The P-Factor Factor

[MIflyer]

I guess that everyone knows what "P-factor" is but first I'll explain it. A prop driven aircraft with a right turning prop (as viewed from the cockpit) generates a spiral of powerful propwash that circles the fuselage in a clockwise direction. When the propwash gets to the vertical fin it pushes the fin over to the right, and the nose tries to go left. This is most noticeable on takeoff and the climb that follows, when the power level is high and the airspeed low, which makes the propwash strong and the directional righting effect of the fin low. In WWII this effect was called "torque" but in fast it was not the gyroscopic effect of the prop.

The way you correct for P-Factor is by applying right rudder. Most airplanes have at least the fin offset to help counter P-factor. Even the Ercoupe has some P-factor, even though steps were taken in the design to reduce it but using two fins outside the propwash and by pointing slightly to the right.

I just read where a P-38 pilot used P-Factor to an advantage. They were escorting bombers in Jan 1944 when they were attacked by 20 or so FW-190's from behind. He had just read an intelligence report that a P-38 pilot was able to evade a German attack by a right corkscrewing climb. So he tried that same tactic. What happened is that he climbed, which of course slowed the airplane a great deal and the Germans climbed too, firing away. The difference was that the P-38 had no P-Factor to contend with,and the Germans had to try to get their noses far enough to the right on him to hit him. He saw one FW-190 after another behind him try to get the nose around, at a high angle of attack and relatively low airspeed, and then flip over to the left when the combination of factors led to a stall and a spin.

When he got home he found that the P-38's left wing had so many holes in it that it had to be replaced. But nothing vital had been hit.. , .

 

[mikewint]

Not any kind/type expert or pilot but my understanding of P-factor is somewhat different.
What you describe above to my knowledge is slipstream rotation. i.e.: The propeller drags some air around with it, and the airplane continually advances through this slipstream of deflected air. The fin, being behind the portion of the propeller disk where the blades are going left to right, feels a push to the right.
P-factor is mostly felt with a high angle of attack as with taildraggers on take off. When an airplane is nose-high, its propeller is tilted a few degrees upward with respect to the direction of its travel through the air, and a downgoing blade has a greater angle of attack than an upgoing one. The downgoing blade is on the right side, and so it tends to pull the nose of the airplane to the left.
Torque is also very real and is an Action/Reaction pair. Torque is the twisting force supplied by the engine to make the propeller spin. The natural effect of torque, if we did not do something to prevent it, would be to spin the airplane in the opposite direction to the propeller in the same way that a helicopter deprived of its tail rotor begins to rotate in the direction opposite to the main rotor's. Torque and slipstream rotation are two sides of the same coin; part of the torque is imparted to the slipstream, making it rotate.
Torque effects can be devastating. Aircraft like the P-38 feel no torque as the propellers rotate in opposite directions. But if one engine get knocked out the sudden torque can and has flipped the entire aircraft over

 

[pbehn]

Not any kind/type expert or pilot but my understanding of P-factor is somewhat different.
What you describe above to my knowledge is slipstream rotation. i.e.: The propeller drags some air around with it, and the airplane continually advances through this slipstream of deflected air. The fin, being behind the portion of the propeller disk where the blades are going left to right, feels a push to the right.
P-factor is mostly felt with a high angle of attack as with taildraggers on take off. When an airplane is nose-high, its propeller is tilted a few degrees upward with respect to the direction of its travel through the air, and a downgoing blade has a greater angle of attack than an upgoing one. The downgoing blade is on the right side, and so it tends to pull the nose of the airplane to the left.
Torque is also very real and is an Action/Reaction pair. Torque is the twisting force supplied by the engine to make the propeller spin. The natural effect of torque, if we did not do something to prevent it, would be to spin the airplane in the opposite direction to the propeller in the same way that a helicopter deprived of its tail rotor begins to rotate in the direction opposite to the main rotor's. Torque and slipstream rotation are two sides of the same coin; part of the torque is imparted to the slipstream, making it rotate.
Torque effects can be devastating. Aircraft like the P-38 feel no torque as the propellers rotate in opposite directions. But if one engine get knocked out the sudden torque can and has flipped the entire aircraft over

Thats my understanding too mike, there is an effect on twin aircraft though, I believe the Mosquito (may be another type) had to change over the engines as the prop swirl made things difficult on take off. Also the engine rotation with an engine out becomes very important.

