Windmilling propellers
Pistons?
Second fiddle, by far, compared to the American designs.
Hmmm... I guess that's why the P51 was eventually fitted with licence-built Merlins, then...
Second fiddle, by far, compared to the American designs.
Hmmm... I guess that's why the P51 was eventually fitted with licence-built Merlins, then...
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Ok, let's use the KISS method
1. The RPM will drop noticeably
2. every other engine parameter will drop with the RPM
Why? no fuel=no combustion=no turn
The propeller will now be driving the engine which will be acting like a brake due to compression in the cylinders.
The RPM will vary depending on airspeed and blade angle and engine type.
What won't vary is that you'll be trying to feather the prop/restart engine and hold the a/c level(twin), might not be as noticeable on a fourbanger but you'll know pretty darn quick.
Hope this helps.
FM
1. The RPM will drop noticeably
2. every other engine parameter will drop with the RPM
Why? no fuel=no combustion=no turn
The propeller will now be driving the engine which will be acting like a brake due to compression in the cylinders.
The RPM will vary depending on airspeed and blade angle and engine type.
What won't vary is that you'll be trying to feather the prop/restart engine and hold the a/c level(twin), might not be as noticeable on a fourbanger but you'll know pretty darn quick.
Hope this helps.
FM
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Funny, when I was doing multi-engine training, in order, it was:
Identify
Verify
Feather
So many of these posts are something along the lines of, "You shouldn't see these confusing indications because you should feather the engine!"
See the list above.
Identify
Verify
Feather
So many of these posts are something along the lines of, "You shouldn't see these confusing indications because you should feather the engine!"
See the list above.
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and that is:
Identify - with the leg.
confirm - with the throttle
feather - with the prop lever.
the instruments CAN be confusing under pressure!
Identify - with the leg.
confirm - with the throttle
feather - with the prop lever.
the instruments CAN be confusing under pressure!
Now which throttle do you pull on a 4-engine prop? Or do you guess on the first pull?
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Now which throttle do you pull on a 4-engine prop? Or do you guess on the first pull?
feather - with the prop lever.
A propellor feathering pump is used to drive the prop blades to the feathered position.
Just....push the feather button.
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No guesswork involved with the large American radial piston engines...you look at the torquemeter (BMEP gauge) and it will tell you straight away, if the engine is developing BHP...or not.
(i.e. It's not as simple as dead foot = dead engine.)
So much theory. Scenario originally given was running a tank dry, not initially changing throttle. With a DC3 THIS is what happens. At 150 knots TAS, there is no change in RPM or MAP. At 90 knots, there is some decrease in RPM and because the supercharger RPM drops with it, some drop in MAP. All of course accompanied by (expletive deleted) as throttle is hastily reduced and fuel tanks changed over.
John Tullamarine is usually right on these matters. It's all about the point at which the prop hits the fine pitch stops when there is no power coming out of the engine. At low airspeeds it must go full fine then it behaves as a fixed pitch prop if speed is further reduced. Also less RPM = less MAP. At higher airspeeds, because the engine is turning and producing oil pressure, the governor/prop combo MAY be capable of constant speed control, depending on TAS.
As an aside, I once had a prop run away to just over 2700 RPM on approach but we got slowed down and found that at 90 knots the RPM was quite controllable with throttle, so we kept it running rather than feather it.
Can't speak for other supercharged types.
Heron with the old Gypsy Queen engines, which were not supercharged (as someone recently tried to tell me - he was confused with the DH Dove): Similar reaction if you switched off the ignition in the cruise. Nothing to see except a gentle yaw. When airspeed reduced the RPM decayed, but now (because it was not supercharged), there was no visible change on the manifold gauge. From memory manifold indication was meaningless on those engines anyway as they had an ingenious single lever control of throttle and RPM - for once the Poms built something that was simple for pilots, if not the engineers. Basically if you could hear noise, the engines were working, and if one failed you knew because it got noisier as they went out of synch.
John Tullamarine is usually right on these matters. It's all about the point at which the prop hits the fine pitch stops when there is no power coming out of the engine. At low airspeeds it must go full fine then it behaves as a fixed pitch prop if speed is further reduced. Also less RPM = less MAP. At higher airspeeds, because the engine is turning and producing oil pressure, the governor/prop combo MAY be capable of constant speed control, depending on TAS.
As an aside, I once had a prop run away to just over 2700 RPM on approach but we got slowed down and found that at 90 knots the RPM was quite controllable with throttle, so we kept it running rather than feather it.
