Pushrod Selection
-----------------

29 Oct 00    MD

My fuselages have the following:

Winching fuse:  0.025" wire for rudder,  0.040" carbon for elevator
Electric fuse:  0.025" wire for rudder,  0.020" wire   for elevator


As I see it, the only advantage of the carbon pushrod is elimination 
of trim changes with temperature.  This is really only important 
to fliers who do maximum-performance winch launches on the edge of stall, 
and do not want their winching pitch trim to change ever.  For sport 
and electric fliers this is not a concern --- a few clicks of the 
elevator trim will take care of it.

I've originally spec'ed a carbon pushrod for the rudder as well, 
but now I think it is not so important.  Rudder trim changes don't 
affect winching significantly, since you're always flying the rudder 
when going up on the line.  And any crosswind has a much bigger 
effect anyway.  The advantage of the wire pushrods is that they are 
tougher, easier to install and replace, and cheaper.  The weights are:

0.020 x 36 wire:  1.5 g  +  0.7 g housing  =  2.2 g
0.040 x 36 CF  :  1.2 g  +  1.0 g housing  =  2.2 g

It's a wash.  Using 0.030 carbon for the elevator would give a weight benefit, 
but this is just too flimsy for this glider.  The airloads on it are tiny, 
but the loads from bumping the stab platform during handling and assembly 
can be significant.


Stretching CST teflon housings
------------------------------
Tie each end to a small rod, and *slowly* stretch until it "hardens" 
and won't stretch any more (like a rubber band).  It should increase 
in length almost 5x, with only a little bit of springback.  The weight
drops fivefold.

The ends need to be tied to something with at least three hitches, 
since the stuff is so slippery it will squirm through your fingers 
or an inadequate knot.


Mounting the stretched teflon housings
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When installing the housings, put the largest K&S piano wire which 
will fit into them.  I used 0.032" wire.  This will minimize waviness
and give minimal friction when the smaller actual pushrods are put in.

Tack the housings into position with small beads of 5 minute epoxy.
Put balsa wedges at each end to hold the housing at the correct location.

Secure them permanently all along their length with 3/8" wide strips 
of 0.75 oz glass cut on a bias.  Brush on a stripe of *highly* thickened
epoxy over the housing and underlying balsa or kevlar, and put down the glass.
Pack the glass around the housing and Kevlar with a piece of 1/32 bass 
or hard balsa.  If the glass doesn't stay down, add more thickener 
to the batch.

4" long glass strips are convenient to handle.  The bias cut prevents 
fraying and makes them conform around the housing more easily. 

Over most of the boom where the housings are together and straight, 
I just used 1/2" wide thin Mylar tape to secure them.  The tape doesn't
work in spots where the housing is curved.  Its adhesive will fatigue
and let go as it's worked by the slightly wiggling housing.