 

[MIflyer]

Actual "Torque" in terms of the twisting of the airframe in reaction to the prop rotation is a concern only at very very low airspeeds when a great deal of power is added very quickly. As I read quite recently, with the P-26 the combination of the actual engine torque when the throttle was advanced suddenly along with the hydraulic shock absorbers on the landing gear could cause the fuselage to twist to the left, the landing gear to deflect, and the left horizontal stabilizer to strike the ground, which would then induce a sharp left turn on the ground. That is actual torque, and it is pretty much not a factor in the air.

The spiraling airflow from the prop is not Torque and its impact on the airframe and especially the control surfaces, is what causes P-Factor.

 

[mikewint]

The spiraling airflow from the prop is not Torque

Precisely correct. Torque is a Newtonian Action/Reaction pair. It is experienced anytime the rotational velocity is changed. We experience the force required to change some aspect of motion. You don't feel air pressure unless and until it changes. You don't feel velocity unless and until some aspect of it changes. Sit in your car and gun the engine. You immediately see and feel the force needed to change the rotational velocity of the engine as one side of the vehicle rises but an engine operating at constant rotational velocity has a constant torque and you feel nothing.

ts impact on the airframe and especially the control surfaces, is what causes P-Factor.

Here is where we part company P-factor and HELICAL PROPWASH are NOT the same thing.
HELICAL PROPWASH
The propeller airfoil necessarily has some drag, so it drags the air in the direction of rotation to some extent. Therefore the slipstream follows a helical (corkscrew-like) trajectory, rotating as it flows back over the aircraft. That means the helical propwash will strike the left side of the tail, knocking it to the right, which makes the nose go to the left, which means you need right rudder to compensate.
You don't notice this in cruise, because the aircraft designers have anticipated the situation. The vertical fin and rudder have been installed at a slight angle, so they are aligned with the actual airflow, not with the axis of the aircraft.
In a high-airspeed, low-power situation (such as a power-off descent) the built-in compensation is more than you need, so you need to apply explicit left rudder to undo the compensation and get the tail lined up with the actual airflow.
Conversely, in a high-power, low-airspeed situation (such as initial takeoff roll, or slow flight) the helix is extra-tightly wound, so you have to apply explicit right rudder.

Now P-FACTOR.
The term P-factor is defined to mean "asymmetric disk loading". For simplicity, assume a single-engine plane with a two-blade propeller. Imagine the aircraft is perfectly level and moving forward through the air. The angle of attack on both blades of the propeller would be the same and thus the thrust is equal on all sides. Now imagine the plane pitches up a bit. One blade's angle of attack will increase and other will decrease. Within reason, the greater the angle of attack, the greater the thrust generated. So one blade (or, to be more precise, a blade on one side of the aircraft) will generate more thrust than the other causing the aircraft to yaw since the descending blade is producing more thrust because of its higher angle of attack relative to the aircraft's motion through the air. So again P-factor or actually (P)ropeller-factor occurs when an aircraft is operating at high angles of attack.

You see essentially the same effect in helicopters: the advancing blade on a helicopter produces more thrust because it's heading upwind. So when a helicopter is in forward flight, the blade on one side has a much higher airspeed than the other. If you tried to fly the blades at constant angle of attack, the advancing blade would produce quite a bit more lift than the retreating blade in effect flipping the helo to one side.

For some reason, P-factor gets blamed for the fact that typical aircraft require right rudder on initial takeoff roll. This is impossible for several reasons.

1. Nearly everybody these days learns to fly in nose-wheel type aircraft, which means the propeller disk is vertical during the initial the takeoff roll. Since there is no angle between the relative wind and the propeller axis, P-factor obviously cannot occur.

