Too much play in rudder linkage?

[astroboy]

The rudder linkage in the Beech 18 I'm finishing up has a lot of play in it. The rudder themselves are about 6 inches high and 2 inches wide; at the aft end, they wiggle about .045-.050" in either direction before encountering resistance. I suspect a lot of the problem is the homemade bellcrank, which is now buried under the stab. The model will not be a slow flyer; the wing loading will be 21-22 oz./sq./ ft. Do I have a problem?

Thanks,
Jeff

[Ollie]

I would fly it slowly at first and gradually increase speed while doing close in fly bys to see if flutter develops. If the rudders don't flutter at full throttle and a shallow dive you may be able to get away with it. If flutter develops slow down. Then you have to do major surgery and replace the bell crank.

[astroboy]

Thanks, Ollie. I thought of a couple of tests that might help me determine the likelihood of flutter developing, before I commit the model to the air:

1) Have my wife drive me down the street at 20-30 mph, while I hold the model(minus the wing) out the window, aligned as best I could with the direction of travel.

2) The rudders are more or less aligned, horizontally at least, with the props. What about a full -throttle static test? I have a large, though not very powerful box fan I could supplement the effect with.

I've never seen flutter before. If you think these tests would be meaningful, would the symptoms of flutter be readily apparent?

Thanks,
Jeff

[Ollie]

With a wing loading of about 22 ounces per square foot, the stall speed will be in the 20 to 30 MPH range. I would try to get the auto up closer to 40 MPH if you can. Flutter will be both audible and visable if you are doing a close fly by. Just pull the nose up slightly and throttle back a bit at the first indication of flutter. However, don't over control or you may stall it. If you do it carefully and methodically, increasing the airspeed slightly with each fly by, you can probably avoid loosing a rudder to flutter. Make sure the rudder hinges are strong and properly secured before each flight test. A strong, metal geared servo on rudder isn't a bad idea either.

[astroboy]

Update:

Thanks again, 0llie. After posting this morning, I got into a little more diagnostic frame of mind(instead of my usual Chicken Little "The plane is falling, the plane if fallling."). Anyway, a closer look at the linkage revealed an easy adjustment that reduced the play to .030-.035" in either direction. So I feel a little better about it now. I'll still take your advice about test flights, though, and try public-street-wind-tunnel tomorrow.

Jeff

[Ollie]

Here is a Rube Goldberg rig that might get rid of most of the slop without major surgery. You could use a piece of Spyder Wire (no stretch fishing line) or kevlar kite string to pull the trailing edges of the rudders toward each other under tension. I bet that would remove almost all of the slop.

[astroboy]

Just thought I'd let you know, Ollie, that we did the "wind tunnel" test today. The large top center section of the fuse had been removed so I could get a better grip on the middle of the model. I think the resulting lack of a streamlined surface was made it difficult to hold the model rock steady, so the tail was moving around a bit, but at 40 mph, I could detect little or no movement in the rudders. I found out later that there was a bit of tailwind, but even if the effective airspeed was only 35 mph, I think the test could still be pronounced a success.

Jeff