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Old 03-04-2012, 09:01 AM   #21
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Re: Dressing & balancing props

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Yes Mr. Wheel Whacker I appreciated that video, I tried to call you so you could post the vid (didn't want to steal your thunder). You musta been on a date!!!!!

So what all should a guy be inspecting for vibration damage? The firewall/fuse joint is the first major structure to be effected, I imagine. On my gas planes, the VS/HS/Elevator/Rudder exhibit the most visable vibs. I imagine the control horn/push rod interface shows a lot of wear too.

A hundred (or two) RPM diffenece near ground idle (static, sitting on the ground) sure appears to make a difference in what shakes. Does having the plane in the air change what is effected by vibration?

I was told to never run a gas plane static without the airfoil attached, the extra mass helps disipate the vibration.
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Old 03-04-2012, 03:52 PM   #22
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Re: Dressing & balancing props

What tree, I have run planes without the wings attached many times while setting up and testing the motor and systems. I can tell you that if the airframe cannot tolerate engine vibration with the wing removed you have a serious structural problem or there is the likelyhood a larger engine than recommended may be attached and the structure may have been borderline to begin with.

Along with adding mass away from the fuselage centerline, adding anything, such as a wing, more significantly alters the resonance of the airframe. As with any structure composed of different elements, each or a combination will be affected by different frequencies and this is why you will notice things like the tail feathers vibrating at certain discreet engine RPM's, (the source). In fact, in helis a common design criteria is to have different structural elements resonate at different frequencies to help dampen each other out or stay away from that single resonant frequency that will quickly multiply and lead to failure.

The famous Tacoma Narrows bridge is a perfect example of this and happens slow enough to see the energy build. By the way, if you watch the vid you will notice the solid side guardrails. With a perpendicular wind they created low pressure cells behind them that acted just like any other airfoil.

Having certain parts of the airframe vibrate is also not a problem if the structure is designed properly. Balsa or any other wood of adequate quality and properties is an excellent absorber of energy, making it a natural damper, but if compromised by engine oil deposits it can lose any or all of those, along with becoming structurally unsound. Other than incorrect assembly or damage during service, fuel/oil contamination will do more to weaken an airframe over time.
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