# inertial coupling



## mikewint (Mar 18, 2010)

does anyone know of a video or video simulation of an aircraft experiencing inertial coupling?
i have found several of a "dutch roll" but have found nothing except verbal descriptions of inertial coupling.
i know it starts with a roll which causes the tail to pitch up followed by gyroscopic precession which yaws the nose. thus the aircraft is moving in all three axis. i just can't picture it occurring and i'm not a pilot
any help greatly appreciated
mike


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## FLYBOYJ (Mar 18, 2010)

Dryden Online Education - Introduction to Flight Testing - Aileron Roll

How's this?


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## mikewint (Mar 19, 2010)

thanks FJB i did find that site. i understand the words and i do know what yaw, pitch, and roll are but i cannot visualize an airplane doing all of them, uncontrollably through the sky.
i had a tough time with "dutch roll" until i foung a site with a computer sim of an aircraft experiencing this.
i guess i need pictures
mike


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## Maximowitz (Mar 19, 2010)

I assume your question pertains to the He 162?

As explained by Graham Boak on the Luftwaffe Experten Message Board:

_ "This is called inertia roll coupling. It was unknown at the time, and the 162 has become the classic case that lead to its investigation and description, though not in time to save the F-100 prototype from a similar fate. When an aircraft has a major angle between its roll axis and its inertial axis (a line joining the major weights of the aircraft) then the inertia forces will dominate the aerodynamic ones, leading to dramatic departures from controlled flight and (in this case) aircraft structural failure under loads it was never designed to take.

Think of a dumbell. You can roll it along the ground quite happily. You can twirl it in your hand, as long as the weights are directly opposite each other at 90 degrees to the rolling axis. But place it at 45 degrees to your wrist and try to twirl it as before - it goes flying out of control."_


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## mikewint (Mar 19, 2010)

max, did not know you had posted here also, i just looked at the plywood post.
i saw your pics there, what is the source? i guess i'm a visual person. i know the words, i know what they mean but i still cannot visualize what the aircraft would be doing in the air and what kind of path its center of mass would follow


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## IvanFP (Mar 19, 2010)

Thanks Maximowitz.

That explanation got through to me. I was thinking earlier it was a side effect of adverse yaw which would be an aerodynamic force, but your description of an inertial effect makes it clear.

Thanks.
- Ivan.


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## mikewint (Mar 20, 2010)

it is indeed an excellent analogy. but as a non-pilot i still cannot picture an actual aircraft rolling, pitching and yawing through the sky. what kind of path would it follow?
the concept seem difficult even for experts to comprehend, i.e. the bell x-1 which crashed and killed its test pilot, and the modified x-1 that almost killed Yeager, the x-3 stilleto and even the super sabre all suffered from the same problem.
if anyone has or can find a video or sim please let me know
mike


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## Ivan1GFP (Mar 28, 2010)

Look at an aluminum soda can with the top facing you.
Imagine a very heavy lead weight glued to the lower side of the top facing you.
Imagine a very heavy lead weight glued to the upper side of the BOTTOM of the can facing away from you.
(Or instead of imagining, go out and do it)
When you roll the can fast enough, it will hop all over the place because it wants to roll about its center of form, but the the lead weights make it want to roll through the axis of its center of gravity.

That is how I am seeing it.
- Ivan.


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## mikewint (Mar 29, 2010)

Ivan, could you please explain the term "center of form" my understanding is that any object, if free to do so, will rotate about its center of mass. a rolling can is not free to pitch
i do understand "moment of inertia" for rotating objects. i can see that short stubby wings make it easier for the plane to roll while the length/engine/tail masses make it harder to pitch thus different inertias for roll and pitch motions.
i'm just not a 3-D person i really need to see these three motions.
my most sincere thanks for trying.


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## awild (Feb 18, 2013)

In Ivan's description, I believe "center of form" refers to the symmetry of the soda can. Imagine a line through the top of the center of the can extending out through the center of the bottom of the can. This would be the axis of roll.

Now, imagine the two weights that he describes as glued to the insides of either end. He doesn't explicitly mention any symmetry or symmetric placement other than at each end. The slightest incoherence between the line through the center of form, and line through of center of gravity will induce an inertial torque when the can gets rolling fast enough. 

In fact, if you have only a soda can that's been opened and its contents removed, the non-symmetric opening in the can will result in an incoherence between the center of form and center of gravity, and this will cause the desired effect once the can is rolled fast enough.

'Regards, Alan


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