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2 Answers

PRECESSION

Asked by: 3326 views ,
Aircraft Systems, Commercial Pilot, Flight Instructor, General Aviation, Instrument Rating

Procession is when you apply a force at a point, its felt 90 degrees in the direction of rotation. This applies to Gyros. Now, what force can that be compared to in flight?

What applies the force? A turning motion? A bank to the left/right or a yaw?

If someone has a simple and imaginable answer to that, I would appreciate it!

Thanks,

David

 

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2 Answers



  1. Mark Kolber on Feb 22, 2016

    Add change in pitch and speed to the ones you mentioned ad you will have all of the adverse yaw items that are based on precession.

    I’ve seen visual demonstrations that bring it home – try searching “gyroscopic precession” on YouTube.

    For the best real world example, consider riding a bicycle or motorcycle. The rotating tires act like a gyro. When you want to turn, you don’t turn the handlebars (unless you are really, really, slow). You lean, placing a force against the rotation of the tires. The force moves 90° to the direction of the tire rotation, resulting in a turn.

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  2. Skyfox on Mar 07, 2016

    It can be applied to the propeller in flight because it’s the exact same force. When the nose pitches down, for example, while the drive shaft is tilting the pseudo-gyroscope of the propeller through its center of rotation, the effective force is just like the tilting force is being applied at the top of the circle of rotation. Gyroscopic precession moves the force to a virtual point 90° in the direction of the prop’s spin, usually clockwise, so it becomes a force on the right side of the prop which effectively makes the aircraft want to yaw to the left. It’s not very noticeable in cruising flight but at slow speeds with high RPM it creates a pretty significant left turning tendency, and in tailwheel airplanes there’s a risk of hard yaw to the left during the takeoff run if the pilot noses the airplane down too fast to get the tailwheel off the ground. The opposite will be true for a pitch up. Yawing left and right can force the nose up and down as well.

    Precession is also why the heading indicator drifts over time, and why the turn coordinator is such an effective instrument.

    Helicopters have a great deal more precession because the rotor is so much bigger than the prop of an airplane.

    As far as why gyroscopes and other spinning objects have a force of precession that transfers the force to exactly 90° in the direction of rotation, I have no idea. It’s one of those mysterious facts of physics.

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