Concorde question

[Shaggy Sheep Driver]

Thanks Brit 312. "Power set" it is, then. I was aware of the '3 reheat' possibility which is decided before T/O depending on T/O parameters ('is this a 3 re-heat day or a 4 re-heat day?').

On the P1 side of the cockpit is a small hinged piece of metal which can be moved to show '3' or '4'. This is set before flight depending on whether 3 or 4 re-heats are the acceptable minimum for take off that day, so if there is a re-heat failure on T/O, a glance at that indicator will show if it's OK to continue with '3 lit' or not.

[M2dude]

Ahhhh... the famous Reheat Capability Indicator. (Yes that was its official title). I seem to remember that before we did the modification to fit the 'RCI' in the late 1970s, the guys used to set an INS CDU thumbwheel as a memo to whether the take-off was a 'go-er' or a 'stopper'.
It seems a million years ago when we fitted this high presicion lump of alluminium. (Hang on a minute, it WAS ).

Best regards
Dude

[skyhawkmatthew]

I've read the entirety of this thread with great interest, having never got to see Concorde in flight, but only visited OAG in Seattle. What a beautiful machine!

My question is: disregarding the certified FL600 / M2.04 / 127ºC restrictions, how high and/or fast do you Concorde builders and designers think she could have gone?

[Quax .95]

Hello skyhawkmatthew!

M2dude gave a good answer on your question in post #1085, so I think I may quote this here again.

Originally Posted by M2dude

As far as the MAX SPEED bit goes, Concorde was as we know flown to a maximum of Mach 2.23 on A/C 101, but with the production intake and 'final' AICU N1 limiter law, the maximum achievable Mach number in level flight is about Mach 2.13. (Also theoretically, somewhere between Mach 2.2 and 2.3, the front few intake shocks would be 'pushed' back beyond the lower lip, the resulting flow distortion causing multiple severe and surges).

The maximum altitude EVER achieved in testing was I believe by aircraft 102 which achieved 68,000'.

[Jane-DoH]

M2Dude

3) A third isentropic fan shock is generated from the progressively
curved section of the fwd ramp

What's an isentropic fan-shock?

5) A terminal shock system is generated by the coalescence of
still supersonic and now subsonic air at the upper section of the ramp
area.

So the lower lip forms a normal shock and the airflow goes subsonic immediately behind it, the supersonic flow above somehow collide and form a shock between the ramps? I understand how the subsonic and supersonic flow coming together would reduce the average velocity -- I'm still surprised the gap between the forward and rear ramps wouldn't act like a divergent surface and cause the supersonic flow to accelerate rather than come down to subsonic speed.

[CliveL]

What's an isentropic fan-shock?

The first bit of the moveable front ramp was carefully shaped to give a sequence of weak shocks that reduced the Mach Number so gradually that shock losses were minimised. This was close to an isentropic process, hence the name. The geometry was arranged so that as the progressive shocks were generated and the Mach angles and shock angles changed the weak shocks tended to 'focus' on a point just ahead of the lower lip. This then became effectively a single 'shock' at that point. Hence isentropic fan shock.

So the lower lip forms a normal shock and the airflow goes subsonic immediately behind it, the supersonic flow above somehow collide and form a shock between the ramps? I understand how the subsonic and supersonic flow coming together would reduce the average velocity -- I'm still surprised the gap between the forward and rear ramps wouldn't act like a divergent surface and cause the supersonic flow to accelerate rather than come down to subsonic speed.

The shock from the lower lip would, on its own, give subsonic flow across the intake, but the change in flow direction where the flow off the solid ramp started to traverse the gap (where Dude's drawing shows the flow going into the void) produced an expansion 'fan' that accelerated the flow in its vicinity and this gave supersonic flow in the upper half of the duct but there was a shear across the height of the duct there. The total effective duct area however was convergent back to about the leading edge of the rear ramp, so the Mach Number reduced continually up to that point. Then the 'terminal shock' brought the flow down to below Mach 1 and from there on the divergent subsonic duct did the usual deceleration job. The whole point of the intake geometry was that the purely aerodynamic boundary between main duct and ramp void was infinitely flexible in shape, which made the design very tolerant of flow disturbances.