Can't speak for other supercharged types.
Heron with the old Gypsy Queen engines, which were not supercharged (as someone recently tried to tell me - he was confused with the DH Dove): Similar reaction if you switched off the ignition in the cruise. Nothing to see except a gentle yaw. When airspeed reduced the RPM decayed, but now (because it was not supercharged), there was no visible change on the manifold gauge. From memory manifold indication was meaningless on those engines anyway as they had an ingenious single lever control of throttle and RPM - for once the Poms built something that was simple for pilots, if not the engineers. Basically if you could hear noise, the engines were working, and if one failed you knew because it got noisier as they went out of synch.
Last edited by Mach E Avelli; 25th Jun 2010 at 06:00.
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simple answer
simple thing is the engine will windmill a little until feathered
any reciprocating internal combustion engine is only around 30% efficient, therefore the other 70% goes in heat...... and the requirement to turn the engine and keep it going.
so if fuel runs out..... no bang no power, just drag......
and a supercharger takes 50% of the power it creates to run itself... its the law of conservation of energy, you don't get something for nothing!!!! - a turbocharger uses wasted energy (heat out of exhaust) to drive a compressor, so it is grabbing some of that 70% waste and using it to increase volumetric efficiency, that is the ability to induct the same air volume as the capacity of the engine
If you want to increase volumetric efficiency over 1, you need forced induction...
anyhow.... a windmilling piston engine that is supercharged may give some MAP but not much, as turbocharged engine wont give any MAP as there is no heat coming to run it..
Cheers
Dash
any reciprocating internal combustion engine is only around 30% efficient, therefore the other 70% goes in heat...... and the requirement to turn the engine and keep it going.
so if fuel runs out..... no bang no power, just drag......
and a supercharger takes 50% of the power it creates to run itself... its the law of conservation of energy, you don't get something for nothing!!!! - a turbocharger uses wasted energy (heat out of exhaust) to drive a compressor, so it is grabbing some of that 70% waste and using it to increase volumetric efficiency, that is the ability to induct the same air volume as the capacity of the engine
If you want to increase volumetric efficiency over 1, you need forced induction...
anyhow.... a windmilling piston engine that is supercharged may give some MAP but not much, as turbocharged engine wont give any MAP as there is no heat coming to run it..
Cheers
Dash
Virgo and Checkboard have got it right, as applied to a supercharged (not turbocharged) engine, and provided the fine pitch limit is not reached. Virgo's reference to submarines suggests that he is an ex Shackleton man, as I am.
If the fuel supply to an engine was cut, there would be a momentary dip in the RPM as the CSU compensated for the power loss, but it would return to its original RPM. The MAP would also dip and return, since the supercharger speed was related to engine RPM. The oil pump being engine driven, the oil pressure would be maintained.
If you missed seeing this brief dip, you were then faced with yaw and reducing airspeed. It would be some time before coolant temperature would show, as the radiator flaps would normally be in auto, and would close to maintain the temperature.
Having decided which side the failed engine was (i.e. dead leg), and after increasing power all round, the procedure was to close the throttle of the
inboard engine on that side, while watching the RPM gauge.
If the RPM dipped, that was the good engine, going from thrusting to windmilling. Restore the power on it promptly, as you are now double assymetric.
If the RPM did not dip, it was because that engine was already windmilling, i.e. it was the failed engine, so go ahead and feather it.
If the fuel supply to an engine was cut, there would be a momentary dip in the RPM as the CSU compensated for the power loss, but it would return to its original RPM. The MAP would also dip and return, since the supercharger speed was related to engine RPM. The oil pump being engine driven, the oil pressure would be maintained.
If you missed seeing this brief dip, you were then faced with yaw and reducing airspeed. It would be some time before coolant temperature would show, as the radiator flaps would normally be in auto, and would close to maintain the temperature.
Having decided which side the failed engine was (i.e. dead leg), and after increasing power all round, the procedure was to close the throttle of the
inboard engine on that side, while watching the RPM gauge.
If the RPM dipped, that was the good engine, going from thrusting to windmilling. Restore the power on it promptly, as you are now double assymetric.
If the RPM did not dip, it was because that engine was already windmilling, i.e. it was the failed engine, so go ahead and feather it.
In the cruise or higher speed range that I describe with the DC3 and Heron ( and I expect most other types) of course, when the engine quits due to fuel starvation or because some mug has hit the magneto master, there will be a MOMENTARY fluctuation downwards in RPM while the CSU makes a correction, but then the indications will be as I describe. Also, although the theory may say otherwise, the throttle needs to be retarded before re-introducing fuel/ignition. Because the pitch has fined off during the power loss, a gob-full of power would cause a momentary overspeed until the CSU gets ahold of it and coarsens the pitch to restore RPM to where it was at the time of the power loss. No theory - simple fact.
