the specific aircraft (owing to variations, modifications, loading, etc).
i remember in the DA-20 katana the best glide was achieved with flaps at T.O. position Per the emergency checklist. I think it was 15 degrees.
I have time in 50+ others and they are all flaps up

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