2. Now let's consider a taildragger, in which the propeller disk is actually non-vertical during the initial takeoff roll. Common experience is that the most right rudder is required at the very beginning of the takeoff, before much forward speed has been achieved. You will often hear/read that this is because P-factor is worst at low airspeeds. This is clearly nonsense, because real P-factor is proportional to airspeed. In the initial moments of the takeoff roll, there is no relative wind, so there can't possibly be any P-factor. Of course, if you are taking off into a headwind, there could be a little bit of P-factor. The real reason that you need right rudder on initial takeoff roll is because of the helical propwash as discussed above. So while P-factor exists in some circumstances, it cannot possibly explain the behavior we observe during initial takeoff roll and we're back to helical propwash.

 

[pbehn]

The torque of a propeller depends on the pitch of the prop and the speed of the plane. Completely feathered on the ground it has no toque effect, if turned at 90 degrees all is torque and no thrust is produced it is a paddle as on an old steamer. As speed increases, the prop develops more thrust with less torque reaction while the wings that oppose this torque moment develop more force as speed increases. The torque reaction does have a major effect on planes but much of it had been designed out by WW2, the Sopwith Camel was famous for rolling much faster one way than the other.

 

[mikewint]

The torque of a propeller depends on the pitch of the prop and the speed of the plane. Completely feathered on the ground it has no toque effect,

I must disagree:
Torque is a measure of the force that can cause an object to rotate about an axis. Just as force is what causes an object to accelerate in linear kinematics, torque is what causes an object to acquire angular acceleration.
Torque is a vector quantity. The direction of the torque vector depends on the direction of the force on the axis.
Consider opening a door. When a person opens a door, they push on the side of the door farthest from the hinges. Pushing on the side closest to the hinges requires considerably more force. Although the work done is the same in both cases as the larger force would be applied over a smaller distance (Work = Force X Distance).

The terminology used when describing torque can be confusing. Engineers sometimes use the term moment, or moment of force interchangeably with torque. The radius at which the force acts is sometimes called the moment arm. Thus the distance from the hinge to the point at which the force to cause the door to rotate is applied is the Moment Arm. As this gets smaller the force must increase to keep the amount of Work constant.

In rotational kinematics, torque takes the place of force in linear kinematics. There is a direct equivalent to Newton's 2ⁿᵈ law of motion (F=ma where F is Force; m is Mass; and a is acceleration)

τ=Iα

Where τ (tau) is torque; I is rotational inertia (which depends on the mass distribution of the system) the equivalent of mass. The larger that I becomes, the harder it is for an object to acquire angular acceleration (a long baton requires more force to rotate than a short one of the same mass); and α (alpha) is the angular acceleration.

The concept of rotational equilibrium is an equivalent to Newton's 1ˢᵗ law for a rotational system. An object which is not rotating remains not rotating unless acted on by an external torque. Similarly, an object rotating at constant angular velocity remains rotating unless acted on by an external torque.

So ANY prop feathered or not REQUIRES a torque to start rotating. Once rotating, a constant though lesser torque is needed to keep the prop rotating (friction must be overcome). Increasing or decreasing the rate of rotation requires a torque since you must apply an acceleration (angular) to change the angular velocity.
Changing the props pitch requires more torque (angular velocity constant) as you are now moving air molecules and friction has increased. At 90 degrees, yes, there is zero thrust and torque is greater since you are striking the greatest number of air molecules at the greatest friction.
Forward motion has no effect on torque and it is the ailerons that would oppose it. A clockwise spinning prop (from the cockpit) produces a counterclockwise roll. At constant propeller RPM the torque is constant (friction only) and the ailerons can be trimmed. Now increasing/decreasing propeller RPM requires a torque (positive/negative) and thus a roll effect would be produced

 

[Shortround6]

Sit on a BMW flat twin motorcycle, rev engine stopped at stoplight. Feel bike try to lean to one side. Torque showing effect. Engine trying to turn one way while turning the flywheel the other way, even if flywheel isn't connected to anything.

Motorcycle engines with crankshafts parallel to axles don't show this anywhere near as well due to longer moment arms of resistance. trying to lift and depress the front/rear suspensions.

