starting a jet engine

[md-100]

Does anybody know where I can find a video showing the start process of a jet engine. ( engne point of view.. not cockpit or outside just noising). I would like to see how the starter work with the bleed air

thanx

[chksix]

http://www.patricksaviation.com/videos/Guest/151/
An F-16 in this case.
http://www.youtube.com/watch?v=pAV1pIsWe-E
SR-71 start at about 4:30 into the video. I added this just for fun

[Bullethead]

G'day MD100,

Have a look here,

http://www.geae.com/education/engines101/

You'll probably find what you want.

Regards,
BH.

[barit1]

The starter (which can be an electric motor, hydraulic motor, or most commonly on today's airliners a small compressed-air turbine) couples to the high-pressure compressor by a gear train. Once it spins up to about 10-15% core speed, fuel and ignition are intoduced to the burner. It lights off fairly quickly, and now both the HP turbine and the starter are supplying torque to the compressor to bring it up to an idle speed of 50-65%. Somewhere along the line the starter will cut out automatically, and the HP turbine carries the load at idle and above.

Does this help?

[md-100]

Thanks

but when you extract air from APU to move the starter, and then it gear to the HP compressor... why dont give that air direct to the compressor to move it???

[Speedbird48]

MD-100,

If you use the APU/Ground air to drive a small highly geared turbine connected to the main engine gearbox you get more revolutions for each pound of air.

The APU would not have enough air to turn the main compressor.

Think of how much air is going down the intake to turn the compressor when you are doing a windmill start after an inflight shut down!!

Speedbird 48.

[lomapaseo]

but when you extract air from APU to move the starter, and then it gear to the HP compressor... why dont give that air direct to the compressor to move it???

If you take the same amount of air as used in a small voulmn starter and deliver it to the compressor intake directly it would be wasted in the inefficiencies of blowing though stators at the wrong pressure conditions. If you've got lots of air to fill the whole compressor under pressure then you can windmill start it that way in flight.

So by using a small amount of air in a starter you can spin up the compressor high enough in speed and get only the rear smaller volumn stages to pressurize the burner.

maybe somebody else can untwist my words a bit here and explain it more clearly

[md-100]

thanks again

I ve got it!!

Another one: what happen if while starting the starter disengage thus the compressor start to deacelerate, and then the starter engage again??

[barit1]

If the compressor is coasting down (aborted start) and you re-engage the starter, it will accelerate fast enough that when the clutch engages, something will probably break due to the high inertias of the rotors. Usually the starter shaft itself will break (it has a "shaft shear section" for just this purpose, to prevent damaging the gears). However sometimes the tower shaft between the gearbox and compressor will fail first.

Either shaft can be changed in short order, but it's an expensive lesson

[Panman]

Originally Posted by barit1

...it has a "shaft shear section" for just this purpose...

Isn't that called a quill shaft?
PanmaN

[barit1]

Yes, or maybe the most logical name might be "torque fuse".

P&W calls the shaft to the compressor bevel gears a "tower shaft".