turning tendencies

[mayday1]

Question on gyroscopic precession..

I have a text that lists gyroscopic precession as a left turning tendency. I've never fully understood this force or tendency (just kinda memorized it). But I recently had an instructor inform me that it is actually a right turning tendency. He further showed me a demo - a spinning bicycle rim with spokes connected to a rod. Sure enough, spin the wheel and angle it upwards to simulate a takeoff angle of attack, and it yaws to the right..

Am I missing something - which is right, the text or the instructor? I tend to think it's a right turning tendency now, following this demo.

Also, does slipstream have an effect, or turning tendency in twins?

 

[pilotman2105]

And if you pitch down, it'll turn you to the left.

It's very noticable in tailwheel aircraft when you bring the nose down "midway" through takeoff roll.

 

[groundpointsix]

Your instructor was correct. For takeoff in a tricycle gear aircraft, when you raise the nose, the nose will want to yaw right. (Take a model of an airplane with a propeller and demonstrate this for yourself. As the move the nose up or down you are applying force to a rotating disc-- the prop arc. When a force is applied to a rotating object, the force will be recognized 90 degrees past that point. So as you raise or lower the nose, the airplane will go right or left respectively.) Upon takeoff in a tailwheel aircraft, the nose will come down as the tail comes up, thus causing you to yaw left. (Again, demonstrate with a model airplane, it's not something that easilly lends itself to being understood in words alone.) The reason you don't see your 172 yaw right at takeoff is because you have three other left turning tendencies that will counteract this right turning tendency.

As for your second question, yes. It's effect can be felt during both single and multi engine operation on a twin.

 

[mayday1]

makes sense now.. thanks for the responses... not sure why some text books place this under the "left turning tendency" headings.. just causes confusion.

thanks again.

 

[ROB-x38]

G'day mayday1

I think the reason the gyroscopic effect is often described in textbooks as a left-turning tendency is because these left turn effects in single engine aircraft are usually related to maintaining runway centreline during the takeoff roll.

So if you're only looking at the takeoff roll a tricycle arrangement has no gyroscopic effect whereas a tailwheel has a left-turning tendency as the tail is raised.

Also a good way of thinking about gyroscopic precession on the takeoff roll...

Picture yourself in a tailwheel aircraft. As you raise the tail (lower the nose) it is similar to pushing with you finger (applying a force) on the propeller disc at the 12 o'clock position (as viewed from the cockpit). Precession says that a force also acts at 90 degrees and ahead of the original force (which is the 3 o'clock position given the usual clockwise engine/prop rotation) which would yaw the aircraft to the left.

 

[minitour]

....Precession says that a force also acts at 90 degrees and ahead of the original force (which is the 3 o'clock position given the usual clockwise engine/prop rotation) which would yaw the aircraft to the left.

Great descriptions and explanations guys. That clears up a lot for me too. I'm trying to put together notes for the CFI gig (what to teach/different ways to teach stuff/etc).

Followup Question:
Why does precession act the way it does? Like, why does it act 90 degrees ahead of the original force? I kind of just memorized that it does that and don't know why. If anyone can answer that thanks! (If you just wanna PM me instead of posting here thats cool).

Okay....sorry for the mini-threadjack. Great question and great answers.

-mini

 

[groundpointsix]

See if this helps you out:

http://science.howstuffworks.com/gyroscope2.htm

 

[Mr. Cole]

I love that website. Just to give you a little more detailed explanation. The spinning disk of a gyroscope or a propeller has an angular momentum vector, which is a function of the angular velocity and is analogous to linear momentum. If you have an object moving with a velocity V, and hence a linear momentum, how do you get it to move faster or slower? You apply an acceleration. If you have an object that spins with an angular momentum L how do you get it to spin faster or slower? You apply an angular acceleration, also known as torque. Like acceleration is the rate of change of velocity, torque is the rate of change of angular momentum.

So if you have a torque, which is a vector, the angular momentum, or rate of spin of the disk, changes in the same direction as the torque vector. So assume you have your spinning clockwise prop. The angular momentum vector points out away from you to the front of the plane. If you apply a force at the top of the disk in the direction the disk is moving, the resulting torque vector will also point straight out, and the change in angular momentum due to that torque will result in a faster spinning disk, but the direction of the angular momentum is still the same. Notice that you applied the force in the direction of motion and the resulting torque was 90 degree away. That's the important piece.

Now apply a force to the bottom of the disk, but perpendicular to it and facing forward. This is the same type of force that would raise the nose when you lift off. This force, applied to the bottom of the prop, cause a nose up rotation. This can be represented by a torque vector pointing from left to right when viewed from the cockpit. Since the change in the angular momentum vector points in the direction of torque, it moves to the right as well. But in this case the magnitude of the angular momentum doesn't change, only the direction.

Dave

See if this helps you out:

http://science.howstuffworks.com/gyroscope2.htm

 

[minitour]

Thanks guys!

-mini

 

[Mr. Cole]

Minitour,

I looked at my post and I don't know if I really answered your question. Given the condition that a moment arm exists, a force will produce a torque 90 degrees out of phase. Since the direction of torque determines the direction in which the angular momentum will move, it too moves 90 degrees out of phase with the applied force. The reason I added the caveat about the moment arm is because a force doesn't produce a torque/moment if it acts through the CG. So a force applied to the edge of the spinning disk will cause precession, but not a force through the center.

Dave