VP propeller question
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VP propeller question
I have been wondering about this for a while:
I fly an aircraft that has a variable pitch (constant speed) propeller. The propeller governor uses engine oil under pressure to drive the piston in the spinner and twist the blades into high pitch (low RPM) and centrifugal forces acting on weights to twist them into low pitch. So my question is: If I loose engine oil pressure but the engine keeps on running the blades would go into low pitch right? (because of centrifugal forces acting ont the weights)
The engine would most! probable quit if the pressure goes down, however if the pressure gauge shows that the pressure is dropping but the blades maintain the same pitch/RPM, it could mean that the gauge is faulty and that the real oil pressure is ok?
One more thing, if all the oil drains out during flight and the engine quits, can i feather the prop? (my guess is no)
hope you understand my questions
p.s. the airplane is a Cessna R172K
I fly an aircraft that has a variable pitch (constant speed) propeller. The propeller governor uses engine oil under pressure to drive the piston in the spinner and twist the blades into high pitch (low RPM) and centrifugal forces acting on weights to twist them into low pitch. So my question is: If I loose engine oil pressure but the engine keeps on running the blades would go into low pitch right? (because of centrifugal forces acting ont the weights)
The engine would most! probable quit if the pressure goes down, however if the pressure gauge shows that the pressure is dropping but the blades maintain the same pitch/RPM, it could mean that the gauge is faulty and that the real oil pressure is ok?
One more thing, if all the oil drains out during flight and the engine quits, can i feather the prop? (my guess is no)
hope you understand my questions
p.s. the airplane is a Cessna R172K
BIRK,
Excellent tutorial here:
Product Support
If you lost engine oil pressure in a 172 with a constant speed propeller, the propeller would not go into low pitch right away. The mechanism for propeller pitch changes is a lot like a car jack, supply pressure to lift, trap pressure to hold, let pressure out to drop. So initially when pressure was lost, and before the engine started making serious metal, oil trapped in the hub would hold the pitch in place, assuming you were just flying straight & level, making no changes to demands of power or propeller pitch. As the engine started to slow down due to impending failure, the govenor would act to let oil out of the hub. The CF and the internal spring forces would push the propeller back to lower pitch. When enough load was removed from the propeller to compensate for the additional load due to no lubrication, the govenor would have the valve closed again and the propeller would stay where it is. Ultimately with a real loss of oil pressure the only way the propeller is going to move is back toward flat pitch, there is no going back up toward feather.
-- IFMU
Excellent tutorial here:
Product Support
If you lost engine oil pressure in a 172 with a constant speed propeller, the propeller would not go into low pitch right away. The mechanism for propeller pitch changes is a lot like a car jack, supply pressure to lift, trap pressure to hold, let pressure out to drop. So initially when pressure was lost, and before the engine started making serious metal, oil trapped in the hub would hold the pitch in place, assuming you were just flying straight & level, making no changes to demands of power or propeller pitch. As the engine started to slow down due to impending failure, the govenor would act to let oil out of the hub. The CF and the internal spring forces would push the propeller back to lower pitch. When enough load was removed from the propeller to compensate for the additional load due to no lubrication, the govenor would have the valve closed again and the propeller would stay where it is. Ultimately with a real loss of oil pressure the only way the propeller is going to move is back toward flat pitch, there is no going back up toward feather.
-- IFMU
One more thing, if all the oil drains out during flight and the engine quits, can i feather the prop? (my guess is no)
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CS-P 220 Feathering Propellers
(a) Feathering Propellers must be designed to feather from all conditions in flight, while taking into account likely wear and leakage. Feathering and unfeathering limitations must be documented in the appropriate manual(s).
(b) Propeller pitch control systems that use engine oil to feather must incorporate a method to allow the Propeller to feather if the engine oil system fails.
(c) Feathering Propellers must be designed to be capable of unfeathering after being feathered for the maximum expected diversion time at the minimum declared steady state outside air temperature.
(d) Where there is a minimum Engine/Propeller rotational speed and/or associated aircraft speed below which Propeller feathering cannot be accomplished, the Propeller type-certificate data sheet must be endorsed
accordingly.
(a) Feathering Propellers must be designed to feather from all conditions in flight, while taking into account likely wear and leakage. Feathering and unfeathering limitations must be documented in the appropriate manual(s).
