The craziness continues!
My continuing flirtation with Aviation:
I've purchased a 90% complete Mitchell U-2 Flying Wing!
(Which in Aviation math usually means, 90% is left to be done.)
Inspecting the Spar Caps
|First we removed a section of the fabric covering - look at the mess we found! At first we attempted to inspect the spar cap construction through the vent holes with the new boroscope. It didn't go too well - I just couldn't see the scarf joint well enough without better light.|
|After cleaning up the mess we discussed the pros and cons of completely re-doing the fabric for the center section of the wing. In the end we decided to just bite the bullet and replace the fabric if we found the spar caps were built correctly.|
|My center-section skeleton with the fabric removed. See the un-repaired hole in the top-center of the frame? We will use that hole along with the vent holes to inspect the spar caps. And then we will have to make a small hole in the D-tube at the left-center of the frame to inspect the other scarf joints. These additional holes allowed us to get better light so the scarf joints could be clearly seen in the boroscope.|
|Finished repair to the inspection hole we had to make in the D-tube.|
Good news!!! We found all four of the spar cap laminated sections and scarf joints to be in perfect condition!!!
Postmortem: Ok, it's time for me to stop being so critical of the the original builder so much! Some things have come to my attention in the last few weeks: 1) The bonding agents recommended by Don Mitchell are no longer available and as noted before, there are some bonding agents that shouldn't be frozen. The builder of this plane may never have intended for it to see freezing temperature until I brought it to Colorado this year. I have it in a hanger but it is not heated. 2) Reading through a long series of threads from the U-2 Yahoo group it would appear that several of the things the builder did (such as beefing up the spar web thickness) were attempts to deal in advance with problems which have been brought to light by other owners.
Rebuild or Replace the Skids
|After a couple of sub-freezing days out at the hanger we were finally able to get the pilot's pod removed from the wing and transported to the warmth of the garage. Here Norman is working on removing the shell from the supporting metal framework.|
|After we were able to get
the skids loose and into better light, we could see the builder had laminated the skids according to the plans.
But he did modify the shape and size of the skids, in addition the bolt
holes for connecting to the supporting metal framework were not all
located at the center of the skid width and all of the .040 steel doublers were
And after close inspection of the crack we have come to the tentative conclusion that the failure is probably the result of "adhesive starvation". There is very little evidence that the laminations were sufficiently glued along the areas that delaminated.
|First we totally separated
the laminations which had started to separate and then we epoxied them
back into place. Here we have one lamination epoxied and clamped and
in the meanwhile Norm is spreading epoxy on the other skid.
At this point we had discovered that the skids were probably made from a Cedar or Redwood like wood even though the drawings clearly call for aircraft quality Spruce or Fir.
|The skid reconstruction is
finished through rough sanding. The sanding seemed to confirm
the builder used the wrong wood for the skids and did not follow the call-out on
the drawings. He also totally supported his seat mechanism on the
skids even though a note on the drawings indicates the seat back should be bolted to
the spar top.
Even though we took some time to repair and sand these skids for the practice, I have decided that I will not use them! We will build new skids using Pultruded Carbon Fiber Rods!