Another APU/jet systems question

When I went through the systems training for the B727 at Riddle, we would go through all of the procedures from APU start to shutdown. After that class I finally realized the answer to my question of what that exhaust out of the tail cones on heavy jets was coming from. Its the APU exhaust. Most aircraft place the exhaust out the back like that, but the exhaust for the B727's APU is out of the top of the right wing (I think it was the right wing).

Stories from my Prof. told us about the cold starts where more fuel was introduced before the APU actually started. You would get a fairly rich exhaust, and sometimes a flame would shoot out the top of the wing. Hopefully none of the pax saw this before they boarded the plane!!

-ColM
 
Well actually, that isn't all that correct, yes they do different things, but a starter aka electric motor and a generator are basicly the same thing. A bunch of wired coils and magnets. Pass electricty through coils, you get magntisum which turns the motor. However, turn the same device, like through an output shaft from the motor, moving magnets past coils generates electricty. In most APUs and a lot of jet engines the starter doubles as a generator. Oh and a generator creates direct current (DC) where as an alternator generates alternating current (AC). Generators are larger in size for the same power output too, I'm guessing that's why you don't see them in cars nowerdays.

mtsu_av8er said:
The starter? Ummm . . . no. An alternator is turned by the engine, producing electricity. A starter TURNS the engine, using electricity to turn. Ever have an electric remote control car as a kid? Ever tear it apart and play with the tiny electric motor inside? Kinda like that!
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Interesting, thanks for sharing. I guess thats why they do voltage checks when starting the APU, both AC and DC, Thats one thing I did read.

I don't know what it is, but the APU is really interesting for some reason.
 
Snow said:
Well actually, that isn't all that correct, yes they do different things, but a starter aka electric motor and a generator are basicly the same thing. A bunch of wired coils and magnets. Pass electricty through coils, you get magntisum which turns the motor. However, turn the same device, like through an output shaft from the motor, moving magnets past coils generates electricty. In most APUs and a lot of jet engines the starter doubles as a generator. Oh and a generator creates direct current (DC) where as an alternator generates alternating current (AC). Generators are larger in size for the same power output too, I'm guessing that's why you don't see them in cars nowerdays.
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A starter doesn't produce electricity, it uses it to turn the engine. A generator helps recharge the battery, but needs higher rpm to do so effectively. The difference between a generator and an alternator is which part is spinning. There is a field and a armature in each, but one spins around the other in the generator, and the opposite for the alternator. (can't remember which one is which). Spinning a magnet or electromagnet around coils will give you electricity output yes, but I don't think its the same unit.

I believe most jet engines/turboprops are started by rotating the turbines and compressors, then introducing the fuel. How they rotate those may be electric or by air, but I believe after the engine is started then they kick in the generators to begin producing the necessary amount of electricity for normal operation. I don't believe the starting mechanism doubles as a generator.

-ColM
 
ColMustard said:
A starter doesn't produce electricity, it uses it to turn the engine. A generator helps recharge the battery, but needs higher rpm to do so effectively. The difference between a generator and an alternator is which part is spinning. There is a field and a armature in each, but one spins around the other in the generator, and the opposite for the alternator. (can't remember which one is which). Spinning a magnet or electromagnet around coils will give you electricity output yes, but I don't think its the same unit.

I believe most jet engines/turboprops are started by rotating the turbines and compressors, then introducing the fuel. How they rotate those may be electric or by air, but I believe after the engine is started then they kick in the generators to begin producing the necessary amount of electricity for normal operation. I don't believe the starting mechanism doubles as a generator.

-ColM
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Almost bud. Typically, turbo jets (737, A320, etc) have a pneumatically driven starter. Simply means, air pressure is used to turn a starter that is mechanically connected to the core section of the engine (N2), tower shaft is the connection. By turning the N2 section, you turn it's respective turbine at the rear of the engine. As the N2 section comes up to speed it starts pulling air through the front of the engine, through the fan (N1). That air is then either routed through the N2 section and then over the N2/N1 turbine or through the bypass (high bypass turbo fans for example or low by pass turbo jet, think DC9) as the air passes over the N1 and N2 turbines their respective sections begin to spin faster and draw in more air. See how the whole thing becomes self sustaining??

Most turboprops (Dash 8, Saab 340, etc) use a starter generator. Simply a DC generator that doubles as a starter motor. The start process here is fairly automated, hit the start button, introduce fuel at a specific N2 speed (NH for most turboprops) and the generator will stop being a starter and be a generator at a specific N2/NH speed.

Most turbojets have a single AC generator per engine, while turboprops tend to have a DC and AC generator per engine. On turboprops the double generator is more of a system design simplicity thing than anything else. It doesn't make sense to use a pneumatic starter on such a relatively small N2/NH section when a DC/Starter generator can do the task for less weight.

Clear as mud yet?
 
Snow said:
Well actually, that isn't all that correct, yes they do different things, but a starter aka electric motor and a generator are basicly the same thing.
Click to expand...

Well, not in the case of an aircraft with a pneumatic starter. If it has a pneumatic starter, the generator is NOT used as a starter. ;)
 
ColMustard said:
.... I don't believe the starting mechanism doubles as a generator.

-ColM
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Actually, they can. It does in the T-37. It draws power from the DC system to motor the engine up to about 25% RPM after which the pilot disengages the starter switch and the rest of the engine start proceeds without the assist of the starter/generator up to idle. The starter generator doesn't begin to produce power (act in its function as a generator) until near idle RPM and above...In the tweet (T-37) Idle is defined as 36-40% RPM, and the generator begins to produce power between 38-42% RPM. So pretty much at idle unless that particular jet has a very low idle, and then you may need to bump the throttles slightly.

Incidently, the tweet has very small jet engines (J-69's) developing in the neighborhood of 1000 lbs of thrust each... That's why it uses an electric starter/generator combination. On bigger engines the power draw required to turn those engines would be huge, so most airliners don't use electric starters, they use mechanical starter motors operated by bleed air... Those small little APU engines however may be started by an electric starter, in which case it will typically do double duty as a generator to provide power on the ground. Hope this clears up questions, and the post a couple above this describing the starter/generator as a coil of wires surrounded by magnets was dead on accurate. You can build your own version of this with Radio Shack's "300 in 1 Electronic Projects Lab".... Just don't try to do much with it. The motor they have you build is pretty small.
 
Most here know that the smaller jet engines on business jets are started by electric starter/generators and most of the larger engines used on RJ's and larger jets use pneumatic starters, but this may be about to change in the near future.

The Boeing 787 will not have any bleed air taken from the engines. Each engine will have two rather large starter/generators and it will be an all electric airplane, including wing anti-ice and pressurization (complete with electric cabin superchargers).
 
Snow said:
Generators are larger in size for the same power output too, I'm guessing that's why you don't see them in cars nowerdays.
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The big drawback with generators on a piston engine is that they do not make any usable power at low rpm. So, at idle you are draining the battery if any electrical devices are in use. While an alternator will usually make enough power to keep everything going and even charge the battery at idle.

On a jet this is less of an issue, as the engines turn at very high speed. So it is possible to use a generator. You are correct about the weight however. Many of the newer aircraft are going all AC because of this.
 
K, I guess I was wrong about that starter/generator thing. I suppose I'm used to the recip engines. Hopefully I'll be able to explain this all forward and backward in a couple months when I'm with a regional.

Thanks for the correction!

-ColM
 
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