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#22
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Feathering an engine
"Flanagan" wrote It is so kind of you to reply, and your explanation is so interesting. Thank you! In addition, I would add that though it seems counter intuitive, a rotating unfeathered prop has more drag (much more) than a unfeathered prop that is not rotating. The feathering has two bonuses, in sorts then. An unfeathered prop has lower drag stopped, so feathering the prop stops the prop for the first reduction in drag, and feathering it after it is (or during stopping it) reduces drag even again. The reasons given about lessening damage, although true, are a very very distant reason on why to feather. The MOST important reason is to reduce drag, increase time in the air and gliding distance, because both of those reasons mean LIFE. Never seen a better reason than that one, yet. g -- Jim in NC |
#23
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Feathering an engine
"Flanagan" wrote It is so kind of you to reply, and your explanation is so interesting. Thank you! In addition, I would add that though it seems counter intuitive, a rotating unfeathered prop has more drag (much more) than a unfeathered prop that is not rotating. The feathering has two bonuses, in sorts then. An unfeathered prop has lower drag stopped, so feathering the prop stops the prop for the first reduction in drag, and feathering it after it is (or during stopping it) reduces drag even again. The reasons given about lessening damage, although true, are a very very distant reason on why to feather. The MOST important reason is to reduce drag, increase time in the air and gliding distance, because both of those reasons mean LIFE. Never seen a better reason than that one, yet. g -- Jim in NC |
#24
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Feathering an engine
On May 6, 6:44*pm, Dave Doe wrote:
In article o_r_fairbairn-B758D1.12591306052009@70-3-168- 216.pools.spcsdns.net, says... In article , *"Tim" wrote: "Orval Fairbairn" wrote in message news In article , wrote: On May 5, 1:06 pm, Scott Skylane wrote: As an aside, the drag created by a windmilling propeller, i.e. one not feathered and attached to a dead engine, creates as much drag as a flat plate the same size as the area of the prop arc. Nope. The blades cannot be everywhere at once, and so the area affected is no larger than the blade area. Dan YES! In aeronautical engineering analysis, a windmilling prop is considered to be a flat disk, with drag numbers to match. Feathering the prop greatly reduces drag. So you are saying if I loose power at high altitude in a fixed pitch prop aircraft, like a Skyhawk, I will have less drag if I stop the prop, as opposed to letting it windmill? That is correct! Please provide some evidence. -- Duncan- Hide quoted text - - Show quoted text - Here is a URL to a thesis that addresses the question. The answer, based on his evidence, is, it depends. http://www.goshen.edu/physics/PropellerDrag/thesis.htm |
#25
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Feathering an engine
Dave Doe wrote:
Q2. I have a prop and I drag it through grease - as I do so it turns to "allow" it it's passage through the grease. Now if I was to hold the shaft so the prop does *not* rotate - surely that would be harder to pull through the grease now. ? Sounds like a slick idea, but I think you'd have to deep fry an awful lot of french fries before you'd have enough to perform the experiment over a reasonable distance. That assumes you don't die from a coronary first, what with having to dispose of all that greasy food first.... |
#26
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Feathering an engine
In article ,
says... Dave Doe wrote: Q2. I have a prop and I drag it through grease - as I do so it turns to "allow" it it's passage through the grease. Now if I was to hold the shaft so the prop does *not* rotate - surely that would be harder to pull through the grease now. ? Sounds like a slick idea, but I think you'd have to deep fry an awful lot of french fries before you'd have enough to perform the experiment over a reasonable distance. That assumes you don't die from a coronary first, what with having to dispose of all that greasy food first.... Well Jim, when yer prepared to give me a scientific evidence based answer.... Hey ok, I admit that, the Reynolds numbers are quite different due to the grease vs air (grease is almost 100% friction based drag, air is largely pressure based drag). And perhaps therein lies the answer - however I wanna see the maths. Reading up on 'a's' link now... http://www.goshen.edu/physics/PropellerDrag/thesis.htm -- Duncan |
#27
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Feathering an engine
In article c8d0834f-8e51-42ad-add6-
