Stalling & Spinning

[whatunion]

I only came onto this forum to talk about some airline matters and to ask a question about how a freind of mine could become a prof pilot (it seems to have changed a lot since my day)

however i have become fascinated by some of the subjects on here and a little rude to some of the respondents
, so sorry about that. I have seemed to take the post on 'serious wing drop at the stall' into a different area so have taken the liberty of starting this new thread on stalling and spinning.

I mentioned several points which some have disagreed with, I will go over them again.

C of G

c of g is a major consideration in stall/spin behaviour & recovery yet v ery rarely talked about , not mentioned at all in the previous thread.

STANDARD RECOVERY
I maintain that it is wrong to blindly say FULL POWER because there are exceptions as to when full power should be used. Does anyone else have any views on this?

WING DROP WITH FLAP DOWN

I contend that the major reason for tip stalling with flap down is that the different Cl over the inboard section caused by change in camber, a of a and possibly wing area allows the tip to stall, first.
Or put another way the transition point moves further back on the inboard section

AILERON RESPONSE AT THE STALL

It is my opinion that some a/c are certified and tested to show that they have aileron response at the stall in certain configurations and ceratin entries. This is chiefly due to WASHOUT along the ailerons and wing on some a/c.

CORRECT DISCRIPTION

I think it is wrong to give a student a description such as "pitch to fix" as a way of teaching stall recovery. I believe that the correct way is to state
'Move the CC forward until flying control is regained.' this then become exactly the same for spin recovery.

[djpil]

I contend that the major reason for tip stalling with flap down is that the different Cl over the inboard section caused by change in camber, a of a and possibly wing area allows the tip to stall, first. But the effect of flap is to raise the local Clmax - Cl at all spanwise sections must be considered in relation to the local Clmax. None of the foregoing explains the reason that you seek.

I refer you to NACA Rept 829 (http://naca.larc.nasa.gov/reports/1945/naca-report-829/) Inboard flaps do not always result in an outboard stall. If student pilots want a simple, general theory that fits in with their trainer's characteristics then try this: "Flap deflection generally tends to aggravate the stall by increasing the upwash over the outer unflapped parts of the wing and by cleaning up the area of separation at the root."

Or put another way the transition point moves further back on the inboard section You've lost me there. Transition point is defined with respect to the boundary layer - transition from laminar to turbulent. Transition point is a significant factor for aerofoils which have a leading edge stall but otherwise it doesn't really fit into this present debate. Stall of a flapped section is generally over the rear part of the wing.

[whatunion]

ok using your theory how do leading edge spoilers prevent tip stalling

[djpil]

I’m afraid that my answer will disappoint you in several ways, whatunion. I potter around the forums for a bit of relaxation and its late at night now. I don’t think I could do your question justice with the time that I want to put into it. I’ve been around long enough that I just do the things that I like to do – this thread seems to be moving rapidly towards something I don’t like so I’m about to lose interest.
I assume that we’re talking about the same thing if I use the term stall strips or wedges. A useful tool or “fix” for a designer to use. I guess that your question is aimed at my statement regarding transition point in relation to the stall strip and the mechanism by which the boundary layer is modified and its effect on the value of the adverse pressure gradient at which flow separation occurs downstream. So yes, transition point is now a factor in the revised scope of the debate.

[whatunion]

amazing how this is a forum for flying instructors but no one can seem to answer any question simply. i admit it is 14 years since i instructed but has in changed so much that you all need to quote advanced aerodynamics or even ask me to download a set of diagrams to explain something so simple?

i appreciate that some of you feel that you have to be seen to be able to be complicated to maintain your status on this forum but is it really all nesecarry?

i aslo notice that some of you are so engrossed in complication and theory you loose sight of the practical world, the can do world!

for instance ga assymetric, loads of complication but no one mentioned ga climb path obstruction, a major consideration with such low climb rates.

stalling, no one mentioned c of g. another major consideration and the cause of many suprise stall accidents.

whatunion says keep it simple

['I' in the sky]

whatunion says keep it simple

- and don't even complicate your sentences with capital letters

[whatunion]

or pointless remarks.

whatunion says, why point out someone else weaknesses when you could be concentrating on your own.

