Hi Allan...
Thanks for the reply. Sorry if I ask one or 2 more questions / make one or 2 more statements:
...We were well aware of this accident and as stated before, this tail didn't fall off. It was sheered in half from surface flutter ...caused by balance issues
Sorry, not my reading. Flutter was a possible factor. The
rudder had possible balance issues, and split in half. The VS remained largely intact (see photos).
and recorded dives in excess 12,000 FPM ... This is far and away beyond the design limits of this airplane
I am not sure this is beyond the design limits of the RV-7? It is 125K in the vertical, something many RV pilots do on many days.
The fractures in the vertical spars occurred just above where the spars fastened to the fuselage. The fracture surfaces were consistent with faiure by overstress. There were no indications of progressive failure.
So the VS Spar(s) were determined the have failed due overstress, not flutter (I am not saying flutter did not occur, but if it did, it was the trigger, but the failure was due overstess).
This failure is in no way even related to the subject of the V/S mounting bracket or the modification we are offering.
OK - what exactly is the bracket offering that now enables you to flick / side-slip as you state?
When in a hard slip, rapid yaw movements, any aerobatic maneuver that ads stress to the V/S like snap rolls or worst case point rolls or knife edge flight this area is really stressed to the limits
Though to our knowledge, no tails have fallen off yet, we think it prudent to be sure this potential point of failure is eliminated
seems incompaible with
...We were well aware of this accident and as stated before, this tail didn't fall off
Maybe I am just unfamiliar with the "structural failure mode" of "falling off" as opposed to failing by overstress (in defined directions / types e.g. shear) or fatigue etc.?
Please do not get me wrong, anything that adds to the safety level of RVs, and in an area I do a lot of flying (aerobatics etc.) is great. The HS/VS attach and structure is clearly one concern. Here in the UK, the RV-8 was delayed aerobatic clearance for exactly such concerns until additonal testing was performed. All I am asking though is exactly what failure mode (and where) has been determined as most vulnerable that this product overcomes? Your text above repeatedly refers to "stress", and the TSB assessed this attachment area failed "due overstress". The 2 therefore seem directly related - but the mod does include the failure point?
Given your
this area is really stressed to the limits....accomplishes this beef-up in just a few minutes
can one ask, in figures or even just %, what the load increase of this "beef-up" is, in terms of the calculated ultimate load for the VS (as a whole)?
Have you approched Vans and asked them what calculations or testing was done on the (somewhat generic) VS attach area, and asked where the failures occurred, or were predicted to occur? The pictures to me suggest a fairly clear story - the VS was subject to a left/right load, and failed (as one might expect) where the structure was at it's weakest. Forward this seems to be the single spar at the rivet hole line (just above your mod area), and the aft spar at the attach bolt holes (where there is no doubling of plates etc.)