Keeping the wings level in a stall

[M14_P]

A&C, I dissagree there. Rudder IS used to pick up a wing, during the falling leaf manoeuvre, this is how you keep the wings level during a stalled condition, with constant rudder input (you need to be quick with your feet), you can keep the wings level and keep aircraft stalled, as it descends, it looks quite spectacular from the ground.

The Extra's spin entry is like any other aerobatic machine, I did some time in the 300L and found it is no different to any other in the entry. All spin entries are similar, however spin recovery, is a very different thing. When teaching inverted spinning in the Pitts I break it right down. Getting the student confident with recovering hands on, then at the very end, when they are proficient with the hands on methods in various attitudes with varying amounts of power and aileron, I show them the Beggs/Mueller method during a full accelerated and flat inverted spin from 7500ft. They are always suprised at how effective this method really is.

Anyway, regarding this "keeping wings level while fully stalled" topic, as stated above, I believe what you are effectively doing, is creating a falling leaf, which you control with rudder, and rudder only. The aircraft slews from left to right when viewed from the ground but in flight it just feels as if it is tipping from left to right (in the tiger anyway).

Interesting stuff.

[bjornhall]

What you say makes sense to me in the main, but I didn't like the idea of the roll damping 'catching' the roll. Unless I'm mistaken, damping is just a resistance to movement (in this case the roll), an inertia if you like. It will not actively restore the wings level, rather make the aircraft less susceptible to the 'fine detail' level of the wings stalling *exactly* at the same time? I'd still suggest they need to stall substantially together.

Yes, I think we more or less agree, when I am saying the wings do not need to stall at the same time I mean not exactly at the same time. A small asymmetry, such as one wing being slightly more stalled than the other, will be damped out by the roll damping. Without roll damping every stall would have a wing drop since there is always some tiny asymmetry present. But I agree with you that if there is any significant asymmetry, such as when stalling out of a slip, the roll damping will not be enough to prevent the roll.

There is a subtle difference between roll damping and roll wise inertia: The latter is resistance to rolling motion starting when a rolling torque is applied, but once a roll has started the roll wise inertia actually keeps the roll going. Roll damping on the other hand is resistance to the roll motion and will tend to stop the roll if it has started.

I'm also interested in the definition of slip and particularly yaw. Does an aircraft in a stable, straight line sideslip have yaw? It definitely has slip, and I know the fuselage is not pointing in the direction of travel, but I'm inclined to suggest it doesn't have yaw, and if it does, it doesn't have a rate of yaw - all parts of the wing move at the same speed, and in terms of direction and space, it is not moving around the yaw axis. In a sense it's no different to an oblique wing NASA AD-1 - Wikipedia, the free encyclopedia flying in a perfectly straight line.

Agreed there as well! When we use rudder to create a slip that will be turned into a bank by the slip roll coupling, we need to yaw... Once the slip is established, there is no yaw. In a dynamic situation where we try to keep the wings level with the rudder there will be altering amounts of slip needed and commanded, thus there will be yaw as well. Overcontrolling and dropping the wing in that situation is likely to carry both yaw and slip into the stall, and that is where the fun starts!

Seriously, great fun at 4000 ft when practicing or just goofing around, knowing we are going to stall and that it might be spectacular. Not fun at 400 ft on a slow final for a short field landing in gusty condition, when all we wanted was to keep the wings level in as safe and effective a way as possible. That is why I suggest and will only use rudder-only to keep wings level as a fun exercise at altitude when in that mood, not as a method for slow flight.

Of course, the rest of you do as you see fit!

[polyfiber]

I have never heard of a school that taught to use aileron to pick up a wing when stalled. However, just to confuse everyone, flaperons stay effective for roll control even in a fully developed stall. The downside of flaperons is the huge induced drag which results in adverse yaw when at low speeds. Example you roll to the left and the nose slews right if you are not on top of it.

Then to compound the problem when you finaly manage to stall the flaperon you get roll opposite to your stick input! So if you were leading with the rudder to control yaw and you stall the flaperon you have just set youself up for a spin Of course the only time you should get into that type of scenario is at altitude practicing slow flight where a spin wouldn't matter, or when you are a few inches off the ground when landing.

