Taking the P
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Taking the P
Has anyone measured P effect?
This is the effect of airflow coming into the prop slightly sideways when you are tail down and moving and alters the thrust from one side of the aircraft to the other. Now intuition says the coarser side will give more thrust but is this so? Part of the blade will be stalled when the aircraft is moving slowly so I wonder how real the actual effect is. Also the amount the airflow bends as it enters the prop disk will reduce the effect.
So any real testing been done?
This is the effect of airflow coming into the prop slightly sideways when you are tail down and moving and alters the thrust from one side of the aircraft to the other. Now intuition says the coarser side will give more thrust but is this so? Part of the blade will be stalled when the aircraft is moving slowly so I wonder how real the actual effect is. Also the amount the airflow bends as it enters the prop disk will reduce the effect.
So any real testing been done?
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????
Is your question - 'is the effect material?' or 'does some one have a specific measured result for a particular set of circumstances?'
Have you started to fly (a single engine aicraft) yet?
The reason for the second question is that everyone who flies a single either knows the effect is real and material or has had some very exciting stall experiences !
If you are looking for a specific number then we need more details? However, if the question is how do aircraft designers measure the effect to confirm their calculations then hopefully one of the engineers will answer (though I suspect that they imperically confirm that the engine cant, vertical stab positioning and trim tab setting provide a neutral rudder in cruise and an acceptable rudder force at power on stall.
Is your question - 'is the effect material?' or 'does some one have a specific measured result for a particular set of circumstances?'
Have you started to fly (a single engine aicraft) yet?
The reason for the second question is that everyone who flies a single either knows the effect is real and material or has had some very exciting stall experiences !
If you are looking for a specific number then we need more details? However, if the question is how do aircraft designers measure the effect to confirm their calculations then hopefully one of the engineers will answer (though I suspect that they imperically confirm that the engine cant, vertical stab positioning and trim tab setting provide a neutral rudder in cruise and an acceptable rudder force at power on stall.
Last edited by mm_flynn; 1st Dec 2012 at 14:35.
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Interesting question. No, I have not done any measurements, but wanted to comment on something else.
I don't agree that "the" (P-) effect is real. I do agree that you need a bootful of right rudder on takeoff (US Lycosaurus talking here) but there are at least four different effects to which this can be attributed:
- Slipstream effect
- Torque effect
- P-factor
- Gyroscopic issues
I have no idea what percentages to assign to each individual effect, and whether one of these effects might be a myth anyway.
The reason for the second question is that everyone who flies a single either knows the effect is real and material or has had some very exciting stall experiences !
- Slipstream effect
- Torque effect
- P-factor
- Gyroscopic issues
I have no idea what percentages to assign to each individual effect, and whether one of these effects might be a myth anyway.
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Unless this is purely a scientific (and perfectly valid) question then surely it's immaterial? Just fly the aircraft.
I remember my first flight for my PPL; coming from an aviation and gliding background my instructor let me try a take off. I asked him whether I needed right or left rudder for take off, 'You need whatever rudder you need to keep straight, just fly it' was his reply.
I remember my first flight for my PPL; coming from an aviation and gliding background my instructor let me try a take off. I asked him whether I needed right or left rudder for take off, 'You need whatever rudder you need to keep straight, just fly it' was his reply.
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I asked him whether I needed right or left rudder for take off, 'You need whatever rudder you need to keep straight, just fly it' was his reply.
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The case to consider it is stable slow flight. In this case you have
1 - high power output
2 - High angle of attack (the principle reason the effect is said to exist)
3 - Should have no different torque effect as high power cruise (as the engine torque and the wing loading are the same
4 - Should have no gyroscopic effects as the aircraft is not rotating about any axis (until it stalls)
So the need for a large boot full of rudder in slow flight is either a slip stream dynamic or P factor.
an experiment to establish the relative magnitude could be
Fly an approach to stall upright and inverted. If the plane needs left rudder upside down then P effect is dominate, if it needs less right rudder then they are both material, if the right rudder pressure is not materially different then slipstream is primary and P factor is minor. I am not an aerobatic pilot so have no practical experience with this one.
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The net torque is neutral, but that's not the same as saying the torque produced by the engine & prop during climbout is the same as the torque produced by the engine & prop during cruise.
