05 Dec 2025, 05:03 [ UTC - 5; DST ]
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Post subject: Re: Reversing Prop on Husky Seaplane- Video  Posted: 12 Apr 2015, 15:33 |
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Joined: 03/19/12
Posts: 4044
Post Likes: +1793
Location: Belton, TX (KTPL)
Aircraft: 1968 Bonanza E33
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Username Protected wrote: A prop going into Beta or full reverse is not "back driving" anything. It is merely changing the thrust vector from rearward to forward, a very simplistic explanation. So this is either pulling forward on the crank against the front of the case or in beta/reverse pushing the crank aft in the engine case.
The engine in both cases is still driving the prop. Uhhh, you'd better give that a little more thought?
The engine is driving the prop not the airflow through the prop driving the engine in Beta and reverse. Any back driving would be in a very small range of use. Should be no more than what our engines see normally.
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Post subject: Re: Reversing Prop on Husky Seaplane- Video Posted: 12 Apr 2015, 19:53 |
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Joined: 12/09/07
Posts: 3836
Post Likes: +1908
Location: Camarillo CA
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Username Protected wrote: The engine in both cases is still driving the prop. Correct. At forward thrust and significant reverse thrust, the engine drives the prop. At very slight reverse thrust, the prop can drive the engine. When the blade angle gets negative, it creates drag that must be overcome with torque from the engine. This is obvious when you consider a prop in fully flat pitch. No way that can drive the engine. Any negative blade angle will require torque. Mike C.
I don't think we're talking about positive and negative ROTATIONAL forces. If we are, then I'll crawl back in my cave! 
Where this subject comes up is the thrust bearing, which essentially keeps the prop from pulling the crankshaft out by the roots. When those forces are reversed, they are trying to force the crankshaft "deeper," or out through the back of the engine. This occurs during reverse operation, or closing the throttle while diving at high speed. This is NOT a problem in our "flat" engines, because they are designed to take it, and the forces are relatively small.
Witness Sean Tucker, who hangs it on the prop in his spectacular airshow routine. His airplane is what, 1,000 pounds, 1,500 maybe? At full power all that weight is on the "Thrust Bearing." That's probably the top "limit." We don't put nearly that much load on it.
However, it IS a problem in the big old radials, and it is a LUBRICATION problem with the Master Rod Bearing. We don't have a MRBs in the flat engines. I can go into that in very great depth, but it's hardly germane. Suffice it to say that with a DC-7, R-3350 engines, a closed throttle descent at 400 knots from 25,000 was melting MRBs at a great rate. It took Western Airlines awhile to figure this out in the fifties, and they spent a lot of money on repairs.
It sure gave a spectacular rate of descent, though!
Even in the big radials, the problem was vastly overrated, though. The common rule of thumb was "Not less than 1" of manifold pressure per 1,000 RPM." That hoary old rule was never technically sound, but it worked for us simple-minded pilots. Some have tried to carry that old rule of thumb over to our flat engines. Like many of the Old Wives Tales, we'll never leave 'em behind.
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Post subject: Re: Reversing Prop on Husky Seaplane- Video Posted: 12 Apr 2015, 20:15 |
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Joined: 12/03/14
Posts: 20797
Post Likes: +26310
Company: Ciholas, Inc
Location: KEHR
Aircraft: C560V
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Username Protected wrote: I don't think we're talking about positive and negative ROTATIONAL forces. "Backdriving" means negative torque, the prop driving the engine. I take it you are referring to negative thrust, which isn't the same thing as negative torque, so it looks like we have a semantic gap. You can easily have negative thrust and positive torque, my engines do that all the time. I don't think there is any real concern with negative thrust with the typical piston aircraft engine in the size range this mod applies to. Mike C.
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Email mikec (at) ciholas.com
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Post subject: Re: Reversing Prop on Husky Seaplane- Video Posted: 12 Apr 2015, 20:40 |
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Joined: 02/01/12
Posts: 498
Post Likes: +63
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Is there a thrust bearing out there for only one axis?