[Jane-DoH]

CliveL

The first bit of the moveable front ramp was carefully shaped to give a sequence of weak shocks that reduced the Mach Number so gradually that shock losses were minimised.

Must have been a highly efficient inlet for a Mach 2 plane: Two traditional oblique waves; a fan-shock (also oblique); a shockwave off the lip that is normal and oblique depending on how far you are away from the lip, and a normal terminal shock.

This was close to an isentropic process, hence the name.

So, isentropic would, in this context, mean that no shock-losses occurred at all?

The whole point of the intake geometry was that the purely aerodynamic boundary between main duct and ramp void was infinitely flexible in shape, which made the design very tolerant of flow disturbances.

Makes sense for an airliner that you would design an inlet this way

[Mr.Vortex]

Thanks for your reply CliveL and thanks M2Dude and CliveL again for the great
reply with detail about the intake.

[CliveL]

Must have been a highly efficient inlet for a Mach 2 plane: Two traditional oblique waves; a fan-shock (also oblique); a shockwave off the lip that is normal and oblique depending on how far you are away from the lip, and a normal terminal shock.

Yes is was very efficient - 94.7% pressure recovery at M 2.0 cruise

So, isentropic would, in this context, mean that no shock-losses occurred at all?

In theory yes, but in practice there was a small loss.

[M2dude]

And a thank you from me CliveL for your superb explanations regarding intake shock structure. It can not be over-emphasised just what an amazing achievement the Concorde engine/intake combo was. I can think of no other design in the world, before or since, civil or military, where a supersonic engine/intake marriage gave such incredidable levels of performance, stability and predictability. I just regard myself as being extremely fortunate to have been able to 'play with' this amazing kit for so many years and see what design excellance really is. (And at least pertly understand it too).

[911slf]

Power limit to 60kt

I believe that engine #4 was limited to somewhat less than max power until 60kt because of a vibration issue. Did this mean that reheat for that engine could not be selected until 60kt was achieved?

[Shaggy Sheep Driver]

All 4 reheats were selected 'on' before take off. They wouldn't actually light until the engine was up to a certain power, so the answer is 'no'. The power-limiting ensures no. 4's re-heat doesn't light below 60kts.

Watch a video of Concorde taking off which gives the view from behind. You'll notice no.4 light up marginally after the other 3 (but there's not much in it as it didn't take the aeroplane long to get to 60kts!).

[Quax .95]

Not quite right: the reheats ignite if

The N1 of number 4 engine is limited to a maximum value of 88% up to 60kts, thus within the operational requirements of the reheat.
(At temperatures colder than -35°C the engine control schedule limits the N1 of all engines to 88% or less.)

Originally Posted by Brit312

Up to 60 kts the F/E could reselect a failed reheat so hoping it would be OK by 100kts

Regards

[Shaggy Sheep Driver]

Thaks Quax. So all 4 reheats should light about the same time, then, regardless of power limiting on #4? It does seem that #4 lags a fraction in vids I've seen.

[Quax .95]

This might be because the #4 engine accelerates less fast than the others due to the limiter, reaching 81% N1 a little bit later. But this thread is too brilliant for presumptions (don't want to repeat the mistake of my first post... ). Let's see what the Concorde-geniuses add.

[Landroger]

Unique design.

I can think of no other design in the world, before or since, civil or military, where a supersonic engine/intake marriage gave such incredidable levels of performance, stability and predictability.

I think Dude's above statement more or less characterises the Concorde design and therefore this entire thread - which I have read, avidly, since post #1. However, since Dude made the statement most specifically about the synergy of the whole intake, engine and nozzles, it is worth reiterating that Concorde's only real peer in her occupation of the very highest and fastest regimes of wing borne flight - the SR71 - initially at least, had a lethal gene. Asymmetric 'Unstart' caused by intake instability.

