superstall
 

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

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?

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.

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

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

vish02
 

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

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

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'.

@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.

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?

@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... ;).

@ 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.

matt
 

Thanks. Well spotted.

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

JF

Definition of (aeroplane) AoA
 

Partial quote from BEA's Final Report on the A320 accident near Perpignan, para. 1.6.6.1:
regards,
HN39

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.

To give the exact definition Davies gives in the book - 2005 reprint

Airplane AoA definition
 

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:
regards,
HN39

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'?

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

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 :).

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.

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?

could someone explain this to a non-pilot ?
 

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 ?)

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.

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.

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

Although the tail experiences an increasing negative incidence due to downwash with i
 

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.

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.

Ah, is that what was meant ?
 

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

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!!

to add insult to injury
 

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

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. :}

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

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.

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.

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

Super Stall
 

DozyWannabe

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

-true, but why "infamous" ?

maybe here ?
 

dixi188
 

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.

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.