Tailwind engine mounts have no offset in theory.  In fact, most of
them are built in space without a fixture of any kind and you can
imagine that many of them must warped out of alignment, one way or
the other. Difficult to see once the engine is installed.   Yet first
flight reports usually say that the airplane flies normally.   I
personally have flown in three TWs that had motor mounts built
without a fixture and all three felt about the same to me and had no
trim issues.  Also, it is quite possible to kick an engine mount one
way or the other with spacers at the firewall . I saw a Cassutt racer
with 3/4" spacers on one side, none on the other.  Apparently helped
turning left.   The Tailwind does off set the leading edge of the
vert. stab 1/2".  I moved this to the opposite side for the rotary.
Jerry

Jerry,

  Your experience seems to fit with Tracy's (no offset required). However,
this doesnt resolve the issue for most builders in attempting to use Paul's
proposed front housing/mount since I beleive most kit manufacturers provide
mounts with thrust offset that goes the wrong way (as Vans does). On an RV,
shimming the mount at the firewall would be a bad option as that would
cause a misalignment of the landing gear. Arent Tailwind gear sockets also
integral to the engine mount? Probably could add shims at the engine end of
the mount which would probably work if the mount were a conical design
rather than Dynafocal.

Mike Wills

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.

Paul Lamar
 
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