P factor
06-03-2004 | 08:48 AM
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From: Montreal, QC, CANADA
P factor
Hello,
I have been reading about the P-factor ( unequal lift caused by ascending and descending propellers) and the explanations given don't seem complete or adequate.
The angle of attack is usually defined as the angle between the chord of the wing and the relative wind direction. We are told that the descending arm of the propeller has a greater angle of attack than the ascending arm thus creating greater lift and a corresponding torque of the airplane toward the left.
How is the chord of the propeller defined ? If we assume an airplane moving horizontally with a pitch up attitude, what is the resultant relative wind direction on the propeller ? Can someone shed some light on this phenomenon ?
I have been reading about the P-factor ( unequal lift caused by ascending and descending propellers) and the explanations given don't seem complete or adequate.
The angle of attack is usually defined as the angle between the chord of the wing and the relative wind direction. We are told that the descending arm of the propeller has a greater angle of attack than the ascending arm thus creating greater lift and a corresponding torque of the airplane toward the left.
How is the chord of the propeller defined ? If we assume an airplane moving horizontally with a pitch up attitude, what is the resultant relative wind direction on the propeller ? Can someone shed some light on this phenomenon ?
06-03-2004 | 10:58 AM
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RE: P factor
The relative wind the propellor is flying in is related to the pitch angle of the plane. The downgoing blade sees a higher angle of attack, the upgoing a lesser. This creates more lift on the downgoing blade everything else being equal.
For single-engine planes it's a problem at high pitch angles.. and even is a major cause of the left turn when starting a takeoff roll, on taildraggers.
For twins, having both props turn "inward" so the down-going blade is closest to the fuselage eliminates the "critical engine" situation.
The P-38 was such that -both- engines were the "critical engine" depending on which stopped turning.
More modern planes have the inward turning configuration.
For single-engine planes it's a problem at high pitch angles.. and even is a major cause of the left turn when starting a takeoff roll, on taildraggers.
For twins, having both props turn "inward" so the down-going blade is closest to the fuselage eliminates the "critical engine" situation.
The P-38 was such that -both- engines were the "critical engine" depending on which stopped turning.
More modern planes have the inward turning configuration.
06-04-2004 | 04:19 AM
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From: Holts Summit, MO
RE: P factor
dtom,
I agree with you that the explination seems flawed and I believe that the right side of the prop (the down going blade) is operating at a lower AOA and the up coming blade is operating at a higher angle of attack (at least that's what I see when I draw it out). That being said, the lift generated by the down going blade will be greater because of the increased velocity moving over the blade. Think about a helicopter (clockwise spin veiwed from below) the right side is seeing higher velocity compared to the left side. In the case of our plane, since V is squared, a little lower AOA at a higher speed will create more lift. I think from what I've read the slip stream is a large factor in the yaw effect. This might help.
[link]http://www.auf.asn.au/groundschool/umodule11.html#tophenomena[/link]
Steve
I agree with you that the explination seems flawed and I believe that the right side of the prop (the down going blade) is operating at a lower AOA and the up coming blade is operating at a higher angle of attack (at least that's what I see when I draw it out). That being said, the lift generated by the down going blade will be greater because of the increased velocity moving over the blade. Think about a helicopter (clockwise spin veiwed from below) the right side is seeing higher velocity compared to the left side. In the case of our plane, since V is squared, a little lower AOA at a higher speed will create more lift. I think from what I've read the slip stream is a large factor in the yaw effect. This might help.
[link]http://www.auf.asn.au/groundschool/umodule11.html#tophenomena[/link]
Steve
06-09-2004 | 01:42 PM
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From: Holts Summit, MO
RE: P factor "Wrong"
Dtom,
The information that I posted before is "Wrong" so I want to appoligize for the confusion. I said:
Well I took a little time and drew it out correctly (not in my head) and the AOA of the downward going blade does increase, but the change in magnitude of the velocity is still what is the main cause of the greater thrust not the change in AOA. In the case of a high pitch prop the change in AOA may actually off set the greater thrust due to the downward prop seeing greater velocity. I've attached some drawings maybe they will help. During a normal takeoff run with a tail dragger, the change in magnitude of the velocity from one side of the prop to the other would be about a 20 to 30 percent. So, 1.2^2 = 1.44 or 1.3^2=1.69 so the velocity alone account for about a 45 to 70 percent difference in thrust. Even with a low pitch prop in a normal takeoff run we are probably only talking about a change of 15 percent (the coeff. of thrust would go from something like .08 to maybe .07).
Steve
The information that I posted before is "Wrong" so I want to appoligize for the confusion. I said:
I agree with you that the explination seems flawed and I believe that the right side of the prop (the down going blade) is operating at a lower AOA and the up coming blade is operating at a higher angle of attack (at least that's what I see when I draw it out).
