How is a jet engine started? What starts the compressor blades turning?
Answers:
Well Rho Zeta, who the hell made you the governing body for these answers?
I would guess that most of your experience is in the commercial airline trade as you seem to have little knowledge in the military sector.
Never heard of a motor/pump?
Never had an APU provide all of the aircrafts electrical and hydraulic requirements as well as providing power for starting?
Yes, we do use a small gas turbine starter on some aircraft, the Harrier being one, and it consists a small gas generator coupled to its own power turbine that drives through a sprag clutch onto the HP compressor auxiliary gearbox ( Plessey Solent).
Sprag clutches are basically free wheel units and are not engaged as you say at 100%. You obviously had not considered torque overload or power turbine overspeed.
Most of the smaller engines are started electrically - Rolls Royce Gem, Gnome, Turbomeca Astazou. The old Rolls Nimbus had a Starter/ Generator that was constantly coupled to the auxiliary gearbox.
Lycoming T55, T712's are hydraulically started either from ground supply or from the on board APU powered hydraulic system on Chinook aircraft, the APU being a direct drive T62 T2a unit providing 95 bhp at 6000 rev/min shaft ouptput being started from a 200 cubic inch hydraulic accumulator feeding aforementioned Motor/pump.
The pump then provides power to the aft transmission auxilliary gearbox which drives the aircrafts own Hydraulic systems ( three in number - No1 and No2 flight boost and auxiliarys, in addition to driving two 400 KvA oil cooled alternators, which have also been used to providing electrical starting power for other aircraft on ground.
Larger engines, for instance the Spey, uses an air start system on both the military Nimrod and the Marine SM1a/ 1c variants comprising of an air start motor driving the HP spool.
Air starters ( in our case) provide LOW pressure, high volume air to turn either an air starter motor on the receiving engine or to provide feed air to turn the HP turbine by direct air impingement.
The last military aircraft to use cartridge or Koffman starters went out in the eighties as the starting fuel was AvPin or IsopropylNitrate and somewhat dangerous, especially on hot days.
Tell you what, you stop generalising for Military and I'll stop for Civil A/c.
Ps if your 747 doesn't have bagged tanks and no low-level pump shut-offs they deserve to have the problems you describe.
IT USES A (APU AIRCRAFT POWER UNIT THAT STARTS THE MOTOR TURNING TO BUILD UP COMPRESSION IN A CAN THEN WHEN THE CAN IS FULL IT OPENS AND QUICKLY GIVES A BLAST OF AIR THAT SPINS AND FIRES THE IGNITEORS AND THE MOTOR STARTS
Hmmm. I thought it was squirrels!
Smaller ones have an electric starter similar to your car, but the larger ones use comprssed air to drive a turbine which spins the primary (high pressure) shaft of a multi-shaft engine. The compressed air can come from an APU, GPU, or another engine on the same plane.
First answer is absolute crap.
Steve is correct, in addition, some turbines are started by smaller turbines attached to them, these are called jet fuel starters or JFS for short.
The JFS starts up (and it is usally started by a small electric motor or by a stored energy source, either hydraulic or pneumatic) and once it is at 100% a clutch is engaged automatically that starts spinning the gear box of the main engine.
These are mainly on military aircraft, more specifically, mostly on fighters and the like.
And on some military aircraft they use starter cartridges, (usually as a backup system) basically an explosive charge that drives a flywheel attached to the gear box.
And if all else fails, most commercial aircraft can be started by running a pneumatic hose from a nearby aircraft to the one that you want to start, kind of like a jump start but using high pressure/ high volume air.
The first answer is correct, as is Steve. It depends on the aircraft setup. I am a UH60 (Helicopter) mechanic, and the UH60 uses an APU (auxillary power unit) to start it's GE701C engines. it does this like this..the apu is started by compressed nitrogen pumping hydraulic fluid through the apu, starting the vanes turning, and lighting off the apu (which in itself is a small engine). The APU sends compressed air to the first engine, which starts, then that engine sends more compressed air to the other engine. We can also use a GPU (ground power unit) to do the same thing, or another aircraft can send the compressed air through a buddy-start kit. In short, all you need is the right voltage in a battery to give you the spark, fuel, and compressed air to start em up.
compressed air
It depends mostly on the electrical system of the aircraft.
DC powered aircraft, use Starter/Generators, somewhat similar to a car. Because they can easily switch role from a starter to a generator. Smaller turbine aircraft tend to be DC powered.
AC powered aircraft generally use air, since AC generators do not change roles, they are allways generators (also called alternators when dealing with low currents). Most airliners, needing the benefits of AC because of their size, use Air to start their engines. Strating pneumatic pressure can come from the APU, a pneumatic GPU (basically a ground based APU) or cross-bleed air from another engine, including one on another aircraft.
steve is correct.
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