Windmilling propellers
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Come on 411........... give us the answer
Twice, I believe.
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411A............your answer is WRONG !
I've been there, I've seen it and I've done it - probably before you !
I've explained what actually happens, which has been agreed and supported by a number of similarly experienced and knowledgeable airmen.
Our experience makes sense............if you rotate a pump, the output varies according to rpm - remember physics at school ? RPM constant = Pressure Constant if nothing else changes.
411A, Re-read your Boyles and Charles Laws and fluid dynamics.
I've been there, I've seen it and I've done it - probably before you !
I've explained what actually happens, which has been agreed and supported by a number of similarly experienced and knowledgeable airmen.
Our experience makes sense............if you rotate a pump, the output varies according to rpm - remember physics at school ? RPM constant = Pressure Constant if nothing else changes.
411A, Re-read your Boyles and Charles Laws and fluid dynamics.
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I have to chime in with SNS3 (love reading your posts on technical matters) and 411A, and especially with the seemingly much overlooked one line post by werbil a way back.
No combustion -> No pressure increase in the cylinder -> less exhaust backpressure -> less remaining pressure in the cylinder as the exhaust valve closes -> less pressure in the cylinder as the intake valve opens -> more air into the cylinder -> drop in MAP, even with everything else remaining constant.
The effect would be there. I have no expertise or experience which enables me to tell if it would ever be noticeable, so there I'll have to trust those who claim it will be.
Exhaust backpressure is often overlooked but can still have some very interesting effects on the performance and behaviour of piston engines.
No combustion -> No pressure increase in the cylinder -> less exhaust backpressure -> less remaining pressure in the cylinder as the exhaust valve closes -> less pressure in the cylinder as the intake valve opens -> more air into the cylinder -> drop in MAP, even with everything else remaining constant.
The effect would be there. I have no expertise or experience which enables me to tell if it would ever be noticeable, so there I'll have to trust those who claim it will be.
Exhaust backpressure is often overlooked but can still have some very interesting effects on the performance and behaviour of piston engines.
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I have no expertise or experience which enables me to tell if it would ever be noticeable, so there I'll have to trust those who claim it will be.
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No combustion -> No pressure increase in the cylinder -> less exhaust backpressure -> less remaining pressure in the cylinder as the exhaust valve closes -> less pressure in the cylinder as the intake valve opens -> more air into the cylinder -> drop in MAP, even with everything else remaining constant.
411A............your answer is WRONG !
I've been there, I've seen it and I've done it - probably before you !
I've been there, I've seen it and I've done it - probably before you !
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one dot right,
the effect is there. The question is on what engines it will be noticeable. All of them? I think not. But on some engines? Weeeeeell... I wouldn't know.
If someone who claims to have extensive experience of operating the Twadoddle PXF38 radially indisposed contraindicating aircraft powerplant says it will show a MAP drop, I'll check on other references to see if that individual is someone I'd generally trust to be correct - until I get the chance to run a Twadoddle PXF38 radially indisposed contraindicating aircraft powerplant in a test bench, or find hard evidence from such bench tests.
In short, I'm only convinced that it will be noticeable as far as my trust in those saying it will goes. I will however not say they are wrong as it can't be - as some people are doing.
If we had several people having operated the Twadoddle, some saying it will show, some saying it won't, then it gets tricky for real - but that does not seem to be the case here. Unless I missed something we have Twadoddle PXF38 flyers saying it will show, and we have people saying it won't based on academical merit (wrong!) or based upon experience of the PXF33.5.
the effect is there. The question is on what engines it will be noticeable. All of them? I think not. But on some engines? Weeeeeell... I wouldn't know.
If someone who claims to have extensive experience of operating the Twadoddle PXF38 radially indisposed contraindicating aircraft powerplant says it will show a MAP drop, I'll check on other references to see if that individual is someone I'd generally trust to be correct - until I get the chance to run a Twadoddle PXF38 radially indisposed contraindicating aircraft powerplant in a test bench, or find hard evidence from such bench tests.
