superstall

[vish02]

Can anybody explain why is 'Incidence' so valuable parameter for Superstall?

[BOAC]

Haven't a clue! Please quote the reference paragraph for this question in full so we can see the context. If you cannot do that, which publication is it from?

[vish02]

I was studying from the book "handling the big jet".
Superstall topic in this book has lot to say about about the angle of incidence.
Even there is some expression on page 120 if you have this book third edition.

[bearfoil]

Holding an a/c in the Stall until established in controllable sink (or loss of a/s) is not unknown. It has been the topic of much discussion in ACM, as well as GA. If in trouble in the small Ercoupe, one can pull back fully, and mush down to land on (hopefully) fairly friendly terrain. In fast jets, it is called 'flat-plating' and can be used to bleed energy at enormous rates while maintaining control (not for the novice).

bear

[Mr.Vortex]

I'm heard about superstall once in Saab draken. Superstall occur at
AoA greater than around 16 degree.

Which at that point the Vortex on the wing which create the lift was start to break down or burst at the outer portion of wing fisrt which will create
the similar "pitch up" characteristic like the swept wing having a wing tip stall.
As the AoA increase the vortex bursting point start to move inward which
causing the aircraft nose to pitch up and also add the enormous drag which
If i recall correctly, the Draken can't increase the speed during the superstall
with full throttle & afterburn.

So to sum this up: higher AoA -> higher drag.

Best regards

[John Farley]

Incidence (sadly) means different things to different people, in different circumstances and different times in history.

Many years ago it was commonly used as a measure of what we would today call Angle of Attack. Indeed Dave's use of it on page 120 is (I suspect) in the Angle of Attack sense.

To an engineer (yesterday or today) Incidence is the physical rigging angle of an aerodynamic surface (usually the wing or tailplane) with respect to some datum line.

To a pilot today Angle of Attack is the angle between the chord line of an aerofoil and the direction of the local airflow as it approaches the aerofoil.

Is that any help?

JF

[bearfoil]

Then there is "Variable-Incidence". From the Vought Crusader (F8U), one garnered an opinion of AOI that remains, and I think of it as important to flight line from the cockpit.

On approach to the Carrier, the Cru's wing would elevate its leading edge to allow the Pilot to see the deck ahead, and the wires.

The only other use I think this old term has is its relationship to AoA at cruise, and only to define the inclination of the aisles relative to Trolley creep.

bear

[BOAC]

I don't have the book, but can only assume that DP refers to an angle of 'incidence' at which the tailplane will be rendered non-effective by the disturbed air from the wing.

'Incidence' is a 'valuable parameter' for ANY stall, ordinary or 'super', hence my confusion with your question.

As J F says, I suspect Mr D should have said 'angle of attack'.

[vish02]

@JF
If you say that the incidence is the taken in AOA sense then its its good, because i was confused about it, because the incidence angle is fixed and according to his explanation, he said that increase in incidence gives progression towards superstall.
But again i am not sure that he is taking the incidence in AOA sense in his superstall explanation.

[BOAC]

Vish - unless someone can access a copy, we will probably never know what you are talking about. Can you scan the page/s and post it?

[mattpilot]

@vish02

page 4 in that book u referenced states what 'incidence' means. Basically the old term for AoA.


I recently finished reading the book, and in all cases where he says incidence, he does indeed mean AoA. Was a confusing read at times with his 70's terms, but what can ya do... ;).

[vish02]

@ matt
oh thanks. yea now even i think that he is talking about the AOA only....and the incidence i am talking about i guess he states that as 'Wing incidence'.
Thankyou guys.

[John Farley]

Thanks. Well spotted.

And as it happens exactly my definition of AoA used above.

JF

[HazelNuts39]

To a pilot today Angle of Attack is the angle between the chord line of an aerofoil and the direction of the local airflow as it approaches the aerofoil.