[paulo]

Troll? (http://members.aol.com/intwg/trolls.htm)

[machonepointone]

A few points on the matters that Whatunion has mentioned on the subject of stalling and spinning.

1. C of G. The position of the C of G makes no difference at all to the stalling characteristics, so why there should be any “surprise stall accidents” I do not understand. Perhaps he would enlighten us mere current mortals by providing some examples. However, the more rearward the C of G the lower the stalling speed will be. The reason is that with a rear C of G the nose down pitching moment of the C of P and the C of G is reduced. This means that a lower downward force from the tailplane is required to balance it. Since total lift in level flight must balance the aircraft weight plus the tailplane down force, a lower total lift from the wings results in a lower stalling speed. As far as spinning characteristics are concerned, a rear C of G can make the recovery more difficult since the moment arms of the elevators and rudder are reduced thereby cutting down their effectiveness.

2. Use of Power. Full power should always be used as a means of reducing the height loss. This is easily demonstrated on the first stalling exercise by showing the height loss on a fully developed stall with and without using any power. On average it will at least halve the amount of height loss. Subject to any engine limitations there is no good reason not to use full power.

3. Wing Drop. The position of the flaps is irrelevant. Given symmetrical wings in balanced flight it does not matter whether the inboard or outboard sections of the wings stall first. The point is that both wingtips should stall at the same time, so there will be no wing drop. The use of stall strips on the inboard leading edge is not necessarily to make the inboard part of the wing stall first. They are very useful for promoting an early breakaway of the airflow and causing the buffet over the tailplane that we use as a recognisable stall symptom. Wing drop will, however, be caused if there is any difference in lift over the wings. Causes of this can be as simple as slight irregularities [dents, dirt, insects etc], and more usually not being in balanced flight. Any sideslip will cause a blanking effect over the downwind wing. It is therefore not so much a case of wing drop as the fact that the downwind wing is producing less lift than the upwind one.

4. Ailerons. Whether or not ailerons will still work at the stall depends on the design of the aeroplane. Vortex generators ahead of the ailerons or a smooth wing surface such as can be provided with composite wings will prevent or delay the transition to turbulence and the separation point. However, a cambered airfoil has a lower stalling angle than a non cambered one, so the application of ailerons in an attempt to raise a down going wing has the effect of putting that wing well beyond the stalling angle of attack. It also increases the coefficient of drag. These two effects give rise to autorotation and a spin, hence the dictate not to use ailerons at the point of a stall.

5. Standard Stall Recovery. Ever since I learnt to fly in 1965 I have been taught, and teach, that the Standard Stall Recovery is “Control column centrally forward to unstall the wings, full power and rudder to prevent further yaw [see previous paragraph].” How do we know that the aircraft is unstalled? Either when the buffet has stopped or, in the case of aircraft such as the Cessna 172 which has no real buffet, when the stall warner has stopped. You cannot use the same argument for a spin recovery because that depends entirely on the characteristics of the aeroplane. I have spun aircraft in which the fully developed spin recovery is merely to centralise the controls. Others require the stick to be moved to the neutral position at the same time as full opposite rudder is applied, others after a pause of about one second. Others need the stick to be progressively moved forward until the autorotation has stopped and some even need inspin ailerons. So to state blandly “Move the CC forward until flying control is regained” is not necessarily correct and most certainly cannot be used as a standard for both stall and spin recoveries.

So, in reply to Whatunion, I would say that nothing much has changed over the years. However, since he has on his own admission not instructed for 14 years, perhaps he could refrain from telling those of us who are still current in the matter all the causes and effects. By and large we already know the answers and are constantly in the habit of teaching them, and being tested on them every three years, practically and without “downloading a set of diagrams.” Nor do we need to complicate the subject in search of an ego trip. We do, however, need to make sure that our students fully understand what is happening, or might happen, and the consequences.

To Paulo I would like to say “Good call.”

