Brian
 

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.

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

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:)

Bearfoil
 

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.

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

Thanks, that's clear
 

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,

Further

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

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

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.

You can see the Draken stall here



And some reading

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