Partial quote from BEA's Final Report on the A320 accident near Perpignan, para. 1.6.6.1:
The aeroplane angle of attack (also called true, real or corrected angle of
attack) is defined by the angle between the relative wind infinitely upstream
and the longitudinal axis of the aeroplane. It is generally noted as α (alpha).regards,
HN39

[bearfoil]

My money's on John. For the simple reason that his answer is correct at all times, and the BEA are correct in only one narrow way. Oh, and no wonder they can't be relied upon in accident investigation. BEA are confusing AoA with Deck Angle. Alpha with Oatmeal.

[Brian Abraham]

To give the exact definition Davies gives in the book - 2005 reprint"Incidence The angle between the wing chord line and the free air stream. (Also referred to as 'angle of attack')"

[HazelNuts39]

BEA are confusing AoA with Deck Angle.Deck angle is the angle between the longitudinal axis and the horizontal plane. For aerodynamicists, performance engineers, and airplane systems, when considering the whole airplane rather than a particular section of the airplane's wing, the longitudinal axis is the reference for AoA and pitch attitude.

Boeing Jet Transport Performance Methods, Chapter 10 - Wings:
Some airplanes have wings which are twisted such that the angle of incidence at the root of the wing is different from the angle at the tip. This is usually done to enhance the stalling characteristics of a wing, or to tailor the lift distribution. The wing of the 747, for example, is twisted by 3.5 degrees, having an incidence of 2 degrees at the root and -1.5 degrees at the tip.regards,
HN39

[BOAC]

Now we've sorted all that out, any chance of dragging this back to the OQ and :finding out why DP (?apparently?) attributed the importance of 'incidence' (valuable parameter) only to 'superstall'?

[bearfoil]

OK I'm talking to France and UK here, and I am a Murican. Incidence was taught to me a very long time ago, it is the discrepancy between Longitudinal axis and the chord line expressed in degrees ('always' positive). The AoA is as John Farley states, the chord line relative to airstream. Let's don't get too precious, What who says is what matters to different people, and let us not wander into standards, Internationale, Eh?

dirty side down, bear

[mattpilot]

Well, vish said that when he subsituted AoA for incidence in the text, and it all made sense to him. So i didn't see a need to reply to the exact reference.

The book is talking about superstalls in the pages leading up to pg 120: comparing low-tail with high-tail planes. THe problem with high-tail planes being that in a stall the downwash of the wings would put the elevator in the turbulent airflow and thus reducing/eliminating its effectiveness and making recovery unlikely. Up until the pre-stall phase, the airplane has no pitch up tendancies (rather the nose would want to drop). Past the pre-stall phase the nose of the airplane has pitch-up tendancies. He then explains:

page 120:

"Many explanations of the super-stall invoke the term 'downwash' and suggest that chagnes in downwash on the tail have some effect on super-stall qualitites. This is not true. A change in downwash angle alone in the approach to the stall does not produce any nose up pitching tendency from the tail. ALthough the tail experiences an increasing negative incidence with increase in attitude this increase is always 'beaten' by the decreasing negative incidence due to the physical change in attitude. If this were not true than the aeroplane would be unstable in the stall approach, which it is not."


Replace references of 'incidence' with 'angle of attack' (as per the books glossary on page 4) , and it suddenly makes sense :).

[Wizofoz]

I think in this case, incidence is meant as the angle between the chord-line and the longitudinal axis.

This is important if, as I suspect, they use the term "Super-stall" to mean what is also called "Deep stall".

This was a problem with some T-tailed aircraft (they lost a Trident in test flying due to it) as turbulent flow from a stalled wind would, at certain angles-of-attack, interfere with the tail and render the elevators ineffective, thus leaving the pilot with no way of recovering.

Changing the incidence of the wing would change the path of the turbulent flow, thus altering the tendency for this to happen.

[BOAC]

It was a 1-11.

Matt, thanks for transcribing that paragraph, although it leaves me slightly confused as to what DP was trying to say. Perhaps if he had inserted the italic words in the text it would have been clearer:

"Although the tail experiences an increasing negative incidence due to downwash with increase in attitude this increase is always 'beaten' by the decreasing negative incidence due to the physical change in attitude."

I guess the original quote by vish (assuming that DP was confused on definitions) is D P's way to lead up to the need to limit 'incidence' (AoA) by means of stick pushers? On its own it made no sense, since 'incidence' (AoA) is a 'valuable parameter' in any stall, is it not?

