What spool is driven while starting?

[betaboy]

A claimed benefit of the RB211 and other 3-spool turbofan engines is that they are "easier to start, since only one spool needs to be turned." (Ref. Handling the Big Jets, Davies).

I presume the spool that is turned, through the accessory gear case, is the N3 spool. So how many spools are normally turned on a 2 spool engine?

[SeldomFixit]

2 spools are turned on a dual spool, 3 spools on a triple spool - the HP directly by the starter and the other/s due to airflow induction.
The easier starting aspect is to do with the stability of the airflow through the engine to the combustors. I triple spool engine will act like a dog if start attempts take place too far below max motoring. You'd expect to see 2/3% N1 ( not too accurate at motoring speeds ), between 5-11% N2 ( the higher the better generally ) and low 30 something %'s from a good starter on a triple spool. Not textbook but it works.

[NSEU]

So how many spools are normally turned on a 2 spool engine?

Not sure what Davies is trying to say, but just the N2 (HP) is driven by the gearbox on a two-spool engine... & just the N3 (HP) on a 3-spool.

I would have said the RB211's were actually harder to start than, say, two-spool CF6's. The RB211 powered 767 even has extra ground cart connections because it is so difficult to start the engines. I would have also said that the CF6 accelerates quicker after light-up (fuel on).

Rgds.
NSEU.

[matkat]

would have said the RB211's were actually harder to start than, say, two-spool CF6's. The RB211 powered 767 even has extra ground cart connections because it is so difficult to start the engines. I would have also said that the CF6 accelerates quicker after light-up (fuel on).

Rgds.
NSEU.

As a licensed engineer on both RB211 and CF-6 NSEU has got it spot on.
matkat

[lomapaseo]

The idea behind starting is to pressurize the combustor section in order to hold the fire in the burner and to expand through and accelerate the turbines. There is not enough horsepower available either on a starter or in windmill to provide this pressure rise by spinning up anything other than the relatively small high pressure compressor. The fact that the other spools do turn is associated with the exit flow generated by the high pressure spool passing through their turbines.

In fact the burner can light off and hold the flame inside in some cases without the other turbines turning (locked up mechanically). However any attempt to accelerate may create a mismatch in pressures resulting in an aerodynamic stall in the compressor.

Starting should have less to do with how many spools than the burner characteristics.

[grease7]

We call the RB211 "kidney stone grinders" because the low noise and vibration you feel going through you, while standing close to a starting RB211. I have always wondered if that's because of the triple spool or the complicated pnuematic system regulating the airflow.

I think it's a great engine btw having seen it stall during a ground run at take-off power and just continuing after that if nothing happened. A CF6 would have been written off.

[lomapaseo]

While burner howl is possible on the RR I always thought the howl was because with a 3 shaft system the shafts pass through a critical speed during spool up to idle.

I suspect somebody on this forum knows for sure.

[idg]

I wonder how RR will get the Trent 1000 (or whatever the 7E7 engine will be) started if they are going to be using the IP shaft to drive the accessories instead of the HP? I would have thought that it would take much more power to do this because you are trying to spin the HP via the IP and also it will be electric on the 'all electric jet'.

[411A]

The odd noise that emits from RB.211 engines during start is a combination of noises from

The N2 compressor
Flame propagation in the combustion section


The latter...mostly.

[Flight Detent]

Hi all,

In my experience with RB211's on the '74 Classics, if I didn't hear that low tone 'rumble' at light-off, I would always fuel chop it, allow it to motor, then select the fuel on again, and if the rumble was there, as it usually was, it would accelerate normally!

I found that, no rumble, no successful start, the engine would always stagnate around 40%.
If about 5 seconds went by with no rumble after light-off, fuel chop.

Doing it that way, would always save a lot of time, since letting it go without the rumble, allowing it to get to the stagnated start point, fuel chop, motoring it for cooling, and then selecting the fuel on again, took significantly longer.
That 'no rumble' happened about 10% of the time, per flight!

Cheers