Positioning the main gear strut

Now that the torsional layup was completed, the strut was ready to be aligned in place. The idea here is to position the strut such that the axles' axes are 14.5" forward of the firewall fwd face (at FS = 110.5) and at WL = -21.6 (drawings say -22, but remember LPC#45 had us move the LMGA holes up 0.4" so this would shift the axle WL up as well). Once the strut is positioned in place, it is Bondo'd on to temporary jigs which allows us to do the layups, using the jigs as templates with the exact hole locations.

Note that the majority of the pictures were taken AFTER I had already Bondo'd the strut to the tabs.

I started out by making the four jig pucks out of 1/4" ply (drawing on page 9-4 in the plans). I inserted them through the LMGA tubes as shown here:

To align the strut, I had to come up with a way to measure all six degrees of freedom:

1. Strut pitch
2. Strut roll
3. Strut yaw
4. Longitudinal translation
5. Horizontal translation
6. Vertical translation

I started out by making sure my fuselage was level on my workbench. I then placed a straight 2x4 on top of the strut and clamped a level to it. This allowed me to measure the strut's roll.

I then tied masonry line from the left axle point to the right axle point. I measured the center point of the line and hung a plumb bob from it. This allowed me to measure two things: 1) the horizontal translation (left or right movement - to make sure the strut's centerline matched up with the fuselage's centerline), and 2) pitching the strut forward in order to get the axle line 14.5" ahead of the firewall.


The longitudinal translation (forward or aft movement) is dictated by centering the strut in the middle of the LMGA tubes - so that's more or less fixed.

I tilted the strut forward until I measured 14.5" from the fwd face of the firewall. I also made sure it was centered with the fuselage's centerline.

To measure the strut's yaw, I hung two more plumb bobs (symmetrically about the fuselage centerline) as close to each axle tip. This allowed me to place a straight edge along the firewall and measure the horizontal distance out to the plumb bob. I yawed the strut until both sides had equal lengths.

As a second way of verifying that the strut was not yawed, I measured the distance from the left axle tip to the centerpoint of the bottom of the F22 bulkhead - I repeated this from the right axle tip and verified that both measurements were the same - indicating a non-yawed strut.

Another shot showing how the centerline plum bob was used to position the strut on the fuselage centerline. Also, notice the level clamped to the 2x4 making sure the strut roll orientation was level.

The vertical translation was adjusted by shimming the strut from the LMGA tubes. I wasn't too worried about the height - as long as the strut cleared the lower longerons by 0.2" I was happy (I'll fine tune the WL = -21.6 location of the axles when I mount them to the strut).

Once all six degrees of freedom (roll, pitch, yaw, horizontal, vertical and longitudinal) orientations were aligned, I Bondo'd the strut to the 1/4" plywood tabs.

I also painted some Bondo on the inside of the tabs...

More Bondo!

After allowing everything to dry, it was time to remove the strut from the fuselage. I was afraid/paranoid that the Bondo would not be able to withstand the forces of yanking the strut out, so I devised a hoist to pull from the LMGA tubes (rather than from the strut which would transfer all the loads to the bondo/tabs).

Here is a close up showing how the rope was holding the LMGA tubes...

I removed the bolts and presto! The strut came out without a sweat.. tabs still in place... bondo still in tact!

The next step will be to remove the LMGA tubes, sand everything nice and smooth, and proceed with the layups that will make up the final gear tabs!