Neumann's Barbell Effect

Gentlemen

I am "pleased" to inform you that ..... I fell victim to the Neumann's Barbell Effect. Actually I was aware of it (learned in Physics, the "polar moment of inertia" I = mr² ), but always thought it was of mere theoretical concern and its effect in our practical CL stunt world would be negligible. Recently I retrofitted a new 60 russian engine to my model. This engine is lighter and, to keep the same balance point without adding weight, I built a 1-3/4" longer nose, the rest of the airplane absolutely unchanged, so its good flying characteristics would be maintained. But - guess what - the airplane became lazy and would not turn corners as it did before. Pushing the CG backwards to restore original agility was of limited success, because this reduced the stability and the airplane would oscilate at every corner. So I had to take the hardest decision: to rebuild the nose again, back to the original length.

Moral of the story:

1) As far as the nose is concerned, heavy and short is better than long and light.

2) Theory sometimes works in practice.

3) Getting hints and ideas at www.clstunt.com is not a bad idea.

jmh

P.S.- Leonard, I would like to have your opínion on another subjet. I read in an european forum that the boost port of a Schneurle engine can be blocked off with an epoxy compound similar to JB Weld. I am concerned about a piece of it breaking off and finding its way to the cylinder and doing a lot of damage. How about it, do you think it's safe?

>...Leonard, I would like to have your opínion on another
>subjet. I read in a European forum that the boost
>port of a Schneurle engine can be blocked off with
>an epoxy compound similar to JB Weld. I am concerned
>about a piece of it breaking off and finding its
>way to the cylinder and doing a lot of damage.
>How about it, do you think it's safe?

If it is "like" JB Weld, it should be OK. If it is "almost like" JB Weld, I don't know. You need to make sure the stuff will stick to the aluminum case and that the heat of the engine will not affect it. After that, it is OK. When Art Adamisin started blocking ports (He used a different compound) he drilled a hole in the side of the port to "lock" the compound in. After a while he discovered it wasn't going anywhere anyway, and quit doing this. If you make sure the case is clean, it will stick. But even if it doesn't stick down, the sleeve locks it into the port so it can't slide out and do any damage.

And, as for a "piece breaking off", well, in the first 6 to 10 or 12 hours you can still work it. After that it gets harder and harder until after about 24 hours, it is not going anywhere. It is not coming loose, it is not breaking apart, it is staying put. The only problem would be if the "similar" stuff wasn't heat resistant. Normal epoxy can be removed with heat. So, if that were the case, it could be a problem.

jmh --

>I fell victim
>to the Neumann's Barbell Effect.
>Actually I was aware of
>it (learned in Physics, the
>"polar moment of inertia"
>I = mr² ), but
>always thought it was of
>mere theoretical concern and its
>effect in our practical CL
>stunt world would be negligible.

As a cohort of mine once said, "everything in the world is hooked together." Swing weights of golf clubs are the same math problem as moments of inertia in airplanes.

Sounds like you figgered it out.

Hi Leonard:

Thank you very much for your reply and all information you give us at your website. I will start experiments with a Thunder Tiger 25GP because I am very familiar with its behavior, so I will be able to better evaluate the effects of blocking the boost port. I will contact you again if I am lucky enough to get any meaningful results.

jmh