The plans are horribly vague in this department (big surprise), but they definitely imply a diagonal strip between the aft stiffener and 2nd rib, and also a spacer strip of some sort over the inspection hole. I had assumed that the diagonal strip would prevent the flop tube from going anywhere near the travel of the trap door, and I assumed the inspection hole strip would keep the flop tube from getting caught up on the screws which hold the plate on. Um, whatever...that sure doesn't seem to be the case.
I did some reading of the RV list archives, and the best (and really only) post I saw on this topic was where a guy described installing the flop tube in the clecoed-together tank, and doing the "Aresti dance." Aresti notation is the symbology used to describe routines in competition aerobatics...and the "Aresti dance" as it were is known as the physical visualization and rehearsal of one's routine with your feet still on earth. Anyway, that made perfect sense...the guy suggested rotating the tank through all possible angles and seeing if the flop tube tended to get caught up anywhere.
Well, I did just that. I clecoed the tank together (yet again) and loosely tightened the nut on the flop tube. I checked the regions where Van's seems to suggest putting anti hang-up strips, and it sure didn't seem to be much of a factor. I actually couldn't find a spot where the flop tube got hung up...at all. Here you can see the tube leaning into the corner where it will spend most of its time during level flight. No hang-up.
This is as close as it'll ever get to the trap door...a few inches away. No factor.
The guy who posted about this said his flop tube would go into the fuel flow hole in the 2nd rib...but I couldn't get mine to actually touch the hole. No factor.
Here's an inverted position, with the tube resting in the top inboard corner. While yes, it does sort of rest above the nutplates, by no means is it hung up. As soon as you rotate the tank a few degrees clockwise the tube flopped back down. Plus, I had to rotate the fitting to get the tube to lay flat like that...wouldn't really be possible with the fitting pointed due aft. No factor.
Here's a close-up on the extreme lower inboard corner, where the tube will spend most of its time. The fittings are too big to go under the nutplate which would even be suspect of causing a problem. No factor.
Ok, finally...here's one case where the tube had any sort of issue, and it's not even really an issue...with the plate off (duh), the tube would flop out of the inspection hole. No factor, since the plate will be on (duh). I'm starting to see the rationale behind the inspection hole retainer strip, though...if the flop tube rubs back and forth over the screws and nutplates, it could conceivably chafe or whatever. So yeah, ok, I'll put the retainer over the hole.
Here's the absolute only case I could come up with where any sort of hanging up was happening. It's when the tube was pretty much dead center of the bay on the bottom. The gap between fittings can rest on the stiffener. But even so, I couldn't manage to really get it jammed. It just rests there. Rotate the tank (and add the sloshing of fuel), and it flops right off. No factor.
Anyway, here's the retainer strip riveted to the inboard rib over the inspection hole. This should prevent chafing in this area.
Even with the strip riveted on, my arm can still fit through the half-holes in a worst case scenario...like if I had to get in there after the tank is on the wing, mounted to the fuse, etc.
