ferpe From France, joined Nov 2010, 1785 posts, RR: 57 Posted (1 year 7 months 3 days 21 hours ago) and read 3532 times:
Watching this beautiful video one suddenly realizes, the bird does not want to be photographed from the right hand side, the reason: there is a big scope type air inlet on the right side of the vertical tail, why?:
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 6, posted (1 year 7 months 2 days 23 hours ago) and read 3051 times:
Quoting HAWK21M (Reply 4): Just an added doubt.....Since the APU will have pneumatic compressors too
Ferpe is right...the 787 APU has no pneumatic capability. There's no point, the airplane doesn't have a pneumatic system. The only pneumatics are on the engine itself and they're not functions that you'd need the APU to provide anyway.
N243NW From United States of America, joined Jul 2003, 1494 posts, RR: 21 Reply 7, posted (1 year 7 months 2 days 16 hours ago) and read 2950 times:
Quoting ferpe (Reply 5):
The 787 APU is all electrical IIRC.
Yep. The APU has two 250kVA generators, the same as each engine. As tdscanuck also mentioned, it's impractical to use the APU to provide pneumatics for engine anti-ice.
DC8FriendShip From United States of America, joined Apr 2005, 242 posts, RR: 3 Reply 8, posted (1 year 7 months 2 days 15 hours ago) and read 2943 times:
Quoting tdscanuck (Reply 6): There's no point, the airplane doesn't have a pneumatic system.
So what type of airconditioning and pressurization system does it have? Is there an air cycle pack at all or is it all compressors and vapor cycle?
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 9, posted (1 year 7 months 2 days 12 hours ago) and read 2852 times:
Quoting N243NW (Reply 7): The APU has two 250kVA generators
Two x 225 kW...not exactly the same as the engines, but pretty close.
Quoting N243NW (Reply 7): As tdscanuck also mentioned, it's impractical to use the APU to provide pneumatics for engine anti-ice.
Not only is it impractical, there's no reason for the capability. The only time you don't have engine bleed air available for engine anti-ice is if the engine isn't running...at which point you don't care if you have engine anti-ice or not.
Quoting DC8FriendShip (Reply 8): So what type of airconditioning and pressurization system does it have? Is there an air cycle pack at all or is it all compressors and vapor cycle?
Four electric compressors feed two conventional air cycle machines.
HAWK21M From India, joined Jan 2001, 31201 posts, RR: 58 Reply 10, posted (1 year 7 months 2 days 6 hours ago) and read 2814 times:
Quoting tdscanuck (Reply 6):
Ferpe is right...the 787 APU has no pneumatic capability. There's no point, the airplane doesn't have a pneumatic system. The only pneumatics are on the engine itself and they're not functions that you'd need the APU to provide anyway.
So since the APU does not provide bleed......Its mearly an electrical back up.
what occurs in case of an IFSD.....The Two Pneumatic compressors on the remaining side is the only source available for Bleed to pressurize.
You start the APU and the APU powers the buses formerly powered by the dead engine.
Quoting HAWK21M (Reply 10): The Two Pneumatic compressors on the remaining side is the only source available for Bleed to pressurize.
No. That's one of the joys of the more-electric architecture...it's really easy to switch electrons around. Any generator can power any bus. Loss of the L engine doesn't mean loss of the L air compressors or pack.
ferpe From France, joined Nov 2010, 1785 posts, RR: 57 Reply 13, posted (1 year 7 months 1 day 6 hours ago) and read 2587 times:
I think I have the reason for the APU scoop being open = APU running.
When you do a photo shoot like this you have to fly pretty advanced formation with the photo plane (a biz-jet with a roof mounted camera it seems). Changing flanks and passing each other requires excess speed and power vs the other plane, further maneuvering around each other like this at close distance is dangerous stuff even for experienced pilots (ref XB70 accident with chase plane), therefore this formation flying is done a pretty low speed where things happen at a slower tempo if someone does a tiny error.
The low speed means the engines are at low RPM, means the alternators are marginal for delivering the A/C electricity, hence the electrical system automatically starts the APU as backup.
BTW why is the keel beam protruding below the wing-root fairings, the only non elegant solution on a beautiful bird .
