daft turbine questions from a piston pilot
11-15-2009, 08:08 AM
#21
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I think that the biggest difference is that with a piston engine, a windmilling prop has to drive the whole engine against it's own compression. A windmilling fan-jet engine normally only turns the N1 section, which produces surprisingly little drag. You have to be going really fast, like above 250 KIAS, to even notice rotation on the N2 gauge.
If you have ever tried to hand prop a moderately large piston engine, you know how much effort it takes to get it to turn. You can walk up to the front of a 737 and start the engine rotating with one hand. If there is any wind, it will probably already be rotating. If there is a tailwind, it will just as easily rotate in reverse.
Joe
If you have ever tried to hand prop a moderately large piston engine, you know how much effort it takes to get it to turn. You can walk up to the front of a 737 and start the engine rotating with one hand. If there is any wind, it will probably already be rotating. If there is a tailwind, it will just as easily rotate in reverse.
Joe
11-15-2009, 10:12 AM
#22
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Pardon the silly question, but apart from the increased weight and complexity, why aren’t jet engines designed to be feathered as a props are? I’d imagine the N1 blades at the front create a lot of drag just windmilling, so why isn’t there a way to reduce drag? Or are jets typically so powerful that engine out climb performance is still healthy?
BAE-4100
11-15-2009, 07:38 PM
#23
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Earlier in the thread, some of you were talking about "P" factor, or the existance of a critical engine on tubofan engines, and I would like to point out that this is no-existant (for practical purposes anyway) due to the air being directed through the nose cowl. Of course, some things will affect airflow at slower speeds (strong crosswind, or high reverse power at slow speeds, for ex.), but there is no differance in power output due to the pitch attitude of the aircaft as the air that is going through the C1 disk is being "straightened" by the nose cowling. Hope this is more helpful than confusing. Some of the bigger engines have 34-48 blades on the C1 compressor, and it would be nearly impossible to build a hub that would withstand the forces imposed on it and accomodate this number of blades.
11-16-2009, 10:42 AM
#25
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I didn't even think about the Jetstream 41, even though I see them all the time. Also, for the OP some t-props (usually the ones made in Europe) have engines that turn counter-clockwise such as the Jetstream 31/32. Critical engine failure in one of those means lots of right rudder, and then start trimming.
11-18-2009, 10:26 AM
#27
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11-18-2009, 11:34 AM
#28
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I'm trying to visualize whether the location of the props makes a difference. My understanding is that the rudder arm acts thru the C.G. This would seem to mean that the fore and aft position of the props should not make a difference, except in that the weight of the engines would tend to give the aircraft a relatively far aft C.G.
Joe
11-18-2009, 12:23 PM
#29
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Hi Rick.
I'm trying to visualize whether the location of the props makes a difference. My understanding is that the rudder arm acts thru the C.G. This would seem to mean that the fore and aft position of the props should not make a difference, except in that the weight of the engines would tend to give the aircraft a relatively far aft C.G.
Joe
I'm trying to visualize whether the location of the props makes a difference. My understanding is that the rudder arm acts thru the C.G. This would seem to mean that the fore and aft position of the props should not make a difference, except in that the weight of the engines would tend to give the aircraft a relatively far aft C.G.
Joe
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