[BEagle]

M1.1 is absolutely spot-on. A clearer and more lucid explanation of stalling I have yet to read.

However, I have certainly found that the departure from controlled flight in a T67A is far more violent when a deliberately provoked dynamic stall is carried out in the turn with an aft CG than with a forward CG. Perhaps not so much the actual stall as the post-departure gyrations!

The point made about dents etc in the wing is very apt. Recently one of our a/c suffered a significant dent in the outboard wing leading edge. After the damage had been assessed by a licensed engineer and the a/c cleared for a flight to the maintnenance organisation for repair, I carried out a low speed handling check down to the stall so that there wouldn't be any nasty surprises on landing at the maintnenance aerodrome. Perhaps unsurprisingly, it exhibited marked wing drop at the stall!

whatunion has stimulated this debate; however, I do share the opinion that those in current instructional practice are better placed to give advice on stall recovery. Centralising the controls in the incipient stage of a spin will normally stop the spin becoming fully developed; however, there is no such thing as a 'standard' spin recovery! The only approved recovery technique is as per the aircraft's POH.

[djpil]

Thanks, machonepointone, looks to be a rational debate at last.
The reason is that with a rear C of G the nose down pitching moment of the C of P and the C of G is reduced. so the pitching moment from elevator deflection can take it to a higher angle of attack. Those aeroplanes where, at forward cg, stall was limited by elevator travel will most likely have quite different stall characteristics at aft cg.
The point is that both wingtips should stall at the same time, so there will be no wing drop Sorry, mach, I disagree with some of your statements here (I'll get back to this another day unless some-one else chips in).

[Oktas8]

CG position does affect the stalling characteristics - partly as BEagle said, and, at a more basic level, simply because the aircraft will stall with less back pressure and take longer to recover if the CG is significantly further aft than usual. This is particularly relevant to students who train in four-seaters with only two POB, as they will get little opportunity to observe MAUW, aft CG stalling characteristics.

Wing flap configuration does make a difference on many aircraft. The theory goes that, when flaps are deployed, most lift is developed inboard, closer to the lateral CG. In the event of a slip developing prior to the stall, there is less restoring moment and decreased lateral stability. If a stall occurs, it is more likely that one wing will have a greater angle of attack than the other, as the inboard, lift-developing, section of wing is more affected by slipstream asymmetries, and hence is more likely to develop asymmetric lift. The wing tips may contribute very little restoring force, as they generate only a small proportion of lift at VS0.

In addition to this, M1.1's comment about being out of balance is possibly the most common reason for a wing drop at any stall.

[Simple bit over, this bit for more advanced pilots.] The C152 seems to demonstrate the best (most sudden, greatest roll rate) wing drop stalls when clean. I speculate that this is because there is very little washout and the wing section, when clean, is such that the separation point moves rapidly forwards with a small change in AoA beyond the critical angle. When a stall occurs with a small asymmetry in lift, the resulting roll increases the angle of attack on the downgoing wing, but with much greater effect on the C152 than on aircraft with different wing sections.

As far as I'm aware, all modern aircraft certified in utility or normal categories must demonstrate aileron effectiveness at the clean (no flap) stall. Although this is certainly not the best way to recover(!), I think it is helpful to make the comment to students; they will observe in their training that the aileron actually does work, and some students may wonder 'why all the fuss' about not using aileron during the stall recovery.

With regard to a post on this forum by whatunion - I have no status on PPRuNe at all. But I do very much appreciate the chance to interact with pilots of the calibre of M1.1 & BEagle - and that's why I post. Whatunion- perhaps you could tone down your insults a little?

regards to all,
O8

[djpil]