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Post subject: Re: Reversing Prop on Husky Seaplane- Video Posted: 12 Apr 2015, 21:56 |
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Joined: 03/19/12
Posts: 4044
Post Likes: +1793
Location: Belton, TX (KTPL)
Aircraft: 1968 Bonanza E33
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Username Protected wrote: "Backdriving" means negative torque, the prop driving the engine.
This is what I take as back driving an engine.
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Post subject: Re: Reversing Prop on Husky Seaplane- Video Posted: 12 Apr 2015, 22:21 |
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Joined: 12/09/07
Posts: 3836
Post Likes: +1908
Location: Camarillo CA
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Username Protected wrote: "Backdriving" means negative torque, the prop driving the engine.
I take it you are referring to negative thrust, which isn't the same thing as negative torque, so it looks like we have a semantic gap. You can easily have negative thrust and positive torque, my engines do that all the time.
I don't think there is any real concern with negative thrust with the typical piston aircraft engine in the size range this mod applies to.
Mike C. I can agree with your final sentence. I'm not sure of the rest. Would you be kind enough to tell me what you would call it in a DC-7, throttle all the way back, props all the way forward, 400 KTAS, and descending rather smartly? I can tell you that the RPM would be at least 2800.
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Post subject: Re: Reversing Prop on Husky Seaplane- Video Posted: 12 Apr 2015, 22:24 |
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Joined: 12/09/07
Posts: 3836
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Location: Camarillo CA
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Username Protected wrote: This is what I take as back driving an engine. A term I've not heard before. "The prop driving the engine," yes.
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Post subject: Re: Reversing Prop on Husky Seaplane- Video Posted: 13 Apr 2015, 00:49 |
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Joined: 12/03/14
Posts: 20797
Post Likes: +26310
Company: Ciholas, Inc
Location: KEHR
Aircraft: C560V
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Username Protected wrote: Would you be kind enough to tell me what you would call it in a DC-7, throttle all the way back, props all the way forward, 400 KTAS, and descending rather smartly? That's any of back driving the engine, the prop driving the engine, negative torque, which all mean the same thing. Note the props are NOT in reverse. The props are still with positive blade angle, albeit with overall negative angle of attack. Mike C.
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Email mikec (at) ciholas.com
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Post subject: Re: Reversing Prop on Husky Seaplane- Video Posted: 13 Apr 2015, 01:05 |
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Joined: 12/09/07
Posts: 3836
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Location: Camarillo CA
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Username Protected wrote: Would you be kind enough to tell me what you would call it in a DC-7, throttle all the way back, props all the way forward, 400 KTAS, and descending rather smartly? That's any of back driving the engine, the prop driving the engine, negative torque, which all mean the same thing. Note the props are NOT in reverse. The props are still with positive blade angle, albeit with overall negative angle of attack. Mike C.
Then you and I are in complete agreement. I guess we gotta find something else to fight about!
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Post subject: Re: Reversing Prop on Husky Seaplane- Video Posted: 13 Apr 2015, 07:53 |
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Joined: 02/14/08
Posts: 3133
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Location: KGBR
Aircraft: D50
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How about 'downshifting'?
Anyway, if I read you right, beta in a piston should be easily achievable without excessive pressure and the engine would not experience and be damaged by excessive back pressure. So why isnt it common?
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Post subject: Re: Reversing Prop on Husky Seaplane- Video Posted: 13 Apr 2015, 10:15 |
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Joined: 02/14/08
Posts: 3133
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Location: KGBR
Aircraft: D50
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Username Protected wrote: It's a lot of complexity and a lot of GA pants already can land in places they can't get out of. Why is it good for turbines and not pistons?
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Post subject: Re: Reversing Prop on Husky Seaplane- Video Posted: 13 Apr 2015, 10:36 |
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Joined: 09/04/09
Posts: 6203
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Location: Doylestown, PA (KDYL)
Aircraft: 1979 Baron 58P
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Username Protected wrote: It's a lot of complexity and a lot of GA pants already can land in places they can't get out of. Why is it good for turbines and not pistons?
I would guess it has to do with popularity. Virtually all turbo props have reverse capability, I'd bet there are less than a couple hundred GA pistons with reverse.
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Rick Witt
Doylestown, PA
& Destin, FL
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