Coffin corner - Concorde

[dynamite dean]

Just reading on Coffin corner and I was wondering how did those designers get Concorde to fly so high without IT getting into that region and come to think of it how do designers get like a high flying bird (eg sr71) up there without it stalling or overspeeding at all - An amatuer pilot wonders....:confused:

[Dan Winterland]

Lots of thrust!!!!

[BOAC]

dd- a simple answer!

'Coffin corner' is where by going a few knots slower you stall, and a few knots faster you exceed your limiting mach number and encounter severe 'compressibility' problems making it virtually impossible to descend without losing control. Concorde was flown at Mach 2 in the cruise (and the SR 71 a 'little bit' faster), so you would have to be VERY high to be anywhere near stall speed, even for those machines, at that Mach. Perhaps some fundi could tell us both the stall IAS of Concorde and the altitude at which this is equivalent to Mach2?

I recall an X-15 getting into a spin at Mach 7 but I guess that was a foot or two higher than Concorde:D

[Bellerophon]

dynamite dean

At slow speed, Concorde’s delta wing didn’t stall in the conventional sense, the drag just kept increasing as you slowed up, until you reached a speed where, even with full power applied, you only just had enough thrust to fly level. This speed, called the Zero Rate of Climb speed, VZRC, could, broadly speaking, be regarded and treated in the same way as the stalling speed on a conventional aircraft, that is, best avoided.

In order to stay well clear of VZRC, operationally another speed was used on Concorde, called the LOwest authorised speed, VLO, which was the slowest speed the aircraft was permitted to fly at.

At high speed, things were more conventional, however, in any significant Mach overspeed, it was likely that the nose Temperature Maximum Operating limit, TMO, of +127°C would be exceeded before any untoward aerodynamic effects were experienced.

Now, to put some numbers to it, at her maximum authorised altitude of 60,000 ft, Concorde cruised at M2.00, around 430 kts IAS. The high speed limit, MMO, was M2.04, around 440 kts IAS, and the low speed limit, VLO, was 300 kts, around M1.41.

How the designers did it, I don’t know, but they came up with a superb aircraft which, even at maximum altitude, had an indicated speed range of 140 knots or 0.63 Mach.

Coffin corner? More like coffin plateau!


BOAC

At a rough extrapolation, VLO, as an IAS, would be equal to M2.00 around 73,000 ft.

Far too high for someone trained at Hamble not Cranditz. :D


Regards

Bellerophon

[SmilingKnifed]

Bellerophon, top explanation, thanks.

(Good Trafalgar reference as well).

I'm struggling to understand though how drag increased with decreasing speed on a delta wing. Would you mind elaborating?

[Dan Winterland]

Any wing will develope more drag once the aircraft is deccelerated below Vimd (Velocity Indicated Minimum Drag). It's just more pronounced on a swept wing aircraft and particularly noticeable on a delta. The 'overspill' of the higher pressure air under the wing starts at the root on a swept wing creating a large vortex above the wing which generates masses of drag. However, this vortex (also known as the ram's horn vortex after it's shape) can be beneficial. The accelerated air in the vortex creates a lower pressure than would have normally been present and this extra lift allows the aircraft to fly slower. Deltas don't have flaps, but this vortex helps nicely. The only drawback is that you need masses of power to accelerate form such a slow speed.

Speaking from experience, the Victor Mk2 could reach FL500. The difference between the slow speed and high speed (Mach) buffet was only a few knots. The Victor's surplus of power allowed it to operate in this regeime.

[BEagle]

Partly correct about the nature of detached vortices at high AoA with a 'slender delta' (i.e. Concorde), but a ram's horn vortex is a differnet beastie not merely associated with delta configurations. Movies of Concorde flying the approach on humid days show the vortex pattern quite clearly.

In the tin triangle, I don't recall any minimum speed at altitude apart from Vapp minus 5 when learning low speed handling quirks with an OCU QFI. Flying slowly at height could leave you unable to accelerate back to normal cruise speed if you flew too slowly. We found this out one day when 'Watt the Plot', our infamous Nav Plotter, requested a speed reduction in order to make a LL entry point time in the south of France after we'd encountered unusually strong winds. Back to 190 KIAS at FL410 for a while - then it wouldn't accelerate out of it even with max continuous. So the Capt sheepishly asked for an early descent and all was well again!

A shame that Skippy killed off your magnificent rocket ship so prematurely, Bellerophon!

[dynamite dean]

Thanks to all especially B. One day when I grow up at get a job at an airline I will fire this question at the interviewee

Watchout Boac I applying in April 2006 just have to get out of Africa first!:E

Certainly smoothed over some theory there for me:ok:

[John Farley]

Smiling

Check your PMs

JF

[Bellerophon]

BEagle

...A shame that Skippy killed off your magnificent rocket ship so prematurely...