[Mr Optimistic]

Although the tail experiences an increasing negative incidence due to downwash with increase in attitude...


Is this saying that the flow behind the wing produces a 'downwash' (deflecting the free stream vector downwards I suppose) at the high tail at pre-stall aoa ( so that the tail is more or less always in perturbed air ?)

[bearfoil]

The Tail must push down at all flying speeds to keep the a/c flying level, and not nosediving. With a T-Tail, at Stall, the a/c can pitch down without this downforce. If the Tail Stalls, the Tail must be flown with a pull to arrest its Stall, then a normal recovery. Let's see, do I have this right? The Tail flies opposite the wing, the lifting surface is the top, not the bottom. Anyway, I rue the attempt to eliminate the use of a perfectly good aviation word (incidence), by combination with its trained for alter ego, AoA.

[rudderrudderrat]

Correct - the tail always exerts a down force to balance the c of g and the centre of pressure on the wings. (Speed Stability)

At high angle of attack, once the wing was stalled, airflow from that wing had less down wash and a more turbulent flow from the top of the wing, which destroyed the "lift" from the elevator of the T Tail which now sat inside that disturbed airflow.

The rear mounted heavy engines means there is a longer fuselage forward of the wing than behind, which hinders a natural recovery.

[bearfoil]

con-pilot has experience (alot) with the DC-8 and the B727, I'll defer to his experience; should we all?

[Mr Optimistic]

but this quote implies pre-stall doesn't it ? At stall I guess the air is disorganised and there is no steady flow so aerodynamically the effect of the tail goes to zero. It was the quote I was interested in which implies that at any (pre-stall) aoa the flow over the high tail is canted downwards relative to freestream on account of the wing. Was just checking that was what was meant as it surprised me (perhaps through nothing more than ignorance).

Con-pilot ? Missed that.

[bearfoil]

The Tail's "downwash" is opposite the wing's downwash, eg, it is an 'upwash'. Like downflow on the wing reduces its AoA, up wash on the tail reduces its AoA.

[Mr Optimistic]

My mistake, figured 'downwash' meant the condition of the airstream as it impinged upon the tail.

[bearfoil]

exactly!! See, that is what happens when unsuspecting aviators are subject to "one word fits all!" That is why the Tail is called a tail, even though it is a wing!!

[Mr Optimistic]

in my day we used 'incidence' for the cord line-free flow geometry (but not an aircraft application): quicker to write if nothing else.

[mattpilot]

I guess the original quote by vish (assuming that DP was confused on definitions) is D P's way to lead up to the need to limit 'incidence' (AoA) by means of stick pushers? On its own it made no sense, since 'incidence' (AoA) is a 'valuable parameter' in any stall, is it not?



I don't think its fair to say DP was confused by definitions. Throughout the book he uses the word 'incidence' to describe what most of us understand as the AoA. If one would read the 'language comparison' page (1 page before intro) and the 'glossary of terms' (pg 4) before reading the meat of the book, things would be much clearer. The 3rd edition was written in 1971, and with him being (or was) a certification pilot for the UK CAA (if i understood that right), i guess it is fair to say he used the proper terms that were known 'back in the days'.

But to be honest, i would of liked a '4th' edition myself that would make use of 'modern' terms when i read this book. :}

[bearfoil]

With the introduction of flight, travel acquired a third dimension, geometrically speaking. So to finalize this, and be clear, it is downwash that the tail experiences, only it is (seemingly) moving up!! Matt is right, each thread should begin with a glossary, "prior perusal prevents ****poor performance."

bear

[DozyWannabe]

(they lost a Trident in test flying due to it)

It was a 1-11.

Correct - BAC lost a 1-11 in testing, in an infamous accident that killed Mike Lithgow and his crew. BEA lost Trident G-ARPI in service due to premature droop retraction and subsequent stick pusher dump leading to deep stall.

[dixi188]

A Trident, G-ARPY, crashed on 3rd. June 1966 at Felthorpe after entering a deep stall and then a flat spin during a production test flight from Hatfield. The 4 crew died.