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Just to add $0.02 to the prop governor discussion:
The governor is designed to this paradigm: To increase RPM, reduce blade pitch angle (i.e. reduce AOA); to decrease RPM, increase pitch (incr. AOA).
As long as the engine is producing positive torque, this paradigm applies nicely.
But if the engine ISN'T producing torque but is instead absorbing torque from the windmilling prop, the above paradigm creates bad results. As the RPM initially drops, the governor moves blades to a lower pitch angle, which now means a NEGATIVE AOA.
The prop now speeds up, and probably passes into a mild overspeed. The governor senses this, and reduces pitch angle some more. This of course means a MORE NEGATIVE AOA, windmilling the prop to an increased overspeed. This cycle repeats until the blade pitch reaches some mechanical stop. It's supposed to be a pitch-lock mechanism, which hopefully contains the problem before something breaks.
The governor is designed to this paradigm: To increase RPM, reduce blade pitch angle (i.e. reduce AOA); to decrease RPM, increase pitch (incr. AOA).
As long as the engine is producing positive torque, this paradigm applies nicely.
But if the engine ISN'T producing torque but is instead absorbing torque from the windmilling prop, the above paradigm creates bad results. As the RPM initially drops, the governor moves blades to a lower pitch angle, which now means a NEGATIVE AOA.
The prop now speeds up, and probably passes into a mild overspeed. The governor senses this, and reduces pitch angle some more. This of course means a MORE NEGATIVE AOA, windmilling the prop to an increased overspeed. This cycle repeats until the blade pitch reaches some mechanical stop. It's supposed to be a pitch-lock mechanism, which hopefully contains the problem before something breaks.
Absolutely incorrect, barit1.
Repeating my post above:
Finer pitch when the engine is producing power increases RPM.
Finer pitch when the prop is windmilling also increases RPM.
In the first case the engine is driving the prop, the airflow is striking the prop on the "back" of the blade (the side the pilot sees) so a finer pitch reduces the blade angle of attack, decreasing the drag and so allowing the engine to spin faster.
in the second case the airflow is striking the prop on the "front" of the blade, so a finer pitch increases the angle of attack from the relative wind, which increases the force that wind is applying to spin the prop, which increases the RPM.
The prop governor thus controls the pitch in the correct sense in both cases.
Repeating my post above:
Prop governors sense prop RPM, and adjust the pitch to compensate. If the RPM drops, the governor commands a finer pitch - if the engine is running, this reduces prop angle of attack, and thus prop drag and the RPM increases back to the governed level. If the engine is not running, the RPM drops, the governor commands finer pitch, this increases angle of attack (relative wind is striking the front face of the prop in a windmilling situation) and the relative wind thus drives the prop faster.
Finer pitch when the prop is windmilling also increases RPM.
In the first case the engine is driving the prop, the airflow is striking the prop on the "back" of the blade (the side the pilot sees) so a finer pitch reduces the blade angle of attack, decreasing the drag and so allowing the engine to spin faster.
in the second case the airflow is striking the prop on the "front" of the blade, so a finer pitch increases the angle of attack from the relative wind, which increases the force that wind is applying to spin the prop, which increases the RPM.
The prop governor thus controls the pitch in the correct sense in both cases.
More modern systems do have all sorts of exotic pitch locks, auto-coarsening, beta backups etc. Not applicable to the original question.
A bit off the topic but to reply to an earlier statement.
stevef
Correct to a point.
It was the supercharger technology that made the Merlin superior to the Allison orginally fitted to the P51. It took the Yanks to refine the Merlin for mass production.
As much as Rolls Royce has everyone believe that they produced the finest engines, they were effectively hand made. Each piston individually fitted to each bore and each crankshaft individually fitted to each journal. You couldn't take a piston out of one cylinder and fit it to another. The Roll Royce produced ones are/were a nightmare to own.
The "Parkard" Merlins were a much better engine. True, a British design at the end of the day.
Back to the original question.
I would expect a supercharged engine to act in a similar manner to a normally aspirated one in the situation described in the question.
That is RPM and Manifold Pressure would not change or if so only momentarily, oil pressure would stay the same also. Temps would drop slowly and of course there would be some yaw and speed would decay.
