P-factor

AggieMike88

AggieMike88

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The original "I don't know it all" of aviation.
I’m familiar with what P-factor is when it comes to airplanes and propellers….

But does the “P” have an actual definition, description, value?

Or is this term a simplified way of saying “Propeller Factor” when applying it to causing the nose to yaw?
 
You could also say it stands for pitch factor.
 
PaulS said:
This guy gives a pretty good overview of the effects that cause it.

Steve's Musings on Life: Steve's Flying Pages: Pilot Courses of Instruction: Aerodynamics: P-factor, Torque, Propwash, and Gyroscopic Precession (localnet.com)
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Thanks! My inner aviation geek is very happy

From that article....

P-factor, also known technically as "asymmetric blade effect" and "asymmetric disc effect," happens when a propeller-driven aircraft is flying at a high angle of attack. The "high" angle of attack occurs, in this instance, when the angle of attack (of the airplane; i.e., the airplane's wings) is great enough to cause the angle of attack of the propeller blades to vary during the course of the rotation of the propeller. The varying angle of attack of the propeller blades results in the center of thrust of the propeller to be offset to one side of the centerline of the aircraft (for single-engine aircraft or centerline thrust multiengine aircraft) or of the nacelle (for multiengine, non centerline thrust aircraft).

Technically, this is referred to an an "asymmetrical relocation of the propeller's center of thrust." This is similar to what happens when a helicopter moves forward in flight, during "translational lift," with the center of lift moving towards the side of the advancing blades, as those advancing blades create more lift than the retreating blades (and more lift is created overall than from a stationary aircraft). Given sufficient forward airspeed the helicopter will roll uncontrollably toward the side of the retreating blades.​
 
P is a Latin letter for pi, which is round, not square, just like a prop-disc. So it really comes from Roman times when P-factor caused their chariots to pull to the left because charioteers were usually right handed and when they'd smack the horses on the back right side, they'd veer left.

... or maybe not.
 
Thanks, Calvin’s dad.

Matthew said:
P is a Latin letter for pi, which is round, not square, just like a prop-disc. So it really comes from Roman times when P-factor caused their chariots to pull to the left because charioteers were usually right handed and when they'd smack the horses on the back right side, they'd veer left.

... or maybe not.
Click to expand...
 
AggieMike88 said:
Thanks! My inner aviation geek is very happy

From that article....

P-factor, also known technically as "asymmetric blade effect" and "asymmetric disc effect," happens when a propeller-driven aircraft is flying at a high angle of attack. The "high" angle of attack occurs, in this instance, when the angle of attack (of the airplane; i.e., the airplane's wings) is great enough to cause the angle of attack of the propeller blades to vary during the course of the rotation of the propeller. The varying angle of attack of the propeller blades results in the center of thrust of the propeller to be offset to one side of the centerline of the aircraft (for single-engine aircraft or centerline thrust multiengine aircraft) or of the nacelle (for multiengine, non centerline thrust aircraft).

Technically, this is referred to an an "asymmetrical relocation of the propeller's center of thrust." This is similar to what happens when a helicopter moves forward in flight, during "translational lift," with the center of lift moving towards the side of the advancing blades, as those advancing blades create more lift than the retreating blades (and more lift is created overall than from a stationary aircraft). Given sufficient forward airspeed the helicopter will roll uncontrollably toward the side of the retreating blades.​
Click to expand...

I'm pretty sure in one of the aviation manuals I read years ago, probably Kerschner, P-factor was described as being due to the force from the gyroscopic precession of the prop, then you had the pro wash effect, and torque was from the asymmetrical thrust due to the difference of the blade angle of attack. But it was a long time ago and I could be wrong.
 
