16 August 2006

Gyroscopic Precession

No, it's not a parade of Greek Sandwiches!
It is, however, the cause of some interesting things in our life, and is VERY important to helicopter pilots. It's also important to the answer of my earlier question about riding a bike.

Some years back, I went to a show put on by the Moody Bible Institute, relating modern day science to the Bible. It was an interesting show- two hours each over three nights, culminating with the host standing atop a 55 gallon drum holding a 2X4 in his hand....... the barrel was then energized with 50,000 volts of electricity, causing the board to sizzle, crack, then catch fire.
In another segment, the host walked to center stage with a large suitcase. He invited a member of the audience onstage with him, then asked him to move the suitcase to the rear of the stage......... a move which required turning with the suitcase. The audience member almost lost his balance as the suitcase would not cooperate with his attempts....... we later found out the suitcase contained a very large gyroscope which had been spun up to 25,000 r.p.m.! Moving it in a straight line was easy, but the gyro didn't want to be turned.

Airplane pilots will be aware of the phenomenon....... along with torque, it causes "P factors" on takeoff.
Did you go out and get on your bike and try my experiment? Were you surprised?
Once again, Jason had the correct answer. If you pull the right handlebar toward you, the bike will lean to, and move left.
Why?
Because of gyroscopic precession!

The theory is, when pressure is applied to a rotating body, that pressure becomes fully realized 90 degrees later in the direction of rotation.
Visualize with me:
Think of the wheels turning beneath you on the bike. If you pull the right handlebar back toward you, you are applying pressure to the right side of the front wheel at it's rearmost point, and to the left side at its most forward point. That pressure becomes most apparent 90 degrees later in the direction of rotation, which results in the wheel leaning to the left, causing the bike to "bank" to the left even though it was a right turn which was seemingly commanded. We never conciously think of what we are doing when we ride a bike, we do this naturally. When I ask students the question I asked you, most students will answer incorrectly. I always send them to find a bicycle!

In order to make the rotor of the helicopter lean in a certain direction, the force necessary to get that result must be applied 90 degrees beforehand. The control systems on a helicopter are rigged to do just that. Next time you are near a helicopter, take a look at the control system......
the control tubes that change the angle of attack of the rotors are mounted ahead of the rotors.

Physics.
Pretty neat stuff, huh?

3 comments:

John said...

Hey Greybeard,

I was taught that P-factor in an airplane is related to asymetric thrust due to the descending prop blade having a greater angle of attack during a climb attitude (and to a lesser extent, the ascending blade having a greater angle of attack during descent).

Gyroscopic precession definitely comes into play in tailwheel aircraft during takeoff when the tail is lifted off the ground.

Greybeard said...

John:
Thanks!
Interesting detailed discussion on the subject
HERE.
As an old friend once said, "that's more than I really want to know about Penguins!"
If you can make clear sense of it, come back and educate me.

Jamie said...

It's always nice to find a fellow blogger I can actually give my url to!