[BOAC]

Thanks dixi - and apologies to wiz for that error. I had forgotten that one.

[petitb]

DozyWannabe (http://www.pprune.org/members/54871-dozywannabe)

"Correct - BAC lost a 1-11 in testing, in an infamous accident that killed Mike Lithgow and his crew".

-true, but why "infamous" ?

[Mr Optimistic]

http://www.pprune.org/aviation-history-nostalgia/350638-mike-lithgow-bac-1-11-crash-site-1963-a-2.html

[John Farley]

I agree your info.

PY had done three successful stalls pusher ON but the fourth was done pusher OFF (I don't know why).

George Errington (40 years a tp) was among the crew although I believe he was a last minute addition.

[Brian Abraham]

Wiki cites "The use of the term "angle of incidence" to refer to the angle of attack occurs chiefly in British usage." and then gives reference to Kermode, A.C. (1972), Mechanics of Flight, Chapter 3.

Perhaps John Farley may be able to provide insight into this British usage. Sorry I don't have a copy of Kermode to see what he has to say.

[John Farley]

Sorry, I don't have a Kermode handy (I was a Piercy man myself - or to be more accurate I was told to acquire Piercy's Aerodynamics by my college in 1950)

Piercy (and it has to be likely that his 536 page tome was typical of late 1940s UK usage) calls the angle between the free stream airflow and the chord line of an aerofoil its incidence and refers to it as alpha. So that seems to support the notion you quote.

Given this and the fact that Dave's book was first published in 1967 it does not surprise me that he uses the word incidence for what we call AoA today.

I had the pleasure of flying with Dave at Bedford when he came to look at our Concorde takeoff director trials in a Vulcan back in Feb 67, but the nature of that flight did not call for a discussion about alpha. It was sufficient merely to refer to high alpha and low alpha. Clearly we both had the same appreciation of what alpha stood for (if not the name of the angle).

Since age matters in this context Dave was 13 years older than me.

[bearfoil]

John Farley

You would seem to be a contemporary of Colonel John Boyd, are you familiar with his work in Air Combat Manouvering? He was instrumental in developing what appears to be discussed here as superstall into a tactic. Energy management was quite the thing in the sixties and seventies. Not a basic manouver by any means, he appears to have constructed a method to integrate the pilot with the machine throughout its performance limits, and past. Sir??

bear

[BOAC]

Groundbreaking stuff, there, bear, but like the Harrier 'Viff', 'flat-plating' a jet in combat, while confusing the **** out of the guy (??girl?!!) on your tail, presents any other 'nasty's mate' around with a low speed target with minimal manoeuvre capability. A brilliant tactic if you are in a 1 v 1 and don't flick/spin in the process with too few feet between you and terra firma:)

[John Farley]

Steady on.

The term superstall is used in connection with a very specific situation where when a wing stalls the tail is unable to provide the nose down force needed to recover from the wing stall.

Please do not start associating the term with all sorts of other high alpha flight. Just because some aircraft can be taken to extreme alphas under perfect control (even up to and beyond 90 deg) that does not mean they are superstalled (indeed quite the reverse).

Re your good Colonel, I never had the pleasure of meeting him but I am familiar with his work.

[PBL]

Mr. Optimistic asked for an explanation of the term "downwash". I refer to the use which DPD makes of the term in his book. I think it is quite clear what DPD means by the terms he uses.

"Incidence" is denoted alpha, and as mattpilot has noted, "incidence" and "angle of attack" are explicitly the same for DPD (quote from p4): the angle between wing chord line and free air stream. Free stream is defined as the air conditions close to but uninfluenced by the airplane. Not all people nowadays use exactly these definitions.

The airstream is turned downwards for various reasons after passing over the wing. The tail sits in the downwards-turned airstream, which is for the tail its "free" airstream. DPD uses "epsilon" for the angle of downwards-turning, aka downwash. Let alpha_0 be the angle between the wing chord and tail chord. Then the incidence (=angle of attack) of the tail is (alpha - epsilon) + alpha_0. This is all on p120. This is all pre-stall terminology.