PaulS said:
I'm pretty sure in one of the aviation manuals I read years ago, probably Kerschner, P-factor was described as being due to the force from the gyroscopic precession of the prop, then you had the pro wash effect, and torque was from the asymmetrical thrust due to the difference of the blade angle of attack. But it was a long time ago and I could be wrong.
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That ain't in anything by Kershner ;)
The left turning(*) tendency due to gyroscopic precession only occurs when there is nose-down pitch rate, like when raising the tail on a taildragger. Torque causes a left *rolling* moment due to the rotation of the prop which *may* couple into yaw due to secondary effects. P-Factor is as has been described in this thread, the difference in AOA between the ascending and descending blades when at high (overall) angle of attack causing a yawing moment. The prop wash is prop wash, that much is OK :)

(*)Assuming a prop turning clockwise as viewed from the cockpit.

Nauga,
back and to the left
 
nauga said:
That ain't in anything by Kershner ;)
The left turning(*) tendency due to gyroscopic precession only occurs when there is nose-down pitch rate, like when raising the tail on a taildragger. Torque causes a left *rolling* moment due to the rotation of the prop which *may* couple into yaw due to secondary effects. P-Factor is as has been described in this thread, the difference in AOA between the ascending and descending blades when at high (overall) angle of attack causing a yawing moment. The prop wash is prop wash, that much is OK :)

(*)Assuming a prop turning clockwise as viewed from the cockpit.

Nauga,
back and to the left
Click to expand...

Thanks Nauga, like I said, it's been a while since I've thought about this. I should quit while I'm behind, but I'm a glutton for punishment. My understanding of prop wash is that it not only moves rearward, but also moves in a spiral fashion around the fuselage, which can cause a torque moment on the wing, empennage and elevator. But I could be wrong.
 
PaulS said:
Thanks Nauga, like I said, it's been a while since I've thought about this. I should quit while I'm behind, but I'm a glutton for punishment.
Click to expand...
No sweat, I can't make it through a 'technical' thread without some sort of brain fart either.

PaulS said:
My understanding of prop wash is that it not only moves rearward, but also moves in a spiral fashion around the fuselage, which can cause a torque moment on the wing, empennage and elevator.
Click to expand...
There may be a misunderstanding due to terminology. *Every* effect in the shopping list of "left turning tendency" causes a torque, it's this torque that causes the yawing motion. When most people (in my GA experience) talk about torque and turning they're talking about engine torque inducing some kind of motion. The reaction to engine torque is along the same axis as the prop rotation (in the opposite direction), so effectively roll. The spiral slipstream due to the prop rotation impinges on the left side of the vertical stab, which then causes a rightward force at the vertical stab and nose-left yaw. Still technically 'torque', as is e.g. the pitching moment induced by deflecting the elevator, but a very different cause and effect.

Nauga,
all coily and <stuff>
 
But what does the P stand for?!? o_O
 
nauga said:
Truth, justice, and the American way!

Or maybe just pee, if you don't correct it.

Nauga,
who knows it's really roll rate ;)
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How comes it's not called Rho-factor?
 
asicer said:
How comes it's not called Rho-factor?
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'Cause Q is pitch rate and R is yaw rate :)

It's not really a factor either, but that doesn't seem to be bothering anyone tonight.

Nauga,
sowing seeds of confusion
 
Beat me up..........go ahead..........But a VERY SMALL effect comes from the jet propulsion from the exhaust. For instance, the exhaust on my Cessna 185 was on the right side which gave some measurable roll to the left.
 
kgruber said:
Beat me up..........go ahead..........But a VERY SMALL effect comes from the jet propulsion from the exhaust. For instance, the exhaust on my Cessna 185 was on the right side which gave some measurable roll to the left.
Click to expand...

I'd be extremely interested to know how you separated "measurable" roll effect from the exhaust over the massively greater left roll effect from prop torque.
 
Early Cezznas had twin exhaust to cut down on the E factor. P factor is all about Probably ending up in the left ditch at rotation when that full right aileron doesn’t bring you back to centerline like your Mazda. You learn these things with experience.
 
nauga said:
No sweat, I can't make it through a 'technical' thread without some sort of brain fart either.