I agree with BEeagle's statement however, there is no such thing as a 'standard' spin recovery! The only approved recovery technique is as per the aircraft's POH however my set of briefing notes describes spin recovery actions which are the same as those you may find described as the "standard method" in the odd American text. The Australian spin endorsement is not type specific so I make it quite clear that my standard spin training course is good only for the (American) types that our Aero Club operates. Exceptions to this on our field are the Chipmunk, Zlin 242 and CT-4.
I don't mention the Beggs-Muller technique at all in my standard briefings although aerobatic students often read about it so we get to discuss it.
The Civil Aviation Safety Authority Australia recently changed its rules on Flight Manuals so our home-grown little black books have been tossed out and we now use the official AFM from the country of origin with whatever else is supplied by the manufacturer in the way of POH or POM.
Rich Stowell's new book (https://www.richstowell.com/stallspinbook.htm) has some interesting comments on instructions found in some of these manuals and inconsistencies with the spin recovery placard. My students usually get to see Rich's video on Stall/Spin Awareness in addition to my briefings.
It is important to know a lot more than what is written in the POH of the older American aeroplanes.

[whatunion]

plenty of stuff out on the internet to challenge most of your theories.

the following might explain stall characteristics that seem to be unknown by some of the high calibre current instructors


TAMING THE STALL

We've gone on a bit about what a stall is, and some techniques to prevent stalling our planes when we don't want to. Now we'll look at how we can tame a planes stall characteristics somewhat.

But first, we need to look at how different wings stall. In general, as angle of attack (AOA) is increased, a straight wing, like on most trainers, will first start to stall inboard, at the wing root near the fuselage. And that's good. Although there is a loss of lift, all the messed up airflow is near the root, and the tips are still flying, meaning we also have aileron control out there where it's most effective. So we get a nose drop, but the plane stays reasonably level and we don't lose control.

But tapered wings are more efficient, and we'll see them on most higher performance planes. And the more highly tapered the wings are, the more likely they will stall first out at the wing TIPS. Stalls on such planes are very different; when a wing stalls first at the tips, a wing drops, you have no aileron control, and a simple stall instantly becomes a snap roll. Your first indication of a stall is when the plane flips onto its back, out of control!

Summarizing, we can see that if the stall begins out at the wingtips, we'll see a wing drop (often violently). But if the stall begins at the wing root, all we get is a nose drop, with the plane still under some control. What we'd prefer should be pretty obvious!

So what's the answer? How do we make a plane do its stalling properly, in at the wingroot?

Washout is one answer. This is best done during the building stage, and involves "twisting" each wing during construction so that the tips "fly" at a slightly lower angle of attack than at the root, near the fuselage. Normally only a matter of about 2 degrees difference, (see fig.) the lower AOA at the tips mean that the root will always stall first. Many kits, even for straight winged planes, include washout in construction - as the wing is laid out on the building board, the trailing edge is set up higher out near the tip of the wing, giving you washout when the wing is complete.

Another way to accomplish the same thing is to use a different airfoil out near the wingtips; one that naturally stalls at a higher angle of attack than the airfoil at the root. This is very easily accomplished when cutting a foam wing, but is seldom seen in built-up construction. "NASA droops" will do this also, and could be added to an existing wing. The NASA droop is a leading edge "anti-stall" modification, usually applied to the outer 35-40% of a wing. (see fig.)

The easiest method to modify an existing plane to cure nasty stalling habits is the use of stall strips. Applied to the inner 20-25% of the wing, the stall strip is a small change to the leading edge that produces turbulence to that part of the wing at higher angles of attack, causing the stall to begin at the root. (see fig) Thus we will have a slightly higher stall speed, but the stall will at least be manageable. Plus - the beauty of the stall strips is that you can just pin them on and experiment till you get the effect you want, then make them permanent!

So if you have a vicious stalling plane on your hands, don't just try to cope till you crash it - add some droops or stall strips - let the plane age gracefully, THEN crash it!

------------------------------------------------------------

Heres another one; notice why stall strips are fitted, nothing to do with your explanation of providing buffetting at the stall!!!!

this is off the web, nothing to do with me


Stall recovery for the Glasair RG is typical of most conventional aircraft; lower the nose with forward stick, and add power. The stall characteristics are predictable in both power off and moderate power on stalls. Just before the stall, slight buffeting is felt, giving an early indication of the stall.

