Gyroscopic effects
 

Good morning,

I was reading a book which tells you about basics of flight. So no Airbus/Boeing but only propeller jets. They talked about gyroscopic effects...

What is it exactly?*
Why does it make the plane go in another direction? A force? What's the 90° thing?*

Could anyone explain?*
Well, I have got a small idea but not sure at all...

Thanks a lot!

Read this, it's all in there: http://www.faa.gov/regulations_polic...H-8083-25A.pdf

Page 4-27 to be precise.

Thanks,

I still don't get it...
What is happening? Ok, your propeller is rotating, you are flying level. You then pitch up.
Air will push the prop upwards. But what happens next? Why do we say that we have changed the propeller's axe of rotation?

I don't get it....

Because you have. This is not referenced to you in the airplane but to an outside frame of reference. Say you are in a plane going down the runway. We'll say the axis of the prop's rotation is parallel to the ground. Then pull the nose up. While to you in the airplane the prop disc is in the same place and the same angle, to someone standing on the ground it changed.

Asking why the force is applied 90 degrees off from the direction of movement is sort of like asking why gravity pulls things together.

I guess you'e talking about the gyroscopic turning tendency for single engine airplanes among other things. The basic physics has it a force placed on a gyroscopic disc such as that formed by a rotating prop, causes a reaction to occur at a point 90 deg. behind the original point in the opposing direction. "Behind" here means going backwards in the circle of rotation.

So, you are sitting in an airplane behind a prop that spins clockwise from where you sit. You place a force on the top of the prop disc somehow- we'll get to the how later- and a force is felt 90 degrees to the left side in the opposite direction (towards you). This reaction causes the prop disc and hence the airplane, to want to turn to the left. It's called left-turning tendency.

So, how did you apply that original force at the top of the prop disc? The usual and most important way is through a sudden raising of the tail on a tailwheel airplane using the stick. There are other ways, but that is the most critical one. It can lead to loss of control during takeoff roll in larger airplanes with big engines.

http://i284.photobucket.com/albums/l...ps3304c558.jpg

There's some nifty mathematics that accompany this subject in math and engineering courses, but pilots do not need to know it to that depth.

No, it will push the prop disc into a right turn and the airplane turns right.

The whole airplane turns- the prop, the prop driveshaft, as well as the axis of rotation of the prop which is along the same line as the propeller driveshaft.

A motorcycle can serve also serve as a nice thought analogy for this subject. You are riding a motorcycle at say, 25 mph and your left turn is coming up. When you are ready to do the turn you do not turn the handle bars to the left- you turn them to the right a little and lean to the left to start the turn. But that's the wrong direction to turn the handle bars isn't it? You were not wanting to turn right at all. The reason you need to turn right a little bit is, that with some kinetic energy built up in the front wheel as it turns at 25 mph, gyroscopic forces are stronger than some other forces (tire to ground friction) and you need to get the wheel disc oriented into a lean in order to turn the bike. You must manage the gyroscopic forces acting on the front wheel.

By turning the handle bars slightly to the right, you are placing a force at the front side of the front wheel pointing to your right. Per the physics of gyroscopes, a reaction is found at a location 90 degrees behind the spin direction of the wheel and in the opposite direction. So, the reaction serves to tilt the wheel to your left at the top side, which is great because the bike can now be leaned to the left, and the ground arc or tire path necessary to go left at the corner is now available.

Get a bicycle wheel.

Thanks a lot!!!

Here is what I understand:

So momentum = Mass*Velocity.
Momentum is always constant.*
Angular momentum = Mass*Velocity of a rotating object.
It is always equal.

When we pitch up, we create a torque so we tend to make the prop rotate ---> we change the velocity of the prop so the angular momentum. The prop tries to keep a constant velocity (mass won't change) and creates a yaw turn...

Am I right now?*

Thanks

The effect is called gyroscopic precession.

Here is a good demonstration video:

http://youtu.be/wt_nYn_XUvE

This is good too:

http://youtu.be/zbdrqpXb-fY

Thanks but am I still right?

JNB's reference to the bicycle wheel is a good suggestion. Take a front wheel from a ten-speed and hold it by the axle. Have someone spin it, and then turn the whole assembly this way or that using the axle. You'll immediately feel the gyroscopic effects.

Some children's museums or science museums have hands-on displays of gyroscopic effects using a spinning weight that you can manipulate.

Pick up a children's toy gyroscope and play with it a little. Spinning, the gyroscope is rigid in space, which is the property we use in mechanical gyro instruments (attitude). When the gyro is displaced, its motion is predictable, and it's that motion that's used to establish rate rate of turn. Gyroscopic effects are applicable to any spinning object, the most obvious of which is a propeller.

I'm not a physicist, so I won't attempt to explain the principle in that matter. When you pitch up, as in a climb, you are changing the PLANE OF ROTATION of the prop. Assuming the prop rpm is constant, the force applied will be shifted 90 degrees in the direction of prop rotation. That shift in force is felt as a left turning tendency. Get a bicycle wheel, spin it fast, then sit on an office swivel chair with your feet up and holding the wheel in front as a propeller. Move the wheel through different planes of motion. The chair will turn.

No, you sound mixed up. Let me try and sort this out.

Yep.

Not necessarily, unless someone told you to assume that for their particular teaching example. It's possible in the case of props and gyros as teaching tools, but actually not true in real life most of the time. Angular momentum is not always conserved in real life.

But let's back up a bit and start simple first. For now, we can assume the speed of a bicycle wheel or a prop is constant, and there is no friction or other acceleration acting on it.

Yes.

Not if it changes. But like I said, we can assume it is constant in the case of a bicycle wheel or prop disc for now as a learning assumption.

Not sure what you mean "make the prop rotate". Assuming we are using props as our example, here's how it goes.

1. A prop is fully spun up to begin with on a running engine. Turning at say 2400 rpm. This is constant speed, no change at any time for that.
2. A torque is applied in the form of a pitch input when the pilot raises the tail of the airplane using their stick. The torque is "seen" or felt on the prop disc at the top and the bottom. Actually it is applied though the engine drive shaft, but it does not matter how it is applied.
3. The airplane (prop disc etc.) responds with a left yaw. This is your gyroscopic precession.

Does this help?

You're reaching beyond where I think you should for now. I would not try and reach too far here by getting into conservation of momentum or anything that deep right now. All you need to know is how gyroscopic precession acts- a prop acts a like a big gyroscope and when it is already spun up to a normal running speed and torque is then applied to it via pitch input, it will react in another plane than the one in which the pitch torque is applied. Simple stuff- specifically, the plane of reaction is always 90 degrees to the upstream side on the disc or gyro where the original torque was applied. In this case the prop-gyro turns pitch torque into yaw torque. Are we ok now? I want to make sure we have that much before we get into it any deeper.

If you insist on going to a (verbal) mathematical level with this topic, see section 19.10.2. on the webpage below. He has a nice vector diagram showing how the various forces add up. Vectors offer an easy way to visualize spatial relationships involving force interactions.

*19**The Laws of Motion