Stalling in a turn

[GogglesOnTarget]

I’ve been reading AFE PPL1 by Jeremy Pratt and in exercise 10b it says this:

“When turning there is an increased tendency for the wing to drop at the stall, but not in the way you might expect. In a balanced level turn, the higher (outer) wing has a higher angle of attack than the lower wing. Therefore the higher wing will reach the critical angle of attack first and stall, actually rolling the aircraft out of the turn.”

I thought that if you’re in a balanced level turn one would only be applying back pressure on the CC (the rudder and ailerons would be central). So how can the outer wing have a greater angle of attack? Surely it’s just higher off the ground than the lower wing.

I thought the lower wing would stall first as it would be going slower (less airspeed due to following a shorter track in the turn).

Jeremy or anyone else got an explanation?

[Ultranomad]

The outer (upper) wing has its aileron deflected down, the inner (lower) one has it up. Thus, assuming the relative wind on both wings roughly the same, the outer wing considered together with its aileron will have a higher AoA. The difference in relative wind between the two wings ("shorter track in the turn" or whatever) is negligible compared to the effect of ailerons.

[172_driver]

Actually, a certain amount of opposite aileron is required to maintain bank angle. Thus, the aileron on the outer wing will deflect up and "reduce" AoA on that wing.

The outer wing's higher AoA can instead be visualised if you take a board, put it across and walk up a spiraling staircase. The rail represent the relative airflow to the board's "chord line". You will se the angle between the rail and the board is greater along the outer rail than the inner.

Same can be said about a descending turn, do the same experiment and you'll see why in a descending turn the inner wing stalls first, flipping you into the turn.

[Flyingmac]

Be aware that this is a general rule. There are exceptions re a/c type or configuration.

[rusty sparrow]

I had this very convincingly demonstrated to me in a Cessna 150, where my instructors way of getting into a spin was to enter a 30 degree banked turn at low power (i.e. final turn setting). As instructed, I held the altitude while banked and with insufficient airspeed and then it flicked into a spin with the higher wing dropping. A good lesson - spinning in from final turn is a killer.

[Halfbaked_Boy]

172 driver,

Without meaning to sound patronizing, every Piper and Cessna (at least) that I have flown requires INTO turn aileron during the turn, due to a little phenomenon called positive static lateral stability.

Therefore, in a left hand turn, the right aileron will be deflected down slightly, increasing the effective AoA on that wing.

[bookworm]

Without meaning to sound patronizing, every Piper and Cessna (at least) that I have flown requires INTO turn aileron during the turn, due to a little phenomenon called positive static lateral stability.

Without meaning to sound patronizing, if you'd used the rudder to avoid slipping out of the turn and keep the aeroplane in balance, you wouldn't have needed the into-turn aileron.

If, in general, into-turn aileron were required to maintain bank, the spiral dive would not be the killer that it is.

[GogglesOnTarget]

Thanks everyone for your responses:
AntonK - surely in a balanced turn, ailerons would not be deflected.
172Driver - Doesn't the spiral staircase analogy suggest a climb or descent?
RustySparrow - I'll give it a go (@ 3000').

[Ultranomad]

GogglesOnTarget, even in a balanced turn, ailerons will still have to be deflected. They will not be deflected only in an aircraft with neutral stability, whereas the vast majority of planes (with the possible exception of fighters and unlimited aerobatic acft) are designed to have a fairly substantial reserve of positive stability.

[Mark1234]

And I'll agree with AntonK here - next time you're in a balanced 45degree turn, look down the wing.. Spiral dives exist because the pitch effect is markedly stronger than the roll stability.

As for stalling in a turn, I find it helpful to consider what is admittedly a fairly far fetched boundary condition - imagine the aircraft is climbing vertically in a spiral with the inner wing anchored to a pole - so the inner wing has zero forward speed. The relative airflow is from directly above, inline with the chord of the wing.. Now consider the outer wing - that has a forward component also, so the relative airflow is from somewhat ahead of that chord line - coming from underneath the wing - it could easily be stalled.