Steve
06-16-2004 | 03:42 PM
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From: Moreland, GA
RE: P factor "Wrong"
You are absolutely correct in your analysis of differential thrust across the propeller disk. There is in fact a left turning moment applied to the propeller due to the asymmetric thrust distribution. The fact that is being ignored is that the rotating propeller is a gyroscope. A force applied normal to the plane of rotation of a gyroscope appears ninety degrees out of phase at the hub. Therefore the left turning moment on the propeller disk affects the airplane as a pitching moment rather than a yawing moment.
Thus when an airplane is pitched upward, the P-factor tends to pitch it up further. This is one of several factors that cause a typical airplane of conventional configuration to be less stable power on than power off.
This gyroscopic effect is particularly noticeable in case of a helicopter, where control inputs provide differential force on the rotor ninety degrees ahead of the required maneuver.
Thus when an airplane is pitched upward, the P-factor tends to pitch it up further. This is one of several factors that cause a typical airplane of conventional configuration to be less stable power on than power off.
This gyroscopic effect is particularly noticeable in case of a helicopter, where control inputs provide differential force on the rotor ninety degrees ahead of the required maneuver.
06-18-2004 | 12:14 AM
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From: Mary Esther, Florida, FL
RE: P factor
There are many misconceptions about “P-factor,†and the term is misused more often than not.
First, what it is.
When the airflow enters the propeller disc at any angle other than 90 degrees, we have P-factor effects.
For purposes of this discussion we will say our taildragger airplane sits on the ground with the nose pointed up at 15 degrees. We will also say the propeller pitch is 15 degrees, just to simplify things. Now, with the nose up 15 degrees, the prop shaft is also pointing up 15 degrees. Now, when the propeller turns, and the airplane starts its takeoff roll, the rising blade has the 15 degree pitch cancelled by the 15 degree up angle of the prop shaft, and the descending blade has that same 15 degree shaft angle added to its pitch. So, in effect, the descending blade has a 30 degree pitch, and the rising blade has zero pitch. The majority of the pulling power is developed by the descending blade, giving off center thrust, and that off center thrust is “P-factor.†The effect is zero at zero airspeed, and the effect builds until the tail wheel comes off he ground. This is why you have to gradually add right rudder as the airplane accelerates, and neutralize it when the tail wheel lifts, disregarding torque.
When the tail wheel comes off the ground and the airplane assumes a level position continuing the take off run, the air flow into the propeller disc is then on center, P-factor no longer has any effect, because it just isn’t there anymore.
With tricycle gear, and the plane sitting level at rest, there is NO affect on the plane from P-factor. It does not exist. If the plane sits slightly nose down or nose up, there is a small amount, but it’s so small it can be ignored. This is one of the reasons why a trike is so popular for training. Both in full scale and R/C. They are just easier on take off.
In normal flight P-factor will never affect the airplane, as the airflow, in relation to the airplane, never gets more than one or two degrees off axis. Key word here is “Normal†flight. Most aerobatics are done in a normal controlled flight regime.
When doing aerobatics that depart from normal flight, 3d, gyroscopic maneuvers, harriers, and so forth, p-factor can rear its ugly head.
But 99% of what people call P-factor in normal flight is truly nothing but torque reaction, and that’s another story for another time.
Bill.
First, what it is.
When the airflow enters the propeller disc at any angle other than 90 degrees, we have P-factor effects.
For purposes of this discussion we will say our taildragger airplane sits on the ground with the nose pointed up at 15 degrees. We will also say the propeller pitch is 15 degrees, just to simplify things. Now, with the nose up 15 degrees, the prop shaft is also pointing up 15 degrees. Now, when the propeller turns, and the airplane starts its takeoff roll, the rising blade has the 15 degree pitch cancelled by the 15 degree up angle of the prop shaft, and the descending blade has that same 15 degree shaft angle added to its pitch. So, in effect, the descending blade has a 30 degree pitch, and the rising blade has zero pitch. The majority of the pulling power is developed by the descending blade, giving off center thrust, and that off center thrust is “P-factor.†The effect is zero at zero airspeed, and the effect builds until the tail wheel comes off he ground. This is why you have to gradually add right rudder as the airplane accelerates, and neutralize it when the tail wheel lifts, disregarding torque.
When the tail wheel comes off the ground and the airplane assumes a level position continuing the take off run, the air flow into the propeller disc is then on center, P-factor no longer has any effect, because it just isn’t there anymore.
With tricycle gear, and the plane sitting level at rest, there is NO affect on the plane from P-factor. It does not exist. If the plane sits slightly nose down or nose up, there is a small amount, but it’s so small it can be ignored. This is one of the reasons why a trike is so popular for training. Both in full scale and R/C. They are just easier on take off.
In normal flight P-factor will never affect the airplane, as the airflow, in relation to the airplane, never gets more than one or two degrees off axis. Key word here is “Normal†flight. Most aerobatics are done in a normal controlled flight regime.