In short, I'm only convinced that it will be noticeable as far as my trust in those saying it will goes. I will however not say they are wrong as it can't be - as some people are doing.
If we had several people having operated the Twadoddle, some saying it will show, some saying it won't, then it gets tricky for real - but that does not seem to be the case here. Unless I missed something we have Twadoddle PXF38 flyers saying it will show, and we have people saying it won't based on academical merit (wrong!) or based upon experience of the PXF33.5.
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A very interesting thread.
Still working the original question through in my head.
I was taught that "supercharging" is an effect of valve timing, rather than just the blower per se. Blowers or superchargers are also used on two stroke diesel engines for more effective scavanging with no increase in MAP.
"Supercharging" is caused by allowing the inlet valve to stay open slightly longer than normal during intake stroke - with the exhaust valve closed, hence not valve overlap as some one has suggested.
Can anyone tell me where the MAP sense line is tapped from on these "big" engines?
My thoughts are that the MAP gauge should be sensing the Blower output.
I find this confusing. Back pressure assists to hold the exhaust valve closed, so I can't see why less back pressure on a dead engine would affect cylinder pressure.
I imagine there would still be some back pressure even with a dead engine.
As the piston swings past the top of exhaust stroke it will induce a slight negative pressure in the cylinder, my thoughts are that this would be the same with a dead engine so there is a nil net affect.
All ways learning.
Cheers
BH
Still working the original question through in my head.
I was taught that "supercharging" is an effect of valve timing, rather than just the blower per se. Blowers or superchargers are also used on two stroke diesel engines for more effective scavanging with no increase in MAP.
"Supercharging" is caused by allowing the inlet valve to stay open slightly longer than normal during intake stroke - with the exhaust valve closed, hence not valve overlap as some one has suggested.
Can anyone tell me where the MAP sense line is tapped from on these "big" engines?
My thoughts are that the MAP gauge should be sensing the Blower output.
No combustion -> No pressure increase in the cylinder -> less exhaust backpressure -> less remaining pressure in the cylinder as the exhaust valve closes -> less pressure in the cylinder as the intake valve opens -> more air into the cylinder -> drop in MAP, even with everything else remaining constant.
I imagine there would still be some back pressure even with a dead engine.
As the piston swings past the top of exhaust stroke it will induce a slight negative pressure in the cylinder, my thoughts are that this would be the same with a dead engine so there is a nil net affect.
All ways learning.
Cheers
BH
Last edited by blackhand; 2nd Jul 2010 at 09:17.
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In short, I'm only convinced that it will be noticeable as far as my trust in those saying it will goes. I will however not say they are wrong as it can't be - as some people are doing.
Whether it is the right explanation remains unclear as all 411A has done is to agree with other posts (mine included) and claim he already knew that.
It may well be the right explanation, but I would rather hear an engineering experts point of view, rather than that of an abrasive, bigoted, self appointed authority on all things aviation. But that's just my opinion.
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Can anyone tell me where the MAP sense line is tapped from on these "big" engines?
My thoughts are that the MAP gauge should be sensing the Blower output.
My thoughts are that the MAP gauge should be sensing the Blower output.
So yes, it senses blower output, one would think.
The MAP is used as a (but not THE) engine power measuring value, hence measuring the blower output will be largely useless to the pilot as variations in throttle setting will not alter the blower output. Only RPM change or (on some) vane angle change will do that.
And PS, this is not just paper wisdom. They let me fly DC-3's every now and then (Pratt & Whitney R-1830's), and every time I move the throttles the MAP (= blower output, see above) definitely changes - a lot. When I move the propeller pitch levers, the MAP also changes - a little.
And before anyone asks, no, I'm not gonna try to switch off the fuel to one engine next time to see what happens
Last edited by MoodyBlue; 2nd Jul 2010 at 20:54. Reason: Managed to insert picture
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this booklet very good on the merlin
Stanley Hooker et al's treatise "The Performance of a Supercharged Aero Engine" is available price £6 from the Rolls-Royce Heritage Trust. It is a facsimile of the original with handwritten emendations.
