Stall stick position

[weasil]

I began an emergency manouvre training course in a Pitts S2A yesterday. Now I have been doing aeros for a few years and have never come across this before.

The instructor is convinced (and was very convincing) that in every aircraft there is a stick position where the foil always exceeds the critical AoA and stalls regardless of speed, flaps, gear, Angle of Bank, etc. A particular elevator position always gives that AoA.

Any comments on this theory? I will expand on this if I have failed to be clear enough.



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"eagles may soar but weasils don't get sucked into jet engines..."
http://users.hunterlink.net.au/~dfckm/weasil.htm

[Croqueteer]

Broadly speaking, you can paint a line on the fusalage wall and if the pole is aft of that line at any speed or attitude the wing will stall. You are flying the ideal aircraft to go and prove this in flight. Sadly there seems to be a lowwer level of understanding of aerodynamics these days.

[browser]

Not in the A320. Perhaps technology has a place after all, and that from one who cut his teeth for many years on 'real aeroplanes' !!!!

[Croqueteer]

I did refer to "the pole" and not to an electrical switch.

This is inserted for testing purposes only

[Note: This message has been edited by Capt PPRuNe]

[weasil]

You're right.. the Pitts is a manouverable little plane isn't it.

We were doing snap rolls (highspeed stall followed by full rudder causing a stalled roll!) it is unreal. and we were doing climbing and descending turns into a stall where one wing stalls first quite violently and a/c spins away out of the climb or into the descent. recovery was quite simple with a little practice. I just hope that it comes back to me if it ever happens for real (touch wood)



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"eagles may soar but weasils don't get sucked into jet engines..."
http://users.hunterlink.net.au/~dfckm/weasil.htm

[Genghis the Engineer]

With all due respect to your esteemed instructor, this is utter balls.

First point is that the aerodynamic stall, and the piloting stall are NOT the same thing. The stuff learned in GS or aerodynamics class is absolutely true about boundary separation at the trailing edge - IN A WIND TUNNEL. From a pilots point of view, the stall is the point where the aircraft does something the pilot didn't command (or fails to do something he did).

To quote a convenient airworthiness standard holding up a table-leg "a stall results as evidenced by a downward pitching motion or downward pitching and rolling motion not immediately controllable to until the longitudinal control reaches the stop" [BCAR S201(a), but JAR-VLA, 22, and 23 use pretty much the same wording I'm certain.]

In something like a Pitts, you've probably got so much elevator authority that the aerodynamic and piloting stall are the same. In plenty of aircraft there is insufficient elevator authority (the CFM Shadow is a classic) to reach an aerodynamic stall and what you get is the classic "mush stall" with the stick on the backstop and the aircraft developing a gentle rate of descent.

But with a reversible control system (e.g. any light aircraft)the elevator authority is a function of CG position, so the stick position to reach the aerodynamic stall on your Pitts will be further back at a further forward CG and visa-versa. I've flown aircraft where at fwd CG you can't reach an aerodynamic stall and only get the "mush", but at aft CG you can get a good solid pitch break, indicative of aerodynamic stall.

For a given wing configuration (i.e.flaps, slats, etc. left alone) an aircraft will always stall at the same AoA regardless of weight and speed, but not at the same stick position. Change the flap / slat settings and the stalling AoA will change too.

I can see however how this could be concluded on the Pitts, it works in a very narrow CG range and has a very powerful elevator - so the stick position to stall probably is damned nigh constant. This is not the case for all aircraft, and only an illusion on the Pitts.

You might like to post the question on the test flying forum, there are one or two people populating that who have done a lot more stall testing than me and might give a different perspective.

G

[weasil]

Thanks for your reply Ghengis. Some good food for thought. Might I ask what you do in the scheme of things?

One question

"For a given wing configuration (i.e.flaps, slats, etc. left alone) an aircraft will always stall at the same AoA regardless of weight and speed, but not at the same stick position. Change the flap / slat settings and the stalling AoA will change too."

Can you clarify this statement... I have been always taught quite firmly that the Critical angle of attack never changes. By extending flap you are simply moving the position of the chord line (the line between the centre of curvature at the leading edge and the trailing edge). You are not causing the wing to stall at a different AoA you are only changing the AoA for a given attitude.

