what is the correct usage of flaps on a sailplane?

[Sanman]

I was wondering about the uses/mixes for flaps on a sailplane. I have watched others use flaps as 'spoilers' when trying to slow for a landing. I have also seen several printed setups, generally for larger sailplanes, where the flaps were used during 'take-off' and 'thermaling'. Then there is CROW, etc.

Anyone care to dribble a short explanation of flap operation?

Thanks ;)

[jlk]

Someone correct me if I am wrong. Flaps when lowered increase the lift of the wing. When put down past about 30 degrees they also start making a lot of drag. The additional lift and drag slow the plane and make it stall at a slower speed.

Lowering flaps slightly during thermaling increase the wing lift and decrease the stall speed allowing the plane to climb faster and spend more time in the lifting air for each circle made while thermaling.

On many of the popular glider airfoils (eg. SD7037) moving the flap up 2-5 degrees is called reflex and allows the plane to fly faster almost like adding ballast, this is sometimes used when in sinking air to move out of it as fast as possible.

Lowered flaps are used on launch to increase lift of the wing and slow the ascent on the winch or highstart to maximize the energy used to pull the glider up.

Flaps can be raised 20-40 degrees and can act like spoilers. This decreases the lift of the wing and causes the plane to sink faster. This doesn't work well on all airfoils and wing positions.

Lastly crow. Crow is a combination of the flaps down and the ailerons reflexed up. This gives maximum drag and spoils the lift causing the plane to decend rapidly without building up any speed and maintaining control. Crow is used mostly for spot landing at the end of a competition flight.

Jordan

[Sanman]

Thanks Jordan!

So if I understand it correctly, < 30% flaps could help create additional lift (depending on situation/conditions) and > 30% flaps would cause a drag situation, useful for slowing down or lowering the stall speed.

Thanks again!

[vintage1]

Flaps down = more lift, a little moredrag and slower stall speed. i.e. you get slightly higher sink rate, but not so much forward speed.

Flaps up = less lift, so feed in up, - more incidence on main wing, so more drag, so airbrake effect. Up flap will just increase sink rate, to kill speed and lose height rapidly.

[jlk]

Yes Sanman, for the most part the first 30 degrees give mostly lift and some drag from there on its all drag. These numbers vary slightly with the airfoil but are a good rule of thumb.

Jordan

[green66]

Flaps increase the wing's lift coefficient, but also generally increase the drag coefficient more, so L/D (efficiency, glide ratio) goes down as flaps are deployed.

Beyond about 10-15 degrees of deflection, control surfaces such as ailerons, flaps, elevator, etc. experience flow separation accompanied by high drag, where flow efficiency around the surface goes down the drain. Flaps are unique in that they can be held deflected for extended periods of flight, e.g. not just during a turn, therefore they should not be deflected more than 10 degrees for any length of time unless braking is more important than glide efficiency.

One thing to note is that ailerons should be mixed into flaps to obtain full-span camber control, in order to avoid an abruptly differing lift coefficient between the flapped and non-flapped portions of the wing.

Another thing to keep in mind (touching on jlk's comment re flaps reducing stall speed) is to be cautious about suddenly pulling flaps up at low altitude. The plane will be flying slowly, then flaps are brought up, and all of a sudden the stall speed is increased, risking a nasty stall :eek: . This is especially true of a Tx programmed to automatically pitch-compensate the flaps ==> the elevator will be moving upward as the flaps are returned to neutral.

For reference, the chart below shows wind tunnel performance of the SD7037, at 0, 5 (red), and 10 (black) degrees of flap deflection, all at a Reynolds Number of about 200,000.

[Sail 'n Soar]

Another way of saying things, for large deflection, flaps increase both lift and drag significantly, enabling slower/steeper landing patterns.

But remember, high performance full scale sailplanes must operate efficiently over a wide speed range, ranging from about 40 - 45 mph while thermaling, to 100 plus or minus while running. For events measured in terms of distance as well as time, the same will hold true for model sail planes. They, too, must operate well over a broad flight envelope.

A high camber, high lift foil is a high drag foil at high speeds and a low camber, low lift foil is a high drag foil at low speeds. Flaps on sailplanes allow you to optomize the foil for minimum drag at each flying condition. A few degrees or so down increases the camber, thus increasing the lift coefficient and DECREASING the drag coefficient at slow speed thermaling conditions. While running between lift conditions, decreasing flaps, to include a few degrees negative, decreases camber, thus decreasing lift coefficient AND drag coefficient while high speed flying. i.e., flaps on sail planes act more as variable camber devices than as high lift devices.