(b) Propeller pitch control systems that use engine oil to feather must incorporate a method to allow the Propeller to feather if the engine oil system fails.
(c) Feathering Propellers must be designed to be capable of unfeathering after being feathered for the maximum expected diversion time at the minimum declared steady state outside air temperature.
(d) Where there is a minimum Engine/Propeller rotational speed and/or associated aircraft speed below which Propeller feathering cannot be accomplished, the Propeller type-certificate data sheet must be endorsed
accordingly.
This is usually done through a separate feathering pump which draws oil from a part of the sump which is not accessible to the engine oil pump.
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Karl,
you want single engine VPs to go to fine pitch in case of pitch mechanism failure. Hence, a single acting prop will have the spring pushing the prop to fine and a dual-acting prop with flyweights will have the weights push the prop to fine.
That way, you can always fly at low airspeeds and have power available if the pitching fails. The only time this will do you in is if you are too far from a suitable place to land for the fuel to get you there at fine pitch. Getting stuck in coarse pitch could mean inability to produce enough power to remain airborne and would certainly hamper the ability to go around and/or climb.
In short, I do not know the answer to your question but I suspect that most single-engine VP props will be non-feathering. The philosophy being that if your engine quits, the propeller drag is usually an academic issue anyway and feathering ability would add complexity and be more likely to do you in than inability to feather.
Rgds,
/Fred
you want single engine VPs to go to fine pitch in case of pitch mechanism failure. Hence, a single acting prop will have the spring pushing the prop to fine and a dual-acting prop with flyweights will have the weights push the prop to fine.
That way, you can always fly at low airspeeds and have power available if the pitching fails. The only time this will do you in is if you are too far from a suitable place to land for the fuel to get you there at fine pitch. Getting stuck in coarse pitch could mean inability to produce enough power to remain airborne and would certainly hamper the ability to go around and/or climb.
In short, I do not know the answer to your question but I suspect that most single-engine VP props will be non-feathering. The philosophy being that if your engine quits, the propeller drag is usually an academic issue anyway and feathering ability would add complexity and be more likely to do you in than inability to feather.
Rgds,
/Fred
A little less conversation,
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Aerobatic props tend to be set up to revert to coarse pitch if oil pressure is lost, because flinging the aircraft around the place can often lead to temporary fluctuations in pressure, and you don't want the potential for frequent engine overspeeds. A classic symptom of "sticky balls" in the inverted oil system is having the prop start to wind back in a vertical up-line (the ball valve is often mounted at a slight angle, so that assuming you do hit a perfect vertical line the system is still working fully in one orientation).
Are there any single pistons that are set up to feather the prop? I never heard of one if there is.
But all the ones I have flown have either electric or manual pitch change, no CSU, and as often as not the choice is fully fine, coarse or feather. We stop the engine to soar, and don't want the drag. Usually land engine stopped, rather than fly around while the engine warms up.
There are a number of electrically activated CS props. Some of these will feather, and some will go negative, so you can reverse!
Just out of interest, what types/ engines are the electric CS props fitted to and what is the mechanism? I have seen a thrush reversing off the pumps, looked like fun but I'm not sure the view was that good
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I know far more about the Rotax stuff then anything else as I have an electric CS prop (which comes as a kit of parts). Mine is an Arplast PV50 with a CSC-1 control unit;
Lyndhurst Touchdown Services Ltd
There are at least six similar products for the Rotax and MT do a full range of very expensive C of A electric props. A friend of mine flew with a reversible prop which gave amazing short landing capability, they are supposedly very popular in float planes.
Rod1
Lyndhurst Touchdown Services Ltd
There are at least six similar products for the Rotax and MT do a full range of very expensive C of A electric props. A friend of mine flew with a reversible prop which gave amazing short landing capability, they are supposedly very popular in float planes.
Rod1
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Depending on the type of propeller, one may or may not see a change as oil pressure is lost. With most conventional Hartzell and McCaulley propellers on light piston airplanes, the propeller will revert to low or course pitch (high RPM position). This is also generally the case with a propeller governor failure. One sees the propeller speeding up. One must keep throttling back to keep the engine RPM under control, because in essence, the propeller has just become as a fixed pitch prop. It's held in the high-rpm (low pitch or course pitch position) by aerodynamic twisting force (ATF), spring pressure, and/or nitrogen pressure in the propeller dome.