When doing aerobatics that depart from normal flight, 3d, gyroscopic maneuvers, harriers, and so forth, p-factor can rear its ugly head.
But 99% of what people call P-factor in normal flight is truly nothing but torque reaction, and that’s another story for another time.
Bill.
06-18-2004 | 12:24 PM
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From: Bloomington, MN,
RE: P factor
There are many misconceptions about “P-factor,†and the term is misused more often than not.
First, what it is.
When the airflow enters the propeller disc at any angle other than 90 degrees, we have P-factor effects.
For purposes of this discussion we will say our taildragger airplane sits on the ground with the nose pointed up at 15 degrees. We will also say the propeller pitch is 15 degrees, just to simplify things. Now, with the nose up 15 degrees, the prop shaft is also pointing up 15 degrees. Now, when the propeller turns, and the airplane starts its takeoff roll, the rising blade has the 15 degree pitch cancelled by the 15 degree up angle of the prop shaft, and the descending blade has that same 15 degree shaft angle added to its pitch. So, in effect, the descending blade has a 30 degree pitch, and the rising blade has zero pitch. The majority of the pulling power is developed by the descending blade, giving off center thrust, and that off center thrust is “P-factor.†The effect is zero at zero airspeed, and the effect builds until the tail wheel comes off he ground. This is why you have to gradually add right rudder as the airplane accelerates, and neutralize it when the tail wheel lifts, disregarding torque.
When the tail wheel comes off the ground and the airplane assumes a level position continuing the take off run, the air flow into the propeller disc is then on center, P-factor no longer has any effect, because it just isn’t there anymore.
With tricycle gear, and the plane sitting level at rest, there is NO affect on the plane from P-factor. It does not exist. If the plane sits slightly nose down or nose up, there is a small amount, but it’s so small it can be ignored. This is one of the reasons why a trike is so popular for training. Both in full scale and R/C. They are just easier on take off.
In normal flight P-factor will never affect the airplane, as the airflow, in relation to the airplane, never gets more than one or two degrees off axis. Key word here is “Normal†flight. Most aerobatics are done in a normal controlled flight regime.
When doing aerobatics that depart from normal flight, 3d, gyroscopic maneuvers, harriers, and so forth, p-factor can rear its ugly head.
But 99% of what people call P-factor in normal flight is truly nothing but torque reaction, and that’s another story for another time.
Bill.
First, what it is.
When the airflow enters the propeller disc at any angle other than 90 degrees, we have P-factor effects.
For purposes of this discussion we will say our taildragger airplane sits on the ground with the nose pointed up at 15 degrees. We will also say the propeller pitch is 15 degrees, just to simplify things. Now, with the nose up 15 degrees, the prop shaft is also pointing up 15 degrees. Now, when the propeller turns, and the airplane starts its takeoff roll, the rising blade has the 15 degree pitch cancelled by the 15 degree up angle of the prop shaft, and the descending blade has that same 15 degree shaft angle added to its pitch. So, in effect, the descending blade has a 30 degree pitch, and the rising blade has zero pitch. The majority of the pulling power is developed by the descending blade, giving off center thrust, and that off center thrust is “P-factor.†The effect is zero at zero airspeed, and the effect builds until the tail wheel comes off he ground. This is why you have to gradually add right rudder as the airplane accelerates, and neutralize it when the tail wheel lifts, disregarding torque.
When the tail wheel comes off the ground and the airplane assumes a level position continuing the take off run, the air flow into the propeller disc is then on center, P-factor no longer has any effect, because it just isn’t there anymore.
With tricycle gear, and the plane sitting level at rest, there is NO affect on the plane from P-factor. It does not exist. If the plane sits slightly nose down or nose up, there is a small amount, but it’s so small it can be ignored. This is one of the reasons why a trike is so popular for training. Both in full scale and R/C. They are just easier on take off.
In normal flight P-factor will never affect the airplane, as the airflow, in relation to the airplane, never gets more than one or two degrees off axis. Key word here is “Normal†flight. Most aerobatics are done in a normal controlled flight regime.
When doing aerobatics that depart from normal flight, 3d, gyroscopic maneuvers, harriers, and so forth, p-factor can rear its ugly head.
But 99% of what people call P-factor in normal flight is truly nothing but torque reaction, and that’s another story for another time.
Bill.
I think you have very nicely and accurately explained what the P-factor is, but I don't quite agree with the consequences you describe.
