FAI Tech Transfer - Motor Tube Alignment

FAI Tech Transfer - Motor Tube Alignment

Post by nal040700-Mcki » Fri, 27 Oct 1995 04:00:00



FAI Technology Transfer - Motor Tube Alignment.

Getting the motor tube correctly aligned on a boost glider or rocket
glider is extremely important.  I am going to pass along how I do
it for my S8e Radio Controlled Rocket Gliders.

Aligning a 2.75" tube to get the correct downthrust and have the
thrust line exactly down the centerline of the boom is very
difficult.  I have tried making alignment tools from old motors and
from dowels, but found them unsatisfactory.  What I have found to work
quite well is to start with  a motor tube that is 18" long but is
scored at the point where you want to trim it to the correct final
length.

I start by figuring the height of the center of the tube at a length
of 18" for the correct downthrust (2-4 degrees, usually).  I then take
a piece of wire (coat hanger will do) and bend it into an "L".  The
long leg of the L should be equal to the correct centerline height of
the 18" tube.  I then glue the short leg of the L into the nozzle of
an expended motor.

Now, score the 18" motor tube at the point where you want to trim it
to final length.  Tack-glue the motor tube on top of and at the
leading edge of the motor tube pylon (or slip it into the rear of
the pod for a StingRay 3).  

Slip the expended motor with the wire*** out of it into the end
of the motor tube.  (You should see where this is headed by now
<G>).  Position the motor tube so the end of the wire is resting on
top of the boom.  Move it back and forth until, when sighting
straight down the boom from the rear, the wire is vertical.  It
should appear to be right on the center line of the boom and on the
centerline of the bottom of the motor tube.  It should also be
exactly centered on the image of the rear of the pylon.  This is
so easy to do you can put a little 5 minute epoxy in the joint
between the motor tube and the pylon when you have verified that
everything will align properly and then align it and hold it there
while the epoxy sets.  

After you re-enforce the joint between the motor tube and the
pylon, slip an old motor down the tube so it is underneath the
score mark.  Trim the motor tube to the correct length.

The motor tube is now perfectly aligned...unless your pylon was
misaligned vertically.  Be sure to level the wings on a flat
surface and use a 90 deg triangle to mount the pylon on top of the
boom. (Or some other good method).

And that is why I am looking for an inexpensive source of thirty
18" BT-50 tubes for a small run (maybe 20) of S8e kits.  Know where
I can get them?

Kevin McKiou