I know the coffin corner is only meant for subsonic aircraft, but if the Concorde is just cruising at Mach 1 and had an engine failure and also lost that afterburner, would it enter a coffin corner situation?
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asked Feb 4, 2020 at 11:13
George Clooney In a MooneyGeorge Clooney In a Mooney
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2$\begingroup$ I don't think thrust influence coffin corner definition. Thus engine out situation would not change anything. $\endgroup$– Manu HFeb 4, 2020 at 11:31
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$\begingroup$ @ManuH how could it not if the engine failure caused the flight to go subsonic $\endgroup$– George Clooney In a MooneyFeb 4, 2020 at 12:09
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2$\begingroup$ Your understanding of coffin corner is incorrect. See aviation.stackexchange.com/questions/70298/…. You are thinking of excess thrust. $\endgroup$– JZYLFeb 4, 2020 at 12:48
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$\begingroup$ Could the afterburner possibly continue to work if the main engine that it was attached to failed? $\endgroup$– FreeManFeb 4, 2020 at 16:32
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3 Answers
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The Concorde doesn't need reheat to cruise supersonically, just to get there (the range would be pretty short if it did). If it loses an engine it's not going to put the other 3 into reheat to hold altitude and speed (fuel burn) and it will do what just about all airliners have to do when at they lose an engine, descend to whatever its engine-out service ceiling is. This is done at some speed near minimum drag that gives the flattest descent, called "driftdown" speed, whatever it is, and it will just pitch down as required to maintain that speed with the available military rating (non-afterburner) thrust. Obviously, it will come out of mach as it does so.
Also I should point out that being a supersonic aircraft, it doesn't really have a "coffin corner" in the normal sense. That applies to subsonic airplanes that are limited to a speed just shy of Mach, and when they get so high they become hemmed in by stall speed on one side and trans-sonic related issues on the other. The upper limit of a supersonic airplane's speed is aerodynamic heating related and is a function of the airframe material's ability to run at high temperature, which limits aluminum structures to below Mach 3. This is why the SR71 is titanium.
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2$\begingroup$ IIRC, Concorde couldn't stay supersonic on 3 engines, and the reduction in speed (and altitude) caused a dramatic reduction in range. $\endgroup$ Feb 4, 2020 at 14:14
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$\begingroup$ Thanks for pointing that out. I should have mentioned that driftdown speeds will be subsonic. I made some edits. $\endgroup$– John KFeb 4, 2020 at 16:39
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2$\begingroup$ @RobinBennett The cruise thrust at Mach 2 was about 10,000 pounds per engine. The engines for production Concordes were rated at 32,000 lb without reheat. So it seems unlikely that cruise thrust couldn't be maintained on 3 engines at 13,300 lb. This is one of the (few) advantages of four engines compared with two, of course. $\endgroup$ Feb 4, 2020 at 20:15
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1$\begingroup$ This is a matter of definition, but the notion of 'coffin corner' can easily be applied to supersonic flight. The nature of limitations doesn't matter much: if you can't fly faster nor slower, you are in the corner. (That said, Concorde's limits in the top-right corner of the envelope (M2, FL600) were mostly certification-related, and it wasn't in the 'corner' at all there in the physical sense). $\endgroup$– ZeusFeb 4, 2020 at 23:40
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$\begingroup$ @Zeus I reckon the difference is that a subsonic plane going too fast risks being instantaneously ripped apart by aerodynamic forces, wheres a supersonic one rather overheats, which is a bit slower and thus allows more time for correction. $\endgroup$ Feb 5, 2020 at 15:50
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The coffin corner is the altitude where your maximum speed (limited by high speed buffetting) is equal to your minimum speed (limited by low speed buffetting / onset to stall).
The Concorde, when cruising at Mach 1, is not in the coffin corner, regardless of its engines health. Both the upper speed boundary and the lower speed boundary* of the flight envelope are not affected by the engine health.
*when taking non-linearity due to high angles of attack into account, the engine thrust has an effect on the minimum speed bound of the envelope. That is not relevant to the coffin corner situation, because of limited angle of attack.
answered Feb 4, 2020 at 11:43
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1$\begingroup$ If it loses an engine, I’d imagine keeping that speed would be very difficult. If the flight goes from supersonic to subsonic $\endgroup$ Feb 4, 2020 at 12:10
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10$\begingroup$ Keeping speed is easy if you allow the aircraft to descent. Anyway it has no effect on the coffin corner, that stays where it was before. $\endgroup$– DeltaLima ♦Feb 4, 2020 at 12:13
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Coffin corner is about speed: the small margin between stall and critical mach.
You don't need engines to sustain speed. You can use descent instead of engines.
As you descend, the margin widens. Soon you find an altitude where your remaining engines can power you at a viable speed.
answered Feb 5, 2020 at 3:05
Harper - Reinstate MonicaHarper - Reinstate Monica
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1$\begingroup$ It's also all about the airframe, and essentially irrespective of thrust. A U-2 at 80,000' with 4 knots between high & low-speed limits, isn't helped by having more & more power available; more thrust to climb much higher & the lines cross & he can be too slow (IAS) + too fast (IMN) at once! The OP's suggestion that engine loss might put one into a coffin corner reflects a complete misunderstanding of what coffin corner is, and isn't. $\endgroup$– Ralph J ♦Feb 5, 2020 at 8:14