Lack of fuel flow, assuming a flow gauge is fitted, would be the major indicator.
stevef
Hmmm... I guess that's why the P51 was eventually fitted with licence-built Merlins, then
It was the supercharger technology that made the Merlin superior to the Allison orginally fitted to the P51. It took the Yanks to refine the Merlin for mass production.
As much as Rolls Royce has everyone believe that they produced the finest engines, they were effectively hand made. Each piston individually fitted to each bore and each crankshaft individually fitted to each journal. You couldn't take a piston out of one cylinder and fit it to another. The Roll Royce produced ones are/were a nightmare to own.
The "Parkard" Merlins were a much better engine. True, a British design at the end of the day.
Back to the original question.
I would expect a supercharged engine to act in a similar manner to a normally aspirated one in the situation described in the question.
That is RPM and Manifold Pressure would not change or if so only momentarily, oil pressure would stay the same also. Temps would drop slowly and of course there would be some yaw and speed would decay.
Lack of fuel flow, assuming a flow gauge is fitted, would be the major indicator.
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It was the supercharger technology that made the Merlin superior to the Allison orginally fitted to the P51. It took the Yanks to refine the Merlin for mass production.
As much as Rolls Royce has everyone believe that they produced the finest engines, they were effectively hand made. Each pistoin individually fitted to each bore and each cranckshaft individually fitted to each journal. You couldn't take a piston out of one cylinder and fit it to another. The Roll Royce produced ones are/were a nightmare to own.
As much as Rolls Royce has everyone believe that they produced the finest engines, they were effectively hand made. Each pistoin individually fitted to each bore and each cranckshaft individually fitted to each journal. You couldn't take a piston out of one cylinder and fit it to another. The Roll Royce produced ones are/were a nightmare to own.
Wikipedia (fwiw) gives the following:
" By the end of its production run in 1950, almost 150,000 Merlin engines had been built; over 112,000 in Britain and more than 37,000 under license in the U.S"
Wikipedia breaks down the numbers:
" Factory production numbers:
- Rolls-Royce: Derby = 32,377
- Rolls-Royce: Crewe = 26,065
- Rolls-Royce: Glasgow =23,675
- Ford Manchester= 30,428
- Packard Motor Corp = 55,523 (37,143 Merlins, 18,380 V-1650s)
- Overall: 168,068"
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Yamagata Ken, check the wikipedia (fwiw) for the Packard V-1650 entry.
I'm no historian in this regard but I'm guessing it went something like this:
Great design+improved production technique=155k engines.
UK/US synergy at its best.
I'm no historian in this regard but I'm guessing it went something like this:
Great design+improved production technique=155k engines.
UK/US synergy at its best.
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One dot right, you are on centreline.
In long range cruise on R2800 there were 2 leaning techniques, "2 bmep drop" and "11 bmep drop". Couple of times a dozey F/E would go past 11 bmep on the lean side, leading to a sudden drop in bmep to almost zero. Prop kept turning same RPM, MAP stayed same unless throttle moved. Capt blood pressure rose. Engineer bought many beers.
I suspect if airspeed low enough, prop would not have enough "fine pitch" available, and RPM would start to decay with decaying airspeed. But that area of flight is academic and of interest only to the very brave.
Use of feathering pump became very apparent when I had a runaway prop on DC-6B. Prop governor(which normally controls RPM ) mechanically broke (governor control spring failure) resulting in loud howl from prop and Capt simultaneously.
Luckily, feathering pump pressurises governor control piston to port oil to "increase pitch" side of prop servo piston. It seemed like an age, but prop eventually slowed a bit, and then feathered normally. Ten more grey hairs.
In long range cruise on R2800 there were 2 leaning techniques, "2 bmep drop" and "11 bmep drop". Couple of times a dozey F/E would go past 11 bmep on the lean side, leading to a sudden drop in bmep to almost zero. Prop kept turning same RPM, MAP stayed same unless throttle moved. Capt blood pressure rose. Engineer bought many beers.
I suspect if airspeed low enough, prop would not have enough "fine pitch" available, and RPM would start to decay with decaying airspeed. But that area of flight is academic and of interest only to the very brave.
Use of feathering pump became very apparent when I had a runaway prop on DC-6B. Prop governor(which normally controls RPM ) mechanically broke (governor control spring failure) resulting in loud howl from prop and Capt simultaneously.
Luckily, feathering pump pressurises governor control piston to port oil to "increase pitch" side of prop servo piston. It seemed like an age, but prop eventually slowed a bit, and then feathered normally. Ten more grey hairs.