P-factor

[RoscoeT]

@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.

So what's your method for separating P-factor yaw from slipstream yaw? Nobody said it's not real, it's just weak. If the prop was really applying as much P-factor yaw force as some seem to think, you fellas would be cracking those little light crank flanges left and right.

 

[Dave Theisen]

P-factor is like pornography, you can’t define it, but you know it when you see it.

 

[dmspilot]

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?

I don't like the term and prefer to use the term "asymmetrical propeller loading" when teaching students.

 

[snglecoil]

So what's your method for separating P-factor yaw from slipstream yaw? Nobody said it's not real, it's just weak. If the prop was really applying as much P-factor yaw force as some seem to think, you fellas would be cracking those little light crank flanges left and right.

So if I’m climbing at a high angle of attack requiring a certain amount of right rudder to correct for left yaw, then lower the angle of attack to establish level flight...without reducing power (fixed pitch), I can reduce/remove right rudder. What has changed regarding spiraling slipstream that allows for the reduction in left yawing tendency? Is it somehow a function of airspeed?

 

[tspear]

Airspeed increases the effectiveness of the rudder, it also decreases slipstream yaw.
So both effects reduce rudder pressure.

Tim

Sent from my HD1907 using Tapatalk

 

[Dan Thomas]

So what's your method for separating P-factor yaw from slipstream yaw? Nobody said it's not real, it's just weak. If the prop was really applying as much P-factor yaw force as some seem to think, you fellas would be cracking those little light crank flanges left and right.

It's there, but it doesn't move the center of thrust out that far. Gyroscopic precession forces are much larger and the crank is designed to handle those.

 

[Dan Thomas]

Airspeed increases the effectiveness of the rudder, it also decreases slipstream yaw.

A low forward speeds the prop is operating at a higher angle of attack due to the low inflow speed. That makes more drag on the prop, and it drags the air with it as it rotates and give that spiralling slipstream effect. As inflow increases, the prop blade AoA decreases and drag decreases and the angle of the spiral decreases.

I once saw a photo of a 172 with its engine running at takeoff RPM, brakes locked of course, and a fellow standing next to it on the left side, holding the end of a long ribbon. That ribbon was following the slipstream and it spiralled just like the drawings show but not as tightly as some of the drawings. Can't find that photo or one like it on the 'net.

 

[Matthew]

A low forward speeds the prop is operating at a higher angle of attack due to the low inflow speed. That makes more drag on the prop, and it drags the air with it as it rotates and give that spiralling slipstream effect. As inflow increases, the prop blade AoA decreases and drag decreases and the angle of the spiral decreases.

I once saw a photo of a 172 with its engine running at takeoff RPM, brakes locked of course, and a fellow standing next to it on the left side, holding the end of a long ribbon. That ribbon was following the slipstream and it spiralled just like the drawings show but not as tightly as some of the drawings. Can't find that photo or one like it on the 'net.

Well, it isn't a 172 but...

 

[snglecoil]

Interesting, this is all good food for thought. Ironically I had an examiner use a picture similar to the one above as an argument against spiraling slipstream as a major contributor to left turning tendencies. His rationale was that the wings interrupted the spiral flow so that aft of the wings the spiraling slipstream was negligible. Of course this was during a multi-engine discussion so the effect of spiraling column of air isn’t exactly equivalent to a centerline single engine.

 

[RoscoeT]

So if I’m climbing at a high angle of attack requiring a certain amount of right rudder to correct for left yaw, then lower the angle of attack to establish level flight...without reducing power (fixed pitch), I can reduce/remove right rudder. What has changed regarding spiraling slipstream that allows for the reduction in left yawing tendency? Is it somehow a function of airspeed?

The reduction in both slipstream and P-factor is absolutely a function of airspeed. The slipstream becomes less dense with increased airspeed, and rudder effectiveness and yaw stability also increase, making the effect less pronounced.

Ironically I had an examiner use a picture similar to the one above as an argument against spiraling slipstream as a major contributor to left turning tendencies. His rationale was that the wings interrupted the spiral flow so that aft of the wings the spiraling slipstream was negligible.

His rationale is flawed. Anyone who's ever done a hammerhead knows how much right rudder is required at the top in order to keep the airplane perfectly vertical in yaw with full power and zero AOA (zero P-factor). Some airplanes require full right rudder. It's pure slipstream, and it's strong.

 

[Dan Thomas]

Well, it isn't a 172 but...

That doesn't represent spiralling slipstream at all. That's the propeller tip vortices becoming visible in high relative humidity. The airflow is moving back, giving the tight helical effect. It's just showing the propeller advance in the air. The spiralling slipstream will be rotating that whole column of air at a slower rate, but without a video it's not visible. If there was some protrusion that the tip vortices struck as they passed it, you'd see the slipstream rotation in the disturbed vortices.
This demonstration, with an RC model, gives a better idea of the angles involved. Start at about the 1:30 mark:

 

[Matthew]

That doesn't represent spiralling slipstream at all. That's the propeller tip vortices becoming visible in high relative humidity. The airflow is moving back, giving the tight helical effect. It's just showing the propeller advance in the air. The spiralling slipstream will be rotating that whole column of air at a slower rate, but without a video it's not visible. If there was some protrusion that the tip vortices struck as they passed it, you'd see the slipstream rotation in the disturbed vortices.
This demonstration, with an RC model, gives a better idea of the angles involved. Start at about the 1:30 mark:

Cool, thanks