Brief Description: Addressing Fin Stress Concentration
The next issue I wanted to address is a stress concentration and poor section property issue I have in the vertical fin main spar where the rudder cables feed through. The issue is that I drilled the cable holes per the plans Page 17 at 1-3/8 offset from the centerline. When I mounted my rudder on the fin and rotated it, the control horn sweep appears to want the cable holes to be much closer together. Additionally, if you look at the dimension between the holes on the Mustang 2 (a very similar design) you will note that the holes are spaced at 1-1/8 from centerline. As a result, I moved the holes to the Mustang 2 location, however, I already had holes drilled at the 1-3/8 location, that meant that I now had a hole with a notch in it. This is a stress concentration and is a bad thing to have whre the spar bending is at a maximum. I did not want to unzip the fin pull out the spar and rebuild it, Instead I decided to reinforce the area. The plan is to mount a 1/8 inch doubler to the aft 164.25 bulkhead then bolt the fin to the doubler and bulkhead when mounting. For proper fit, this also requires that a shim be fabricated and riveted to the fin spar. In the end this interface adds a bit of weight to the tail, but is also fairly robust. I am not taking the weigh addition lightly (pun intended) Aircraft weight creeps up in unnoticable steps. I will need to be vigilent in the future.
Original Fin Spar With Oblong Hole in Critical Area
Spar with Shim Installed Showing Bulkhead Location
Showing Doubler Plate that Rivets to Bulkhead and Bolts to Spar