, says... On May 6, 6:44*pm, Dave Doe wrote: In article o_r_fairbairn-B758D1.12591306052009@70-3-168- 216.pools.spcsdns.net, says... In article , *"Tim" wrote: "Orval Fairbairn" wrote in message news In article , wrote: On May 5, 1:06 pm, Scott Skylane wrote: As an aside, the drag created by a windmilling propeller, i.e. one not feathered and attached to a dead engine, creates as much drag as a flat plate the same size as the area of the prop arc. Nope. The blades cannot be everywhere at once, and so the area affected is no larger than the blade area. Dan YES! In aeronautical engineering analysis, a windmilling prop is considered to be a flat disk, with drag numbers to match. Feathering the prop greatly reduces drag. So you are saying if I loose power at high altitude in a fixed pitch prop aircraft, like a Skyhawk, I will have less drag if I stop the prop, as opposed to letting it windmill? That is correct! Please provide some evidence. -- Duncan- Hide quoted text - - Show quoted text - Here is a URL to a thesis that addresses the question. The answer, based on his evidence, is, it depends. http://www.goshen.edu/physics/PropellerDrag/thesis.htm Thanks, so it really depends on the pitch of the propellor. qv... you have a prop on the end of a shaft that has no engine, just a braking mechanism (this is where I find it hard to get my head around the maths! ... You're say gliding through the air, the prop is freely spinning. Now, we apply some braking to the shaft and slow down the prop. Basically (and according to the maths you've shown), the drag will (dependent on pitch, but for most fixed pitch props), increase. And "at the other end" the brake will produce heat. The prop will slow and I would expect the drag to *increase* and the aircraft attitude will need to be lowered to maintain the same airspeed. But... according to the maths you've shown, this is all dependent on the pitch of the prop. And, I *assume* that fixed pitch props are too fine in pitch to be good windmills. According to the maths, I assume that wind turbines are more efficient if built really large, and spin slowly, rather than fast (which kinda makes sense - certainly in known (expected) wind strengths. -- Duncan |
#28
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Feathering an engine
In article c8d0834f-8e51-42ad-add6-
, says... On May 6, 6:44*pm, Dave Doe wrote: In article o_r_fairbairn-B758D1.12591306052009@70-3-168- 216.pools.spcsdns.net, says... In article , *"Tim" wrote: "Orval Fairbairn" wrote in message news In article , wrote: On May 5, 1:06 pm, Scott Skylane wrote: As an aside, the drag created by a windmilling propeller, i.e. one not feathered and attached to a dead engine, creates as much drag as a flat plate the same size as the area of the prop arc. Nope. The blades cannot be everywhere at once, and so the area affected is no larger than the blade area. Dan YES! In aeronautical engineering analysis, a windmilling prop is considered to be a flat disk, with drag numbers to match. Feathering the prop greatly reduces drag. So you are saying if I loose power at high altitude in a fixed pitch prop aircraft, like a Skyhawk, I will have less drag if I stop the prop, as opposed to letting it windmill? That is correct! Please provide some evidence. -- Duncan- Hide quoted text - - Show quoted text - Here is a URL to a thesis that addresses the question. The answer, based on his evidence, is, it depends. http://www.goshen.edu/physics/PropellerDrag/thesis.htm Oh, BTW, it is not correct to for Orval (according to your link) to *simply* say "that is correct" - as it is not. Just looking at the conclusion... .. Conclusions: From the experimental results reported here we can conclude what we could have figured out with a little thought: drag force increases with length and wind velocity, and decreases with pitch. What is less clear is how the drag force increases and decreases with these variables. Does it increase linearly or quadradically with length and wind velocity? When considering the pitch, does the windmilling drag force also follow a cosine-squared curve? More accurate data are needed to determine the characteristics of the crossover point. Does it depend on wind velocity? Arguments both for and against rely on data that could be drastically changed if just a couple of data points were moved. Further work in this topic should begin either with an increase in the range of the variables, or increasing the precision of the data. Improving either one of these will help answer all of the questions posed above. The main goal of this investigation was simply to determine whether a stationary or a windmilling propeller has more drag. The answer is complicatedly simple: it depends. It is clear that it depends on the pitch and length of the propeller, and it is probably independent of the wind velocity. A crossover point was discovered where the drag forces for the windmilling and stationary states were the same. This crossover point is also dependent on the pitch, the length, and probably independent of the wind velocity. -- Duncan |
#29
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Feathering an engine
In article ,
"Tim" wrote: The point was how much do you gain by stopping a fixed pitch prop, as opposed to letting it wind mill. A lot. You can actually do this experiment in a plane with a variable-pitch prop. Idle the engine, and play with the prop control. I did this years ago in a 182RG during dead stick landing drills. The difference in glide performance between the two extreme prop settings was quite dramatic, almost like having an extra set of very fast acting flaps. There's also the conservation of energy argument. If the engine is turning, the energy to overcome friction and compression has to come from somewhere. rg |
#30
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Feathering an engine
In article ,
Ron Garret wrote: There's also the conservation of energy argument. If the engine is turning, the energy to overcome friction and compression has to come from somewhere. I don't think this works. In the non-spinning case, you're dissipating all that energy into the air, and there's no real limit as to how much that could be. Now, it would seem that the conservation-of-energy argument gets you the right answer, but IMO not for the right reasons. -- Mike Ash Radio Free Earth Broadcasting from our climate-controlled studios deep inside the Moon |
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