my235
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Aircraft Lateral Shimmying?

Sat Dec 03, 2011 1:17 am

On takeoff, just at the point of rotation I've often felt the aircraft shimmying laterally back and forth quite vigorously as the weight shifted to the wings. What would cause this?

I've felt this both on commercial (757, 747, 737 etc) aircraft and bugsmashers flown by yours truly.
 
BMI727
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Aircraft Lateral Shimmying?

Sat Dec 03, 2011 1:25 am

A crosswind maybe? On the ground it will cause the plane to weathervane into the wind but on takeoff the aircraft will be blown along with the wind as well. That might be the acceleration you feel.
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my235
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RE: Aircraft Lateral Shimmying?

Sat Dec 03, 2011 3:53 am

Quoting BMI727 (Reply 1):
A crosswind maybe

Hmm perhaps, but the feeling I've felt on more than one occasion is a back and forth lateral motion. Like the plane got up on it's main gear and became unstable as it's weight shifted to the wings from the gear. A crosswind would push you just in one direction (obviously)
 
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Starlionblue
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RE: Aircraft Lateral Shimmying?

Sat Dec 03, 2011 4:55 am

I think that is caused by crosswind gusts. Crosswinds aren't always even.
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tdscanuck
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RE: Aircraft Lateral Shimmying?

Sat Dec 03, 2011 5:07 am

Quoting my235 (Thread starter):

On takeoff, just at the point of rotation I've often felt the aircraft shimmying laterally back and forth quite vigorously as the weight shifted to the wings. What would cause this?

There's a shift in what's providing directional control at rotation. As long as the nosewheel is down it's providing some lateral control. As speed goes up the vertical fin/rudder do more but it's the sum of nosewheel friction, steering, and aerodynamic force on the vertical fin that's all adding up to the total lateral control force.

As soon as the nosewheel leaves the ground, that force goes away. All lateral control abruptly shifts to the vertical fin and the aircraft needs to adjust (weathervane and/or put in some rudder) to the new neutral point. If there's even a little crosswind or the nosewheel/rudder were a little off center at rotation the neutral point is at a different orientation and/or rudder setting, so you get a little lateral bobble until the transient dies down and everything gets stable again.

Tom.
 
musang
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RE: Aircraft Lateral Shimmying?

Sun Dec 04, 2011 4:03 pm

Quoting tdscanuck (Reply 4):
There's a shift in what's providing directional control at rotation. As long as the nosewheel is down it's providing some lateral control. As speed goes up the vertical fin/rudder do more but it's the sum of nosewheel friction, steering, and aerodynamic force on the vertical fin that's all adding up to the total lateral control force.

As soon as the nosewheel leaves the ground, that force goes away. All lateral control abruptly shifts to the vertical fin and the aircraft needs to adjust (weathervane and/or put in some rudder) to the new neutral point. If there's even a little crosswind or the nosewheel/rudder were a little off center at rotation the neutral point is at a different orientation and/or rudder setting, so you get a little lateral bobble until the transient dies down and everything gets stable again.

Thats all true if no crosswind control effort has been used during the take-off roll. If the into-wind aileron and downwind rudder inputs happened to be just right, the lateral shift is eliminated.

Regards - musang
 
roseflyer
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RE: Aircraft Lateral Shimmying?

Sun Dec 04, 2011 4:29 pm

I think you are feeling the rudder kicking back and forth as it gets more authority as the pilot keeps putting in inputs on takeoff roll while dealing with a crosswind. In strong crosswinds, pilots are often unable to maintain a smooth continuous position of the rudder but rather have to actively modulate rudder position with their pedal input.

One thing that does change when the airplane rotates is that there is no longer a pivot against the gear, so directional control of the rudder will feel a little different and the airplane will start crabbing and using aileron input for directional control. However, I think you are just noticing it more on rotation since the rudder inputs have sharper impacts at higher speed.

Quoting tdscanuck (Reply 4):

There's a shift in what's providing directional control at rotation. As long as the nosewheel is down it's providing some lateral control. As speed goes up the vertical fin/rudder do more but it's the sum of nosewheel friction, steering, and aerodynamic force on the vertical fin that's all adding up to the total lateral control force.

Analysis shows that the nose wheel loses effectiveness on steering between 60 and 80 knots. Above 100 knots, full deflection of the nosewheel will have no impact on steering capability since a combination of low forces on the tire and airplane momentum. The nose gear just skids with rudder input. That's why pilot training says stay off the tiller on takeoff since if a pilot needs to start using it, they likely are so out of control that they will end up off the runway regardless.
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Goldenshield
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RE: Aircraft Lateral Shimmying?

Sun Dec 04, 2011 6:51 pm

The same also happens in reverse as the aircraft transitions from flight (Aircraft CG) to ground (rotation around one, or both main landing gears, and then as the nosewheel touches down.
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bristolflyer
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RE: Aircraft Lateral Shimmying?

Sun Dec 04, 2011 8:07 pm

I've noticed this, particularly on Frontier's Airbusses. Feels like yaw that comes from lots of rudder movement. I'm sure it would feel a lot worse if you're at the back of the plane, furthest from the center of rotation.
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my235
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RE: Aircraft Lateral Shimmying?

Sun Dec 04, 2011 10:43 pm

Quoting RoseFlyer (Reply 6):
However, I think you are just noticing it more on rotation since the rudder inputs have sharper impacts at higher speed.

And once the aircraft gets up on it's mains, the rudder inputs actually cause the aircraft to move laterally with the flexibility of the tire rubber. Makes the utmost sense.

Thanks everyone for the information!

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