Now, if you move it out to more sensible climb angles, and more sensible sizes of circle, the same relative effects remain - if the inner wing is at 0 deg AOA, the outer will be at some +ve AOA; both wings climb at the same rate. Add in the aileron effects too, and it should be clear why the outer wing goes first.

[bookworm]

[The ailerons] will not be deflected only in an aircraft with neutral stability, whereas the vast majority of planes (with the possible exception of fighters and unlimited aerobatic acft) are designed to have a fairly substantial reserve of positive stability.

The stability that you describe is lateral stability originating from roll-slip coupling. A slip causes a rolling moment. There is no stability in roll without slip -- the aeroplane has no inherent way of knowing which way up it is, so it can't provide a rolling moment in response to an angle of bank.

Spiral dives exist because the pitch effect is markedly stronger than the roll stability.

Presumably you'd agree that the bank angle increases in a spiral dive. How does "the pitch effect" cause the aircraft to bank into the turn?

In a turn without slip, the yaw-roll coupling causes an into-turn rolling moment. In essence, the outer wing is moving faster and generates more lift than the inner wing.

[BackPacker]

RustySparrow - I'll give it a go (@ 3000').

HOLD IT just a second.

ONLY do this if you are current in aerobatics (or under competent supervision) and if you are in an aircraft that's cleared for spinning. And even then the POH might limit the ways a spin may be entered.

This is definitely not something you will want to experiment yourself while you're still flight training (which seems to be the case here if you're reading that book.)

[IO540]

There is no stability in roll without slip

I thought that wing dihedral provided some stability in roll. The wing which - as a result of a disturbance around the roll axis - becomes more horizontal is going to produce more lift, and thus counter the roll.

It's not a strong effect though, and every plane I know of (except some model planes, with massive dihedrals) will go into a spiral dive eventually.

[Lister Noble]

Entry into a spin is sudden,scary and disorienting,even if you've got your licence.

As already mentioned.
If you are learning DO NOT TRY THIS ON YOUR OWN.

(sorry to shout,but it can be a killer)

When you have a few hours it's worth having some spin training with a qualified instructor,in a spin certified aircraft.

Lister

[Mark1234]

Originally Posted by IO540

I thought that wing dihedral provided some stability in roll. The wing which - as a result of a disturbance around the roll axis - becomes more horizontal is going to produce more lift, and thus counter the roll.

Y'know, much as I hate to admit it, I'm getting bookworm's point here - in a co-ordinated turn, the force is still directly perpendicular to the wing / parallel to the vertical stab, so the wing is equally loaded, and there is no 'more lift' from the more horizontal part of the wing. Seems dihedral would only help when the 'weight' vector is off axis. Hmm.

Come to think of it, I'm sure I can recall gliders which required positive out-stick to stop their tendancy to roll into the turn, however, that doesn't explan why all the short winged noisemakers I've flown *do* seem to require a little in-turn stick... and I do know what a co-ordinated turn looks like

WRT spinning - what BackPacker, and everyone else said! Mind you, I think rusty sparrow is overstating the case rather.. never met an aircraft that could enter a spin instantly yet; it may flick at the point of stall, but if you stop yanking back on the controls it'll stop long before a spin develops Still, don't mess without adequate supervision and knowing what might bite you. Please!

[rusty sparrow]

I 've done scores of intentional spins in gliders so am happy with them. But I was surprised how quickly the C150 flicked over into a spin - we were at 4000 over the sea at the time and when it ran out of flying speed, the upper wing lost life and dropped us into a spin. Others who know more about aerobatics than me can comment more.
.
Spinning is no longer in the syllabus - it was when I converted my licence to a UK one. But the method of getting into a spin then was the unreal one of stalling and then kicking in rudder. Then you expect to spin.

