Silly question I am sure but was wondering why when in cruise that the autopilot used so much left rudder correction. I can see the trim move half way to the left but cant figure out why.

 

Most single engine propellor driven aircraft have the vertical fin offset to help counter the left turning tendency at takeoff and low speed climb.

An single engine prop aircraft with a large airspeed difference between takeoff speed and cruise speed will need left rudder for coordinated flight at high speed cruise, so it seems like the autopilot is doing its job.

Joe

 

To the left of center or the left of the green dot on the trim indicator? When the trim is "centered" for take-off (nose of the plane on the green dot of the indicator) it's actually about 15 degrees right to offset the p-factor of the 1200 hp the pt-6 puts out. In cruise the trim centers up inline with the longitudinal axis aircraft.

 

It is a silly question!

 

TBM's share this same trim concept. There's quite a bit of movement, and thankfully the new 850's with G1000's will auto-rudder trim with the YD on.

 

Alrighty but I still dont understand it....I get the concept but its not clicking. I dont think i understand why it needs so much left rudder when we have so many left turning tedencies....

 

It's speed! The plane is set-up to handle the left turning tendencies a the most critical speeds at take-off and landing. As you speed up, it's time to neutralize those forces, because now you're going fast!!!!! Now you have to center the rudder trim, or even sometimes go slightly left of center.

The take-off setting for a TBM on the rudder trim has a green-mark on the trim, and a center mark in white. I don't know the exact degree difference, but it takes about 10 seconds of holding the switch to get a full difference between the two.

 

Remember the two most critical aspects of left turning tendency - high power and high angle of attack. It is negligible in straight and level flight because all forces are in equilibrium.

Because single engine turbo-props have such high levels of power output (the PC-12 is 1270 ESHP I thnk) it is necessary to trim the airplane to the right to compensate for all the torque and p-factor, otherwise it would be a wild ride down the runway. Once you've leveled out and the forces are all neutralized the rudder centers up for coordinated flight.

If you started a climb from cruise you would see the rudder move to the right to keep the ball centered, but when you land it's centered up because there is no left turning tendency when the engine isn't accelerating the plane.

The green marks on the trim indicator are for take-off only.

 

And the light has been SEEEEEEN Thank you guys. I get it...I makes sense now...