p3pilot

A discussion came up the other day in the ready room, and I wanted to see what you guys think. Why is there a tendency for a left yaw in the flare or landing in a twin engine Kingair. P-factor or Gyroscopic precession.

Thanks

Ewfflyer

Torque! Personally I never noticed it in the King-Air, but in the last few days I've been reading the TBM700 POH and it mentions a caution about left turning during t/o and ldg due to the torque of the engine in either situation. Just stating what it said.

P-Factor would really have no effect in this regime of flight, nor would the gyroscopic "forces"(Not precession) acting 90-degree's of the rotation of the props. The only way the gyro-force would be if you had some extremely high AoA and slammed the nose down, which I think you'd have other issues.(remember this force is particular to tail-draggers when the tail is coming off the surface prior to actual rotation/lift-off. Test it using the bicycle wheel test like most of us did in school)

Obviously the 4th-left turn tendency of slip-stream is negligable(sp) since you are probably in reverse thurst and it's a twin anyways.

I'm sure I got something bass-ackwards, and it is 1:30am, but I'll leave that to chance.

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p3pilot

I was always taught that P-factor had to do with the increase of torque. IE: pushing the power lever up for takeoff or touch and go you get a left yaw, but does it still do it on landing. The yaw I'm talking about is just before landing when the power is slowly coming off while in the flare.

EvilGN

you sure the prevailing wind in corpus hasnt been out of the east lately? pulling power off should induce a right yaw, but not nearly what putting power on would do as far left yaw, so if the wind has been out of the east lately as opposed to 130, that would put force on the tail causing a left yaw

just a thought

usmc-sgt

one thing that I have found that fights that left turning tendency specifically in twins on landing is that the students/new twin pilots are tensing (sp) up subconciously and applying light pressure with their left foot and their controlling hand (left) and they are putting a little rudder pressure in because they are nervous. It is not scientific but I have found that when I tell them to be more concious of the possibility it seems to go away. This has worked in light twins and for some reason after 10 or so hours the plane no longer yaws to the left

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the King

Perhaps your engines spool down at slightly different rates leaving the right engine at a higher power than the left. That's a wild theory on my part. In terms of precession or P-factor, precession wouldn't cause that. The pitch up moment would cause a right yaw, not left. So precession is out.

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Ewfflyer

P-Factor is the left turning tendency caused by the rotational angle of the blades in reference to the planes actual flight path. The only reason it's left-hand is because 99% of props spin clockwise(in reference to viewing them from the cockpit). The right-handers are the DeHavilland Chipmunk, a bassackwards spinning prop on a inverted inline-4 engine, but I digress.

As the blade on the right hand side is going down, it is taking a greater "bite" out of the air in reference to the flight path plus+ the angle of attack. The left hand blade at the time is coming up, so you minus the angle of attack. So the right is giving more pull than the left, and hence the right force steering the plane to the left. Obviously this situation only occurs during flight, and usually when at lower airspeeds(when the AoA is at it's greatest in terms of difference to actual flight path).

Now for the same theory, except on landing, the AoA is still high in reference to the flight path(figure 0 degrees attitude, but the flight path is -3 degrees). Everything still adds up appropriately It could cause a little bit of influence, but that would be felt down the whole slope until you suddenly decrease/idle the power, which have a little to do with p-factor, but more with torque.

Now torque is strictly rotational(twisting motion) in reference to the mass associated with the internal guts of the engine that turn, and obviously the prop that's attached at the end of it all also. Think about that big V-8 car reving it's engine. Have you ever seen a car that can twist it's whole frame just by revving it's engine? That's power. Planes don't have that kind of ratio, but it can add up especially with rapid to extreme power contol movement(ie going into reverse/flat pitch).

Once again, just my stab at it, feel free to correct me. I think we need to find some aerodynamisist to really break it down in laymans terms.

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Ewfflyer

P-Factor is the left turning tendency caused by the rotational angle of the blades in reference to the planes actual flight path. The only reason it's left-hand is because 99% of props spin clockwise(in reference to viewing them from the cockpit). The right-handers are the DeHavilland Chipmunk, a bassackwards spinning prop on a inverted inline-4 engine, but I digress.

As the blade on the right hand side is going down, it is taking a greater "bite" out of the air in reference to the flight path plus+ the angle of attack. The left hand blade at the time is coming up, so you minus the angle of attack. So the right is giving more pull than the left, and hence the right force steering the plane to the left. Obviously this situation only occurs during flight, and usually when at lower airspeeds(when the AoA is at it's greatest in terms of difference to actual flight path).

Now for the same theory, except on landing, the AoA is still high in reference to the flight path(figure 0 degrees attitude, but the flight path is -3 degrees). Everything still adds up appropriately It could cause a little bit of influence, but that would be felt down the whole slope until you suddenly decrease/idle the power, which have a little to do with p-factor, but more with torque, because once the wheels are on the ground, AoA and flight path are equal.

Now torque is strictly rotational(twisting motion) in reference to the mass associated with the internal guts of the engine that turn, and obviously the prop that's attached at the end of it all also. Think about that big V-8 car reving it's engine. Have you ever seen a car that can twist it's whole frame just by revving it's engine? That's power. Planes don't have that kind of ratio, but it can add up especially with rapid to extreme power contol movement(ie going into reverse/flat pitch).

Once again, just my stab at it, feel free to correct me. I think we need to find some aerodynamisist to really break it down in laymans terms.

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Ewfflyer

Well I can now back up my left-turn theory. The TBM's do steer left on the landing roll in Beta and Reverse. The Meridian doesn't as noticiable, but it's there too. Now on Take-Off, the TBM's really going to let you know it's got 700HP out front.

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tsween

rudder trim jk