Oh and cheers Checkboard, see you soon!
Oh and cheers Checkboard, see you soon!
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Originally Posted by blackhand
I find this confusing. Back pressure assists to hold the exhaust valve closed, so I can't see why less back pressure on a dead engine would affect cylinder pressure.
Less back pressure means the piston will have to work less as it empties the compression chamber on the exhaust stroke. This translates to lower pressure in the combustion chamber all through the exhaust stroke as it vents easier into the exhaust manifold.
I imagine there would still be some back pressure even with a dead engine.
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Let me add that the average piston aircraft engine turns rather slowly in relation to the speed of sound in hot exhaust gas and the length of the exhaust system. You shouldn't have much back pressure unless there is a turbo, a PRT or a silencer.
After an excellent landing you can use the airplane again!
After an excellent landing you can use the airplane again!
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Back pressure in fact acts against the spring
Thanks for the correction. Back pressure does in fact slow the scavenging of exhaust gasses.
In practice this pressure is not measured at an intake port, but at the rim of the supercharger collector. It is known as manifold pressure,
So this would infer that regardless to what is happening during the Otto Cycle(modified) the output of the blower would stay the same as long as RPM and throttle position remained constant.
Cheers
BH
A good landing is one we can walk away from
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I have never been involved in the operation of UK piston engines, but I was always told that the main difference was that UK engines were "boost limited" whereas US engines were not. What I understood that to mean was that the pilot was in charge of max boost on a US engine, and it was possible to overboost the engine by advancing the throttle. The UK engines reached max boost and were automatically limited, permitting no overboost. How all that was achieved is a mystery to me, but maybe some UK engineer will remember.
On the MAP issue, my only experience of windmilling was at cruise ( around 33" MAP on a P&W R 2800 cb3/4 (single speed supercharger) or cb15/16 (two speed supercharger), and scavenge back pressure would have been close to atmospheric so no noticeable change in MAP took place.
Possibly at high altitude in "high blower" (CB 15/16) the low outside pressure would affect scavenge back pressures and thereby indicated MAP.
I am trying to remember what happened on DC-6 when RPM switches were toggled up for approach. If my memory is correct MAP decreased with increasing rpm and vice versa. (Throttle not moved)
Thing is, it is all back in early 70s, so it is a bit hazy.
On the MAP issue, my only experience of windmilling was at cruise ( around 33" MAP on a P&W R 2800 cb3/4 (single speed supercharger) or cb15/16 (two speed supercharger), and scavenge back pressure would have been close to atmospheric so no noticeable change in MAP took place.
Possibly at high altitude in "high blower" (CB 15/16) the low outside pressure would affect scavenge back pressures and thereby indicated MAP.
I am trying to remember what happened on DC-6 when RPM switches were toggled up for approach. If my memory is correct MAP decreased with increasing rpm and vice versa. (Throttle not moved)
Thing is, it is all back in early 70s, so it is a bit hazy.
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My gawd 95 posts arguing about what happens when you shut off an engine in flight?
How about a raise of hands...everyone in here with an MEI that has actually taken up a student, shut down an engine on a muti-engine turbocharged aircraft, feathered a prop, then started it back up....
(my hand is up)
How about a raise of hands...everyone in here with an MEI that has actually taken up a student, shut down an engine on a muti-engine turbocharged aircraft, feathered a prop, then started it back up....
(my hand is up)
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You shouldn't have much back pressure unless there is a turbo, a PRT or a silencer.
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By way of comparison, IIRC the boost figures (in inches of mercury) were:
Neptune, Wright Cyclone - 61"
Shackleton, RR Griffon - Low Gear 58" High Gear 81"
Neptune, Wright Cyclone - 61"
Shackleton, RR Griffon - Low Gear 58" High Gear 81"
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Could it be that when an RR engine is deprived of fuel in flight that there is a drop in boost pressure that is detected by the boost regulator which immediately restores it by opening the throttle (a Corliss valve on the Griffon, by the way)?