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"eagles may soar but weasils don't get sucked into jet engines..." http://users.hunterlink.net.au/~dfckm/weasil.htm


[This message has been edited by weasil (edited 19 December 1999).]

[This message has been edited by weasil (edited 19 December 1999).]

[BEagle]

The lift curve slope (dCL/d alpha) will vary with flap and or slat extension. Stalling AoA may possibly be the same for a given aeroplane with flap/slat as without - but that is unlikely. The lift curve slope is the 'fingerprint' of the wing and is configuration-specific - as is stalling AoA.

[Genghis the Engineer]

I'm an airworthiness & flight-test Engineer by profession, I survey, test and approve / disapprove new and modified aircraft designs from drawing board to C of A. In between I do about 70 hrs pa of either private flying or low grade airworthiness test flying as either pilot or observer.

Critical AoA doesn't change by and large (actually it does at very low or very high airspeeds, but most aircraft never reach either end of the scale) with either loading or airspeed unless you have a particularly flexible wing. However, by deploying flaps or slats you change the aerofoil section, and almost inevitably the critical AoA with it.

G

[Checkboard]

Does the aircraft stall at the same stick position?

The factors affecting the stall speed by changing the critical angle of attack are:
Flap configuration
Ice/damage to the wing surface
and the factors affecting the stalling speed, without changing the critical angle of attack are:
Power
Weight
Centre of Gravity position
Load Factor

Now the question concerns a Pitts special, in aerobatics, and you instructor was really referring to the stick position in the straight and level stall (initiating a spin), compared to a 2-3G stall (flick/snap roll) to a 4g stall (say, entering a loop) being the same.

Considering the above, flap configuration doesn't matter (as the Pitts doesn't have flaps), hopefully the wings haven't suffered damage and you are not performing your routine in icing conditions! As all of these change the critical angle of attack the stick position is definitely changed.

I think that (for an individual sortie) the CofG and the weight are pretty constant so in this context the question really is: Does the stick position change significantly with power and load factor in a stall?

Now the stick position refers directly (via cable and push rod) to the elevator angle. This question is a little like the statement "You don't trim the aircraft to an attitude, you trim it to an airspeed."

If you set up a particular airspeed, say 80 knots, in S&L, trim then reduce the power the aircraft will start descending (in trim) at around 80 knots. The key is the "around" bit. As the power is reduced the power/drag force couple has been changed and the aircraft is descending so the Lift/weight force couple has changed (as the lift required in a descent is less) so the elevator (which acts to balance these force couples) needs to be in a slightly deferent position to maintain 80 knots exactly. If you try this in the air, it should prove to you that the elevator (and stick position) change with power application - i.e. a stall with no power and a stall with power will require a different stick position.
http://members.theglobe.com/checkboard/forces.gif
Is the elevator position required to produce the critical angle of attack at 100 knots (4g) the same as it is at 50 knots (1g)? The Lift varies with V², as does the Drag and as can be seen from the pic, these two force couples roughly oppose each other, the force produced by the tailplane (moving the tailplane down) will also vary with V² , as will the "weather cocking" effect trying to force the tail plane up. In practice it seems that these effects all roughly cancel out. The stick position is roughly the same at the critical angle of attack with changes in load factor.

Was you instructor literally correct in teaching this? No - but instructing isn't about being literally correct all the time, it is about achieving results. I used to teach the same concept when I was teaching aeros (but a little more accurately *wink* :)) and the effect is that the power, weight, CofG etc., don't change much so the stick position is roughly in the same position for the stall. The purpose of the instruction is to make a student aware of how close the aircraft is to the stall, when that student has probably only ever seen S&L stalls before, and is hung up on "stalling speed". It is a situational awareness thing, and also a handy technique when teaching how to accelerate the flick roll - but that is another story :)

[BEagle]

Good explanation!! But I do remember being taught when flying the Gnat that control column position (mil-spec. QFIs never call it a 'stick'!!) was a very good indicator of the incipient stall at low speeds. However, that was in an aircraft with a very unusual and highly complicated longitudinal control system!!.

[Genghis the Engineer]

With a reversible control system (and many powered systems) the stall requires considerably more back-stick than would normally be used in flight. So, the stick position can be a very good stall warning.

[bizjet pilot]

Interesting but: Don't you think that most pilots fly on stick pressures rather than positions. True on aerobatics, true on slow flight. Am I wrong? In case I'm not, why bother with the stick position hypothesis.

