ENG START A320
 

Hi..
during engine start N1 was rotating by windmiling effect , are there any limit for this phenomenon to avoid any damage to the rotor shaft and the starter??

Not really. Remember that N1 is essentially a free turbine, and will start to rotate in the correct sense once the N2 has started to build up, there is a lot more force from the flow induced by the N2 than from the airflow over the N1 fan blade. The windmilling of N1 is normally in a negative direction if the wind is from the front rear of the engine.

The starter is connected to the N2, not N1.

Surely you mean from the rear.

downunder... d'oh

Thanks a lot for your reply sir
yes I know that starter is connected to N2 shaft
my doubts how to prevent any probability for cause damage for the tower shaft or starter splince since the rotation is in the opposite direction.

sure sir.

[QUOTE=fdr;11438255]Not really. Remember that N1 is essentially a free turbine..
Negative , N1 not free turbine in my case CFM engine.

Sorry don't know the 320, but I do know there are tail wind limits for starting on other a/c.

Shatwa07: the N2 shaft is connected to an ancillary gearbox and the load/drag from that will stop any possible rotation of the N2 simply from wind blowing through it.

As others have mentioned the N1 shaft is free to rotate and will do so with even quite light winds since it is so well balanced.

Tailwind restrictions on starting have little to do with N1 twisting in the wind. Rather, the problem is that the effective back-pressure that a tailwind presents to the core can cause a hung or hot start.
Used to be a big problem with the JT9D engines (which didn't have a whole lot of stall margin to start with).

This thread might be the appropriate time to ask.

What about the 20% N2 limitation for starter engagement? After having heard the terrible sound of engaging the starter on a car when the engine is running, one has a tendency to avoid ever wanting to do it again.

Obviously this is a much different design than a car but an explanation of why it is OK up to 20% but not above would be interesting.


As for tailwind on start issues on the JT9D, I have seen 50 knots on a CF-6. Better design?

The starter uses a 'one way' clutch to engage/disengage with the rotor (the one I was familiar with was a Spraque type, but I don't know if that's the standard). The problem with a higher speed N2 engagement is the shock load to the starter shaft when the clutch engages - at higher N2 speeds that shock can shear the starter shaft. I worked one engine that had a 'smart' starter that somehow modulated the starter speed to match the N2 when there was a 'crash' starter engagement - sorry I don't remember which engine type that was or how well it worked in service.

At least with respect to compressor stability - yes, the CF6 had a far better design than the JT9D. JT9D stalls/surges in service were pretty common - fortunately the compressor was robust enough that they seldom did any damange. The CF6 rarely surged, but when it did it probably meant an engine replacement (insert light bulb joke here :)).

Reingestion of exhaust gases with a tail wind not a big thing with fixed wing td, other factors override?

I suppose in a severe case reingestion could present issues, but I'm not aware of any actual problems on large commercial aircraft. The engine nacelles are rather large and long - and with huge bypass ratios - so any exhaust would likely be highly diluted before it made it's way back into the core inlet.

Thanks td, reingestion was an issue with helos I flew on occasion, had to get tarmac to turn them so wind wasn't blowing up the tail.

When Boeing pulled the first 747 (RA001 - currently on display at the Museum of Flight in Seattle) out of mothballs to flight test the 777 engines, it still had JT9D's on the other three positions. At the time I'd been involved in ground engine runs, and was onboard when they were doing engine "trim" runs on the JT9Ds. We had about a 10-knot tail wind when they were trying to start the JT9Ds - and they couldn't get two of them started until they moved the aircraft to get rid of the tailwind...
Granted, they were old, rather tied JT9Ds, but still :=

Negative, nor have I seen any limitation for that on any other engine.

your comments and suggestions on my thread is highly appreciated..
thanks for sharing your experience.

It used to be common practice for the headset Eng to call out "positive N1" when starting into a tail wind. Particularly on RR RB211/Trents. During manual starts (old school) we used to have to wait for the call before opening the fuel cock.