I agree that there is no yaw torque due to the P-factor when the airspeed is zero and/or when the angle to the oncoming air is zero. However, there is another, usually stronger, source of yaw torque, which is most evident at zero or low airspeed, and which is almost indepent of the angle of attack. This is the yaw torque which is caused by the spiraling slipstream from the prop striking a vertical stabilizer which has a lot more area above the thrust line than below it. Most vertical stabilizers are like this. The effect that we rely on to counteract yaw torque from both the P-factor and from the spiraling slipstream is the 'weathervane' effect of the vertical stabilizer, which becomes stronger as the airspeed increases. As a result, a taildragger, once the tail lifts up, will tend to steer to the left due to the slipstream, and this will be most noticeable before the airspeed gets high enough to give a strong weathervane effect. As you mention, once the tail lifts up, the P-factor becomes quite small, but the spiral slipstream remains. On a tricycle gear plane, the nose wheel resists this slipstream-induced steering for the most part.
In my own experience, most people refer to all of these effects as 'prop torque', even though they are not in the correct axis for that to be the case.
banktoturn
06-18-2004 | 12:58 PM
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From: Mary Esther, Florida, FL
RE: P factor
BTT-Jon:
You are perfectly correct on the circular flow of the slip stream. And that's why it is not uncommon on a single engine prop plane to find the leading edge of the vertical fin offset to the left.
This effect also is greatest immediately after take-off, while on the ground plain old "Ground Effect" neutralizes it somewhat, and during acceleration with the tail still low p-factor is of much greater effect.
In the air, as the plane approaches pitch speed, the effect diminishes, Also, with a longer tail moment arm there is less effect from the spiral flow, as it tends to straighten in proportion to the distance from the propellor.
But torque is always with us, so long as you are feeding power to the propellor.
The note on p-factor above is a copy of one post in a series I did on basic design considerations, someday I'll go back and finish it. The series starts with the third post here:
http://www.rcuniverse.com/forum/Wher...1063414/tm.htm
I invite yoour reading the series, and yoour comments.
Bill.
You are perfectly correct on the circular flow of the slip stream. And that's why it is not uncommon on a single engine prop plane to find the leading edge of the vertical fin offset to the left.
This effect also is greatest immediately after take-off, while on the ground plain old "Ground Effect" neutralizes it somewhat, and during acceleration with the tail still low p-factor is of much greater effect.
In the air, as the plane approaches pitch speed, the effect diminishes, Also, with a longer tail moment arm there is less effect from the spiral flow, as it tends to straighten in proportion to the distance from the propellor.
But torque is always with us, so long as you are feeding power to the propellor.
The note on p-factor above is a copy of one post in a series I did on basic design considerations, someday I'll go back and finish it. The series starts with the third post here:
http://www.rcuniverse.com/forum/Wher...1063414/tm.htm
I invite yoour reading the series, and yoour comments.
Bill.
06-18-2004 | 07:38 PM
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From: Holts Summit, MO
RE: P factor
Bill,
I question this statement:
By pointing the nose up doesn't the angle of attack of the up or down going blade stay the same? The plane that the prop is rotating in and the direction of the thrust and drag will change. (put a ruler and prop on a screw driver, spin them together in any direction (direction of the nose) ...does the angle between the prop and the ruler change? The ruler represents the plane of the spin) As you increase forward velocity it will change the pitch of the prop. The faster you go the bigger the effect. If the prop is "nose up" then as you increase speed you will also increase the thrust differance between the two sides mainly because of the change in the realitive wind speed....change in AOA has little to no effect.
Steve
I question this statement:
For purposes of this discussion we will say our taildragger airplane sits on the ground with the nose pointed up at 15 degrees. We will also say the propeller pitch is 15 degrees, just to simplify things. Now, with the nose up 15 degrees, the prop shaft is also pointing up 15 degrees. Now, when the propeller turns, and the airplane starts its takeoff roll, the rising blade has the 15 degree pitch cancelled by the 15 degree up angle of the prop shaft, and the descending blade has that same 15 degree shaft angle added to its pitch. So, in effect, the descending blade has a 30 degree pitch, and the rising blade has zero pitch.
Steve
06-19-2004 | 11:42 AM
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RE: P factor
Hello, DipStick,
I think you 2 are talking about the same thing, just nomenclature difference, like AOA.
You are right the 2 props have identical AOA as viewed from the shaft shaft axis.
But, the different airflow vectors that the upward prop sees vs. downward prop sees are the result vectors of props AOA w/r/t shaft axis PLUS the forward velocity of the plane. So as viewed from the ground, the downward prop will have a higher resultalt AOA.
This effect will happen only during transient stage of a maneuver, when the thrustline point higher than plane's motion, with airspeed significant fraction of prop speed. For a 3D plane doing a harrier, I don't think the airspeed is not quite enough to have great enough P-effect.
This brings up another point. For planes w/ massive built-in down thrust, like some trainers, won't the P-factor pull the plane to the right during steady-state forward flight?
Another thing I am curious about is the P-factor of a 3-bladed prop. I was told that a 3-bladed prop has much greater P-effect during pull-ups. It was an experiment by Andrew Jesky on a F3A plane. This probably requires more than hand calculation to understand the effect due to a lot of non-linearity involved. All the IAC planes use 3-blade, I am sure for ground-clearance reason. I wonder if it's an issue for them.