[Pilot DAR]

Though I don't disagree with the references to flaperon behavior, I can't think of a certified aircraft which empolys them. I flew the first CH701 many years ago, and experienced what has ben described. It was not good! I ran out of aileron control with flaps extended, and had to supplement with rudder (beyond co-ordinated flight) for standard manuevering. I did not have the opportunity to fly later versions, which I trust were improved!

Getting back to the use of aileron to pick up a wing at the point of a stall... I agree that a deflected aileron does have the affect of changing the local chord line, so as to "increase" the local angle of attack very slightly. However, does not deflected aileron also deepen the camber of the wing locally and thus increase the lift which that portion of the wing will produce, and its resistance to stalling? Would this not offset the affect of the increased angle of the chord line? I do agree that this would vary by aircraft type.

All that said, I continue to read posts, and hear about using rudder in isolation to control roll during a stall, but I do not read about it in any authoritative document. I do read about "normal" use of the control through the stall (which I tak eto contradict the rudder only technique). Why the discrepancy in information? Usually our industry is all about consistancy and traceability (of information, in this case). It seems not so much in this case...

[polyfiber]

Good question DAR, remember we are talking of a wing that is already stalled. It has reached its burble point and the airlow is in a seperated state over the top of the wing , no laminar flow at all. Any increase in the ailerons deflection is only increasing drag and, of course, increasing yaw.

Yaw + stall = spin. You cannot autorotate without yaw. Fat high lift wings with washout, that you find on most GA planes, are tollerant of sloppy handling to a point. Most will even recover from a spin by relaxing the controls if they were originaly trimmed for level flight and it is only in the insipient phase. Try putting a Mooney in a deep stall and use aileron to level the wing without rudder. You will see what I mean.

I think that POH's don't bother to tell you NOT to level your wings in a stall with ailerons for the same reason they dont' tell you NOT to push the stick forward to climb. It is something that you should have caught onto a long time ago.

Helicopters put an end to the developement of fixed wing STOL in GA aircraft, and there were certified AC with flaperons.

[IO540]

If the wings are completely stalled then the rudder is the only way to keep them level, but at the onset of the buffet only the inboard part of a wing should be stalled (on a certified / properly designed plane) and the ailerons should still work.

[englishal]

Isn't there the risk then that excessive aileron use may cause the outside of one wing to then stall by exceeding AoA and the drop to happen (i.e. one wing stalled inner and outer and the other only stalled inner)? Especially with types like the Cessna which don't have the stall strips on the inner leading edges and a lower dihedral wing?

[Mark1234]

To your specific question PilotDAR - when I was taught to fly the first time, in gliders in the UK there were a couple of relevant exercises that were marked off as part of the syllabus, rather than simply passed on by the instructor; they were the changing effects of rudder and aileron near the stall (predominance of secondary effects I mentioned previously, which I've experienced in a K13), and a spin entry off aileron alone, by holding off bank in a shallow turn.

I've tried to find a reference to these, as I'm guessing they must be in some sort of training manual, but can't find anything. http://www.lasham.org.uk/members/man...20syllabus.pdf does make a minor reference, but that's all.

I wonder if the reason there's not much firm guidance is a combination of factors: 1) behaviour in this regime is probably rather type specific 2) We're a rather litigous society so nobody wants to stick their neck out (normal could mean anything!) 3) appart from a lunatic fringe, aviation in general attempts to stay well away from exploring these corners, and when encountered get out of them fast. A quick unload to make the wing fly again, and you're away. No need to worry. Personally I think that's to the detriment of handling skills, but that's another arguament.

I also suspect that the 'increasing the AOA of the wingtip' explanation for an aileron induced spin is in the category of 'lies to children' - a vast oversimplification of what actually happens, but adequate for the audience. I can't argue with the principle having exprienced it, though I'm sure it's type specific.

On the flipside as someone mentioned pitts's (I couldn't resist), to my embarrassment I've so far failed all (3) attempts to spin one inverted - if you rudder at any time other than right at the break of stall it seems to just yaw with no appreciable roll developing, even when fully stalled. Ok, I'm probably a hamfisted oaf, but did I mention type specific? Go figure!

And finally englishal - I'm under the impression that things like stall strips tend to be bolted on as an afterthought to 'improve' stall behaviour of aircraft which don't play nice.. I'm more inclined to worry about the behaviour of aircraft that have them, than aircraft that don't!

