Taguilo From Argentina, joined Aug 2005, 72 posts, RR: 0 Posted (3 years 5 months 3 days 9 hours ago) and read 1674 times:
Just for airliners:
When descending from cruise level at idle power, do the engines windmill?
If so, is there an increment in the N1 values shown on EICAS, compared to their standard values for an idle condition, as a result of the windmilling effect?
SlamClick From United States, joined Nov 2003, 9994 posts, RR: 72 Reply 1, posted (3 years 5 months 3 days 9 hours ago) and read 1660 times:
I've been told, specifically for the P&W JT-8D as mounted on the DC-9 that at some point just above idle the engine produces zero thrust and that at the speeds we'd be flying, at an idle the engine actually produces more drag than if it fell cleanly off the airplane.
So yes, it would appear in that case the engine is windmilling in the sense that the first stage fan, at least, is being pushed along by the airflow.
However, I have noticed that descending with a flamed-out engine the N1 is higher with the engine idling than with it shut down.
I think either way it would be hard to prove except in engineering terms.
Broke From United States, joined Apr 2002, 1289 posts, RR: 3 Reply 2, posted (3 years 5 months 3 days 7 hours ago) and read 1621 times:
If the engine is running, it is not windmilling. Flight idle RPM is higher than ground idle RPM in order to be able to accelerate quickly to high engine power when needed. It may not be developing a lot of thrust, but it is producing some.
Air2gxs From , joined Dec 1969, posts, RR: Reply 3, posted (3 years 5 months 3 days 7 hours ago) and read 1613 times:
Quoting Broke (Reply 2): If the engine is running, it is not windmilling. Flight idle RPM is higher than ground idle RPM in order to be able to accelerate quickly to high engine power when needed. It may not be developing a lot of thrust, but it is producing some.
Let's think about this a second. The engine is developing thrust, but is it developing enough thrust? I know that at top of descent a JT9 will "produce" an EPR that is less than 1.00. To me that means pressure at the inlet is greater than pressure at the exhaust (this is why the bleeds on a high bypass are so critical). That means the thrust level that is being produced is not enough to overcome the ram air effect in the inlet. Therefore, you have drag.
777DadandJr From United States, joined Feb 2005, 1501 posts, RR: 15 Reply 4, posted (3 years 5 months 3 days 6 hours ago) and read 1596 times:
To go a bit off topic, but related,
I was thinking the other day, after watching the doc about AirTransat A330 gliding to Lejes:
If properly engineered, would a windmilling engine be capable of being substituted for or capable of enhancing the purpose of a RAT to suppliy emergency electrical power?
Would it be possible to attach a generator to the engine that would produce the same effect in the event of an engine failure?
Am I making sense?
Just curious.
Thanks in advance!
Russ
My glass is neither 1/2 empty nor 1/2 full, rather, the glass itself is twice as big as it should be.
SlamClick From United States, joined Nov 2003, 9994 posts, RR: 72 Reply 5, posted (3 years 5 months 3 days 6 hours ago) and read 1587 times:
Quoting 777DadandJr (Reply 4): would a windmilling engine be capable of being substituted for or capable of enhancing the purpose of a RAT to suppliy emergency electrical power?
I've seen a windmilling engine's generator produce 115V but not the 400Hz. At windmilling RPM the generator RPM was just below the operating range of the CSD.
I'd believe that it might but with nowhere near the efficiency of a RAT in which everything is optimized for the production of (hydraulic) power.
Okie From United States, joined Jul 2003, 985 posts, RR: 2 Reply 6, posted (3 years 5 months 3 days 4 hours ago) and read 1546 times:
Quoting 777DadandJr (Reply 4): If properly engineered, would a windmilling engine be capable of being substituted for or capable of enhancing the purpose of a RAT to suppliy emergency electrical power?
One thought would be that the main reason, other than fuel shortage, for deployment of the RAT would be to engine failure of some sort damaging the engine to the point that windmilling would not be a sure source of emergency power.
Thought two would be you would not want the parasitic rat load on the turbine if you were trying for a windmill restart.
Air2gxs From , joined Dec 1969, posts, RR: Reply 7, posted (3 years 5 months 3 days 4 hours ago) and read 1545 times:
Quoting 777DadandJr (Reply 4): Would it be possible to attach a generator to the engine that would produce the same effect in the event of an engine failure?