But flicking into a spin out of a turn is a powerful lesson.

[bookworm]

I thought that wing dihedral provided some stability in roll. The wing which - as a result of a disturbance around the roll axis - becomes more horizontal is going to produce more lift, and thus counter the roll.

A wing that's "more horizontal" doesn't produce more lift. It just produces its lift in a direction closer to the vertical. But if you take both wings together, the direction of the resultant simply rotates with the aircraft. There's no net moment about the roll axis.

You can also envisage angular displacements that are not strictly about the roll axis but about an axis displaced from it by the angle of attack. Such displacements do result in a rolling moment, but only by introducing slip. The fundamental issue is that an airframe (with side to side symmetry) has no way of knowing which way up it is, only of its slip or pitch angle, and how much it is yawing, rolling or pitching.

[Shunter]

In most Pipers/Cessnas a stall on base/final turn config isn't going to dump you straight into a spin. Low speed, high nose, 20deg bank with flaps the picture is going to be completely wrong and the stall warner will (or should) be blaring way before anything untoward happens. However if you ignore all the above, the high wing can drop and it most likely won't be a nice gentle drop, it'll be sudden and surprising but still gives you an opportunity to recover before it all goes pear-shaped.

The same recovery is required as for any stall - controls centrally forward, full power, rudder to prevent further yaw. Don't be trying any of that "pick the wing up with rudder" bollocks. If you don't do anything about the drop (or do the wrong thing) that's when you're into spin territory. A quickly recovered wing-drop on base/final turn will probably not kill you, a spin probably will. If you've ever managed to drop a wing unintentionally in the average Piper/Cessna you're doing something seriously wrong.

As mentioned by many other wise and experienced posters, do it with an instructor. From a sensible altitude. It's worth it, and probably very different to how you might perceive it.

[bookworm]

Come to think of it, I'm sure I can recall gliders which required positive out-stick to stop their tendancy to roll into the turn, however, that doesn't explan why all the short winged noisemakers I've flown *do* seem to require a little in-turn stick... and I do know what a co-ordinated turn looks like

I have a theory but you might not like it.

You don't need to a balance a turn to make an aircraft yaw. You can leave an aircraft slipping out of the turn and rely on the directional stability to yaw it round. That's what happens if you don't apply the rudder necessary to balance an already-established turn -- note I'm not talking about the much more significant rudder input often required rolling into/out-of the turn because of adverse yaw.

The amount of slip for a given turn-rate or bank angle depends on the airspeed and the coefficient of directional stability. In a glider, trying to drag it around a turn in a slip is an ugly, noticeable process, and you probably use rudder carefully to balance it. In a short-winged noisemaker, it's much more difficult to detect the slip, and less rudder is generally required to balance it, so little in fact that we often don't use it. And the balance ball is not very sensitive. So we actually tend to fly them around with a little out-of-turn slip, which produces an out-of-turn rolling moment, which at low bank angles outweighs into-turn roll of the differential wing speeds. I reckon if you used a slip-string to balance the turns, you'd find a tendency for the aircraft to roll into the turn.

As you implied Mark, one good test is worth a thousand opinions. I'd encourage everyone to look at the aileron input required (i.e. control-wheel/stick position) next time they try a steep turn. I'd be surprised if you find that in-turn aileron is required to maintain the bank angle.

[172_driver]

If I read the arguments correctly, I am with bookworm here. Coordinated turn, no slip, no positive lateral stability effects at all.

I am sure there are people with more experience than me, but about 1000 hrs in C172 I say you definately need a small amount of opposite aileron pressure on the yoke not to cause overbanking (due to outer wing moving faster). This becomes more dominant in climbing turns when the airspeed is lower and the relevant velocity of the two wings is higher. Try a coordinated climbing power-on stall and you'll se what I am talking about.

GooglesOnTarget, indeed the analogy suggests climb/descent. Me not reading the question properly