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View Poll Results: What do you consider the fuction of the flaps and elevator
elevator controls AOA which requires different flap settings to be efficient 4 66.67%
flaps (flight modes) control the speed with the elevator controlling Angle of Attack 3 50.00%
Multiple Choice Poll. Voters: 6. You may not vote on this poll

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Old Aug 29, 2012, 04:57 PM
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Elevator: Controls airspeed or Angle of Attack

I've been thinking a bit about the function of the elevator and flaps in a sailplane I think it selects the angle of attack which in turn changes speed that requires different flap setting to be efficient. But isn't that what people use different modes for, IE (Launch, Run, thermal, landing)? I really have no idea so I want to know what do you consider the function of the elevator and flaps?

B36
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Old Aug 29, 2012, 08:34 PM
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The elevator is a device to control the wings angle of attack. In trimmed flight in Cruise Mode, for example, the elevator provides a down force and nose up pitching moment around the CG that just balances the nose down pitching moment of the airfoil section being used. That said, you can trim in Cruise at various speeds depending on where you set the elevator (thereby changing the angle of attack of the wing and its lift).

Lowering or raising the flaps changes the lift, drag and moment characteristics of the airfoil. It is likely that going from Cruise to Thermal or Speed modes will require a small elevator trim change. In practice I trim a little nose up from the Cruise setting in Thermal to slow down and try to get close to the max L/D of the airfoil (Max Endurance). And I trim a little nose down in Speed to push the speed up towards Max Range. Elevator trim is being used as a speed control.

In landing, typically large flap deflections are used. This is mostly to increase drag, but also increases lift which is handy as the airspeed falls off on approach. The commonly used sailplane computer radios provide an elevator to flap mix that allows elevator deflection with flaps. But, as always, the elevator is controlling the wings angle of attack and airspeed by putting in a nose down pitch to offset the increased drag of the wing.

So, to me, elevator trim is speed control while flaps are a wing efficiency control mechanism. They are complementary, but have different functions.

Hope this helps a little.

lc
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Old Aug 29, 2012, 10:47 PM
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Originally Posted by b36roxs View Post
...requires different flap setting to be efficient.
I don't see flaps as an efficiency thing. Deflecting the flaps is always less efficient than not.

They just let you choose between the "most efficient" balance, trading lift for speed, or trading speed for lift.

Once that choice is made, you still use the elevator trim for speed control within the range the adjusted airfoil provides.
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Old Aug 30, 2012, 12:22 AM
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I don't see flaps as an efficiency thing. Deflecting the flaps is always less efficient than not. .....
That is incorect. For each lift coeficient, there is an ideal flap (camber) setting. More or les cammber than the ideal gives more drag.
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Old Aug 30, 2012, 09:10 AM
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Elevator definately controls AOA. In a glider this will have clear impact on air speed.

Flaps modify the wing profile for greater lift at the cost of drag or to provide a lot of drag to slow the plane while lowering stall speed.

Their function is separate but they are often used together.
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Old Aug 30, 2012, 09:38 AM
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A great mind once said: "For every action there is an equal and opposite reaction".
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Old Aug 30, 2012, 12:21 PM
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Originally Posted by Phil Barnes View Post
That is incorect. For each lift coeficient, there is an ideal flap (camber) setting. More or les cammber than the ideal gives more drag.
Ok. How does one select lift coefficient?

I thought that you chose it with your flap deflection, but that's not correct?
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Old Aug 30, 2012, 12:38 PM
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Originally Posted by rdeis View Post
Ok. How does one select lift coefficient?

I thought that you chose it with your flap deflection, but that's not correct?
Coefficient of lift (Cl) is a function of the current airfoil configuration (including camber) and the angle of attack (alpha). Angle of attack is most directly determined by the horizontal stabilizer/elevator position, but it's not quite that simple, of course.
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Old Aug 30, 2012, 03:31 PM
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Originally Posted by Wazmo View Post
Coefficient of lift (Cl) is a function of the current airfoil configuration (including camber) and the angle of attack (alpha). Angle of attack is most directly determined by the horizontal stabilizer/elevator position, but it's not quite that simple, of course.
I see, and that is what drives the original question.

Restated with simplifying assumptions:

Do you "select Cl" by setting AoA with the elevator and then match the flap setting to that, or do you set the flaps first and match the Cl to them by setting AoA with the elevator?


I'd assert that if you want to fly at max efficiency all the time you'd couple the two together with a mix so that it doesn't matter-- they always match.
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Old Aug 30, 2012, 03:35 PM
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I'd assert that if you want to fly at max efficiency all the time you'd couple the two together with a mix so that it doesn't matter-- they always match.
Correct, though a mix typically won't be exactly right for all circumstances. Some people will control camber manually with the "throttle" stick or a slider, but I find that's too much work load for me. I usually use a linear or bi-linear mix from elevator to flap, so I get more camber with up elevator and less with down.
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Old Aug 30, 2012, 03:42 PM
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Remember that deflecting the flaps actually changes the angle of attack because that angle is between the center of the leading edge and the tip of the trailing edge. Dropping the flaps 10 degrees increases the AoA by an angle less than that, depending on the wing/flap chord numbers. This all happens independently of any elevator/stabilator input.

i.e. Your poll needs more choices...
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Old Aug 30, 2012, 04:24 PM
R2R
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Originally Posted by schrederman View Post
Remember that deflecting the flaps actually changes the angle of attack because that angle is between the center of the leading edge and the tip of the trailing edge. Dropping the flaps 10 degrees increases the AoA by an angle less than that, depending on the wing/flap chord numbers. This all happens independently of any elevator/stabilator input.

i.e. Your poll needs more choices...
+1. Good point.

To the question of whether to apply flaps or elevator first: the best thing IMHO is to apply the correct amount of flap and elevator simultaneously to change or maintain the desired glide path (L/D). That combination can change non-linearly, which is why we love our multiple-point curves in our mix profiles.

Sometimes we want most efficient; sometimes we are willing to incur a little more drag to increase lift in a thermal or launch; sometimes we want to slow the speed and have a steeper glide path for landing.

Not an exact analogy, but ... what's the best way to slow down a car: down shift, step on the brake, let your foot off the gas? It depends on what you are trying to accomplish and what you intend to do in the next phase of the car ride -- accelerate around a curve, park, keep on cruising.
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Old Aug 30, 2012, 05:51 PM
mosquito303
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I have the answer. You always should say "it's the flow over the tail" that causes it. Whatever it is.
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Old Aug 31, 2012, 11:10 AM
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Quote:
Originally Posted by rdeis View Post

I'd assert that if you want to fly at max efficiency all the time you'd couple the two together with a mix so that it doesn't matter-- they always match.
Max efficiency is a pipe dream. You will never attain MAX efficiency.
Airspeed has a lot to do with efficiency of the airfoils we use. Each airfoil has a different efficiency level. Some fly faster and some fly slower. Since your airspeed is not constant the efficiency is different and with the camber changing wing configuration it is up to the pilot to choose the camber setting for the speed or conditions present.
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