"Gyroscopic effects" when taking about right rudder aren't talking about the rotation of the airframe, they're talking about the high-speed rotation of that big noisemaker up front (assuming single engine.)
"High power output" is irrelevant if you aren't talking about torque and gyroscopic effects. A rocket also has high power output.
"Gyroscopic effects" when taking about right rudder aren't talking about the rotation of the airframe, they're talking about the high-speed rotation of that big noisemaker up front (assuming single engine.)
"High power output" is irrelevant if you aren't talking about torque and gyroscopic effects. A rocket also has high power output.
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I would suggest that the 'P' effect is very real - consider the issue of critical engine on twins.
For a discussion on this (including other factors in addition to P effect) see: Critical_Engine_1.pdf
Just my ha'penny worth (which is probably an accurate judgement on it's worth).
OC619
P.S. Heard during a briefing to someone converting to a Tiger Moth (anti-clockwise prop)
Q: Which way does a Tiger Moth swing on take off?
A: It doesn't if you're doing it correctly
For a discussion on this (including other factors in addition to P effect) see: Critical_Engine_1.pdf
Just my ha'penny worth (which is probably an accurate judgement on it's worth).
OC619
P.S. Heard during a briefing to someone converting to a Tiger Moth (anti-clockwise prop)
Q: Which way does a Tiger Moth swing on take off?
A: It doesn't if you're doing it correctly
Last edited by OpenCirrus619; 5th Dec 2012 at 07:08.
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I got my pencil out to see how big the p-effect should be. I made the following assumptions
300 hp engine
84 inch prop
53 inch effective diameter (the distance we between the point we assume is the centre of effort on each blade)
80MPH forward velocity,
10 degree angle of attack of the aircraft (say 13 degrees for the wing)
400 MPH blade velocity at the centre of effort
20 degree pitch (resulting in a blade angle of attack of 9 - about 2.5 times that in cruise)
This seems to result in a 650 N-M torque.
Something else I noticed was that at cruise (say double the speed), if you temporarily generate the same 10 degree angle of attack (which will make you go up at an astounding rate), the torque will only be 50% because the thrust = power (constant) * velocity (therefore the thrust of the engine will be halved at double the speed - assuming constant power) and the rudder effectiveness will be increased by a factor of 4 (lift being proportional to V^2), so the rudder deflection required to counter act a 10 degree angle of attack at cruise should only be 1/8 that at slow flight (just considering p factor here).
I am further dubious of the slipstream effect as wings (which are all prop blades are) don't normally impart a large forward velocity to the air, which in a prop would be necessary for the rotational effect argued in the slipstream case.
They do cause a significant tip vortex which would seem to be shed symmetrically around the aircraft and I think (if I have the mental picture correct) these would marginally slow the airflow over the left side of the vertical stabilizer and increase it over the right).
As a final note - the engine torque will be 'by definition' equal if the high speed cruise is done at the same power/rpm setting as climb out. 'High Power' was specified in my example to eliminate the differential lift (and hence drag) that is without doubt required to compensate for different engine torque in flight.
As Silvaire1 indicated, gyroscopic effect normally refers to the yaw generated by pitching a rotating mass about a perpendicular axis.
300 hp engine
84 inch prop
53 inch effective diameter (the distance we between the point we assume is the centre of effort on each blade)
80MPH forward velocity,
10 degree angle of attack of the aircraft (say 13 degrees for the wing)
400 MPH blade velocity at the centre of effort
20 degree pitch (resulting in a blade angle of attack of 9 - about 2.5 times that in cruise)
This seems to result in a 650 N-M torque.
Something else I noticed was that at cruise (say double the speed), if you temporarily generate the same 10 degree angle of attack (which will make you go up at an astounding rate), the torque will only be 50% because the thrust = power (constant) * velocity (therefore the thrust of the engine will be halved at double the speed - assuming constant power) and the rudder effectiveness will be increased by a factor of 4 (lift being proportional to V^2), so the rudder deflection required to counter act a 10 degree angle of attack at cruise should only be 1/8 that at slow flight (just considering p factor here).
I am further dubious of the slipstream effect as wings (which are all prop blades are) don't normally impart a large forward velocity to the air, which in a prop would be necessary for the rotational effect argued in the slipstream case.
