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P-factor and gyroscopic precession

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Question about P-factor and gyroscopic precession: In all flight training manuals discussing p-factor (at high AOA down swinging prop blade develops more thrust...thus a left yaw...part of left turning tendencies....this is logical and I understand this)...but what about gyroscopic precession...shouldn't the increase thrust take effect 90' degrees later in direction of rotation...resulting in the aircraft pitching up? I have been searching for an answer and I have not found one yet. thanks.

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5 Answers

  1. Brian Konsko on Aug 10, 2010

    “what about gyroscopic precession”

    Gyroscopic precession is caused when a force is applied to a rotating disk. P-factor is an entirely different concept and the two are not coupled (connected).

    P-factor is a result of the aircrafts relative wind effecting the propellers relative wind. The effect is, as you mentioned, creating more lift on the down going than up going blade. This is a result of the propeller blades being at different angles of attack because their relative winds were tilted.

    So you see, no force is being applied by p-factor. Sure the propeller is experiencing its own forces as it rotates. However, gyroscopic precession refers to an outside force acting on a disk.

    There are 4 common forces we can apply on the propeller disk through control inputs, only two of which the average pilot must know. The four actions are rudder left/right or pitch up/down. The rudder actions can be ignored for non aerobatic pilots.

    In either of these 4 actions we are driving a section of the blade into the wind and another away from, thus causing two forces to act on the disk. For example, let’s consider a pitching action typical of a tailwheel aircraft rolling down the runway. When a tailwheel aircraft picks up enough speed its tail is lifted off the runway by pushing the nose forward.

    Let’s apply our two forces from this action: 1) We have taken a stick and pushed it against the top of the spinning propeller, that is our force to push the nose forward. 2) We have tied a string around the bottom of the spinning propeller and are pulling on that. So our forces, assuming sitting in the pilot seat, are a push on the top and a pull on the bottom.

    Since the propeller rotates these forces will not act at the point of application, but instead 90 degrees (in the direction of disk rotation) from the point of application. From sitting in the pilot seat the propeller turns to the right and we know we have a push force on top and pull on the bottom currently. Let us rotate them 90 degrees to the right, giving us this result:

    1) Push force on the right side of the disk
    2) Pull force on the left side of the disk
    3) A left yaw has been produced

    Hence the grouping of gyroscopic precession with “left turning tendencies” even though this action is not seen in the more commonly flown tricycle geared aircraft.

    Any forward push on the yoke must be accompanied by a right rudder application (in flight also) to keep coordinated. Vice versa for a pull on the yoke. These actions are so subtle in a typical trainer that you will likely never recognize them during typical flight. I haven’t flown anything but aerobatic airplanes where I really noticed gyroscopic precession from pitching.

    Gyroscopic Precession (A different look.)

    Consider sitting in your chair and reaching out with your right hand to grab the top propeller blade. Act as though that is the yoke. Now place your left hand as if it were holding the bottom blade. Push forward with the right (top or yoke) hand, pull back with your left hand. Now rotate 90 degrees to the right. There you have it, your right hand pushed forward and left hand back (or a left yawing tendency). You can use your hands to simulate forces on the propeller and then rotate them, this provides a easy way to always remember/figure out gyroscopic precessions. (Source: Emergency Maneuver Training by Rich Stowell)

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  2. Bret J. on Sep 17, 2010

    ‘Push forward with the right (top or yoke) hand, pull back with your left hand. Now rotate 90 degrees to the right. ‘

    Becareful how you say (90′ degress to the right) best would be stated: 90’ degrees in direction of rotation again.

    otherwise they will take the bottom hand (left hand) holding the bottom of the propeller blade (simulating a pull and go the the right (instead of rotating to the left in the direction of rotation) to understand better

    Best Regards

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  3. Wesley Beard on Nov 09, 2010

    Gyroscopic Precession is more prevalent in tailwheel airplanes than in tricycle gear airplanes.  You are correct in your assumption with tricycle gear airplanes that gyroscopic precession will actually make the airplane yaw right.  However, the change in “plane of rotation” is minimal compared to a tailwheel airplane.  So minimal in fact as to not be perceivable.
    Tailwheel airplanes move the propeller through a large “plane of rotation” and results in a large left yaw.  When you start flying tailwheel airplanes, it will become apparently obvious how gyroscopic precession affects the airplane.

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  4. Steve Pomroy on Jan 15, 2011

    Hi Student Pilot.
    There are two thing to keep in mind about precession of the propeller.  First, it only kicks in if we actually allow the prop to move out of it’s plane of rotation.  Second, it is only strong enough to be important at very low airspeeds where the controls are less effective for correcting.
    So, in the case of asymmetric thrust, if we correct with rudder, and the aircraft doesn’t yaw, there will be no precession effect.  But because of control effectiveness in flight — even at low airspeeds — the effect of precession in the absence of a rudder correction is negligible.
    I can only think of two contexts in which precession is important:  takeoff in a taildragger and some aerobatic maneuvers.
    During takeoff in a taildragger, we raise the tail prior to flying speed, but at a high (usually full) power seting and with a fairly high pitch rate.  The result is notable precession to the left, which we can correct with right rudder.  Some aerobatic maneuvers are done a low airspeeds with high pitch or yaw rates.  When this is the case, precession can become important.
    In your question, you mentionjed precession causing pitch.  This happens, for example, during hammerhead turns — which are done at very low airspeeds and high power settings an involve fairly high yaw rates.  A hammerhead to the left requires forward stick to correct for the precession.  A hammerhead to the right will require less forward stick, possibly none at all or even a little back stick.

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  5. ATCguy on Sep 11, 2011

    Brian Konsko – thank you for a great explanation. I’m in ATC school and a pre-req is Priv Pilot Ground; I have been scouring the internet for an explanation of the effect for my exam this week, and yours is by far the best I have found. Makes perfect sense to me now.

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