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Also how engine power changes throughout flight.


thanks

2007-02-20 03:17:10 · 10 answers · asked by fanta m 1 in Cars & Transportation Aircraft

10 answers

You're talking jet engines, right? The fan is rotating slow when the engine starts because the engine is not up to full speed yet. It's only at "idle" speed"
The engine power changes throughout flight depending on if the plane needs to climb or accelerate or decelerate or descend. When more fuel is added to the engine, the flame burns hotter, resulting in more gases across the turbine, and everything speeds up. It's much more involved than that, but those are the basics.
Of course this all depends on what kind of engine it is. Some have the fan directly coupled to the compressor, others have it driven by the power turbine.

2007-02-20 03:30:44 · answer #1 · answered by M333 6 · 1 0

Big high bypass jet engines have more than one shaft running through them, if you look around you'll find mention of N1 and N2 speeds. The core of the engine is effectively a gas generator, it spins at very high speed when it is running and forces hot, high speed, gasses out of the back of the core. These gasses impinge on the turbine of the inner shaft which connects the power turbine at the back of the engine to the fan and low pressure compressor disks at the front. So the core of the engine can spin up and start while the fan is still moving relatively slowly. Most of the power of the jet is developed between roughly 90 and 100% of the maximum high pressure section speed.

Have a read of the Wikipedia article below

2007-02-20 11:30:01 · answer #2 · answered by Chris H 6 · 0 0

The start-up of a jet engine is as follows: engage the starter motor, be it electric or pneumatic. When engine rpm reaches a certain percentage of rpm (about 17% for USAF fighters), thrust levers are advanced from idle cut-off and fuel is introduced into the combustion chamber. Large spark plugs ignite the fuel/air mixture and the expanding gases begin to create the turning force on the turbine. Just like in a car, the more gas you give it, the faster it turns, the more power is produced. At 100% power, the turbine is turning at about 17,000 rpm. Unlike a car, when you advance the throttle(s), power is not produced immediately. The engine has to have a few seconds to increase the rpm (spool up), thus increasing the produced thrust. Shut down is simply a matter of bringing the thrust levers to idle cut-off. This stops fuel from flowing into the engine, and it stops turning (eventually).

2007-02-20 04:24:17 · answer #3 · answered by Anonymous · 1 1

the process of startup was descriped clearly above but the question of why the fan spins slowly at the "gate" as we call it is because the there is such low friction in jet engines that after shut-down they may turn for several minutes... not to mention that even a light breeze can turn the fans...

2007-02-21 10:50:33 · answer #4 · answered by ALOPILOT 5 · 0 0

Most front wheel drive cars have an electric fan. It doesn't move until the thermostat gets hot enough to close it's switch and start the fan to cool.

Rear drive vehicles have the fan connected directly to the motor.

Now we have two options.
1. The fan has a built in thermostat that is friction based, and will spin freely till the heat around it closes the clutch to engage the fan.
2. The other is a direct drive, which sounds like what you are talking about. When your engine is started it is turning at a lower RPM than when running. Your idle speed is around 700 RPM, with a top RPM of 5,000 - 8,000. Driving is 2,000 - 3,500 RPM, usually 2/3 throttle.

Cranking RPM is around 100 RPM, so that is why it would rotate slower when cranking.

2007-02-20 03:34:23 · answer #5 · answered by Bigdog 5 · 0 4

On an aircraft engine, the fan or prop has pitch control. When it is in the stand the pitch control is usually disconnected unless that is the problem being investigated.

An aircraft has two ways to add power: By increasing the rpms or by increasing the pitch of the prop or a combination of both. When the pitch is at max, the prop is appling all the pressure it can to the air stream until the rpms are increased.

As you know, the configuration of the aircraft plays a part in all of this.

2007-02-20 03:33:09 · answer #6 · answered by Sophist 7 · 0 3

he fan is used to cool down the engine to prevent overheating

2007-02-22 14:53:41 · answer #7 · answered by jerry 7 · 0 1

THE BLADES ISN,T THE FIRST THING TO TURN THE ENGINE REALLY STARTS IN THE BACK ,IN THE CONBUSTION CHAMBER AND THEN IT TURNS THE HIGH TURBINE THEN THINGS START TO MOVE FORWARD.THE BLADES IS ALMOST THE LAST TO TURN

2007-02-20 04:16:09 · answer #8 · answered by AIR HEAD 2 · 0 1

the fan is run by an automatic clutch that is controled by the thermostat. when the motor heats up the thermostat opens and engages a switch to operate the fan

2007-02-20 03:23:50 · answer #9 · answered by cavebyrd 2 · 0 5

the fan is used to cool down the engine to prevent overheating thats all i know

2007-02-20 03:20:47 · answer #10 · answered by mssweetblue 1 · 0 5

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