Strut-to-wing attach update

Several months ago I posted photos of the replacement strut update I made on my AirBike. In that post, Scott Peterman noticed an installation arrangement that would cause a bending moment on the bolt holding the assembly to the wing. The strut centerline was not in line with the spar brackets but was offset. Taking his observation and concern seriously, I redesigned that installation.

Replacing the block that formed the "universal joint" with rod end bearings brought the strut centerline in line with the spar brackets. Due to differences in the rod end bearing size and the bolt holding the assembly to the wing, some additional hardware was needed.

The rod end bearing's I.D. was and An-5 while the bolt holding the assembly to the wing was an AN-4. Also, the rod end bearing's thickness was less than the aluminum block initially used. To fill the remaining space, on my lathe, I machined a spacer from aluminum rod stock having  a spherical radius on one end, and after facing the other end, a chamfered edge. A 3/8" hole was then drilled through its center. I did not drill out the strut bracket to 3/8" and replace the AN-4 bolt with an AN-5 bolt because that would have resulted in insufficient edge margin on the bracket.

A bushing made from 3/8" O.D. and 1/4" I.D. steel tubing was cut to a length equal to the combined length of the rod end bearing's width and the aluminum spacer.

During assembly, the AN-4 bolt and flat washer is inserted into one strut bracket, through the bushing, spacer and rod end bearing, then if needed, additional washers then through the other strut bracket. Then the protruding bolt could be secured with another flat washer and a Nylock nut.

Notice that the first photo below shows the attachment in the position Scott observed to cause the bending moment. Other photos are of the update from that condition. Notice in the last two photos that the diagonal brace wire has not been taughtened, once is has been, it clears the strut.

Update on the AirBike has been made.

Thanks for your sharp eyes Scott.

Paul Fiebich
a.ka. AirBike Ace

Paul,
Thanks for the kind words.
The new struts sure look good. Have you noticed any performance gains with the new struts as compared to the old set with custom fairings on them?

Scott Peterman

Hi Paul,

I would like to see that eyeball shank thread be screwed a bit more into the aluminum bushing here. My concern is the aluminum bushing thread strenght. What do you think?

Scott, I really don't notice any difference in performance, perhaps with specific instruments and a wind tunnel test that would be possible. I am just happy with the clean shape and them being adjustable.

Jacob, the thread engagement is 2 x the bolt diameter. I would think that would be enough. I could provide more thread engagement by unbolting the barrel and moving it further out along the strut. That would then mean two bolts instead of three holding the barrel to the strut. That would give me about 3/4" more of thread engagement. I may do that during my next Annual Condition Inspection.

Thanks for your observation.

Paul Fiebich

Looks good, Paul.

But I would be a bit concerned that you are using a (ball joint) thread in tension. The threads are a potential source of cracking when loaded this way.

The fitment as drawn on my Eros is an aluminium block to take out the change of angles. In this way, the two bolts that hold the wing on are loaded in double shear - a much happier mode.

Just sayin'. . .

Love your pics.

Bruce