My Impact tail broke off yesterday at a pattern meet. I had installed the rear bulkhead and also had a piece of foam mounted horizontally in the tail from behind the canopy to the rear bulkhead. It broke right at the front edge of the rear bulkhead.
(Post #661)
Hello BillB.
It appears to me from the way you are describing your fuselage reinforcement, that you may have created a significant stress riser (stress concentration) just at the rear former, where the foam plate ends. In my opinion, (as a structural/mechanical engineer) it is very important that the plate/shear web is continued all the way back to the sternpost, or at least to the stab tube. Think of the fuselage as a beam in bending, where the fuselage sides are the flanges, and the foam plate is the shear web. If the shear web was to suddenly stop, then you would have a very high stress concentration at this point, and local buckling would be likely to occur.
Eric Henderson wrote in post "676:
I noted that in mine the logitudinal former is glued to the former in front of the stab. Thus making continuous force transference to the stab tubes etc. Could or would this make the difference? I don't know.
I just looked at CARF's Impact Instruction Manual Update, and this is the way they (now) recommend that the fuselages are reinforced, and is also the way new kits are being supplied. The longitudinal former behind the bulkhead prevents a sudden stress concentration. (Assuming that the foam plate is in place)
It would be interesting to categorize the failures uccurred so far, i.e.
- No modification at all
- Modified with what I call stress risers
- Modified with continuous plate/shear web
- Any other modification
Any reports of failures with the recommended fix? (I have not read all 691 posts in this thread, sorry.)
Just my observations and opinion, I could be wrong.
Magne