There may be a misunderstanding due to terminology. *Every* effect in the shopping list of "left turning tendency" causes a torque, it's this torque that causes the yawing motion. When most people (in my GA experience) talk about torque and turning they're talking about engine torque inducing some kind of motion. The reaction to engine torque is along the same axis as the prop rotation (in the opposite direction), so effectively roll. The spiral slipstream due to the prop rotation impinges on the left side of the vertical stab, which then causes a rightward force at the vertical stab and nose-left yaw. Still technically 'torque', as is e.g. the pitching moment induced by deflecting the elevator, but a very different cause and effect.

Nauga,
all coily and <stuff>
Click to expand...

Good summary Nauga, I was thinking about saying pretty much the same thing about torque last night, asymmetrical thrust causes torque which results in the left turning tendency, the slip stream on the elevator causes a roll moment, the slip stream on the rudder causes more yaw tendency. There are a lot of torque forces on an airframe, then start analyzing what goes on inside the engine, fugetaboutit. It's amazing these machines get off the ground.
 
texasclouds said:
Early Cezznas had twin exhaust to cut down on the E factor. P factor is all about Probably ending up in the left ditch at rotation when that full right aileron doesn’t bring you back to centerline like your Mazda. You learn these things with experience.
Click to expand...

P-factor is a yaw effect, not a roll effect and in any airplane with a centerline rudder, the spiraling propwash is a way more dominant left yaw force than P-factor. But pilots LOVE talking about P-factor LOL.
 
nauga said:
'

It's not really a factor either, but that doesn't seem to be bothering anyone tonight.
Click to expand...

Why not? Seems to fit definition 1 below. P-factor is an influence that contributes to a tendency to yaw left when at higher angles of attack. I agree it doesn't fit definition 2.

noun
1. a circumstance, fact, or influence that contributes to a result or outcome.
2. a number or quantity that when multiplied with another produces a given number or expression.
 
Torque reaction causes the fuselage to rotate opposite the prop rotation, and in the takeoff roll it puts a bit more weight on the left wheel (with clockwise prop) which causes more rolling resistance on that side. In the typical GA airplane it's minimal.

Most of the right rudder needed is due to the P Factor (asymmetric propeller thrust due to the rotation axis not being parallel to the flightpath) and the spiralling slipstream off the prop. Gyroscopic yaw forces in takeoff are normally noted in taildraggers, and only then if that prop is big and heavy enough.
 
Dan Thomas said:
Most of the right rudder needed is due to the P Factor (asymmetric propeller thrust due to the rotation axis not being parallel to the flightpath) and the spiralling slipstream off the prop. Gyroscopic yaw forces in takeoff are normally noted in taildraggers, and only then if that prop is big and heavy enough.
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IMO P-factor is a way smaller force than slipstream, to the point of near insignificance in most airplanes. In all the tailwheel airplanes I've flown, I've never noticed much of a difference in the amount of right rudder pressure needed on the take off roll whether the tail was jacked up to a level attitude, or held just off the runway in a very tail low attitude. In a level attitude, there's no P-factor, just slipstream. In a tail low attitude, there's both P-factor and slipstream...but yet little difference in yaw effect that I've ever observed - maybe just a touch more rudder needed in a tail low attitude, largely for rudder blanking effect.
 
Matthew said:
P is a Latin letter for pi, which is round, not square, just like a prop-disc. So it really comes from Roman times when P-factor caused their chariots to pull to the left because charioteers were usually right handed and when they'd smack the horses on the back right side, they'd veer left.

... or maybe not.
Click to expand...


I take it you’re the guy who writes the PPL test questions.....
 
