AF330

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!

awax

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

Page 4-27 to be precise.

AF330

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....

Twin Wasp

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.

Cubdriver

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.

Cubdriver

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.

Cubdriver

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.

Cubdriver

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.

JamesNoBrakes

Get a bicycle wheel.

AF330

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