I think you 2 are talking about the same thing, just nomenclature difference, like AOA.
You are right the 2 props have identical AOA as viewed from the shaft shaft axis.
But, the different airflow vectors that the upward prop sees vs. downward prop sees are the result vectors of props AOA w/r/t shaft axis PLUS the forward velocity of the plane. So as viewed from the ground, the downward prop will have a higher resultalt AOA.
This effect will happen only during transient stage of a maneuver, when the thrustline point higher than plane's motion, with airspeed significant fraction of prop speed. For a 3D plane doing a harrier, I don't think the airspeed is not quite enough to have great enough P-effect.
This brings up another point. For planes w/ massive built-in down thrust, like some trainers, won't the P-factor pull the plane to the right during steady-state forward flight?
Another thing I am curious about is the P-factor of a 3-bladed prop. I was told that a 3-bladed prop has much greater P-effect during pull-ups. It was an experiment by Andrew Jesky on a F3A plane. This probably requires more than hand calculation to understand the effect due to a lot of non-linearity involved. All the IAC planes use 3-blade, I am sure for ground-clearance reason. I wonder if it's an issue for them.
ORIGINAL: DipStick
Bill,
I question this statement:
By pointing the nose up doesn't the angle of attack of the up or down going blade stay the same? The plane that the prop is rotating in and the direction of the thrust and drag will change. (put a ruler and prop on a screw driver, spin them together in any direction (direction of the nose) ...does the angle between the prop and the ruler change? The ruler represents the plane of the spin) As you increase forward velocity it will change the pitch of the prop. The faster you go the bigger the effect. If the prop is "nose up" then as you increase speed you will also increase the thrust differance between the two sides mainly because of the change in the realitive wind speed....change in AOA has little to no effect.
Steve
Bill,
I question this statement:
For purposes of this discussion we will say our taildragger airplane sits on the ground with the nose pointed up at 15 degrees. We will also say the propeller pitch is 15 degrees, just to simplify things. Now, with the nose up 15 degrees, the prop shaft is also pointing up 15 degrees. Now, when the propeller turns, and the airplane starts its takeoff roll, the rising blade has the 15 degree pitch cancelled by the 15 degree up angle of the prop shaft, and the descending blade has that same 15 degree shaft angle added to its pitch. So, in effect, the descending blade has a 30 degree pitch, and the rising blade has zero pitch.
Steve
06-19-2004 | 12:54 PM
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From: Mary Esther, Florida, FL
RE: P factor
Sean:
Thanks for coming in here regarding Steve the DipStick. I've been pondering exactly how to answer since he made the post, perhaps I over simplified the thing. But I've had too many positive comments from people who now understand exactly what p-factor is to think it's really unclear. Even had an instructor come back and say using my explanation made his ground school classes go much easier.
A three blade prop with equivalent engine load as the two blade will have very close to the same p-factor effect when operated at the same rpm.
Bill.
Thanks for coming in here regarding Steve the DipStick. I've been pondering exactly how to answer since he made the post, perhaps I over simplified the thing. But I've had too many positive comments from people who now understand exactly what p-factor is to think it's really unclear. Even had an instructor come back and say using my explanation made his ground school classes go much easier.
A three blade prop with equivalent engine load as the two blade will have very close to the same p-factor effect when operated at the same rpm.
Bill.
06-19-2004 | 03:24 PM
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From: Holts Summit, MO
RE: P factor
Sean,
If you read LOUW's post because the plane is pulling right from unequal thrust it will also have a nose down moment because of gyroscopic precision. Overall, it would cause it to pull down and right (i think).
I also think it brings up some interesting implications regarding putting down thrust in speed planes. Even though the angle of down thrust may be much less, because of higher speed the thrust difference that would be created my be fairly large. Overall, wouldn't this reduce top speed?
Louw,
Since on a tail dragger the plane would not have the freedom to pitch up (until the tail came of the ground) wouldn't there still be left yaw or would this be dampened out by the gyroscopic precision? Does gyroscopic effect require and actual change in the direction before it takes place? Does the rate of change effect the strength of the effect? Since, the thrust difference is going to be related to the forward speed would a gradual change in the thrust difference lesson the gyroscopic effect?
Bill,
If it works for you great, there are many aerodynamic web site that report the same information as you...that angle of attack is responsible for the thrust difference. I'm just trying to learn a little more about aerodynamics and since I'm not aerodynamicist, I figured I would run my understanding past more learned people to see if I was missing something....ie gyroscopic precision as an example....or if I had screwed up in my conclusions. Simplification has its place and if it helps then I'm all for it.