The Glasair RG should not be intentionally stalled with any heavy baggage in the baggage compartment unless it is securely fastened down. When practicing stalls, be sure to check the air space for any conflicting traffic.Stall recovery for the Glasair RG is typical of most conventional aircraft; lower the nose with forward stick, and add power. The stall characteristics are predictable in both power off and moderate power on stalls. Just before the stall, slight buffeting is felt, giving an early indication of the stall.

The Glasair RG should not be intentionally stalled with any heavy baggage in the baggage compartment unless it is securely fastened down. When practicing stalls, be sure to check the air space for any conflicting traffic.

NOTE
Stall strips are mandatory on the Glasair RG to induce the wing roots to stall first. Without these stall strips properly installed, the stall is unpredictable and can be rather erratic.
----------------------------------------------------
i fly a large jet transport which has leading edge spoil strips. it is part of the pre-flight check to ensure these strips are present. this strip has nothing to do with stall buffet, it ensures that the roots stall before the roots to prevent wing tip stalling. on T tail swept wing a/c it is a requirement that a stick shaker and pusher is fitted. the stick shaker provides the stall warning.

I flight tested a C152 with stall strips attached to the leading edge in 1980 at wichita(cessna) i can assure you that there was no wing drop in the app config. with the same a/c with the strips removed it reverted to the normal wing drop.

so beagle i am afraid m1 was very far away from being spot on!

[BEagle]

Bit of a 'Boys Book of Aeromodelling' extract you quoted there.....with its references to 'cutting foam wings', 'built up construction' and the astonishing "So if you have a vicious stalling plane on your hands, don't just try to cope till you crash it - add some droops or stall strips - let the plane age gracefully, THEN crash it!"

Incidentally, only non-pilots talk about 'planes'!

Swept wing behaviour at high AoA isn't the same as that of a straight wing - the spanwise flow even before the stall being somewhat more complicated. The heavy jet transport I used to fly also had a stick push system; we used to take it down to the stick shaker during air tests only - but if buffet ever occurred, the exercise was discontinued. AoA probes needed to be carefully set up and the whole system was vital for safe flight. I've had the stick pusher go off on a flapless approach due to incorrect maintenance and faulty 'lift rate modifier' input to the system - quite thought provoking!

The small inboard 'toblerones' on many military trainers were purely there to induce buffet over the elevators at high AoA and had no significant effect on the stall; the devices fitted to the Glasair are totally different in nature, as whatunion states.

[djpil]

The stall strips are used as a final fix once the prototype is flying. The Airtourer is a good example. Without stall strips (per the T6 variant) it has quite a sharp stall and based on one example of the type, even flaps up & power off, the stall is accompanied by an uncommanded roll to a bank angle of around 20 deg or so. Not so with other Airtourer variants. With stall strips the resulting early separation at the wing root causes sufficient buffet that it is accepted as natural stall warning - the tail happened to be in just the right position for the wing wake to impinge on it - the buffet still has to occur within 5 to 9 kts of the stall to be accepted as appropriate warning.
I don't recall any examples (I lead a sheltered life so only know details of those types that I've worked with) where the strips have been added just to generate sufficient buffet for natural stall warning - but no reason why not if that is the requirement.
The last military trainer wing that I designed (unfortunately only got to prototype stage and not into production) had wash-out plus different aerofoil sections root to tip. During wind-tunnel tests we saw the need to modify it to ensure stall characteristics would be acceptable - so we added a wing root cuff (it had been on our list from when we did the early analysis anyway). We didn't want any more wash-out nor did we want to change the aerofoil sections - one factor being that detail design was well advanced by then.

Reminds me of the MB326 fleet with the wing rebuild. Afterwards they needed "sharks teeth" on the leading edges to eliminate a leading edge laminar separation bubble and it then reverted to the turbulent separation further aft on the wing as per original production.

I've also added stall strips further outboard from the root to eliminate wing drop at the stall. It was enough to induce earlier separation at mid-semispan - less effect on stall speed. And that was with a constant chord wing, no wash-out and same aerofoil.

