Wing Drop stalling with flaps - why??

[wheels up]

Could someone explain why lowering flaps aggravates wing dropping tendencies at the stall. Possibly I am just stupid but I would have expected the opposite.

Lowering flaps will increase the effective angle of attack as well as the camber of the inboard sections of the wing. Surely then one would expect the inner wing sections to stall (because of the greater AOA and camber)before the tips thus promoting stability at the stall? Why the wing drop then??

[john_tullamarine]

The problem, which can be quite frightening, relates to asymmetric lift distributions, however caused. Some of the light aircraft LE VG mod kits cause considerable problems in this respect. I guess that most of us have seen the world substantially upside down due to this phenomenon.

It does not necessarily follow that flap extension will cause the inboard sections to stall before the outer in the ordered manner which you describe, although that is usually desirable to meet the aileron requirements in the stall regime. There are many detail design matters involved here. I suspect that you might be forgetting that the CL-alpha curves vary quite a bit as LE or TE devices are employed.

I have always suspected that much of the realworld problem relates to

(a) manufacturing tolerances
(b) small rigging variations
(c) in-service surface deterioration, contamination, and damage

- one doesn't need too much of a lateral lift asymmetry to generate significant rolling moments.


[This message has been edited by john_tullamarine (edited 24 May 2001).]

[little red train]

if you draw out the CL/alpha curve, you'll see that over the stallnig angle, say 14deg. increase in AoA results in a reduction of lift. Therfore in the stall, you are CL/Alpha Unstable.

Wing drops (due to minor differences ect)
AoA increased - over stall angle so lift further reduces.
Wing drops more.

I think the dragy barn door type flaps on some trainners exentuate the loss of lift and drag, also at a slower speed, a reduction is a bigger percentage of total airspeed giving the drop more gusto.

[robione]

Lets give u a straight answer.Lowering flap reduces lateral stability.The centres of pressure move inboard,therefore reducing the moment arm.
BINGO, now we are really unstable laterally at the stall.The result is to change the rolling moment produced by wing dihedral.When a reduced lateral stability exists[FLAPS LOWERED AT THE STALL]a wing which tends to drop,will drop further and faster

[Gerund]

Wheels up (and flaps down) -

After drafting up my answer in Word to paste in, I find robione has beaten me to it! Still, here is my farthing's worth:

In answering this query I am assuming that we are looking at basic aerodynamics and considering a standard training aircraft with inboard trailing edge flaps.

When we talk about a wing drop at the stall, we are talking about a sudden loss of lateral stability.

The severity of the wing drop depends on two factors:

a) The imbalance in lift between the wings

b) The distance of the centres of pressure from the longitudinal axis.

In an unflapped wing, if one wing starts to stall at the tip first, the centre of pressure will start to move inboard. Not only has the lift decreased, but so has the length of the arm to the longitudinal axis. A double whammy!

If, by the use of washout, or root spoilers etc we can ensure that the wing root stalls before the tip, we can ensure that as a wing starts to stall, the initial loss of lift occurs inboard. The centre of pressure will then move out towards the wingtip. This increases the length of the arm and the turning moment derived from the remaining lift.

Right, down with those flaps!

Firstly, you are bang on right that the inboard sections will stall before the outboard sections. This would seem to imply that this would lead to improved lateral stability at the stall. However, there are other factors at work:

a) With flaps extended the centres of pressure move inboard due to proportionately more lift being produced on the inboard section of the wing.

b) At the stall, or even at angles of attack past the stall, the flapped aerofoil can still produce more lift than the unflapped aerofoil, even though the latter is unstalled. What happens is the the coefficient of lift/angle of attack curve moves substantially upwards and slightly to the left. So, even with the inboard, flapped, wing sections stalled, the centres of pressure are still further inboard when compared with the unflapped wing. This leads to decreased lateral stability.

So, what it all comes down to is the lateral instability caused by the inboard movement of the centres of pressure.

Doesn't add a lot to robione's answer really, who put it much more succinctly!

[john_tullamarine]

People,

Although there might be a few subtleties missed in regard to the stability comments above, the problem really is one of in-service stall character deterioration for whatever reason.

At certification, the stall character was shown to be reasonably benign, and the lateral control capability to be reasonably effective. Yet, here we are, with an in-service aircraft which now exhibits most undesirable characteristics - I can recall one PA31 mod program when, to my surprise, I ended up quite inverted during a very docile approach to the full flap stall - interesting. Subsequent testing demonstrated that the problem was not mod related and it was hand balled back to the local regulator - I have no idea what happened in regard to its resolution.

I suggest that a simplistic playing about with CL-alpha curves and thoughts of pressure derived forces moving nicely inboard is just that - far too simplistic ....

And .. when the talk gets around to consideration of each wing's operating to a different side of the stall alpha, then a consideration of Cd asymmetry results in that condition known as autorotation ... which is not nice in little trainers not designed for that sort of thing.

[This message has been edited by john_tullamarine (edited 25 May 2001).]