Yes, sadly we are now just another Boeing Airline, with nothing left but happy memories. Still, it was a privilege to fly her.

Regards

Bellerophon

[mighluss]

another question...

Wich Was the VLO in a normal approach? (light weight at sea level, for example approaching to JFK after crossing the pond), the AoA on approach seems quite large!

and related...

The increase in drag at slower speeds have direct relation to the increase of AoA?

Thaks!

[swampthing]

Just a quick reply on the question a very good friend of mine .. Dynamite Dean asked about coffins corner and wing design..

As we know , the higher the aircraft flies the thinner the air gets and the more an aircrafts performance is affected..

This in short means that the thinner the air gets, the faster through the air you have to move your wing to produce the correct amount of lift for your aircraft at a given weight and configuration.

For example.. lets say you are cruising at M0.84 at FL370, you start your climb to FL410 at a constant mach number of 0.84.. in this case you will agree that your TAS is decreasing... eventually you will reach a piont where your mach cruise is not sufficient to produce enough lift for your wings, but you are at Mmo.. this is shortly what coffins corner is.. The U-2 had the worst coffins corner of them all, in a steady left turn at 70 000+ feet the airflow over the left wing will be slower than the airflow over the right wing.. this will result in the left wing going into a slow speed stall and the right wing approaching the high speed stall... this is one reason why the U-2 has such a great turning circle, with a coffins corner of only 5 knots, a steady hand was needed.

As far as your question about the wing design, you have to remember that it is vital to keep the wing tips inside the shock cone.. having a wingtip extend into the shock wave produced by the nose of the aircraft can cause serious buffet and tip stalling due to flow upset..
For this very reason the AVRO company when designing the Vulcan had to camber the wing tips to a more agressive angle to keep it inside the shock wave... the vulcan had a much larger delta span wise than the concorde if you compare wingspan to fuselarge length.. and a much thicker wing.. thus the vulcan would reach the Mmo much quicker than the Concorde, but its slow speed stall was retarded due to the low wing loading it had.

Hope this helps a bit..
:ok:

[Shaka Zulu]

May i dare to change one thing in swampthings post which is the correlation between TAS and Mach. I think what you meant to say was IAS and Mach, since that is what the coffin corner charts are based on. The correlation between TAS and Mach is merely dependent on temperature and yes in an ideal atmosphere the temperature would go down between FL370 and FL410 so on constant Mach the TAS would decrease. But if we are talking northern hemisphere the tropopause would typically set in at an altitude equivalent of the FL's we are talking about now.
The IAS would decrease anyway and that's where Mmo starts going towers V(IAS)stall

[ORAC]

Which brings to mind the origins of the phrase, "behind the drag curve", and the number of early deltas lost on approach.

[Jhieminga]

Someone mentioned the SR-71 in an earlier post. While not Concorde, this page from its operating handbook might help to clarify things.

The full manual is available here. (http://www.sr-71.org)

http://www.sr-71.org/blackbird/manual/5/5-9.gif

[used2flyboeing]

THe Delta is a nifty high speed design, but terrible for low speed control ( speed unstable ) hence the need for an extra human on the COncorde flight deck to work the throttles - in case the analog autothrottles gave up.. We also saw an example of the delta low speed performace with the last fateful Concorde accident. Boeing recognised this commercially unacceptable penalty & went with the swing-wing on the SST baseline configuration which ended up on the the B-1 & F-111 bombers. But AirFOrce experiance was that a redundant autopilot weighed alot less than the clap trap & hugh swingwing bearing & led the Boeing sonic cruiser back to the delta configuration.. Boeing developed an integrated autothrottle - autopilot suite called "total-energy" that allowed pilots to fly in the "behind the power curve" speed unstable regime with docile speed stable - conventional characteristics .. Space Shuttle is also delta ..

[Bellerophon]

used2flyboeing

...hence the need for an extra human on the COncorde flight deck to work the throttles...

What are you on about?

Concorde had a standard crew of three, a Capt, F/O and F/E.

On a non-autothrottle approach, the pilot flying handled his, or her, own throttles.


mighluss

…Wich Was the VLO in a normal approach…

VLO below 15,000 ft was V2 or VREF.


...increase in drag at slower speeds have direct relation to the increase of AoA?...

No, not if you mean direct relation in a mathematical sense.

The drag rise with increasing AoA was more exponential than linear.


Regards to both

Bellerophon

[Ranger One]

used2flyboeing:

I've never been near one operationally, but I seem to recall hearing that the principal duties of the 'extra human' on the Concorde FD concerned shuffling the fuel load around the aircraft to maintain trim at M2.0...