[24Carrot]

Just one point about exceeding the AoA and getting a wing drop.

Near the stall, lift hardly depends at all on angle of attack, so the ailerons have very little impact on wingtip lift. In particular the stalled wingtip continues to produce very significant lift, but has become unstable in the sense that any further increase in angle of attack reduces lift, which usually increases angle of attack because of increasing roll.

In the turbulent conditions of a stall, almost anything could get this started.

As an aside, this instability approach explains why stalling the whole wing while landing is so much gentler than stalling at height - the wheels stop any rapid descent and so the just-stalled wing still produces lots of lift.

[n5296s]

I've been following this thread with a lot of interest, but not chiming in because I'm not sure what I have to contribute, But what the heck...

My plane (TR182) will fly in the stalled regime using ailerons for gentile roll. I'm sure that if you oput in full aileron it would probably snap - I haven't tried it. But *tiny* amounts of aileron do what it says on the tin.

Just the other day I flew a falling leaf (or what I call falling leaf)... stall, hold the yoke in your lap, and fly using rudder to keep the wings level. No sweat. sometimes you put in a bootload of rudder but overall it flies very nicely. The key thing is to stay ahead of the game and relax rudder as soon as it starts to respond.

I also did the same thing in the Pitts recently. Hold the stall, stick full back, and keep wings level (more or less) with rudder. No problem.

I've only tried this in one plane where it didn't work - that was a Citabria that was clearly badly out of trim. After I got into two incipient spins, my instructor (who seriously knows what he is doing) said "let me show you" - and promptly did the same thing. We agreed that something needed adjusting, and did something else instead.

Bottom line - ailerons SHOULD work but rudder definitely DOES work.

John

[bookworm]

Near the stall, lift hardly depends at all on angle of attack, so the ailerons have very little impact on wingtip lift.

That doesn't follow. The ailerons don't work by changing the AoA of the wing. The aileron is a trailing edge flap that is deflected. If you look at the lift plots for such devices, you'll see that they are effective in increasing lift coefficient all the way to stall, but that the AoA (measured by reference to the original chord line) for maximum lift is marginally reduced.

There is a subtle difference between roll damping and roll wise inertia: The latter is resistance to rolling motion starting when a rolling torque is applied, but once a roll has started the roll wise inertia actually keeps the roll going. Roll damping on the other hand is resistance to the roll motion and will tend to stop the roll if it has started.

But what you don't mention, is that roll-damping depends on the positive slope of lift vs AoA. In the stalled regime, where lift decreases with AoA, this mode becomes an instability. And it's this that makes spin entry a significant risk if high roll rates develop at or beyond stall, probably even more significantly than yaw rates, although the two are of course linked.

[24Carrot]

ailerons don't work by changing the AoA of the wing. The aileron is a trailing edge flap that is deflected. If you look at the lift plots for such devices, you'll see that they are effective in increasing lift coefficient all the way to stall, but that the AoA (measured by reference to the original chord line) for maximum lift is marginally reduced.

Not sure I follow this.

Angle of attack is defined as the angle between the relative wind and a line from leading edge to trailing edge. What else could it be? Ailerons and flaps move the trailing edge, so their main function is to change the AoA of the wing section. They also increase camber, but that is incidental.

The stall AoA is a maximum lift AoA. You know it is a maximum because both reductions in AoA, and increases to AoA, lower the lift. Which implies that the the graph is pretty flat, ie effectiveness is pretty low. One side is stable, the other isn't.

I should make the usual caveat: even if the inner wings are stalled, the outer wings might not be, etc. I am talking abut stalled wingtips (which modern designs try to avoid). Perhaps you were talking about aileron effectiveness when only the inner section is stalled?

[bookworm]

Angle of attack is defined as the angle between the relative wind and a line from leading edge to trailing edge. What else could it be? Ailerons and flaps move the trailing edge, so their main function is to change the AoA of the wing section. They also increase camber, but that is incidental.

There are two points here -- one is about convention, the other is about the relative magnitude of two effects.

Conventionally, when the effect on a wing section of flap is measured for various AoAs, the AoA is measured by reference to the original chord line. But I agree that you could look at the section with flap deflected as being a new wing section, and that the chord line has changed to accommodate the flap, changing the "effective AoA". That's just a definitional thing. What's more interesting is what proportion of the change in lift comes from the new shape, and what proportion from the new effective AoA, to the extent that it's possible to partition the effects.