(I apologise in advance to anyone who thinks that my students get this sort of detail at a standard briefing)

[Oktas8]

I've also added stall strips further outboard from the root to eliminate wing drop at the stall. It was enough to induce earlier separation at mid-semispan - less effect on stall speed.

djpil - could you please go into a little more detail? I have observed the mid-span stall strips on PA28-161's, and have wondered about the various reasons that they might have been put in that position. For example, to provide separation over ailerons to demonstrate buffet, to provoke separation outboard of the stabilator to minimise fatigue, to use a channel of turbulence at the mid-span position to reduce spanwise flow at the stall... But I'd like something that is less speculation, more wisdom!

If you prefer, PM me.

thanks in advance,
O8

[whatunion]

agree that spanwise movement may be different but seperation before centre of pressure movement is surely the same on conventional or swept wing

my point is that leading edge spoilers are used to encourage the tips to stall before the root and that is their primary purpose. (remember with buffet,it depends were the tailplane is sitting, eg high mainplane to talplane or conv mainplane to t tail)

in the usa many cessnas use this mod for just that reason.

so to recap we have some aggrement that
leading edge spoilers cause early seperation at the root(djpil)
so now may i ask if any of you belive that flaps down config delays seperation???

beagle
model a/c may be boys own stuff but most protoypes start life as a model.

by non pilots did you mean people like barnes wallis!!???

[djpil]

could you please go into a little more detail? I have observed the mid-span stall strips on PA28-161's, and have wondered about the various reasons that they might have been put in that position. Oktas8, as I said, I lead a sheltered life so if I responded it would just be more speculation that you don't want as I know little about the Warrior's wing aerodynamics. I've found something which serves as an online whiteboard and I have a bottle of red wine here to prompt the brain but I doubt if I could answer your question well enough.

(I wouldn't use a stall strip to prevent spanwise spread of separation.)

If my whiteboard could show the real spanwise lift distribution for the Warrior wing planform then I'd be happy to continue the discussion. Have a look at Drawing 3 here. (http://www2.arnes.si/~lzs1/aerodynamic/userguide.htm) It shows the stall patterns of 3 wings:
- constant chord - stalls first at wing root
- straight taper with outboard stall
- one something like a Warrior planform with both an inboard and outboard stall. Do you think that it could do with stall strips just inboard of mid-semispan so that it gets more of the inboard wing to stall before the tip stalls?

[whatunion]

stall strips dont prevent spanwise movement thats what the dirty big wing fence is for, next to the stall strip on most swept wings.

other consideration is, section affects whether stall develops from leading edge or trailing edge.

[Chuck Ellsworth]

whatunion :

Interesting reading all this fancy hi tech info you are pounding out on your key board.

But can you fly an aircraft as good as you think you understand how they should fly? :D

[whatunion]

Of course not, thats why I am on here!

by the way Chuck did you check out the fatal at humberside. it sadly lends some weight to my point that its one thing to talk about single engine performance and entirely different doing it near the ground for real!

ps have you seen the size of the stall strips on a aa5 traveller there are 2 and there the total length of an aileron!

[Oktas8]

Thanks very much djpil - that website has some excellent diagrams. Yes, it seems that the stall strip on the Warrior would ensure that the stall progresses steadily from root to tip instead of jumping root --> tip --> midspan. I might use those diagrams in future. ;)

Many pilots over-simplify stall strips by stating that they cause the wing root to stall before the tip. However, a more correct explanation: "They are designed to modify the stall characteristics of an airplane by inducing a stall at a controlled location along the wing.” (NTSB Safety Recommendation A-97-41 through -45) leads to questions that aerodynamicists are best equipped to answer!

Seems a shame that the aircraft manufacturers are unwilling to assist flight instructors with actual data for their aeroplanes...

Thanks again,
Oktas8

[whatunion]

stall strips or more correctly leading edge spoilers encourage stall at the root before the tip, isnt that what i have been saying all along.

so now we have got some agreement on that, perhaps we can agree that washout allows aileron control at the stall in some a/c in some stall configs. and encourages the root to stall before the tip.

that only leaves slots and slats!!!