James Robinson wrote in
:

Bertie the Bunyip wrote:

Big John wrote:

Bertie

I did some service testing at Edwards on the T-28A. Things found
there. Bird had a bad 2P vibration in cockpit. Was caused by each of
the big wide blades of prop blowing big chunk of air back on the
original canopy which was much taller that the one ending up on
bird. Felt like sitting inside a base drum.

OK, you're going to have to tell me what 2P means...

These are terms typically used in diagnosing engine vibration
problems.

1P vibrations are those which are at the same frequency of the engine
RPM. 2P vibrations are those which are at twice the frequency of the
engine RPM. 1/2P vibrations are those which are at 1/2 the RPM of the
engine.

If you have 1P vibrations, you look at things like prop balance or
that you have all the right size pistons in the engine.

With 2P vibrations, you look at things like the effect of two bladed
props, or anything that might be attached to the engine with 1:2
gearing.


OK, got it. Thanks

Bertie