The whole thing about certification authorities is they want to see a nice, stiff, stick force gradient so you can't apply enough of a g load when you're fast to reach stall AOA. Again--pressures are part of the approved design. Reason being that we rarely are conscious (rarely need to be, shouldn't be) of stick position.

Don't mean to be grumpy. All the best.

[Genghis the Engineer]

Couldn't agree more Bizjet; by and large stick forces are far more important than stick displacements.

However, you still need reasonable stick displacements with most control system designs otherwise you start to encounter fine-control problems as soon as normal wear and tear cause a bit of freeplay in the system.

However, most pilots will be aware if there is a lot of back-stick, so it does no harm to identify that as one of the stall warning clues.

G

[lomcevac]

Weasil
I think that for the Pitts, your instructor is right. I had to do Cof A test flights this summer on three of those naughty biplanes and they have enough control authority to make this happen. (Like any good aerobatic machine) There is a word about stalling on my website www.tsr3.freeserve.co.uk (http://www.tsr3.freeserve.co.uk) at the bottom of the first page.

[Checkboard]

For a Civil type with a good force gradient, stick (I'm sure everybody knows what I mean ;-) ) pressure provides a fairly good situational awareness (except the A320?? :)), but in an aircraft with minimal longitudinal stability (like the Pitts) there can be very little force gradient, especially at low g/airspeed. I think that stick position is more definitive.

Stick force is too trim/airspeed/artificial feel dependant.

[This message has been edited by Checkboard (edited 22 December 1999).]

[E cam]

If you pitch-up about 60 degrees and centralise the elevator, the aircraft will stall.

[Low 'n Slow]

E-cam, not neccesarily, this would depend upon the stability of the aeroplane. It might well pitch down as it decelerates before it reaches the stall.

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say nothing and watch your ****

[E cam]

I also reckon that the stick position to stall will vary between power on/off and with CofG location.

[FU2]

Stick position?
What's your stall speed at zero g? Where's your stick position?
I refer you to the Vn diagram

[Low 'n Slow]

FU2
At zero g an aerodynamic stall will not take place as the wings are not supporting the mass of the aeroplane, but then you knew that.

[FU2]

Yeah low and slow that's correct, now lets ask the question what is the stall speed at -1g and what is the stick position? seems to me there are different stick positions!


[This message has been edited by FU2 (edited 27 December 1999).]

[lomcevac]

FU2
Can't disagree with that. I think the way to look at this is that what the Weasil was told was an aide to training that was helpful in a given situation, rather than gospel. I remember using lots of 'instructors porkies' myself in the past to keep the students brain free of technocrap at important moments.

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[4dogs]

Folks,

In a previous life free of significant CofG, configuration and propwash impositions, the stick position was an aid to identifying a pitch rate that was at the stall boundary and, once prestall buffet was identified, a stick position that meant that all further manoeuvring for max performance could only entail lateral control movements. If it happened that your life (in the particular environment) depended on manoeuvring at Cl(max) then it was a useful teaching point.

It demonstrated that, in a fixed configuration, changes in airspeed were irrelevant to maintaining Cl(max) as the horizontal stabiliser/elevator encountered the same relative airflow as the wing - once established, all the niceties of wing downwash etc were self-compensating!

Why was it useful?

Nobody got out a ruler - the message simply was:

Stop pulling!

In all things aviation, there are two major crimes:

+ "chopping wood with a scalpel", and

+ "conducting brain surgery with an axe".

Our primary responsibility (after identifying that the above techniques are mutually exclusive) is to apply the correct approach to the circumstances.


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Stay Alive,

[email protected]

[Genghis the Engineer]

I think also that anybody using such approximations to explain something should make it clear that it is a useful model or explanation, not the gospel truth.

Most scientific or explanations do contain a lot of approximations and innacuracies - but are good enough. The problem comes when somebody does not understand that it is an approximation, and thus can't necessarily be applied to other circumstances.

G

[Wee Weasley Welshman]

I must say this is all jolly interesting stuff. I'm going to turn this into a briefing for my students.

WWW

[weasil]

Well I have been following this thread with great interest. I appreciate all the comments folks as it has opened my eyes up a little.