 

[pbehn]

Sit on a BMW flat twin motorcycle, rev engine stopped at stoplight. Feel bike try to lean to one side. Torque showing effect. Engine trying to turn one way while turning the flywheel the other way, even if flywheel isn't connected to anything.

Motorcycle engines with crankshafts parallel to axles don't show this anywhere near as well due to longer moment arms of resistance. trying to lift and depress the front/rear suspensions.

Oh S/R I was composing a reply, including moments and vectors and all sorts of stuff. But I had a brother in law with a Moto Guzzi 850 Le Mans and a friend with a BMW R60. The Guzzi was the best, and I rode it. Parked at the lights blipping the throttle it kicked quickly to the right and returned more slowly left. A more considered reply including resolving turning moments of Merlin Engines especially the 2000BHP 133/134 versions where one type has all engine rotating components turning in the same direction as the prop and the other is doing the opposite will follow.

 

[gumbyk]

The torque of a propeller depends on the pitch of the prop and the speed of the plane. Completely feathered on the ground it has no toque effect, if turned at 90 degrees all is torque and no thrust is produced it is a paddle as on an old steamer. As speed increases, the prop develops more thrust with less torque reaction while the wings that oppose this torque moment develop more force as speed increases. The torque reaction does have a major effect on planes but much of it had been designed out by WW2, the Sopwith Camel was famous for rolling much faster one way than the other.

Feathered is the blades at 90º to the plane of rotation - you don't have the prop in this condition with the engine running. It's only there for engine failures on multi-engine aircraft. Flat pitch (blades at 0º to rotation) gives no thrust, but some aircraft use it as an airbrake.

 

[pbehn]

Feathered is the blades at 90º to the plane of rotation - you don't have the prop in this condition with the engine running. It's only there for engine failures on multi-engine aircraft. Flat pitch (blades at 0º to rotation) gives no thrust, but some aircraft use it as an airbrake.

OOps, I leapt into civilian mode there.......too much time watching civilian turbo props spool up.

 

[mikewint]

Flat pitch (blades at 0º to rotation) gives no thrust, but some aircraft use it as an airbrake.

I doubt that this would produce a brake effect. Braking requires a Negative Pitch. At flat pitch the propeller is going round but producing zero thrust. This is zero pitch angle. Increasing the pitch from zero starts to produce thrust, which moves the aeroplane forwards. By changing the pitch to less than zero, a negative pitch angle, the thrust from the propellor is directed forwards and it slows the aircraft down after landing. This is reverse pitch.

 

[pbehn]

I doubt that this would produce a brake effect. Braking requires a Negative Pitch. At flat pitch the propeller is going round but producing zero thrust. This is zero pitch angle. Increasing the pitch from zero starts to produce thrust, which moves the aeroplane forwards. By changing the pitch to less than zero, a negative pitch angle, the thrust from the propellor is directed forwards and it slows the aircraft down after landing. This is reverse pitch.

Yes mike reverse thrust is reverse thrust and obviously acts against the direction of movement. Zero pitch does not produce ant thrust but a massive amount of drag to slow the plane down.

 

[gumbyk]

I doubt that this would produce a brake effect. Braking requires a Negative Pitch. At flat pitch the propeller is going round but producing zero thrust. This is zero pitch angle. Increasing the pitch from zero starts to produce thrust, which moves the aeroplane forwards. By changing the pitch to less than zero, a negative pitch angle, the thrust from the propellor is directed forwards and it slows the aircraft down after landing. This is reverse pitch.

Even a windmilling prop with positive pitch produces braking. Zero pitch effectively turns the prop into the equivalent of a flat disc worth of braking.
I've used the prop as an airbrake during an engine failure, and it is amazing how effective it is, even without being able to go to zero.

 

[V-1710]

I have heard the story of the P-38 right corkscrew too. I think the tactic was commonly used in the PTO. A little asymmetrical throttle improved the maneuver.