Considering the term to use for a superstall, DPD says "Any title, such as super-stall, deep stall or a locked-in condition, is acceptable because they all refer broadly to the same thing; the only point to be made is that the locked-in description should be applied only to those types on which recovery is not possible", p115.

He says that there are two characteristics which affect whether an airplane has a superstall, namely the pitching tendency of the airplane at the stall, and tail effectiveness in recovery from the stall.

First, some swept-wing airplanes have a pitch-up tendency at stall. This may be due to a number of factors: if tips stall first in a swept-wing aircraft, then center of lift (CoL) moves forward, and thereby closer to CoG, reducing any nose-down moment caused by weight x arm (measured to CoL). Further, the lift generated by the forward fuselage of the aircraft usually continues to increase until well past the stall, contributing to the pitching-up tendency past the stall (I presume he is presuming that the lift generated by the fuselage forward of the -changing- CoL is greater than that generated by the fuselage rear of the CoL, even as the CoL is moving). Third, the aircraft will start to sink significantly, increasing alpha even without change of pitch angle. This "compounds the entry to, and the progression of, the super-stall."

If the tail then sits - quite quickly, as inboard airflow separates - in turbulent airflowthe from the stalled wing rather than its usual free airstream, then elevator effectiveness is - quite quickly - much reduced. In some aircraft, the tail is positioned such that this can happen. And if you are in a high-alpha position with much reduced elevator effectivess, it can be quite difficult to impossible to get out of this situation.

DPD is, as John Farley says, referring to a very specific form of behavior of some commercial swept-wing jets at the unaccelerated stall when he talks about "super-stall".

I note that current airworthiness certification requires that it be demonstrated that a commercial aircraft be not able to enter a superstall. Aircraft whose aerodynamics and geometry render them possibly susceptible to one must be fitted with automatical devices which preclude it (stick pushers).

PBL

[Mr Optimistic]

The airstream is turned downwards for various reasons after passing over the wing. The tail sits in the downwards-turned airstream, which is for the tail its "free" airstream. DPD uses "epsilon" for the angle of downwards-turning, aka downwash. Let alpha_0 be the angle between the wing chord and tail chord. Then the incidence (=angle of attack) of the tail is (alpha - epsilon) + alpha_0. This is all on p120. This is all pre-stall terminology.

[Brian Abraham]

Thanks JF. I just dragged out AP3456 (which I'm sure you could almost quote verbatim :p) and found this in CHAPTER 2 - AERODYNAMIC FORCE, Symbols and Definitions,

i. Angle of Attack ( a ). The angle between the chord line and the flight path or RAF. In many textbooks this is referred to as Incidence.

j. (Rigger’s) Angle of Incidence. The angle at which an aerofoil is attached to the fuselage. The angle between the mean chord line and the longitudinal fuselage datum. The term is often used erroneously instead of Angle of Attack.

Further

a. Free Stream Flow. Air in a region where pressure, temperature and relative velocity are unaffected by the passage of the aircraft through it. Sometimes called relative airflow (RAF).

Not a day you don't learn something new. Hence, Davies was quite correct in his usage IMHO.

[bearfoil]

Brian The second quote rings true. It has a special application in aircraft that have tandem, or in line MLG (eg B-52). The B-52 does not rotate, it lumbers off the runway at a virtual level Pitch, relying on its AOI to get airborne. Other Freighter types in some configurations had sufficient AOI to land on their nosewheel, an odd and rare, though acceptable landing.

John Thanks for the great reply. Flat Plate is not advised in Air Combat, for reasons BOAC has stated, but was taught at Nellis while Boyd was Instructor.

The manouver is FIPO, for instructional purposes only.

bear

[Jane-DoH]

Mr.Vortex,

That's what a superstall was in the Saab Draken? I'm surprised the vortex would burst so early, especially with the rear-delta having a 57-60 degree sweep.

[Brian Abraham]

You can see the Draken stall here

tZeehlhC6T0

And some reading

http://ftp.rta.nato.int/public/PubFullText/RTO/TR/RTO-TR-AVT-113/TR-AVT-113-23.pdf
http://ftp.rta.nato.int/public/PubFullText/RTO/TR/RTO-TR-AVT-080/TR-AVT-080-06.pdf