Mike Wills wrote:

The loads on the mounts are higher so it will probably be
necessary to go up a size or two on the rubber mounts.


Paul Lamar

Paul,

  For the semi-ignorant among us could you please explain this comment? Why
are the loads higher and how much higher?

  I'm still playing with ideas for my mount for the RV 4. The appeal of
this idea is that it gets the rubber away from the heat source.
Implementation would be similar to Tracy's with a much abbreviated main
mount with gearleg sockets bolted to the firewall and this structure bolted
to the engine with the bushings between the two and located as close to the
corners of the main mount as possible.

Mike Wills

This is best illustrated with a load diagram from the ACRE NL web site.
This is for a typical composite airplane in this case a Lancair.
My KIS loads are similar. The rubber bushings would be placed at
points K, F, C & I. These loads are 1715# while the loads at points
A & B total 1800 pounds or only 900 pounds each. The rubber
bushings used with a conventional mount normally go at points A & B 
where the loads are lower. 

By the way it is quite practical to have the 
nose landing gear welded to this mount. This means the load from 
the landing gear goes directly into the engine first and then 
(much reduced) into the fuselage. The same idea is used on the
Questair Venture and all rear engine race cars built in the last
thirty years. 

Having said all that I don't recommend this idea for an RV4 because
Van has designed it for point contact loads on the firewall. If
you started cutting out the longerons behind the firewall to 
accomodate the two to three inch diameter rubber bushings
required you would seriously weaken the structure. Composite
airplanes have a simpler firewall to fuselage attachment and it
is much easier to find room for these large rubber bushings
behind the firewall. this idea is therefore very well suited
to composite airplanes only.
 
Paul Lamar
 
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