John Slade wrote:

Yikes! I would not want my engine to fall off. If it throws
a blade I think you might want larger bolts. Kind of hard to glide
with the missing weight of the engine.

A Cozy IV driver recently lost part of his exhaust through the prop and took
out a blade. Shaking was so bad that he couldn't see the canard. The plans
1/4 bolts held just fine. I'm sure Burt and Nat did the math before
recommending this bolt size. Did you, before suggesting it was too small?

Burt may have got it right on the bolts John but the aluminum angle
that the mounts are bolted to in the Vareze fatigue and crack :-)

That is one isolated instance. Why take chances to save ounces? Makes
no sense. Yes if you look at a bolt strictly from a tensile or shear
strength point of view they are way over kill. When I was a young
engineer I calculated that two 1/4 bolts would hold a 800 HP big
block Chevy in a Chaparral 2F and they did fine if it were not for
the mechanics torqeing them off. Colin Chapman of Lotus designer fame,
when he was alive, made the same mistake. It just does not pay off in 
terms of weight saved to use the "proper" size of small bolts on 
motor mounts. If you are bolting the skin on instead of riveting
then yes, by all means use the right size as you will save a bunch of
weight. 

BTW Look at the size of the bolts holding the 13B in your RX7.

Are the forces on the bolts greater because of the mounting method?

Now to answer this question yes they are. The bolt is in bending
somewhat as the radial rubber loads are centered somewhat along the shank of
the bolt. The bonded in steel spacer will take most of this load IF THE BOLT
IS TORQUED PROPERLY! These bolts are loaded more akin to the bolts used in
a conventional Lycoming dynofocal mount. Note, those are not 1/4 inch.

Paul Lamar
 
The Aircraft Rotary Engine Newsletter.              Powered by Linux.

http://home.earthlink.net/~rotaryeng/            http://www.linux.org