They do cause a significant tip vortex which would seem to be shed symmetrically around the aircraft and I think (if I have the mental picture correct) these would marginally slow the airflow over the left side of the vertical stabilizer and increase it over the right).
As a final note - the engine torque will be 'by definition' equal if the high speed cruise is done at the same power/rpm setting as climb out. 'High Power' was specified in my example to eliminate the differential lift (and hence drag) that is without doubt required to compensate for different engine torque in flight.
As Silvaire1 indicated, gyroscopic effect normally refers to the yaw generated by pitching a rotating mass about a perpendicular axis.
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P effect
Yes I have started flying a single. I started in 1984 and have 960 hours, an AFI rating a check pilot rating and a full test pilot rating on aircraft up to 450 kgs.
Bronze C Silver C and went solo at 4.5 hours. I got all my licences on the legal minimum hours. I have 150 hours on the Auster and have yet to ground loop it.
So the question is "Has anyone measured P effect on take off?" As one other contributor pointed out we have slipstream in there which may mask any real P effect. The fact that more of the coarser blade will tend to be stalled at low airspeed means this is going to be different compared to a twin with one engine out as hopefully he will still be flying.
Some years back I did static thrust tests on a number of props. We found the finer pitch props at the same rpm as the coarser ones gave more static thrust due to less of the blade being stalled. However as the coarser props had slightly less diameter the calculations got a bit problematic. These included the loss of disk area and the variation in tip speed at the same rpm due to the reduction in diameter.
Hence the question to which the answer looks to be no.
Bronze C Silver C and went solo at 4.5 hours. I got all my licences on the legal minimum hours. I have 150 hours on the Auster and have yet to ground loop it.
So the question is "Has anyone measured P effect on take off?" As one other contributor pointed out we have slipstream in there which may mask any real P effect. The fact that more of the coarser blade will tend to be stalled at low airspeed means this is going to be different compared to a twin with one engine out as hopefully he will still be flying.
Some years back I did static thrust tests on a number of props. We found the finer pitch props at the same rpm as the coarser ones gave more static thrust due to less of the blade being stalled. However as the coarser props had slightly less diameter the calculations got a bit problematic. These included the loss of disk area and the variation in tip speed at the same rpm due to the reduction in diameter.
Hence the question to which the answer looks to be no.
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I should have been more polite in my answer.
The difference in blade angle is rather small (+/- 0.5 degrees at a 10 degree angle of attack). 2/3s of the effect in the example I worked is from the different velocity of air over the climbing and descending parts of the blade. Once you are into the takeoff roll (probably less than 50% of Vso) most props appear to function close to the idealised model (i.e. they are not partially stalled and are operating at some reasonably large fraction of their design efficiency).
Static thrust is quite heavily influenced by disk diameter, hence, your observations
I would have thought P-effect is negligible in the takeoff roll (and certainly has nothing to do with the sudden left turn at the start of the roll!), measurable in a Vx climb, minimal in a Vy climb and quite significant in slow flight. I have never seen a sensible argument for why the torque resulting from the slipstream effect would change materially with angle of attack or the relatively modest speed difference from Vy to Vso - so measuring the rudder deflection over that speed range (and the angle of attack required for unaccelerated flight) you could determine the P-effect (as it seems the only effect dependent on AoA).
The difference in blade angle is rather small (+/- 0.5 degrees at a 10 degree angle of attack). 2/3s of the effect in the example I worked is from the different velocity of air over the climbing and descending parts of the blade. Once you are into the takeoff roll (probably less than 50% of Vso) most props appear to function close to the idealised model (i.e. they are not partially stalled and are operating at some reasonably large fraction of their design efficiency).
Static thrust is quite heavily influenced by disk diameter, hence, your observations
I would have thought P-effect is negligible in the takeoff roll (and certainly has nothing to do with the sudden left turn at the start of the roll!), measurable in a Vx climb, minimal in a Vy climb and quite significant in slow flight. I have never seen a sensible argument for why the torque resulting from the slipstream effect would change materially with angle of attack or the relatively modest speed difference from Vy to Vso - so measuring the rudder deflection over that speed range (and the angle of attack required for unaccelerated flight) you could determine the P-effect (as it seems the only effect dependent on AoA).
