Stalling in a turn
 

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?:confused:

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.

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.

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

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.

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.

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.

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').

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.

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.

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.

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.

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.)

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.

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:)

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 :p

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!

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.
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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.

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.

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.

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.

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 :ugh:

Thanks everyone some very interesting responses here.
Don't worry - I wont be doing any practicals by my selvsy for quite some time.

I WOULD like to experience this however so I can appreciate it for what it is. Maybe I'm trying to walk before I can run; but I certainly don't want to find out what it's like when turning off base, onto finals:eek:

Did my annual spin check in the Puchacz glider last weekend. The guy in the back wanted a spin to the right; so, told me to put on a few degrees of right bank and slowly bring the stick all the way back. It seems I cheated by keeping the yaw string straight as the Puch spun left.

Perhaps we should have tried a bit of left bank to get a right hand spin -- or a bit of skid.

Of course the classic spin turning final is a shallow banked turn from a tailwind on base "helped" by a bit of rudder. A well banked turn goes a long way to avoid that.

Trust me, nobody here wants to find that out right there and then.:ok:

I don't know if your club/school offers "unusual attitude" training. Mine does. It starts in the morning with a thorough briefing about stall/spin awareness and execution, and then each participant does a 45-minute flight in an aerobatics airplane (R2160 in our case), loaded within aerobatics limits, to experience precisely these things. As this day is taught by normal qualified instructors, you can already attend while training for your PPL (although you'd have to be quite at the end of the course to get meaningful value out of it) and the hours count towards your PPL experience requirements. (As far as I'm concerned, they should even count double...:)) So check around to see if something like that is offered where you are training.

Alternatively, places like Ultimate High offer Advanced PPL training which includes stall/spin awareness and such.

Final Turn
 

An old ditty around Aero Clubs used to be :

" I watched him spin, I watched him burn, he held off bank in a gliding turn."

Tmb

To the original question from goggles on target. A picture is worth a thousand words, and it's difficult to describe/explain, Hopefully this makes sense:

The angle of both wingtips in space is the same.
The wingtips climb at the same rate, if they were not, you would be rolling.
The inner wingtip moving slower than the outer - otherwise you would not be turning.

As the inner wingtip is slower, takes longer to move a given distance, so will climb more in that given distance than the outer would in the same distance - it is rising at a steeper angle. So, it's angle to the relative airflow is less.

As to experiencing this - I recently did a skills test (converting my license from another contracting state), and was required to demonstrate a stall and recovery off a climbing turn, so I suspect it may well be an exercise somewhere in your training. You'll most likely sit there in a ridiculous attitude waiting for the stall and wondering how on earth anyone manages to do this accidentally.

@bookworm - I know what you mean, and I'm going to have to experiment, but I'm pretty (very) sure i'm not skidding turns! and to 172 driver, I have to say my experience in the 172 is the opposite, perhaps down to rigging differences and / or perception.

And to spinning - glider spinning is a rather different, and more gentlemanly game - the wing drop you get from a C150 is quite violent I'd agree, however, a spin it is not. Provided you're not too shocked (you will be unless it's something you've seen a bit), it can be stopped very rapidly with standard stall recovery and minimal altitude loss - you just fly the wing out of the stall and the problem goes away; it takes quite a while to turn into a spin proper. If however you should start trying to pick up the wing with aileron, and persist in holding the stick back, they you truly are in trouble, ditto if you happen to wait until you're upside down before doing anything about it. Personally I subscribe to the glider stall recovery of getting the nose down pronto, I don't like this powered idea of keeping a relatively flat attitude and hauling it out with power.

That's true enough for a climbing turn, Mark -- it's the "staircase" that was used as an analogy earlier -- but here's the passage Goggles quoted from Pratt (I don't have the book so I can't verify it)

The claim is that the outer wing has a higher AoA than the inner wing in a level turn. I don't believe that it's true.

Ah, yes.. RTFQ. Doh!

Goggles on Target

Suggest you google: See How It Flies--J.S.Denker
Read Sections 8 & 9 for the answer to your question.

If you're not aware of this internet resource it is an online book written by a flight instructor who also happens to be a Physicist.It is the finest account of the fundamentals of flight that I have read (and I've read quite a few.)Aimed specifically at us (amateurs at the light end of aviation) so nothing on mach effects/jet turbines etc. Since I have only a rudimentary grasp of mathematics, physics and aerodynamics I am not in a position to be critical of the text but practising Aerodynamic Engineers that have posted on these forums in the past have recommended it.

Read and understand why a statement like:
" 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"

is wrong.

This text coupled with Stick and Rudder by Langewieschie should be required reading in any flight training.

TIM

• BackPacker - Thanks, I'll look into some advanced SSA when I have a few more hours in my log book, I think it will be invaluable.
• BookWorm - Thank you, You got it :D You indeed, RTFQ:}
• RansS9 - That's an awesome resource, thanks.

So the quote from PPL1 could read:
“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 because the pilot has to deflect the ailerons against the turn due to the faster (higher wing) producing more lift than the inner. Therefore the higher wing will reach the critical angle of attack first and stall, actually rolling the aircraft out of the turn.”????

