Yet another Concorde question

[Nick Thomas]

I have just watched the ITVV video on Concorde. Before departing JFK the flight engineer on starting no 3 engine holds it a sub idle to debow the engine shaft. This was done as the engine had been shut of in the last four hours. I understand that it's to even out the the internal temps of the engine to I presume ensure that any bowing of the shaft is removed. He then starts no 2 engine yet he says that debowing is not required for that engine. Is this because he can tell the temp of the engine before starting? or is it possible that this engine had been shut down for a longer period possibly because some work had been down to no 3 engine. Or is there another explanation?
Am SLF but am interested in aircraft design and it's differences and similarites to my design work as an architect.
As two other recent concorde questions have been answered so clearly I thought this would be the the best place to ask.
Thanks
Nick

[M2dude]

Hi Nick, a very good question. DEBOW was instigated into the design of Concorde (so legend has it) following an engine failure in the Vulcan Olympus 593 flying test bed in the late 60's (on the ground at Filton). The engine had been undergoing repeat stop/starts over a period of time; unfortunately this finally resulted in a failure of the engine. Investigators concluded that un-even cooling of the rather long LP/HP shafts resulted in a slight 'bowing', and accelerating this engine in this condition resulted in extreme shaft vibrations, that could be 'hazardous to engine health'.
Debow was fairly simple in concept, in that when you started the engine, part of the starter interlock circuit required you to select the 'DEBOW' switch (located directly below the start switch) to ON. This switch would then input a discreet signal to the electronic Engine Control Unit that would have the effect of limiting engine N2 to 30 %. (This you quite rightly stated is a sub-idle condition). Setting the switch also started an electronic timer, that after 1 minute caused a light set into the DEBOW switch to illuminate, this indicating that the switch could now be set to normal, as the temperatures of the shafts would have now been equalised, and the discreet was therefore removed, and the engine allowed to accelerate to normal idle.
Now if you were undergoing a 'normal' start ie. a cool engine, then all the F/E would do was as the start switch cut-out and sprung back to normal (@30% N2) then the DEBOW switch was manually returned to normal also, thus allowing the engine to rapidly accelerate to it's normal idle of about 68% N2. Now to fully answer your question, if an engine had been shut down within 15 minutes, it was deemed that bowing of the shafts had not had a chance to occur, and so a normal start could happily be performed. (I seem to remember that the limits are 15 minutes to 4 hours, for a debow start). So it is possible that a check start might have been carried out on #2 engine, so a debow start was not required. More likely, if the aircraft was a night-stopped BA003, departing JFK as the morning BA002 back to LHR, then all engines would be cool, and so a normal start could be carried out, unless a ground run had been carried out that morning on #3 engine. (ie. less than 4 hours previous).
As a matter of interest, when you started a 'warm' engine, you could feel the vibrations for several seconds, even at debow. These were not violent, but certainly noticeable. Also, after engine starting, if the DEBOW switch had been inadvertently set back to debow, nothing would happen, because as the engine passed 58% corrected N2, then the debow function was inhibited.
I hope this all helps Nick, and thanks for an interesting post

Dude :O

[Capn Bloggs]

And RR is still at it: the EEC on the BR715 in the 717 spins the engine at max cranking for 30 seconds to reduce the rotor bow effect before introducing fuel. :ok:

[Nick Thomas]

Thanks M2dude for such a complete and clear answer. Concorde is indeed a very interesting and complex plane. Especially when you consider that it was conceived about 55 years after the first powered flight!
Regards
Nick

[howiehowie93]

That's a very interesting subject.

I used to work on Industrial Olympus - all are based on the 200 series. There is nothing about DEBOW in the manual or any such function in the Fuel Control System. There is a critical speed band though where the LP Spool shouldn't be run at for any length of time. As I recall the Service Bulletin is based around fatigue of the LP Turbine Disk and it was recommend to note the MW output in that band and either set below or above it.

The function of the ones I worked on was Peak Lopping or Black Start; 0 to 25MW (at least) in two minutes from hitting the button was the requirement. And they still are doing that 35+ years after installation !

regards
Howie

[M2dude]

Nick Thomas
You are most welcome Nick, she was an amazing aircraft wasn't she. :)
howiehowie93
It's so easy to forget the 'other' applications of this incredible engine. As welll as the industrial applications, the 'OLY' was of course also used on the Type 42 Destroyers and Invincible class carriers. The Olypus 593-610-28 used on Concorde also had a 'verboten' sustained N1 band, again due to turbine fatigue reasons. (I seem to remember that it was a resonance issue).

Dude :O

[gas path]

'verboten' sustained N1 band,
Not only the 'OLY' but also the RB211-524 and the Trent although in this case it is the fan that has a resonance band.
The GE engines also suffer from shaft bow. Ground running a GE90 above idle requires a 20 min idle to let the shafts settle..... and they DO vibrate if shut down 'hot'!

Sorry thread drift.

[M2dude]

Gas Path
Yes, I've noticed that the GE90 shakes a bit when shut down, less so the Trent I've found. :)
wetbehindear
No honestly, it was just that, a very slight sideways bowing of the L/P and H/P shafts. This was slight of course, but still enough to cause a fair amount of vibration on a hot start. These shafts were relatively long inter-bearing, particularly in the 'hot bit', and uneven cooling after shut down caused this phenomenon. (I know it sounds weird, but it was a fact). :ok:

Dude :O

[M2dude]

wetbehindear
You are very welcome. As far as the bowing goes, I think it is just down to expansion coefficients; differening shaft temperatures/expansions on critical parts of the rotating assembly. (At least that's the way I've always understood it, but as oh so often these days, I could be wrong :)).

Dude :O

[rudderrudderrat]

Hi wetbehindear,

We had a similar problem on RR Tynes in 1971. I was told that bowing was due to a compressive load on the length of the shaft - due to the different expansion rates & temperatures between the casing and shaft.

[M2dude]

rudderudderrat
Wow, the Tyne as well eh. Was this a Brit'/Vanguard by any chance?

Dude :O