Steve
This brings up another point. For planes w/ massive built-in down thrust, like some trainers, won't the P-factor pull the plane to the right during steady-state forward flight?
I also think it brings up some interesting implications regarding putting down thrust in speed planes. Even though the angle of down thrust may be much less, because of higher speed the thrust difference that would be created my be fairly large. Overall, wouldn't this reduce top speed?
Louw,
Since on a tail dragger the plane would not have the freedom to pitch up (until the tail came of the ground) wouldn't there still be left yaw or would this be dampened out by the gyroscopic precision? Does gyroscopic effect require and actual change in the direction before it takes place? Does the rate of change effect the strength of the effect? Since, the thrust difference is going to be related to the forward speed would a gradual change in the thrust difference lesson the gyroscopic effect?
Bill,
If it works for you great, there are many aerodynamic web site that report the same information as you...that angle of attack is responsible for the thrust difference. I'm just trying to learn a little more about aerodynamics and since I'm not aerodynamicist, I figured I would run my understanding past more learned people to see if I was missing something....ie gyroscopic precision as an example....or if I had screwed up in my conclusions. Simplification has its place and if it helps then I'm all for it.
Steve
06-19-2004 | 03:52 PM
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From: Mary Esther, Florida, FL
RE: P factor
Steve:
Sorry, as I said in my earlier post I was trying to figure another way to re-word it. Basically, p-factor is an effect of air entering the prop disc at an angle to the axis of the propellor. Greater angle greater effect, lesser angle lesser effect.
But on gyroscopic precession, there are no precessive forces until the axis of the gyroscope is disturbed. So there are none on a straight take-off roll until the tail rises, and then only as the tail rises. With a new equilibrium, i.e. accelerating with the tail up the precessive force disappears once again, until the nose is pulled up breaking ground. And yes, the amount of the precessive force is dependent on the rate of change.
Further, the rotating mass (prop and engine internals) is such a small percentage of the total airframe weight precessive forces are usually ignored anyway.
Now if you go back to the WW I planes with the leRhone, Gnome, Bentley, Oberussel, and other rotary engines precession is a large consideration, many pilots were killed by forgetting about the big gyroscope they had on the nose. If one wanted to turn right at low altitude, for example, he hauled back on the stick. If he gave a lot of right rudder input he found his engine would stop when it hit the ground. Even this was different from one rotary engined plane to the next, as reversing the engine rotation also reversed the effect.
Hope this helps clear things up. If not, yell at me, make me explain things more clearly.
Bill.
Sorry, as I said in my earlier post I was trying to figure another way to re-word it. Basically, p-factor is an effect of air entering the prop disc at an angle to the axis of the propellor. Greater angle greater effect, lesser angle lesser effect.
But on gyroscopic precession, there are no precessive forces until the axis of the gyroscope is disturbed. So there are none on a straight take-off roll until the tail rises, and then only as the tail rises. With a new equilibrium, i.e. accelerating with the tail up the precessive force disappears once again, until the nose is pulled up breaking ground. And yes, the amount of the precessive force is dependent on the rate of change.
Further, the rotating mass (prop and engine internals) is such a small percentage of the total airframe weight precessive forces are usually ignored anyway.
Now if you go back to the WW I planes with the leRhone, Gnome, Bentley, Oberussel, and other rotary engines precession is a large consideration, many pilots were killed by forgetting about the big gyroscope they had on the nose. If one wanted to turn right at low altitude, for example, he hauled back on the stick. If he gave a lot of right rudder input he found his engine would stop when it hit the ground. Even this was different from one rotary engined plane to the next, as reversing the engine rotation also reversed the effect.
Hope this helps clear things up. If not, yell at me, make me explain things more clearly.
Bill.
06-19-2004 | 10:56 PM
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From: Canton,
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RE: P factor
My understanding of gyroscopic procession comes from a demonstration in college freshman Physics class. A student is holding a spinning unicycle while sitting on a swivel chair. As he changes the axis of the spinning wheel, his body would rotate on the swivel chair. And I only remember the right hand rule. Assign right thumb to the spinning wheel axis, align the right index finger to the axis of the disturbance torque, then point the right middle finger downward to get the axis of resultant torque. For the plane w/ spinning prop up front, the thumb would point foward to the nose of the plane. Then a pitch-down disturbance motion (with axis of rotation aligned to the wing tube) would align the index finger to the left wing tip. Then middle finger pointing upward toward canopy would dictate a torque yawing plane to the left. So as the tail dragger pitches up during take-off while the tail lifts, it would cause the plane to yaw to the left, right? The take-off yaw-left effect I've seen is most severe in Cub. I am not sure if it's caused by something else.
06-21-2004 | 12:32 PM
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From: Holts Summit, MO
RE: P factor
Sean,
We must of had the same freshman physics teacher....did the same deminstration anyway. As the tail comes up off the ground then there would be a nose down and left movement. Never did quit get the thumb, finger thing. My five year old son has a pull string "engine" pack for his power rangers car and I have just been cranking it up to see the qyroscopic effect.