If I look at Abbott and von Doenhoff figure 100 (page 195), it shows experimentally measured lift coefficients for different AoAs and different flap deflections of a 20% chord sealed plain flap for a NACA 66(215)-216 aerofoil.

The plain section, with no flap, has a lift coefficient of 0.2 at zero AoA, 0.92 at 8 degrees, and maximum of 1.42 at 16 degrees.

Deflecting a flap 15 deg gives a lift coefficient of 0.85 at zero AoA, 1.4 at 8 degrees, and maximum of 1.75 at 15 degrees, where those AoAs are measured in the conventional way with respect to the original chord line of the section.

Looking at the effective AoA, dropping a 20% flap by 15 deg lowers the trailing edge by an amount equivalent to about 3 degree tilt of the chord line. So you could argue that those numbers are 0.85 at 3 deg "effective AoA", 1.4 at 11 degrees, and maximum of 1.75 at 18 degrees, where those "effective AoAs" are measured with respect to the new chord line of the section with the flap deflected.

You can compare those numbers with the effect of changing the AoA of the entire original unflapped section by 3 degrees. At 3 degrees the lift coefficient is 0.5. So one could roughly apportion the effect on the original unflapped section of extending 15 degrees flap at zero AoA as being 0.3 from tilting the original chord line, and 0.35 from changing the shape (camber) of the section.

That's actually more to do with the "tilt" than I had suspected, and more to do with the "camber" than you had suspected.

[24Carrot]

To be fair there are various Angle of Attack definitions, though I think all are as above plus or minus some angle of incidence or similar.

IMHO the definition I give is the most helpful when understanding the lift of a wing section.

[24Carrot]

bookworm, I posted #54 before reading #53.
Your analysis looks good to me and I agree with your final line.

[whowhenwhy]

From an instructional (TMG) perspective, ease the stick centrally forward to adopt the recovery attitude (slightly lower than approach attitude). Once you have safe flying speed (5kts over stall) then you can worry about rolling the wings level, prior to resetting straight and level flight using PAT!

[Pilot DAR]

Please forgive my not knowing who PAT is, copilot?

then you can worry about rolling the wings level, prior to resetting straight and level flight using PAT!

The exercise is though to demonstrate that the bank angle can be controlled through the stall (the aforementioned design requirement that we are testing for). For this reason, it's not an option to let it get away, then get it back later.

In a more practical sense, pilots would like to feel assured that if they foolishly allow the aircraft to stall (short final perhaps) they can have some faith that when they recover it, it's still pointed more or less where they started, with the wings more or less level, presuming they put the proper corrective effort into the recovery.

So why are some pilots trained that the proper recovery effort does not include the use of ailerons, when the design requirement says it should?

[Pilot DAR]

I was just up stalling my 1975 C-150M. It is equipped with a Horton STOL kit, and has the taller rudder of the later 150's (more authority than earlier 150's). During the approach to stall it was not possible to maintain roll control with rudder alone. I reached the rudder stop on each occasion, while the gentle rolloff continued - damped, but not overcome. When allowed to break in the stall under these circumstances (roll within 15 degrees of level, hardly controlled, but lots of yaw), the predictable spin resulted, but had not been deliberately entered.

[bjornhall]

Doing some substantial googling, one finds a number of references to the rudder-only method on "my personal thoughts of flight instruction/stall training" type sites...

Whereas all authoritative sources one finds on flight training and instruction standards (from US, Canadian and Australian civil aviation authorities for example) say aileron should be used for roll control up to the stall.

Seems pretty clear what "normal use of the controls" means in the modern world.

[whowhenwhy]

Roll control up to the stall - yup agree with that bjornhall!

As far as using the ailerons during the stall and immediately after to affect the recovery is concerned, I was always taught that you may end up spinning the aircraft. The aircraft may drop a wing during the stall, but if you immediately try to correct it with aileron before attaining safe flying speed, all you'll do is enter a deeper stall and potentially spin the aircraft.

PAT = Power Attitude Trim. The theory is that there must have been some reason that you entered or approached the stall, which would probably be an incorrect P A or T setting. Once you've recovered from the stall, reset PAT and continue onwards to destination, seeing and avoiding as you go!