If this level turn is balanced it would indicate that some power & trim had been applied to maintain this balance. If the turn is balanced the aircraft must be travelling at a constant airspeed. Therefore although the outer wing is describing a greater diameter than the inner wing, the two wings are balanced by nose up trim & power+. How can the a/c stall in such a balanced condition? & why would the outer (faster) wing stall first in the unlikely event that the laws of physics have been broken?
I have set my Emeraude up in a balanced level turn & taken the hand off the stick to take photographs, it takes a bit of care to get it balanced well enough & ensure it is going to stay there, but not a problem. I must try it one day to see how long it will stay balanced before it falls over!

I've been required to demonstrate a stall and recovery off a descending turn, the spin-in-from-final-turn scenario, and I was left wondering how on earth anyone manages to do this accidentally.

But I know that they do, and that they die.

What this has done to me is make me very careful when I misjudge the wind on base leg and overshoot - I try to recognise the scenario and positively think to myself "shall I chop the power and tighten the turn, or shall I do something more sensible?".

Oh dear, is power really needed for a balanced turn ? That's me screwed in my glider then ;-(. Being serious, constant speed can be achieved by attitude and not just throttle. Furthermore a wing stalls when the critical AoA has been reached and this is expresssed as AoA relative to airflow not necessarily direction of travel. Also a wing is 'stalled' when the drag it generates exceeds the lift it's generating. So, it's entirely possible for a wing that's travelling physically faster (but with a higher AoA) to be generating an excess of drag over lift than the 'slower' wing. Throw in the square rule for lift and drag and you'll see that any situation where drag > lift is further exacerbated by speed!

Stalling in a turn is no different than stalling with the wings level; recover the same, fly the same; it's the same airplane. It hasn't become a different airplane.

You shouldn't need stick into the turn our agaist the turn. In fact, in most light general aviation airplanes, a standard demonstration to the student should be a hands-off steep turn in which the instructor puts the airplane in a 45 degree bank, trims it for pitch, then lets the airplane fly around the turn hands-off.

That said, the aircraft, loading, degree of bank, and center of gravity do play a roll, and not all aircraft respond the same, or are flown quite the same.

For most light production airplanes, a stall in a turn is a non-event, every bit as much as it is straight and level.

Keep the inclinometer ball in the center with your feet, and fly the airplane, Whether it spins or not is entirely up to you. So far as the 150 and 172 previously mentioned...one has to work to make them spin. They don't bit agressively out of a simple turn unless the pilot isn't coming close to doing his job.

No, throttle isn't needed in a balanced turn, but it is in a level balanced turn. A level balanced turn is rather difficult in a glider unless there is lift from some other source?? In which case you would be unscrewed.:ok:
Attitude can control speed, but not level flight. I had a glider once, a K6E, it would only fly downhill:confused:

Crash one, gpn01 - I think in general parlance, and almost certainly in the example cited, a 'balanced' turn is taken to be one where you are neither slipping, nor skidding - i.e. the longitudinal axis of the aeroplane is aligned with where it is going, no particular consideration of airspeed.

Sorry but I disagree just a bit. The original question referred to a balanced level turn. This would imply a constant airspeed, it would have to be in order to be both level & balanced. More power would be reqd than for the same airspeed in a balanced level cruise.

Isn't the manuevor as described by the OP an FAA accelerated stall? (A standard PPL requirement)

My recolection was simply a faster break with a bit more pronounced wing drop (dependent primarily on how well coordinated the turn was), but at quite a bit higher airspeed (exactly as described by Denker)

Please explain this then guys.

Wings stall at an angle of attack, not a particular speed.

An aircraft in a steady balanced level turn is developing the same amount of lift from both wings, otherwise it would be rolling.

The outer wing is travelling further than the inner wing (in the same time, of course,) therefore it is going faster.

Since the amount of lift developed by a wing varies according to speed and angle of attack (and a constant) , if both wings are developing the same lift and one is going slower than the other the slower wing must have a greater angle of attack.

If speed is decreased the angles of attack must increase to maintain level flight The wing with the greater angle of attack, the slower one, must stall first.



During the fifties and sixties both Cessna and Piper were spending vast amounts of time and money to develop aircraft that were easier to fly, ( 'A plane in every garage,') and less easy to crash. Along the way some of the aircraft gained unusual handling characteristics.

I did turning stalls as a part of the FAA CPL in a 172SP, at many angles, along with all sorts of other stalls like accelerated stalls. I never spun once, recovery is the same - reduce the AOA of the wings and don't hit the ground.

Perhaps post-PPLs in the UK should be encouraged to explore the envelope rather than scare them off with scare stories of spinning and death?...that is if they can find an instructor who is not too scared...?

Couldn't agree more - both that it's harder to spin than most think, and that spinning should be experienced at some point. Pretty sure nobody said don't, but don't go and do this on your own. 90% of folks have no idea where the edges of the envelope are, or what they feel like, which is both a waste of capability, and in my opinion dangerous.

However, this is another pprune debate which borders on holy war. Expect someone from the anti-spinning brigade along shortly using words like 'macho rubbish', etc.