 

[pbehn]

I must disagree:
Torque is a measure of the force that can cause an object to rotate about an axis. Just as force is what causes an object to accelerate in linear kinematics, torque is what causes an object to acquire angular acceleration.
Torque is a vector quantity. The direction of the torque vector depends on the direction of the force on the axis.
Consider opening a door. When a person opens a door, they push on the side of the door farthest from the hinges. Pushing on the side closest to the hinges requires considerably more force. Although the work done is the same in both cases as the larger force would be applied over a smaller distance (Work = Force X Distance).

The terminology used when describing torque can be confusing. Engineers sometimes use the term moment, or moment of force interchangeably with torque. The radius at which the force acts is sometimes called the moment arm. Thus the distance from the hinge to the point at which the force to cause the door to rotate is applied is the Moment Arm. As this gets smaller the force must increase to keep the amount of Work constant.

In rotational kinematics, torque takes the place of force in linear kinematics. There is a direct equivalent to Ncrankshaftewton's 2ⁿᵈ law of motion (F=ma where F is Force; m is Mass; and a is acceleration)

τ=Iα

Where τ (tau) is torque; I is rotational inertia (which depends on the mass distribution of the system) the equivalent of mass. The larger that I becomes, the harder it is for an object to acquire angular acceleration (a long baton requires more force to rotate than a short one of the same mass); and α (alpha) is the angular acceleration.

The concept of rotational equilibrium is an equivalent to Newton's 1ˢᵗ law for a rotational system. An object which is not rotating remains not rotating unless acted on by an external torque. Similarly, an object rotating at constant angular velocity remains rotating unless acted on by an external torque.

So ANY prop feathered or not REQUIRES a torque to start rotating. Once rotating, a constant though lesser torque is needed to keep the prop rotating (friction must be overcome). Increasing or decreasing the rate of rotation requires a torque since you must apply an acceleration (angular) to change the angular velocity.
Changing the props pitch requires more torque (angular velocity constant) as you are now moving air molecules and friction has increased. At 90 degrees, yes, there is zero thrust and torque is greater since you are striking the greatest number of air molecules at the greatest friction.
Forward motion has no effect on torque and it is the ailerons that would oppose it. A clockwise spinning prop (from the cockpit) produces a counterclockwise roll. At constant propeller RPM the torque is constant (friction only) and the ailerons can be trimmed. Now increasing/decreasing propeller RPM requires a torque (positive/negative) and thus a roll effect would be produced

Yes Mike but in terms of the two sides of the equation in an aeroplane the prop is attached/connected to the crankshaft. At any change in the system the angular momentum of the prop and everything attached to it. When everything is in balance in level flight, the force applied by the prop on the air, is balanced by the forces applied by the wings trim settings. However when the aeroplane stalls it stops providing any force to oppose rotation. The engine applies a force which acts equally in one direction on the prop and the other on the plane. The problem comes on planes such as a Sopwith Camel. The rotary engine weighed 160kg and the total airplane weighed 680 kg.

 

[mikewint]

massive amount of drag

Ah...OK I understand. The props turning at 0 degrees are producing 0 thrust either pos or neg but are being dragged through the air increasing air friction. My interpretation of the prop statement was that for the props themselves to produce a brake-effect they would have to produce negative thrust. Gumbyk's statement also now is understandable as a small degree of pos-pitch would produce some pos-thrust but less than the amount of negative frictional drag producing a braking effect. Thank you gentlemen

have heard the story of the P-38 right corkscrew

The P-38 was always one of my favorites and I have built several models and done some research. Capt. Robin Olds with 13 kills stated that he had gotten those kills NOT because of the P-38 but in spite of it. The problem was that while American pilots were generally well trained, they weren't well trained for a very complex twin-engine fighter. Its Allison engines consistently threw rods, swallowed valves and fouled plugs, while their intercoolers often ruptured under sustained high boost and turbocharger regulators froze, sometimes causing catastrophic failures.

Arrival of the newer P-38J to fill in behind the P-38H was supposed to help, but did not help enough. The J model's enlarged radiators were trouble-prone. Improperly blended British fuel exacerbated the problems: Anti-knock lead compounds literally seethed out and became separated in the Allison's induction system at extreme low temperatures. This could cause detonation and rapid engine failure, especially at the high power settings demanded for combat.