It wouldn't be able to produce the 400hz and any load put on the generator (if the system would allow it to come online) would cause the voltage to drop below usable levels.
Hydraulics would still be there and at 3000psi, but the volume wouldn't be there. Any use of the hydraulic system would bring the system down. Small, very judicious control movements may be possible, but I certainly wouldn't count on anything big, like gear and flaps. They'll come out, but very slowly and awkwardly. In fact, on modern aircraft with electronic asymmetry systems, you may actually cause the flaps to lock out.
Lightsaber From United States, joined Jan 2005, 3926 posts, RR: 71 Reply 8, posted (3 years 5 months 3 days 1 hour ago) and read 1487 times:
Quoting SlamClick (Reply 1): at an idle the engine actually produces more drag than if it fell cleanly off the airplane.
True. Engines at "flight idle" induce some drag. Note that the FADAQ runs "flight idle" higher than ground idle. (There is a need to keep the combustion stable with the colder air of flight and extra air due to the plane moving.)
Quoting Broke (Reply 2): If the engine is running, it is not windmilling.
Correct. Technically (in engineering terms) windmilling is when no fuel goes to the engine and the conditions in the engine are purely a function of ambient conditions (pressure & temperature) and flight mach number.
777WT From United States, joined Jan 2005, 835 posts, RR: 3 Reply 9, posted (3 years 5 months 2 days 23 hours ago) and read 1466 times:
Quoting SlamClick (Reply 5): I've seen a windmilling engine's generator produce 115V but not the 400Hz. At windmilling RPM the generator RPM was just below the operating range of the CSD.
I'd believe that it might but with nowhere near the efficiency of a RAT in which everything is optimized for the production of (hydraulic) power.
The 747 does not have a RAT. It was determinded that 4 windmilling engines provide enough hyd pressure.
Matt72033 From United Kingdom (England), joined May 2005, 1617 posts, RR: 4 Reply 11, posted (3 years 5 months 2 days 11 hours ago) and read 1336 times:
Quoting HAWK21M (Reply 10): BTW Abong the Commercial Airliners which have the RAT Installed.Im aware of A320 & B757.
regds
330/340, 767, 777 i think.....correct me if i'm wrong
just one quick question slightly off topic, but has any engine manufacturers ever designed an engine with a fan that can be feathered, like a prop in the case of engine loss? would it be worth it? i guess more so on the higher end of the high bypass engines.
F14D4ever From United States, joined May 2005, 311 posts, RR: 9 Reply 12, posted (3 years 5 months 2 days 6 hours ago) and read 1287 times:
Quoting Matt72033 (Reply 11): just one quick question slightly off topic, but has any engine manufacturers ever designed an engine with a fan that can be feathered, like a prop in the case of engine loss? would it be worth it? i guess more so on the higher end of the high bypass engines.
Designed? Yes, the GE36 UDF blades were variable pitch.
I can't imagine that any conventional turbofan engine would have had this feature. The weight and complexity would far outweigh any advantage. To add a pitch change mechanism while maintaining blade retention robustness would be a daunting task.
Air2gxs From , joined Dec 1969, posts, RR: Reply 13, posted (3 years 5 months 2 days 4 hours ago) and read 1263 times:
Quoting HAWK21M (Reply 10): Is this a general statement or Specific
That can pretty much be applied to any engine. You can see this quite clearly if on the B747 classics. As the engine spins up through about 35% n2 you will see voltage come up to 115v but frequency doesn't come to 400hz until around 50% or so.
I'm sure on the newer aircraft the same thing happens, but you would have to be monitoring the electrical page instead of the engine page during start. Not very smart.
777WT From United States, joined Jan 2005, 835 posts, RR: 3 Reply 14, posted (3 years 5 months 2 days 4 hours ago) and read 1268 times:
Quoting F14D4ever (Reply 12): I can't imagine that any conventional turbofan engine would have had this feature. The weight and complexity would far outweigh any advantage. To add a pitch change mechanism while maintaining blade retention robustness would be a daunting task.
P&W is working on an variable pitch turbofan engine, not sure if it feathers but sure does change pitch from low to high.
Sponsor Message: 
From Argentina, joined Aug 2005, 72 posts, 
From United States, joined Nov 2003, 9994 posts, 
From , joined Dec 1969, posts, 
From India, joined Jan 2001, 26086 posts, .gif){kind=small}
From United Kingdom (England), joined May 2005, 1617 posts, 
Printer friendly format
