Steep Turn wing drops

[aviationcynic]

Probably an easy question, but why when stalling an aircraft in a steep turn, would you have a wing drop towards the top wing instead of the bottom wing?
My reasoning says it should be the inside wing that stalls first, due to holding off bank with aileron.

Any good explanations out there??

One text book I have glossed over it by saying "Either wing may drop regardless of the direction of turn, but generally a slight sideslip angle predominates just prior to the stall, causing yaw opposite to the direction of turn, and this leads to the higher wing stalling first." (Flight Instructors Manual, R.D. Campbell)

I can't quite grasp that for as far as I know the inherent directional stability of the aircraft would cause a yaw in the direction of the turn, due to sideslip, not in the opposite direction.

[Dick Whittingham]

I don't think the sideslip issue is relevant, for it assumes sideslip is present, which is not always true, and should not be present anyway in a balanced turn. If a little sideslip were present just prestall, then whether the aircraft rolled in or out of the turn would depend on the balance between directional and lateral (sideslip) stability.

Basically asymetric aircraft like the Harvard, with big props going round one particular way, would usually always flick in the same direction at the stall. Clean, symetric jet aircraft in a properly balanced level turn should stall without wing drop, but if there is yaw on, most likely because of a rudder input, then right rudder will depart you to the right, and vice versa.

In a descending turn, however, typically on finals, the slower inside wing will likely stall first, for the descent rate is producing a greater increase in alpha on the slow wing. This rolls you down and into the turn, with usually fatal consequences.

Dick W

[batty]

The inside wing has a greater alpha and therfore is likely to stall before the outer wing.

[Dick Whittingham]

Not too sure about that. In a level turn both wings have the same alpha, but the inner wing is marginally slower than the outer wing. This gives more lift on the outer wing and thus a tendency for the aircraft to roll into the turn. This is not a stall, and I have only felt it on a big wingspan glider in a stonking turn.

In a descending turn a vector of vertical speed is added, which increases alpha, but increases alpha more on the slower wing. At near stall alpha this can push the inner wing over the edge.

Dick W

[batty]

Even in a level trun the inside wing will have a higher alpha, if you think of the vector diagram the the inner wings velocity component is slightly less than the outer wings and thus the equivelent alpha is higher on the inner wing. Similar in effect to the different alphas on upgoing and downgoing prop blades. :8

[aviationcynic]

The only reason the inside wing will have a greater angle of attack in a level turn is because of the use of opposite ailerons to hold off bank. ie the inside wing will have the aileron deflected downwards, which is effectively raising the angle of attack.

[Dick Whittingham]

Nope. Alpha is not directly determined by forward speed. In a level turn there are vectors of diferent magnitudes hitting the wing, but all of the same direction, horizontal. It is only when the relative airflow is determined by a vector sum of a vertical and a horizontal vector that you get different answers for alpha.

I accept that aileron affects the local alpha, but repeat that it is not possible to say in isolation if aileron in or out of the turn or neutral will apply. It depends on compex balances of directional and lateral stability. Modern transport aircraft are required for certification to have positive spiral stability, that is that they will roll out of turn if left to themselves.

Anyway, since no one knows whether an unspecified aircraft will flick one way or the other in a stall I'm going to have a beer and chill out. Best advice is not to stall at all.

Dick W

[MLS-12D]

Basically asymetric aircraft like the Harvard, with big props going round one particular way, would usually always flick in the same direction at the stall.I only have about 7 hours on type, but that has not been my experience. More often than not it is the right wing that drops, but certainly not always.

[Dick Whittingham]

Which is why I said "usually" and not "always"

Dick W

[flystudent]

search "stall outside wing" enter carriage return...

my internal data base has come up with the following..

The outside wing is travelling faster and thus creating more lift , as a result also creating more induced drag, drag = bad, a/c want to roll towards outer wing at higher speed as in adverse aileron yaw. (overcome by use of spoilers)

wahhhhh my god I must have learnt something in the last 5 months !!

of course it could all be pants, go back to school:eek:

Flystudent

[Dick Whittingham]

Steady on! In a turn the outer wing is going faster than the inner, but in a turn of 2nm radius at 250kt, for example, the percentage difference in forward speeds is tiny. In practice the effect only shows up in very small radius turns at low speed, so it is not a big aircraft problem.

If the outer wing is going faster it is again, technically, producing more drag, but not induced drag. Induced drag is a function of Cl, effectively of angle of attack, and is only directly related to speed when you look at the drag in straight and level flight, for in level flight angle of attack is related to speed squared.

Aileron drag only ocurs when aileron is applied, and while you may be holding small amounts of aileron on in the turn this is insignificant compared to the big drag input as you apply a lot of aileron to roll in or roll out. In fact, if our theory is right, and the aircraft is trying to roll into the turn because of the faster outer wing then you would be holding out-of-turn aileron which would give yaw into the turn, not out of the turn as does aileron drag as you try to roll in.

Don't get too tied up in this. Most of these effects are tiny (outer wing going faster) or are corrected by various fixes (adverse aileron yaw) The only critical one is - do not pull to the stall in a descending turn on finals.

Dick W