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 14, posted (1 year 7 months 1 day 2 hours ago) and read 2547 times:
Quoting ferpe (Reply 13): The low speed means the engines are at low RPM, means the alternators are marginal for delivering the A/C electricity
That's part of the reason aircraft don't use alternators...modern large aircraft generators either have internal transmissions to get the right RPM (IDG's) or put out variable frequency (VFG's). Either way, you get full power from the generators regardless of engine speed.
Quoting ferpe (Reply 13): BTW why is the keel beam protruding below the wing-root fairings, the only non elegant solution on a beautiful bird
It's always been that way...I always assumed it was to cut down the weight of the wing-body fairing. It would be silly to expand the entire fairing just to make an aesthetic change to cover an inch of keep beam.
ferpe From France, joined Nov 2010, 1785 posts, RR: 57 Reply 15, posted (1 year 7 months 22 hours ago) and read 2483 times:
Quoting tdscanuck (Reply 14): modern large aircraft generators either have internal transmissions to get the right RPM (IDG's) or put out variable frequency (VFG's). Either way, you get full power from the generators regardless of engine speed.
It is intuitive that an IDG keeps the power constant, the ID sucks more tourque to keep the RPM constant. To keep the VFD power constant the generators must also suck more mechanical torque from the engine at low RPM, IMHO this should be a bit tricky to handle for the engine guys. On the electrical side how is this torque vs RPM transfer happening?
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 16, posted (1 year 7 months 13 hours ago) and read 2375 times:
Quoting ferpe (Reply 15): It is intuitive that an IDG keeps the power constant, the ID sucks more tourque to keep the RPM constant.
Not really...the IDG alters its gear ratio to keep the generator spinning at the correct RPM regardless of engine RPM (within the range of operation). Torque at the generator depends on generator load...torque to the engine is the generator torque times the current gear ratio of the IDG.
Quoting ferpe (Reply 15): To keep the VFD power constant the generators must also suck more mechanical torque from the engine at low RPM
IDG's and VFG's aren't constant power devices...they only extract as much energy as the electrical load they're feeding at the time. At lower RPM, for a given power, both the IDG and VFG require more torque.
Quoting ferpe (Reply 15): IMHO this should be a bit tricky to handle for the engine guys.
On large engines, the generator extraction is typically a relatively small fraction of the overall power moving through the spool (and there are other loads on the gearbox besides the generator) so it's not as bad as you might think. The FADEC will control idle speeds (and VSV schedules on engines so equipped) to keep the gearbox supplied at adequate RPM.
Quoting ferpe (Reply 15): On the electrical side how is this torque vs RPM transfer happening?
The engine is controlled, basically, on RPM. Pratt & Whitney and RR control on other parameters (EPR and TPR respectively) but those boil down to RPM normalized by outside conditions. So, for a given thrust lever setting, the engine is going to run at a given RPM. Essentially all engine gearboxes are fixed ratio, so that means the generator input is spinning at a particular RPM. The load on the generator determines how much torque in on the generator shaft. As load changes, torque changes. This feeds back into the engine (via the gearbox) and tends to try to slow down the spool that's driving the gearbox. The engine controller will see that slowdown and increase fuel supply to maintain the target RPM.
ferpe From France, joined Nov 2010, 1785 posts, RR: 57 Reply 17, posted (1 year 7 months 4 hours ago) and read 2311 times:
Thanks for an excellent answer Tom. I guess I assumed a certain load from the A/C but who cares, good explanation of what counts.
So why the APU was running is still not obvious. My observations on the formation flying is valid however, normal formation flying is straight forward, sweeping around each other like they did is not, requires 100% discipline (who moves and who fly straight etc).
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 19, posted (1 year 7 months 1 hour ago) and read 2279 times:
Quoting ferpe (Reply 17): I guess I assumed a certain load from the A/C but who cares, good explanation of what counts.
It's not a constant load...how hard the compressors work depends (a lot) on what the required airflow is, altitude, and what temperature the packs are trying to produce.
Quoting ferpe (Reply 17): So why the APU was running is still not obvious.
In flight testing there are a lot of reasons to leave the APU running for an entire flight...I don't know why they did it this particular time but I have no idea what other testing they were doing that day
Quoting nomadd22 (Reply 18): I thought generator speed was important for frequency, not power. Do they put out DC that gets converted to AC?
Generator speed isn't important for power, just the product of speed and torque. IDG's regulate speed to put out constand frequency. VFG's put out AC with a wide frequency range. Modern generator architectures like the A380 and 787 just use wild-frequency AC on the primary buses and skip the whole issue.
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