Bill,
Thanks for the info....the more I look at whats happening during takeoff it is suprising that we ever get planes off the ground.
Steve
We must of had the same freshman physics teacher....did the same deminstration anyway. As the tail comes up off the ground then there would be a nose down and left movement. Never did quit get the thumb, finger thing. My five year old son has a pull string "engine" pack for his power rangers car and I have just been cranking it up to see the qyroscopic effect.
Bill,
Thanks for the info....the more I look at whats happening during takeoff it is suprising that we ever get planes off the ground.
Steve
06-21-2004 | 02:26 PM
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From: Canton,
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RE: P factor
Dipstick,
I forgot the name of the professor, but it was in Harvey Mudd College.
Most of the planes I have are tail dragger, except for my Delta Vortex (it's a tri, but it lands like a tail dragger, if you know what I mean). None of my current planes has much of the yaw-left effect during take-off, because all are so terribly over-powered that the tail reaches flying speed the moment I gun my left thumb. Even for my profile Edge w/ a very high sitting AOA, the transient pitch during take-off is so instanteous that there's not enough time to yaw. The only plane I ever had that I recall having difficulty taking off was a Sturdy Bird (like Dura Plane, except it was tail dragger).
Another factor that I think affect the take-off P-effect is the placement of the main gears. If the main wheels are way forward of the CG, that means it takes much more yaw torque to yaw the plane, than say if the main wheels are right on the CG. I think parallel axis theorem can quantify this. My profile Edge's wheels are like 35% chord-width forward of the LE.
I forgot the name of the professor, but it was in Harvey Mudd College.
Most of the planes I have are tail dragger, except for my Delta Vortex (it's a tri, but it lands like a tail dragger, if you know what I mean). None of my current planes has much of the yaw-left effect during take-off, because all are so terribly over-powered that the tail reaches flying speed the moment I gun my left thumb. Even for my profile Edge w/ a very high sitting AOA, the transient pitch during take-off is so instanteous that there's not enough time to yaw. The only plane I ever had that I recall having difficulty taking off was a Sturdy Bird (like Dura Plane, except it was tail dragger).
Another factor that I think affect the take-off P-effect is the placement of the main gears. If the main wheels are way forward of the CG, that means it takes much more yaw torque to yaw the plane, than say if the main wheels are right on the CG. I think parallel axis theorem can quantify this. My profile Edge's wheels are like 35% chord-width forward of the LE.
06-23-2004 | 07:24 PM
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From: POSEN,
MI
RE: P factor
interesting posts, remember, angle of attack is simply the difference between wich direction the airplane is pointing and wich way its going. i love the info from robison, you sound like a full size flyer too, as am i, (1943 stearman). it will make a pilot outta ya. all physics aside, you need as much or as little rudder as it takes to keep it pointed down the runway. one thing i must disagree with you though is the point of real aircraft having two engines. you see, on a twin engine airplane when one quits, the good engine simply carries you to the seen of the crash!
06-29-2004 | 08:14 PM
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From: Moreland, GA
RE: P factor
There has been some good information in the previous posts and some not quite accurate. I will try to summarize.
Gyroscopic precession affects all airplanes with a turning propeller. The effect may be significant or not depending on the relative size of the rotating mass. Obviously the old WW I aircraft with their light fabric construction and heavy rotary engines, and the WW II fighters with their large propellers powered by thousand plus horsepower are significantly effected. On the other hand for modern light aircraft with a six-foot propeller twisted by a small engine the effect is rarely noticeable. Significant or not, the effect is always there. For models, whether the effect is significant likewise depends on the relative size of the rotating mass. It is usually not significant except in 3D flight where the aerodynamic forces are overshadowed by thrust and power.
Of course any maneuver that changes the direction of thrust will involve gyroscopic forces. However there are mainly two conditions where gyroscope forces may be noticed. First, if the tail of a tail wheel configured aircraft is raised rapidly, the nose will tend to swing left. This has not been noticeable in the full-scale aircraft I’ve flown, mostly because rapidly raising the tail is really poor technique. Normally the tail is allowed to rise on its own accord as the speed builds and any gyroscopic forces are not apparent.
The other gyroscopic force arises from the so-called P-factor. The asymmetric thrust due to the propeller disc not being perpendicular to the direction of motion (like in climbing flight), exerts a left turning moment on the propeller disc (as previously described). Gyroscopic precession converts this to a nose up pitching moment on the airplane. It is not necessary for the axis of rotation to be displaced for the gyroscopic precession to occur. A force on the rim produces a force ninety degrees out of phase at the axis, even when the axis is not moved.
