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 :).