Vibration

Thread view:

Enable EMail Alerts

Start New Thread

Thread rating:

Jim - 01 Apr 2004 03:42 GMT

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 this Message

Ted Brindle - 01 Apr 2004 04:17 GMT

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.

> Hi there,
>
[quoted text clipped - 19 lines]
>
>

Reply to this Message

Dan Thomas - 02 Apr 2004 03:21 GMT

> > 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?

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 this Message

Gord Schindler - 01 Apr 2004 13:47 GMT

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

> Hi there,
>
[quoted text clipped - 17 lines]
>
> Comments? Thanks...Jim

Reply to this Message

Model Forum / Radio Controlled / Air Models / April 2004

Vibration