None of this explains the tendency of an airplane of conventional configuration to turn left when climbing at high power and slow speed. There are two other forces that must be considered. First there is torque. The torque driving the propeller produces a rolling moment in a direction opposite the propeller rotation. For high-powered aircraft with relatively short spans (such as WW2 fighters) it can loom pretty large. The pilot resists this rolling force with the aileron control, which produces a yawing moment (due to adverse yaw) requiring rudder input to compensate. For small low powered aircraft it isn’t much of a factor.
This leaves us with the twist of the propeller slipstream, which is the primary source of the left turning tendency. The air effected by the propeller is accelerated aft producing thrust and at the same time is given a slight twist in the direction of rotation. The twist is not a tight spiral like the threads on a screw, but rather a gentle twist more like the grooves in a rifle barrel. A typical light aircraft has the fin/rudder area well above the thrust line and this twist changes the angle at which the slipstream passes the tail causing it to move to the right, swinging the nose to the left. Many old free flight models (Zipper, Playboy, etc.) took advantage of this twist to control the climb profile. A large sub rudder and a few degrees of down thrust and there was about as much area below as above the thrust line which effectively eliminated the left turn tendency. The high pylon on which the wing was mounted was effected by the slipstream twist by tending to roll right when climbing under power. With this setup, the airplane was easily adjusted to climb in a tight right hand spiral under power and glide in a wide circle to the left in the glide. Some pattern ships have a low profile fin/rudder with a sub rudder, which not only minimizes the left turn tendency in a climb, but also reduces roll coupling in knife-edge flight.
In the final analysis, with the exception of some high powered aircraft with large propellers where torque may be a factor, the tendency to turn left when in high power, low speed climb, is due to slipstream twist, period.
Gyroscopic precession affects all airplanes with a turning propeller. The effect may be significant or not depending on the relative size of the rotating mass. Obviously the old WW I aircraft with their light fabric construction and heavy rotary engines, and the WW II fighters with their large propellers powered by thousand plus horsepower are significantly effected. On the other hand for modern light aircraft with a six-foot propeller twisted by a small engine the effect is rarely noticeable. Significant or not, the effect is always there. For models, whether the effect is significant likewise depends on the relative size of the rotating mass. It is usually not significant except in 3D flight where the aerodynamic forces are overshadowed by thrust and power.
Of course any maneuver that changes the direction of thrust will involve gyroscopic forces. However there are mainly two conditions where gyroscope forces may be noticed. First, if the tail of a tail wheel configured aircraft is raised rapidly, the nose will tend to swing left. This has not been noticeable in the full-scale aircraft I’ve flown, mostly because rapidly raising the tail is really poor technique. Normally the tail is allowed to rise on its own accord as the speed builds and any gyroscopic forces are not apparent.
The other gyroscopic force arises from the so-called P-factor. The asymmetric thrust due to the propeller disc not being perpendicular to the direction of motion (like in climbing flight), exerts a left turning moment on the propeller disc (as previously described). Gyroscopic precession converts this to a nose up pitching moment on the airplane. It is not necessary for the axis of rotation to be displaced for the gyroscopic precession to occur. A force on the rim produces a force ninety degrees out of phase at the axis, even when the axis is not moved.
None of this explains the tendency of an airplane of conventional configuration to turn left when climbing at high power and slow speed. There are two other forces that must be considered. First there is torque. The torque driving the propeller produces a rolling moment in a direction opposite the propeller rotation. For high-powered aircraft with relatively short spans (such as WW2 fighters) it can loom pretty large. The pilot resists this rolling force with the aileron control, which produces a yawing moment (due to adverse yaw) requiring rudder input to compensate. For small low powered aircraft it isn’t much of a factor.
This leaves us with the twist of the propeller slipstream, which is the primary source of the left turning tendency. The air effected by the propeller is accelerated aft producing thrust and at the same time is given a slight twist in the direction of rotation. The twist is not a tight spiral like the threads on a screw, but rather a gentle twist more like the grooves in a rifle barrel. A typical light aircraft has the fin/rudder area well above the thrust line and this twist changes the angle at which the slipstream passes the tail causing it to move to the right, swinging the nose to the left. Many old free flight models (Zipper, Playboy, etc.) took advantage of this twist to control the climb profile. A large sub rudder and a few degrees of down thrust and there was about as much area below as above the thrust line which effectively eliminated the left turn tendency. The high pylon on which the wing was mounted was effected by the slipstream twist by tending to roll right when climbing under power. With this setup, the airplane was easily adjusted to climb in a tight right hand spiral under power and glide in a wide circle to the left in the glide. Some pattern ships have a low profile fin/rudder with a sub rudder, which not only minimizes the left turn tendency in a climb, but also reduces roll coupling in knife-edge flight.
In the final analysis, with the exception of some high powered aircraft with large propellers where torque may be a factor, the tendency to turn left when in high power, low speed climb, is due to slipstream twist, period.
