Regarding Erik's vibration problem.



During the initial flight testing of a friends IV the test pilot
reported a loud "raspberry" noise and vibration at speeds over 170
indicated. An investigation under the cowling showed that a portion of
the silicone air seal was prolapsing under pressure, causing the noise
and vibration.



Erik's vibration seems to be related to speed and not power so it
likely relates to a component that changes as a function of speed.  The
two areas I would look at first are engine compartment component
interference and control surface "flutter".



In the engine compartment, you must consider that the cowling changes
shape with speed as the differential pressure increases. This can cause
engine components to come into contact with the airframe. Also, the
engine mounts sag over time, which may be the reason the vibration has
only developed recently.  Have a look at the spinner to cowling
alignment. Has it sagged? It may be time to shim the mounts or replace
them. Also consider that the cooling air flowing through the engine has
drag and this drag results in a pressure differential between the top
and bottom of the engine.  The thing is that the area the pressure is
acting on is large, around a 1,000 square inches, so even a small
pressure drop of  1/4 psi will develop 250 pounds of down force on the
engine mounts. Add the prop "pulling" the engine with a few hundred
pounds force and you can see that the engine does not want to stay put.



The other thing to check is if the trim tabs could be fluttering. Does
the vibration go away if you drive the trim tabs "out of trim"?



Let us know what you find.



Regards

Brent