Aileron control with negative flaps or spoilers on initial roll

I would like to understand why the use of negative flaps or spoilers provide better aileron control on initial roll (low air speeds). It is not intuitively obvious why to me. Please point me to past publications or articles that cover this subject. Thanks.

If you can accept a layman description... Regarding the negative flaps, it seems that when you're on the ground the wing's angle of attack is very high, so that at low air speed it's actually stalled. By using negative flaps you 'reduce' the angle of attack, thus somehow anticipating the moment where the wing starts flying.

About the spoilers the point is different, and probably easier to figure out. The spoilers are actually 'destroying' the lift from an area of the wing, conseguentely transferring it to the remaining of the span. So the ailerons are more charged e more effective. You can note this transfer of lift if you observe a glider on final, preferably one with a large span or a flexible wings (like an ASW20): everytime the spoilers are extended the wings bend up, and if retracted they flat down, and this withoud any change on the g's.

Of course now I'm looking forward for a really technical description...

If you can accept a layman description... Regarding the negative flaps, it seems that when you're on the ground the wing's angle of attack is very high, so that at low air speed it's actually stalled. By using negative flaps you 'reduce' the angle of attack, thus somehow anticipating the moment where the wing starts flying.

I agree with most of your explanation except the discussion above of angle of attack. Stalling is a function of true angle of attack, not airspeed. My approach to the use of negative flap reduces to the same argument you used regarding spoilers: i.e. that the lift of the wing is reduced, thus any changes at the aileron produce a bigger *proportional* effect.

To get back to the stall argument, there is a belief that there is a 'stall speed' below which the wing will be stalled. This can be demonstrated. However this speed is dependent on the wing loading (All-up weight of the glider/pilot combination). What actually happens is that as you reduce the speed, to keep a reasonable glide angle, you are increasing the angle of attack until at some speed the required angle of attack reaches the limit and the stall occurs. Remember that it is the true angle of attack we are talking about, not the angle made between the wing chord and the apparent (to the pilot) path of the glider. In other words, as the glider gets slower, the sink rate starts to increase, so the actual air flow direction is pointed further upwards, giving a larger angle between the direction of airflow and the wing chord than you might expect.

Discussion anyone ?

Cheers, John G.

How about this... at zero G you cannot stall any wing. Think about it. 0G and 0kts = wing not stalled. 0G and Mach3 = wing not stalled.

You are absolutely correct in your statement about a 'stall speed'. This speed is only relevant in a given flap/spoiler configuration at a fixed wing loading (read g-load). Depart from any of those parameters and the wing will stall at a different speed. I've stalled an aircraft at Va to prove a point to a student.....+6G was not fun for either of us!

Miriano -

Your layman description is quite excellent. Don't change a word.

John,

Taking the example or situation originally described, ie, aileron control at takeoff, the glider (as far as wing loading is concerned) is at 0 g's... there is no wing loading initially. It only gradually builds as the glider picks up speed.

Larry Goddard '01' USA

I disagree with both explanations.

For the case of flaps: although the wing's angle of attack is high, it is below stall angle of attack. Stall is not a question of speed but of angle of attack. Precisely the stall angle of attack is the angle of angle of attack where the lift coefficient reaches its maximum value, so that a further increase in angle of attack results in a decrease of the lift coefficient. The notion of stall speed is only related to this as far as the plane is airborne, since any decrease of speed must be compensated by an increase in lift coefficent in order that the lift balances the weight of the aircraft, and when the angle of maximum lift coefficient is reached, no further reduction of speed is possible while maintaining the balance of forces. No sailplane when sitting on its landing gear on the ground has an angle of attack equal or very close to the stall angle. However, as the angle is nevertheless higher than in normal flight, the increase of lift coefficient as a function of aileron deflection may be not so steep as at lower angles of attack, so reducing it may help. This is especially true on sailplanes where a coupling between flaps and ailerons let the ailerons drop with the flaps, although with less deflection, or on sailplanes with flaperons. But even with ailerons independant from flaps, negative flaps decrease drag and so allow a better acceleration and the speed where ailerons work is reached earlier. Another reason for negative flaps during initial roll with ailerons coupled to flaps is that in this lowered position increased by the attitude on the ground, moving them, beside generating changes of lift, also genarate huge changes of drag and so much adverse yaw. I experienced personnally on a LAK-12 during landing that with flaps in landing position, this effect is so important that the rudder is unable to counter it.

For the case of spoilers: again this explanation is only valid when the ship is airborne, so that the lift must balance the weight. It then happens because the lack of lift produce an increased sink, i.e. an increase of angle of attack until the balance of forces is reached again. On the ground, if you cancel some part of the lift, you simply loose is, nothing compensate it. The explanation is probably rather in the fact that the airstream is forced to accelarate around the spoilers and some of this accelarated airstream flows along the ailerons. However this may not be true for all ships. I read in the flight manual of the ASW24 that extending the airbrakes helps the efficiency of the ailerons, but nothing similar is said for the LS4 and my own feeling is that the effect is opposite, during the ground roll after landing I find it is easier to keep its wings level with airbrakes retracted. This is consistent with the fact that on this glider there is some overlap between airbrakes and ailerons, so with airbrakes extended, some part of the ailerons is in turbulent flow behind the airbrakes.

My conclusion is that the reasons may be various or inexistant and only the flight manual of the glider gives the good answer and maybe the explanation. E.g. the flight manual of the LS6 says keep flaps at +5 from the beginning of the ground roll when taking off.

A STRONG WORD OF CAUTION.

I own an LS3. This is the ship with full span flaperons. For some time, I used the negative flap method and moved the flaps from -7 position to zero, then to +10 degree. The +10 degree is the recommendation in the manual.

I really couldn't tell you why I did this, other than other people telling me that this was the thing to do. Then, I had a life changing experience. As I transitioned from -7 degrees, one wing dropped and hit the ground. Before I could realize it, I was sliding backwards down the runway. I came up with a couple of presumptions as to why, but the 'why' part doesn't matter. I now follow the instruction manual and do not have a problem with a lack of control.

Marty Pautz