REFLECTOR: Door strut review

Tue Oct 6 20:30:40 CDT 2009

This is just a story about different door strut configurations and issues
which may be of interest to builders who have not yet installed them, or
those who may not like the way the door works.  I’ve now had experience with
original version of the strut, my modified original version, and the new
version.

First I’ll mention a couple of things I really like about the new door and
hardware.  In fitting up the new door, I was pleasantly surprised and
impressed that its shape matched up very nicely to the fuselage – much
better than the original had.  Remember all that discussion years back about
heating the doors to get them to fit right? And the over-center spring on
the latch crank is much stiffer than the earlier version – good idea.

Those of you that built your plane some years ago with the elite doors know
that there were some issues with the door struts.  They fastened to the door
up at the back corner, tended to twist the door toward the back, and caused
distortion so the top rear of the door tended to stick up above the fuselage
surface. Disappointing after you had carefully fit the door.

My solution to that was to install a new hardpoint further forward in the
doorframe; just in front of the rear hinge.  This took away the twisting
force by putting the force between the hinges and perpendicular to the hinge
line (See photo 1). Of course this didn’t eliminate the force tending to
bend the door up in back. Noting that most of the bending was between the
top frame and the hinge line, I reinforced that area with about 5 plies of
BID, with a ply of fine BID over (photo 2).  This eliminated most of the
problem, and with a little filler on the fuselage, all was copasetic. The
behavior of the door was nice, and had a slight closing force near the end
of travel.

After that someone came up with the idea of a longer strut, connected
further down on the door, along with the claim that it eliminated the
distortion of the door, worked better, etc; and the design was adopted by
Velocity for subsequent kits. So naturally in replacing my door I chose the
new approach so I wouldn’t have to install a new hardpoint and reinforce the
hinge area – even though it wouldn’t match the other side.

After fitting up the new door and hinges to nicely fit the existing opening;
I installed the strut per the current instructions.  Yikes – the bottom
front corner was now coming to close position almost ¼ - 3/8” too far
forward.  And from about the 2’ open point it wants to SLAM shut.  If person
had their fingers over the edge and wasn’t sure to restrain the door you
could loose a finger. And now the downward force on the rear hinge, and the
resultant distortion, was about the same as in the original top corner
approach – except downward instead of upward.

So a caution to those still building: install the strut BEFORE doing the
final fitup of the door to the fuselage.

Had I reviewed the strut installation geometry, and the resultant force
components, before installing; I would have realized the likely result, and
done as I did with the original.  With the current strut (photo 3, yes, I
painted it the match the interior), the forward component of the force on
the door during the stroke is nearly 30 lbs, pushing forward, which is why
the door gets so twisted out of alignment toward the front.  The new
geometry gives the strut much more mechanical advantage on the door, which
could be a good thing for holding the door open, and allowing a lower strut
force rating; except when the door is open a good percentage of the force is
directed forward instead of upward. That same advantage requires much more
force to open or close the door, and makes it want to slam.  I’m sure one
can get used to that; if that’s a characteristic you like.

The door frame is very rigid longitudinally, as down the sides of the door;
but the force in the hinge area is transverse to the upper beam, and
although there are some carbon plies in there, the distortion is basically
between the upper beam and the hinge line.  So again I reinforced the hinge
area as before; which solved most of the problem of the distortion of the
hinge area.  Nonetheless, the 60 lb strut still has about 12-15 lb in excess
of that needed for my door.

So the unshod is that I definitely prefer my original modified strut and
orientation.  Putting a new hardpoint into the upper door frame to install
that strut is fairly easy – drill a couple of holes about ¾” dia just
forward of where you want to mount the bracket, make a fairly still mix of
epoxy and 50/50 mico and milled fiber and fill in the space behind where you
want to mount the bracket. Light and strong.

The replacement for the original door is readily available from
McMaster-Carr (part #9416K17, $12.85).

Before I consider a rework to my earlier version of strut; I’d like to try a
45-50 # strut in the new configuration.  So far I haven’t found any vendor
for a strut with 23” extended length with the lesser rating – which may be
why Velocity supplies the 60#.  Anyone know a source I should check? (e.i.;
if you happened to be interested enough to read this far:-)).

Al 

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