Especially the comments regarding stick pressures as opposed to positions etc. There is obviously still so much more to learn in flying for me and I can't wait. I'm going to the USA inabout 3 weeks to complete my commercial and a degree. I seriously hope to do some aerobatic flying whilst there also.

Happy New Year's All.


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"..You must ensure you don't cross his nose, give him a shot at you. That is critical!" Malan

[chicken6]

Here's one from the archives! It looked interesting when I saw the "useful links page" so I replied to it. Didn't realise how old it was. :rolleyes:


I have been using the 'stick position' idea for the last couple of days to teach budding new instructors how far to check forward in a spin recovery. There are a few who are much more scared of spinning than the others, so in order to keep their mind at ease I demonstrate that if we do a max rate turn or slow flight, there is a postion on the control column at which the stall warning will come on, and if you pull a wee bit further the a/c will stall.

"That position is as far as you need to check forward to unstall" I tell them, and then they know what to do, and feel confident enough to have a go.

I thought up until now that it was the gospel - I got it from the late Tom Middleton and he was clever so it was right, until I've started reading this thread and now I'm not sure again.

But I won't be putting any doubt in their head until they pass! As long as they don't dig themselves into having to explain it in the briefing room or in the a/c, they can do it if they like. Expediency.


Sidebar - Genghis, what do you mean by 'reversible controls'?:confused:

[John Farley]

Chicken

It is rude to reply to other people’s mail, but reversible controls are ones you can move normally during the walk around. Irreversible controls are those that are fully power operated, which appear locked solid during walk around.

For what you are discussing (spin recovery technique) reversible controls allow any aerodynamic loads on them to be felt at the pole. Indeed, as I am sure you brief your students, it may be necessary to apply considerable force in the cockpit to stabilise (reversible) controls that want to blow about which way. To a pilot used to flying only with respect to forces, having the stick or rudder thrashing about of its own accord can be quite off putting. This is when it is useful to encourage them to look down at the controls and physically put them in to whatever position is appropriate until more normal circumstances prevail.

While I think it is open to some doubt whether the precise position you are briefing them to move the stick to is absolutely correct in all circumstances, the notion of getting them to look at the stick and moving it by numbers in spin recovery is very important. Make them use two hands too!

I was always taught to apply opposite rudder (and really force same against the stop), then after a pause (of about half to one turn – which is just long enough to shout out loud PAUSE) PLACE the stick in the centre position (controls neutral when on the ground) and from there on move it progressively forward (not by feel but by position) until recovery starts (this does mean that if recovery does not start you will eventually be holding fully forward stick). As soon as recovery does start, really force the controls to central (especially the rudder, about which there must be little delay) and then execute a normal dive recovery.

This has worked for me since 1955 and since I have just about quit now all looks set fair for a 100% record.

Cheers

[RatherBeFlying]

For one example of stalling and unstalling at the same stick position, I was demonstrating a spin in a Blanik glider. Shortly after entering the spin and still holding full back stick before doing the recovery procedure, the airspeed began increasing rapidly and we quickly decided an expedited spiral dive recovery better fit the situation -- low VNE and Rapid downhill acceleration.

Because of our light weight, the Blanik was close to forward CG limits. Heavier people may find different behavior.

[Genghis the Engineer]

I agree about 90% with JF, my disagreement being that the level of identification of the spin mode (particularly direction) that John has, and so-far I've always managed to demonstrate, may not be shared by every PPL - particularly given the modern refusal to teach spinning.

For that reason, when certifying a new non-aerobatic type (which I do a couple of times a year) I always look first for a controls centralised recovery. If I get a 100% success rate from that (pretty much guaranteed in anything with cruciform tail for example), I'll always aim to publish that as the recommended recovery for that type, for the following reasons:-

(1) It doesn't require enough ability to identify which way the aircraft is spinning.
(2) It can be performed in the incipient spin, whereas the convention that John has described requires the spin to be developed enough to identify the spin direction.

Expanding on (2) slightly, the RAF has always taught a "loss of control" drill preceding the spin recovery drill - that loss of control drill is what I always hope will be an acceptable spin recovery. Civil GA teaching is to perform a full spin recovery once the spin mode is identifiable. In my opinion this leads to unnecessary loss of height when recovering from an inadvertent spin.

