Generator Altitude Limitations Question

[pmcustom]

you said the main generators would have the same limitations (if there were any) as the apu, but since they're obviously not the same generators, then they most likely would not...but either way it's besides the point, and this argument still does not help, so again, nevermind.

 

[bocefus]

Why are they "obviously not the same"? Because the engine has air starters? Bad assumption. You are better off asking the Dassault rep if you are trying to find out info on the engine driven generators, they are installed by the airframe manufacturer, not the engine manufacturer.

 

[pmcustom]

hey i'll be the first to admit that i don't know much about the plane...i work on them here and there, but not very often. i'm taking a flight safety class for the new 2000EX EASy. the instructor either came from dassault or still works for them...i'm not sure. you mentioned a brushless generator...could that be what he was talking about? maybe he just used the term "brushes" for convenience.

 

[bocefus]

Most likely, the engine driven generators are brushless, probably made by Thales. Can't speak for the Honeywell APU

 

[pmcustom]

Ok this is how it goes. The main engine generators are actually brushless alternators. They are brushless because at high altitudes the air has less resistance, and arcing became a problem between the brushes and slip ring (arcing is obviously not good, and greatly reduces life). The APU, though, has a standard starter/generator (not brushless, probably because it would cost more and no one cares if they can use the APU at higher altitudes) and therefore cannot be operated above a set level. In actuality, it can run just fine as high as the engine was designed to go, but it would need overhauled all the time I imagine.

 

[PHX767]

Why indeed?

If I may chime in...

I believe the original question is why the APU is limited to FL 350.... Most APU's have a start/run limit at high altitudes. E.g, they won't start when: 1) Cold soaked 2) At high altitudes outside of their normal operating envelopes. This is unrelated to generator cooling limits which vary with altitude due to air density.

An operational reason for needing an APU at FL350 is for dispatch with a generator MEL'd. Not uncommon when you need to get a plane back from the wrong side of the pond. In this condition we would have to leave the APU running for the whole trip, even the high altitude portion across the ocean.

 

[bocefus]

There might be an operational need for a large, transport aircraft, but never for a corporate aircraft.

 

[Snoopy58]

Corporate aircraft never need to fly with an engine-driven generator inop???

Even if you never (!hah!) dispatch without everything working in "good as new" conditions, generators can fail enroute and Mr Big in the back might rather you fired up the APU and put it on the bus & continued on to his destination instead of either continuing with degraded equipment (which generator powers his laptop power port & entertainment center, btw?), or diverting to whatever Podunk is closest to wait until a replacement generator gets FedEx'd to you.

Who needs an operational APU above FL350 anyway?

It seems to be news to some here that this is a common procedure, but it is. Running APUs in-flight, even at high altitudes, as a common non-normal procedure (i.e. what you do if a generator is inop or fails) in many jets.

 

[bocefus]

The post was in reference to corporate aircraft. Yes, who does need an apu above 350 in a corporate aircraft? Nobody, this is why Dassault didn't bother going to the extra expense in certificating it to operate above 350. And, you might be surprised to know that there are plenty of transport aircraft that are NOT approved for in flight use of the APU, and some don't even have an apu.

 

[A Squared]

Getting back to the commutator question. It doesn't have to do with arcing betwen the commutator and the brush, it has to do with arcing between commutator segments. A Commutator is a segmented slip ring. Each segment must be insulated from the adjacent segment, as it voltage is always different than the adjacent segments. At higher altitudes the air has *less* resistance, not more. Actually, it has a lower dielectric coeficient, but "resistance" captures the general drift. The higher you go, and the thinner the air, the more likely you are to get electricity jumping from one segment to another. The fact that the brushes have coated everything with cinductive carbon doesn't help the situation.

Now, is this *the* reason the APU on the 2000EX has an altitude limitation? I don't know, and I'm not going to enter that pissing match. I do know that generators with commutator will get arcing at higher altitudes.