Some propellers need oil pressure to feather, and some don't. The Hamilton Standard Hydromatic propeller needs oil pressure to drive the propeller into feather. Hartzell props don't, and will feather with a complete loss of oil pressure. Some will feather automatically, some will not, depending on the control system in use. In turboprop installations, some will feather, some won't.
I experienced a complete oil pressure loss last year in a piston installation, and the propeller was featherable afterward. I experienced a complete oil loss in a TPE-731 turbine installation in a single several years ago, and the engine ran as though there was nothing wrong (honeywell says it will run a half-hour in that condition)...but had no torque because I had no control over the propeller...which did not feather.
Generally, that is not the case, and I have found the opposite to be true in actual practice.
With any demand on the governor to alter the pitch of the propeller (which occurs constantly in flight), the oil pressure will be released but not resupplied, and a change in RPM will occur. The governor, flyweights, and propeller pitch doesn't stay constant inflight; these vary position to maintain a constant RPM. This works, until oil pressure is no longer available to continuously supply the propeller via the governor.
Again, what happens in a given airplane and propeller/engine installation depends on the installation; some will autofeather, some will revert to high pitch, some will revert to low pitch.
Some propellers need oil pressure to feather, and some don't. The Hamilton Standard Hydromatic propeller needs oil pressure to drive the propeller into feather. Hartzell props don't, and will feather with a complete loss of oil pressure. Some will feather automatically, some will not, depending on the control system in use. In turboprop installations, some will feather, some won't.
I experienced a complete oil pressure loss last year in a piston installation, and the propeller was featherable afterward. I experienced a complete oil loss in a TPE-731 turbine installation in a single several years ago, and the engine ran as though there was nothing wrong (honeywell says it will run a half-hour in that condition)...but had no torque because I had no control over the propeller...which did not feather.
If you lost engine oil pressure in a 172 with a constant speed propeller, the propeller would not go into low pitch right away. The mechanism for propeller pitch changes is a lot like a car jack, supply pressure to lift, trap pressure to hold, let pressure out to drop. So initially when pressure was lost, and before the engine started making serious metal, oil trapped in the hub would hold the pitch in place, assuming you were just flying straight & level, making no changes to demands of power or propeller pitch.
With any demand on the governor to alter the pitch of the propeller (which occurs constantly in flight), the oil pressure will be released but not resupplied, and a change in RPM will occur. The governor, flyweights, and propeller pitch doesn't stay constant inflight; these vary position to maintain a constant RPM. This works, until oil pressure is no longer available to continuously supply the propeller via the governor.
Again, what happens in a given airplane and propeller/engine installation depends on the installation; some will autofeather, some will revert to high pitch, some will revert to low pitch.
Avoid imitations
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As already stated, on some aerobatic types, the prop goes fully coarse if engine oil pressure is lost. The Bulldog is one such type.
I recall a landing accident to an RAF Bulldog at Cosford, during my time as QFI on a University Air Squadron, probably in about 1990.
A student pilot bounced very hard on the nosewheel, after suffering PIO during his landing attempt. He damaged and displaced the nosewheel strut upwards into the engine bay. On a Bulldog, the top of the nose strut sits just below the oil pump for the CS prop. The pump was knocked and stopped, the oil pressure was lost and the prop went fully coarse as advertised.
Unfortunately, by now the student had given up trying to salvage the landing and opened the throttle to go around. The engine couldn't rev up due to the coarse prop setting and so the aircraft couldn't climb, drifted on down the full length of the runway, overshot the airfield. Eventually, the old Bulldog decided enough was enough and put itself down in a field, in a heap!
I recall a landing accident to an RAF Bulldog at Cosford, during my time as QFI on a University Air Squadron, probably in about 1990.
A student pilot bounced very hard on the nosewheel, after suffering PIO during his landing attempt. He damaged and displaced the nosewheel strut upwards into the engine bay. On a Bulldog, the top of the nose strut sits just below the oil pump for the CS prop. The pump was knocked and stopped, the oil pressure was lost and the prop went fully coarse as advertised.
Unfortunately, by now the student had given up trying to salvage the landing and opened the throttle to go around. The engine couldn't rev up due to the coarse prop setting and so the aircraft couldn't climb, drifted on down the full length of the runway, overshot the airfield. Eventually, the old Bulldog decided enough was enough and put itself down in a field, in a heap!