R1

[used2flyboeing]

when I said extra-human I did not mean 4th crew member - I recall a BA flight instructor told me that the flight engineer or FO flew throttles w/autothrottle unavailable b/c it was Sooo speed unstable .. "a real hand full" ..

[AAIGUY]

US AIRFORCE abandoned the swept wing design as it made to aircraft too heavy in Air to Air in the late 70's. F15 and F16 were the result.

AS much as I don't like Airbus, I think the Concorde worked fine with its wing. Swept wings were a neat idea, but not great in reality.

[Bellerophon]

used2flyboeing

...I recall a BA flight instructor told me that the flight engineer or FO flew throttles w/autothrottle unavailable b/c it was Sooo speed unstable .. "a real hand full" ..

I am very surprised you say a BA instructor told you this, but it is still rubbish.

Concorde was not ...a real hand full... nor was she ...Sooo speed unstable... that she required ...an extra human on the COncorde flight deck to work the throttles....

Concorde was speed unstable on approach, and a manual throttle approach was not as easy as on, say, a B747. However, manual throttle approaches, on four, three or two engines (both on the same wing) were all manoeuvres that were part of the initial conversion course and many recurrent checks thereafter.

Once trained for, such approaches were well within the capabilities of the average line pilot, and I can’t imagine why anyone would claim otherwise.


Regards

Bellerophon

[mfaff]

I might be speaking out of turn here but I seem to recall that the Concorde designers were more aware of the material temperature limitations as a primary limiting factor in the upper speed ranges.

If so then they would probably have designed the airframe to achieve its performance in a speed zone convergent with the temp limits.

As in designing the high speed, high alt 'coffin' corner to be wide enough to be acceptable in all control configurations at or near materila temp limits...yet not so wide as to provide excess redundancy. No point in having a limiting M of 2.5 if you know that in pretty much all coniditions the max temp limit of the pitot is exceeded and hence you will damage it.

However the U-2 and SR-71 were max performance driven designs... the highest possible, the fastest possible, hence in the SR-programme the development of a whole range of new materials, production methods and assembly technology...to say nothing of mechanics tools and so forth.

It then follows that to reach those extremes the margins are far slenderer and the aircraft will be operated far closer to those limits.

As for the low speed behaviour of the Concorde ogive, initially the on paper design was not brilliant. The prototype wings were revised to produce the pre production items which were further refined to give the definitive production items. Each iteration was primarily aimed a improving the low speed performance whilst retaining the lowest possible effect on the high speed performance. If you get a chance look at them look particularly at the camber and twist of the outer portions of the leading edge.. on 002 a Yeovilton its a pretty simple transformation, slightly more complex on 01 at Duxford and a whole heap more complex on the production wing.

Great stuff empirical design.....

Just on another note...swept wings/ swing wings....The F-15 and F-16 are almost a generation later in engine design than both the F-111 and the F-14, both swing wings....hence the additional power made it possible to achieve the limited high performance required of the airframes without resorting to swing wings or true swept wing (the leading edge is swept but trailing pretty much straight).

And the F-14 can certainly give the F-15 a run for its money in a dog fight...and can certainly out last an F-15 in terms of loitering performance... as it was designed to do.

However I agree that with improving engine technology and computer enhanced stability the purely mechanical advantages of the swing wing design and true swept wing design will be questioned, certainly in smaller agile combat aircraft. It really boils down to the size of the perfrmance envelope demanded by the aircraft. As it expands in all directions the advantages confered by one or other physical configuration will be diminshed to a point where it is no longer justified.

Conversely a very focused and defined envelope (like a subsonic or even supersonic commercial airliner) will benefit from a physical configuration tightly tailoered to the envelope, afterall pulling 6 g at high Mach low alt is not on the cards....but efficient cruising and 'stack' performance are..

[turbynetrip]

Re swing-wing v. delta for SST,

True, Boeing's initial design for the 2707 used a swing-wing, great numbers on paper but the hardware to secure and move 'em weighed something like 40,000 pounds- they never did get the weight under control, went with a more conventional delta in the redesign prior to its cancellation.

I talked with a fellow that was on Pan Am's SST selection committee who told me that his favorite was the Lockheed design for that reason...in the end Boeing's was quite similar to Lockheed/Tupolev/BAC-Sud Ouest. At least the engines were completed, really impressive numbers, something like 67,000lb st w/reheat/AB

Shame it was never built, I suppose if the US hadn't wasted so much money in 'Nam it might have been a reality. Hats off to the Anglo-French alliance for seeing it through!

TT