The P-38's General Electric turbo-supercharger sometimes got stuck in over-boosted or under-boosted mode. This occurred mainly when the fighter was flown in the freezing cold at altitudes approaching 30,000 feet, which was the standard situation in the European air war. Another difficulty was that early P-38 versions had only one generator, and losing the associated engine meant the pilot had to rely on battery power.
At 26,000 feet over Germany, pilots shivered in bitterly cold cockpits, their hands and feet became numb with cold and in some instances frost-bitten; not infrequently a pilot was so weakened by conditions that he had to be assisted out of the cockpit upon return
The P-38 engines produced a tremendous amount of torque and therefore had counter-rotating engines to overcome left-turning tendencies caused by its 1,600-hp engines.

The engines rotated outward from the cockpit. This made the platform more stable for shooting guns, however, if the pilot lost an engine, the operating engine was so powerful that it could uncontrollably roll the aircraft inverted.

 

[MIflyer]

The P-38J and later models did not have enlarged radiators. The radiators were the same size but they moved them further out into the airstream and added a lip to separate the slower boundary layer air from the faster air that did more cooling..

The leading edge intercoolers of the earlier models were not only a challenge to build but had poor heat transfer characteristics; the H model was horsepower limited due to the inadequate intercooler. But the new intercoolers they added under the front of the engine had no regulation of the temperature and often over-cooled the air, resulting in failure to vaporize the fuel properly. At the 9th Photo Recon they solved the problem by blocking off some of the outflow from the intercooler, since their missions were pretty much all at high altitude. They finally added adjustments on later P-38's to enable pilots to control the cooling airflow through the intercooler, similar in concept to cowl flaps.

Another factor was that on the later models they added a manifold pressure regulator to the V-1710, which might have been a good idea for the P-39 and P-40, but interfered with smooth operation of the turbo. On the photorecon models the manifold pressure regulator was removed.

 

[mikewint]

The problem comes on planes such as a Sopwith Camel.

Again, if anything I am less knowledgeable about WWI aircraft but The dynamics of aircraft flight are/were the same then as now.
So very much IMHO: Rotary engines (prop and engine rotating in the same direction) didn't necessarily make monstrous torque. Rather, the gyrations of the engine (torque, helical slip stream, P-factor, gyroscopic precession, etc.) coupled with the design of certain airframes made the effects of torque, precession, P-factor, spiraling slipstream, etc., act upon the airplane in such a way that large control inputs were necessary to maintain stable flight.

This happens these days as well. Many aerobatic aircraft are "riding the ragged edge" aerodynamically, and are quirky as hell during takeoff and maneuvering. This isn't necessarily because of the motor's torque, but rather the way the airframe is balanced and how the aircraft responds to the torque produced. The Camel was designed in such a way (perhaps by accident) that it was riding this ragged edge, much like a modern aerobatic plane. I'm sure Nieuports were the same way to some extent. A modern Pitts Special doesn't necessarily have gobs of power, but it weighs very little and responds to the gyrations of the engine in a pronounced manner. A Pitts can perform some outstanding maneuvers, but will quickly get out of control if not handled with experienced control inputs. The Camel was just like this. The motor didn't necessarily make gobs of torque; it's just that power inputs were felt in a drastic fashion through the controls.

 

[gumbyk]

Ah...OK I understand. The props turning at 0 degrees are producing 0 thrust either pos or neg but are being dragged through the air increasing air friction. My interpretation of the prop statement was that for the props themselves to produce a brake-effect they would have to produce negative thrust. Gumbyk's statement also now is understandable as a small degree of pos-pitch would produce some pos-thrust but less than the amount of negative frictional drag producing a braking effect. Thank you gentlemen

The Camel was just like this. The motor didn't necessarily make gobs of torque; it's just that power inputs were felt in a drastic fashion through the controls.

110 hp and 700+ftlbs of torque at 1100 RPM... (at least that's what the new build ones are making)