REFLECTOR: Door strut review

Tue Oct 6 21:08:02 CDT 2009

Al, 

I too was not too keen on such beautiful aircraft having the aft potion of the doors bending under the load & heat. My approach was slightly different than yours: 

I purchased some gas springs that had half the force of the factory originals (mine are 30 pounds). Then I ended up working on the attach points to stiffen things up on the door, and find a place that would work for mounting the new springs. 

The spring source I used is : http://www.spdhardware.com/catalog/list.asp?id={A818FF64-E51A-422F-A59A-4E2E52A3BDA2}   and the springs I used are the 5300-30's. 

I made a quick look over the site and didn't see a 23" extension spring. I am sure you measured attach point to attach point, but the closest they have is a 20" one on their website. 
You can always call them, but just make an inquirey and write down the part number if they have what you need. It is much cheaper to order off of the website. I think they have a minimum dollar amount on phone orders. 

The good news, My doors do not bend up at the back and they don't twist the door. The not so good news is that the door opens about 4" shorter height than the factory ones. Works for me since I'm short :) 

I also tried the "new method" while I was playing aroud and really did not like the front-door-twist, or the slamming at the lower end. Same results that you had. 

Kurt Winker

-----Original Message-----

From: Al Gietzen <ALVentures at cox
.net>
To: reflector <reflector at tvbf.org>
Sent: Tue, Oct 6, 2009 7:30 pm
Subject: REFLECTOR: Door strut review

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 perpend
icular 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 origina
l 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.  Nonet
heless, 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 farJ).

Al 

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