Without proper scheduling, disturbances inside the inlet could result
in the shock wave being expelled forward--a phenomenon known as an
"inlet unstart." That causes an instantaneous loss of engine thrust,
explosive banging noises and violent yawing of the aircraft--like
being in a train wreck. Unstarts were not uncommon at that time in
the SR-71's development,

This quote is from a much longer article quoted in this thread, about a test flight by Bill Weaver, a Lockheed development pilot, in which Weaver was, quite literally torn out of the aeroplane at Mach 3.2, as was his back seater who, sadly, did not survive the incident.

Basically, a relatively small failure within the intake/spike structure of the SR71 engine, was enough to simply tear the airframe apart within seconds of onset. The scale of forces within these structures therefore, must be almost beyond imagination and yet the Concorde design was such that she did not suffer such destructive failures.

My admiration for everyone who worked on her is endless.

[SundayForever]

there were already several conduits through tank 11, such as hydraulics for the tail wheel, various electrics, and the 'backbone' fuel manifolds, that ended at the fuel jettison port in the tailcone.
A couple of fairly substantial air ducts would only have displaced a few hundred kgs of fuel at the most, out of the more than 10,000 kgs in tank 11.

[Jane-DoH]

M2Dude

I can think of no other design in the world, before or since, civil or military, where a supersonic engine/intake marriage gave such incredidable levels of performance, stability and predictability.

Well, the XB-70 had an inlet with an efficiency in the 90% range but it wasn't as stable/predictable (it suffered unstarts).


911slf

I believe that engine #4 was limited to somewhat less than max power until 60kt because of a vibration issue.

What kind of vibration issue occurred?

[M2dude]

Jane-DoH
One of the real beauties of the Concorde intake was that it was completely self-startiing, and so unstarts as such were never heard of.
Regarding the vibrations thing, here is my post #80:

The reason that #4 engine was limited to 88% N1 on take-off was an interesting one, down to something known as 'foldover effect'. This was discovered during pre-entry into service trials in 1975, when quite moderate levels of first stage LP compressor vibrations were experienced at take-off, but on #4 engine only. Investigations revealed that the vibrations were as the result of vorticies swirling into #4 intake, in an anti-clockwise direction, coming off the R/H wing leading edge. As the engine rotated clockwise (viewed from the front) these vorticies struck the blades edgewise, in the opposite DOR, thus setting up these vibrations. The vorticies were as a result of this 'foldover effect', where the drooping leading edge of the wing slightly shielded the streamtube flowing into the engine intake. #1 engine experienced identical vorticies, but this time, due to coming off of the L/H wing were in a clockwise direction, the same as the engine, so were of little consequence. It was found that by about 60 KTS the vorticies had diminished to the extent that the N1 limit could be automatically removed. Just reducing N1 on it's own was not really enough however; some of this distorted airflow also entered the air intake through the aux' inlet door (A free floating inward opening door that was set into the spill door at the floor of the intake. It was only aerodynamically operated). The only way of reducing this part of the problem was to mechanically limit the opening angle of the aux' inlet door, which left the intake slightly choked at take off power. (The aux' inlet door was purely aerodynamically operated, and diff' pressure completely it by Mach 0.93).

I seem to remember that Rolls Royce proposed a solution of their own, whre the right hand pair of engines would rotate ant-clockwise (viewed from the front) rather than the clockwise norm for just about any 'Roller' that I can think of. Although this would have completely solved the vibration problem, and was great business for the folks at RR in Patchway (just about doubling the required number of engines) it was a pretty lousy idea if you were an airline and required a much latger holding of spare engines.

[CliveL]

Investigations revealed that the vibrations were as the result of vorticies swirling into #4 intake, in an anti-clockwise direction, coming off the R/H wing leading edge.

Only one comment Dude; as I said to you in a PM the vortices came off the intake sidewall leading edge rather than the wing. If you think about it, the highly swept, sharp leading edge of the sidewall looks just like a delta wing on its side, so that flow coming on to the sidewall leading edge from the outside generates a vortex just like that above the main wing, but now going inside the intake. At low speeds the engine is sucking in air from everywhere it can, so there is a substantial flow entering from the side of the intake. As you increase speed the potential air supply coming from the streamtube directly ahead of the intake increases enormously so the 'sidewash' onto the intake sidewall diminishes and the vortex is suppressed. On the other side of the aircraft of course the sidewall vortex was handed the other way.