Concorde / NEO thermal de-bow

[rse]

Interested in the de-bow function of Concorde & similar - long shaft, variety of exotic alloys & fine tolerances for max perf meant a current (?) was applied to the shaft pre-start & post-cutoff to even the thermal gradients and stop the shaft bowing. The NEO now has a pre-start equalisation cycle for the same reasons.

Was Concorde the pioneer for needing de-bowing?
Was this a current or just a bleed spin-up?
Do any other powerplants do this today?

[Jump Complete]

Garrett turbo-props (fixed turbine) need turning by hand (15 blades / 3 complete rotations on the 5 blade Jetstream 41) after shut down for de-bowing.

[bigduke6]

GE-Nx on B-748 do a 'bowed rotor" procedure if they have been run between 30 min and 6 hrs prior to start. Essentially, they motor to around 30% N2 (approx 30 seconds) before start. This reduces/eliminates the noticable vibration that had been felt prior to this procedure.

[tdracer]

Bowed rotor is an issue on most current generation turbofan engines - it's just the severity that varies.
All HP rotors bow a bit after shutdown due to the differential cooling of the engine parts (i.e. heat rises). The exposure window varies a bit with different engines, but if you perform a re-start somewhere in the 30 minutes to 6 hours window after shutdown, the HP rotor will have some bow in it. Best case, you'll get higher than normal vibes during the start - worst case you can rub the compressor - possibly even damaging blades - and cause permanent performance loss. No first hand knowledge, but scuttlebutt is that the starting issues Pratt's having with their geared fan on the NEO and C-series are related to bowed rotor...
The first engine I was aware of with significant bowed rotor start issues was the GE90 - IIRC if you start a GE90 in the bowed rotor window, it'll max motor for ~20 seconds to let the HP rotor thermally stabilize a bit before fuel-ON. The problem got worse on the GEnx - there were instances on the GEnx where a bowed rotor start took chunks off of some compressor blades . During the bowed rotor window, the GEnx will motor for ~40 seconds - but it's not "max motoring' - that's too fast and still can cause damage. So on the 787 the electric starter reduces the torque to motor ~20-25 % N2. The pneumatic starter on the 747-8 can't do that so it holds the variable vanes open to keep the motoring N2 down.
The LEAP engine on the MAX will pulse the starter air valve open/closed/open to keep the motoring N2 in the desired range during a bowed rotor start.
The GE9X on the 777X has the most interesting solution - there will be a small electric motor on the gearbox (aka "the rotisserie") that will use aircraft power to slow turn the HP rotor after a shutdown to prevent bowed rotor from occurring.

[rse]

Excellent answers. So it's still a feature & done pneumatically.

The Concorde's Olympus evolved from the Vulcan & TSR-2 - but was the Concorde the pioneer in de-bowing?

[megan]

that will use aircraft power to slow turn the HP rotor after a shutdown

Long used on steam turbines, where they refer to the motor as "jacking gear".

[EXWOK]

Debow on Concorde was done differently to the above examples (as always!) insofar as it wasn't motored by the starter, but the engine was started then run in a sub-idle condition.

The gens wouldn't be online so you'd keep ground elex attached but didn't need ground air.

[stilton]

Probably missed this but how long was the debow run for ?


Was the 'signal' to go to normal idle based on an elapsed time or
engine vibration monitors falling below a certain value ?

[EXWOK]

Elapsed time: 1 minute (with a maximum of 3 mins).

IIRC, procedure required if previously shutdown for >10mins and <5hrs.

Edited to add that I've never flown an aeroplane where you used/trusted vibration monitors.

[noflynomore]

Originally Posted by Jump Complete

Garrett turbo-props (fixed turbine) need turning by hand (15 blades / 3 complete rotations on the 5 blade Jetstream 41) after shut down for de-bowing.

Isn't it on the subsequent preflight rather than on shutdown? Surely you aren't interested in de-bowing if you don't intend to start it again.
On the Do228 with those nasty little high-speed Garrets you often saw pilots grab a blade and fling it to spin the turbine - when the point of that exercise was to start movung the propellor tip very slowly - very slowly indeed while listening for the turbine scraping. If no scrapes then continue turning at whatever rate was suggested - I forget.
Difficult (impossible) procedure to achieve on a normal noisy apron!

[megan]

Isn't it on the subsequent preflight rather than on shutdown

You do it following shutdown. The point behind the exercise is to rotate the shaft while it's cooling so that it doesn't take a "set" ie bow, or at least minimise the degree of bow.