RoscoeT said:
IMO P-factor is a way smaller force than slipstream, to the point of near insignificance in most airplanes. In all the tailwheel airplanes I've flown, I've never noticed much of a difference in the amount of right rudder pressure needed on the take off roll whether the tail was jacked up to a level attitude, or held just off the runway in a very tail low attitude. In a level attitude, there's no P-factor, just slipstream. In a tail low attitude, there's both P-factor and slipstream...but yet little difference in yaw effect that I've ever observed - maybe just a touch more rudder needed in a tail low attitude, largely for rudder blanking effect.
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It’s more noticeable in slow flight, just shy of stalling, when you’re at high rpm and steep angle of attack.
 
  • P-factor: yaw due to difference in angle of attack of prop blades when relative wind not aligned with prop axis
  • Torque: roll of aircraft in reaction to rotation of prop mass in opposite direction
  • Slipstream: yaw due to propwash pressing left side of tail and fuselage
  • Gyroscopic precession: yaw or pitch in reaction to changing the plane of a rotating mass such as a prop disc. Force felt at 90 degrees after point of application.
A hammerhead is a really great maneuver for teaching and experiencing all four of the forces discussed here.

As you pull to vertical, P factor causes left yaw. Apply right rudder to counteract.
As airspeed decays, torque causes left roll. Apply right aileron to counteract.
As airspeed decays further, slipstream causes left yaw. Apply increasing right rudder to counteract.
As airspeed reaches zero, slipstream impacts side of fuselage and causes vibration and noise. This is signal to initiate 180 yaw turn. Wait past this and you begin tailslide.
Initiate turn using full left rudder. Yawing left uses slipstream to assist. Yawing right works against slipstream, slowing rotation and causing aircraft to fall through rather than pivoting in place.
As aircraft yaws 180 in place, right wing moves through space while left wing is stationary, generating lift on right wing and causing left roll. Apply full right aileron to counteract.
As aircraft yaws 180 in place, gyroscopic precession causes nose to rise. Force is applied to right side of prop disc, and felt at bottom edge of prop disk. Apply forward elevator to counteract.
When aircraft pointed vertically down, center stick and briefly apply right rudder to stop rotation and counteract slipstream.
 
Half Fast said:
It’s more noticeable in slow flight, just shy of stalling, when you’re at high rpm and steep angle of attack.
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It's still mostly slipstream effect rather than P-factor. Fly a parabolic arc (zero AOA / zero P-facto) at high power/RPM and you'll notice just about the same amount of rudder is required around that same speed you're slowing flying at high AOA.
 
snglecoil said:
Interesting...I would argue the opposite.
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It's easier to see the difference between P-factor and slipstream effect when flying aerobatics and tailwheel airplanes. Slipstream is way more powerful. I already mentioned the example doing a tailwheel takeoff with and without P-factor. Not much difference in rudder required since you're mostly countering slipstream. And lots of aerobatic maneuvers are done from slow to zero airspeed at full power and zero AOA (no P-factor), which produces only propeller torque (roll) and slipstream (yaw). You begin to get a feel for what the dominant forces really are.
 
Ed Haywood said:
  • P-factor: yaw due to difference in angle of attack of prop blades when relative wind not aligned with prop axis
  • Torque: roll of aircraft in reaction to rotation of prop mass in opposite direction
  • Slipstream: yaw due to propwash pressing left side of tail and fuselage
  • Gyroscopic precession: yaw or pitch in reaction to changing the plane of a rotating mass such as a prop disc. Force felt at 90 degrees after point of application.
Click to expand...

Nicely listed.

Hey @AggieMike88 all of the above was on my multi oral — as well as demoing each to a student for the multi CFI oral — and talking about how spiraling slipstream changes on one particular engine in a twin with a “critical” engine...

... just to drag your other thread into this one and simultaneously terrify you for your orals.

LOL LOL LOL

Okay probably not THAT likely on yours but it’s one of our local examiner’s many methods available to dig into your aerodynamic knowledge in your head and also see if you’re one of those sorts who’ll hang yourself with bad terminology or slang that’s not from the books.

Hahaha.
 
@RoscoeT, I’ll take your word for it as I have no aerobic experience. I just know from teaching in 172s all day...the P-factor is real.
 
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