Subject: engine mount offset
From: ACRE NL
Date: 9/17/2002, 7:18 PM



I am not following you Mike. Why is what we are doing any different
that what Tracy did using the stock dynafocal mount that came
with the RV4? What would you like us to do different other than use
the conical mount?

Paul Lamar



Paul,

  Tracy didnt use a stock RV4 Dynafocal mount. He cut it up and then
rewelded the Dynafocal cups to the firewall/landing gear structure that was
left of the original mount. The result is a much modified mount with no
offset to which he attached a submount (also with no offset).

  Jerry indicates that the typical Tailwind mount is also built with no offset.

  This is not true of Vans mounts and I'd bet most other kit manufacturers
also provide mounts with an offset that goes the wrong way for a 13B with a
2.17 PSRU. I wouldnt know if all manufacturers use the same amount of
thrust offset.

  The new front cover will need to be manufactured in a way that removes
this wrong way offset to at least get us back to zero offset. I take back
what I said previously about shims being a reasonable solution on the RV.
Simply adding shims at the isolator will remove the thrust angle. But the
prop hub will no longer be centered on the firewall or in the cowl as
viewed from the top. Vans mounts are designed with both a thrust angle and
an offset to center the prop hub. If you remove just the angle(with shims)
and leave the offset, then the prop is no longer centered.

  Am I more clear now? If not I'll try to remember to bring in a drawing of
the stock RV-4 conical mount to illistrate the problem.

  Of course you could ignore all of this, build the airplane with thrust
offset the wrong way, and add a huge trim tab or vertical stab offset.  :-)

Mike Wills


Or use a 2.85:1 gear box.



What have I missed here? What is the difference in the PSRU's with different ratios?
Do they run in opposite directions?
Royce


Yes.

Paul Lamar
 
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