Gyroscopic Precession

C

clestudentpilot

Well-Known Member
Hey guys, question for you that I have wondered and the other thread about p-factor got me wondering is, what is gyroscopic precession and how does it work? I have read all the book definitions, I have talked to flight instructors, including my old flight instructor, along with family members who are flight instructors...but I still don't really understand it. The diagrams on websites still don't really help me out. Haha, I guess looking back on it I should be glad I wasn't asked about it on my checkride a few years ago.

Can someone try to explain it in a very simple manner, because like I said, I have yet to find an explanation that makes sense to me, and I can't imagine it is all that difficult of a concept, but nothing has ever clicked with me as to what the force really is.

Thanks in advance for the replies.
 
Thanks tgrayson, I guess I should have searched. I remember that thread, but gave up on it after I still didn't understand it, but it now does make some more sense reading the whole thing, and the explanation someone used with a ball attached to a string. Thanks for providing the link even though I should have not been so lazy and should have searched myself.

I still question though, I kind of get the whole 90 degree force involved, but why is it a left-hand turning tendency? I understand the force is 90 degrees off, but it still doesn't make sense why this tendency is left, and not right, or up, or down, why it is only left
 
clestudentpilot said:
Thanks tgrayson, I guess I should have searched. I remember that thread, but gave up on it after I still didn't understand it, but it now does make some more sense reading the whole thing, and the explanation someone used with a ball attached to a string. Thanks for providing the link even though I should have not been so lazy and should have searched myself.

I still question though, I kind of get the whole 90 degree force involved, but why is it a left-hand turning tendency? I understand the force is 90 degrees off, but it still doesn't make sense why this tendency is left, and not right, or up, or down, why it is only left
Click to expand...

While precession is part of what makes a left turning tendency, it's a small one. Much more important is torque and p-factor. but to answer your question, here's how I think about it. Imagine a taildragger. During the takeoff roll, the tail will come up before the plane lifts off the ground. How does that motion translate to the propeller? At first, the prop is at a laid back angle, so that the top of the prop arc is "behind" the bottom of the prop arc. As the tail lifts, it will move so that top part moves forward until the prop arc is perpendicular to the ground. Essentially, you're putting a force on the top of the prop (to move it from tilted to straight up and down). Since the prop rotates clockwise (from the cockpit), that force will then precess 90 clockwise, so that it's acting on the right side of the prop arc. A force pushing forward on the right side of the aircraft's centerline will cause the aircraft to turn/yaw left (just like failing the left engine in a multi...since the right engine is still producing power, you'll yaw left). If it helps, use your hands to visualize this, by using your flat left hand as the prop and your right index finger as the "force".

Have you actually messed with any gyros to feel precession yourself? It would be best if you could get a bike tire with a couple hand holds, but if you cant do that, I routinely use a wheel off of my roller blades. Really, any wheel that you can spin up and still hold the axle of will work. Simply take the wheel off, hold it by the axle (stick a pen through the middle of the hole if you don't have anything else), spin it up by rolling it on a table, and then move it around a little. You'll figure it out soon enough by doing that.
 
I second the bicycle demonstration. I used a tire off one of my kids' bikes (it was an old one), to make one of these that I use for tail wheel pilots.
 
I will yield this knowledge to Rick Stowell in his Emergency Maneuver Training book and slightly tweak it to how I like to explain it.

Forget what you think you know, forget about taildraggers and conventional gear, forget it all except: when a force is applied to a rotating disk it acts 90 degrees from the place it was applied. We can build off of that.

The propeller is, for all intensive purposes, a rotating disk. When we pitch up, down, or apply rudder left or right we put a force on that disk.

Let's make a picture out of it, go get a stick, bat, broom, golf club, or any other stick type of object. Yes, I am serious, go get one before you continue reading. Now with your 'stick' sit in the chair and hold it out in front of you. Put your right hand on the top and left hand on the bottom, holding it vertically.

