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