G

Note 1: For the benefit of instructors, on most light aircraft all the primary controls are reversible, but trimmers are generally irreversible - that is you can waggle it out on the airframe all you like and nothing happens in the cockpit.

Note 2: By co-incidence, I've just read this whilst taking a break from writing a section on stalling and spinning of small aeroplanes in what I hope will eventually become my PhD thesis. I believe that in the trade, this is known as "Doing a Darrol".

[John Farley]

Sorry Genghis, but I do not see how comments about the merits of the centralisation technique (as a reaction to an inadvertent loss of control for inexperienced pilots flying non-aerobatic aeroplanes) has got much to do with Chickens question about teaching spin recovery to would be instructors!

Cheers

[Genghis the Engineer]

I hope that anybody being taught a spin recovery, whether a baby pilot or baby QFI will be being shown the POH recovery for the type being flown, even if that differs from the SSR. If there are other recoveries used on other types, I'd suggest that's a subject for ground-discussion or when flying that other type.

However, my point was not about training QFIs, it was primarily about the best spin recovery not being universal, and that what is given for any particular aeroplane should be the best available for that aeroplane.

My secondary point is that I disagree with the teaching only of a spin recovery drill which can only be executed (in the case of an inadvertent spin) once the incipient phase is sufficiently progressed that the spin direction is known - by which 250-500 ft height loss is likely in the most benign light aircraft.

G

[John Farley]

Agree all that 100% Genghis

Cheers

[chicken6]

Thanks Genghis and John

Always good to get some things reinforced and learn some new stuff! I have made sure they know Rudder PAUSE check forward in that order is the start of recovery, but min. height loss dictates a rather rapid dive recovery without a secondary stall.

The bit about horses for courses and read the POH is also there, although I haven't spun many (C152, PA38 and DH82a) different recoveries.

Genghis - I have told them the difference between what we're doing here (in their spin recovery training for an instructor rating) and what they'll actually do in the a/c with Mr Uncoordinated is when it's the real thing they'll realise how little height they've got, and those magic three words will come back to them...

"I HAVE CONTROL"

Although if I can talk our committee into getting an aerobatic aeroplane over summer it will be much more fun (for me at least).

Ta
c6

[TyroPicard]

This is nowhere near complicated enough! How will the original question (or indeed answer)vary for:

a) Trimmable tailplane with elevator e.g. jet airliner
b) Fixed tailplane with elevator with trim tab e.g. Jet Provost
c) All-flying tail e.g. Gnat, Lightning (sigh)

As an aside, am I the only idiot who used to loop and barrel-roll a JP using the trim wheels?

Cheers
TP

[Genghis the Engineer]

There is an aircraft, built in this country, whose Dutch Roll characteristics are a function of the colour of the wing undersurface.

Having grasped that, it is easy to accept that on an aeroplane virtually any change, can affect virtually anything else.

I have seen stalling characteristics change due to trim-tab setting, weight, CG, deceleration rate, sail tension, introduction of a jury strut, etc. etc.

When considering a question like this, the only way to answer it is to work on the assumption that nothing else is changing at the same time.

G

[18greens]

Ghengis/John,

I now understand what reversible controls are but why are they called that ?

Its a great word and I can just see all of those bar conversations starting "Of course when you have reversible controls....."

Ghengis, which aircraft is affected by its wing colour?
Excellent thread btw.

[John Farley]

18greens

Perhaps because you (or the air) can operate them the reverse way from what one might expect. Like a car road wheel jerking the driver's steering wheel when it hits a curb.

Certainly irreversable controls cannot be moved in the reverse way. Come to that why are you called 18 greens.......

[18greens]

John, Ah, Sound of hand hitting head!!

To a neophyte the word reversible controls conjurs up images of stick forward, houses get smaller, stick backward houses get bigger. It brings me back to the realistically possible nightmares of a glider with crossed control rigging.

As for 18 greens, well its better than 3... or none.

Thanks for the explanation.

[Genghis the Engineer]

The aircraft is a microlight called the "Pegasus XL-Q". A particular multi-coloured undersurface they called the "Rainbow-Q" version was found, due to the dieing process altering the stiffness of the lower surface fabric to suffer a divergent DR at high speed, which on a more soberly coloured aircraft remained (just) neutral, becoming convergent at lower speeds. It also I found the couple of times I have flown one, had about double the roll-mode time constant compared to the "standard" wing.

G