Vibration

Hi there,
I bench ran in my DH2 Beaver in my shop as the motor had not been broken
in yet.
I put the 8 foot wingspan bird on its floats on the table after sticking
my "ground fork' into the table to fit right in front of the stabilizer to
prevent the bird from shooting foreward while I ran in the motor.
I am getting vibration deflections of possibly 1/4-3/8 inches in my
wing struts at a certain RPM (wide open). Wing struts are about 22 inches
long. The prop has been balanced. Do you think the vibration could have
something to do with the fact that the floats and plane were sitting on a
hard table and the vibration would dissapear or not be as bad if the plane
were airborne?
Anyways...I ran the plane for about an hour and a half total varying the
RPM and everything worked fine. I checked the plane out and I have no cracks
in anything due to vibration. BTW... the motor is a new .90 Super Tiger 2
stroke with a pitts style muffler.
Comments? Thanks...Jim
Reply to
Jim
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My guess is that in the water or flying with weight on the wings/struts that you will not notice the vibration. Reaction from the hard surface was adding to the vibration.
Ted b.
Jim wrote:
Reply to
Ted Brindle
It sounds as though the struts are vibrating only at certain frequencies(sympathetic vibration?)...probably helped along by sitting on a hard surface. I would not worry too much as long as the prop is balanced and nothing else is twitching. You will rarely fly that machine at full throttle anyway. My Beaver has a 120FS in it, weighs 19 lbs and flies at about 1/3 throttle. Takes full power to get off though. A word of warning...don't try and force the Beaver off the water prematurely or it will drop a wing. I take off with about 10' of flap down. I let it run until I see light under the floats and then commence the climb. This is not unique to my Beaver. I've seen it happen on quite a few of them. Great 'plane though. Gord Schindler MAAC6694
Reply to
Gord Schindler
I would suggest that the prop slipstream is disturbing the struts. There is always some vibration, even with a perfectly balanced prop, and it comes from the firing pulses of the engine. This vibration could set the wings and struts to oscillating, and the airflow might do the rest by causing a bit of strut flutter. I'd fool with a couple of small trim tabs on the struts to see if some small deflection might stop the resonance. If the struts are close enough to the prop, the pulsating airflow off the props blades can cause vibration of any airframe parts in the propwash. This is the biggest source of noise inside full-scale single-engined airplanes, as the prop blast drums the windshield. The vibration will likely, sooner or later, cause strut or attachment failure. Vibration tends to be much more noticeable in flight. That's my experience, anyway, as a pilot and aircraft maintenance engineer.
Dan
Reply to
Dan Thomas

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