What do we know about the propeller when we are sitting in the aircraft? We know it rotates to the right? If you have to, spin the stick to the right to imagine that and then reposition it vertically the way you started. Let us apply some forces and see what happens:

Pitch Up: With your right hand, pull back on the stick and with your left push forward. Now rotate the stick 90 degrees to the right (horizontal), your right hand will now be on the right side of your body, pulled back, with your left hand to your left and pushed forward. The result in a pitch up is a right yawing tendency, NOT a left yawing tendency.

Weird, why do they call it a left turning tendency when it is a yaw and to the right? Introduce the tail dragger! A tail dragger, as it begins its takeoff roll will raise the tail (pitch down) which we will now discuss.

Pitch Down: Go back to the vertical starting point with your stick and this time push your right hand forward and pull your left hand in (your right hand is still on the top). Now rotate 90 degrees clockwise again and notice your right hand on the right side is pushed forward and your left hand on your left side is pulled back. The result, a yaw to the left.


What does this mean to the aerobatic pilot? Well each time they push forward they must apply some right rudder and each time they pitch back they must apply some left rudder. Learning to do this accurately is what many don't realize, people think an aerobatic guy just does a lot of yanking and banking with a really cool airplane. Not the case, and it even goes further than this.


Let's look at the application of right or left rudder. For this you need to turn your stick horizontal and grab it with both hands again, right on the right and left on the left. It is easiest if your knuckles face the ceiling, palms facing the floor.

Right Rudder: Pull back on your right hand and push forward on your left to demonstrate applying right rudder. Now move this force 90 degrees clockwise and notice your right hand, on the bottom, is pulled back. Your left hand, on the top, is pushed forward. The result, a pitch down moment when right rudder is applied.

Left Rudder: Return to horizontal and pull back with your left hand, push forward with your right. Rotate 90 degrees clockwise again and notice your left hand, on top, is pulled back. Your right hand, on the bottom, is pushed forward. Result, a pitch up moment when right rudder is applied.


An aerobatic pilot will have to think about all of these possibilities. It is what makes the 8 point roll so incredible when you realize the precision that is involved. Their big engines exhibit a very noticeable yaw and pitch with application of pitch or yaw described above.


For the GA pilot, in a trainer aircraft this information can basically be ignored. A full forward or full aft pitch in your typical 172 will cause the ball to move about half to a full ball movement from one side or another. It is rare you will ever pitch that extreme. Your typical pitch will move less than a half a ball.

As you move into higher performance aircraft, if you make quicker pitch actions then it might be worth while to consider learning how to use the rudder with pitch. Just like adverse yaw while rolling though, it depends on how intense the pitch is. A smooth application of pitch in almost any non aerobatic aircraft will not need rudder.

Moral of the story? Apply pitch smoothly and unless you intend on being an aerobatic guy you can ignore gyroscopic precession. IMO it is an overly stressed term to private pilots that has little, if any, effect in the world they operate.


xcaliber: Torque is a left rolling, not yawing tendency. P-factor is a left yaw, but not nearly as significant as slip stream. Here is the list I use if it helps:

Torque = left roll (right yaw...adverse yaw)
P-factor = small left yaw
Slip stream = largest left yaw
Gyro Prec = In taildraggers a significant left yaw when the tail is raised. For trainers and most other GA aircraft almost not noticeable and depends on the force applied discussed above.
 
clestudentpilot said:
why is it a left-hand turning tendency?
Click to expand...

It isn't. Only in a taildragger when you lift the tail is it a left-turning tendency. Raising the nose in a trike is actually a right turning tendency, but it's overshadowed by other left turning tendencies, so you don't see it.

And if you yaw the airplane left, there is a pitch up; yaw it right, there is a pitch down.
 
OK guys, thanks a lot for the replies, this is now making much more sense to me hearing it in more plain English. I never did understand many "book" definitions because I think many people who write that stuff act as though everyone has a fairly thorough understanding of this, and it was like I was reading a foreign language. The other 3 turning tendencies always made perfect sense, but I never understood this one. Also, no one had ever told me that there was a difference with this one between tricycles and conventionals. I guess I should have realized that if I thought about it, it just never clicked. Anyway, thanks for the responses!
 
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