Flop Tube Mods?

RV7Factory · #1 08-07-2006, 04:19 PM

My A&P neighbor was over the house yesterday while I was wrestling with the "black death" (I was sealing the tanks).

He took a look in the tanks and started asking questions about their configuration, specifically the flop-tubes and the associated mods. We talked about the trap door which we both understand the reasoning behind, but he then asked why the plans don't call for also closing off the 2" hole in the middle of the first interior rib. I responded... "I dunno"

So now I am puzzled... why don't the plans also want us to close off that hole in addition to the trap door on the lower hole?

Thanks.

Captain_John · #2 08-07-2006, 04:34 PM

I am thinking it is a less is more thing. Ya don't need to because ya don't need to.

Why would ya need to? I mean, it ain't broke...

CJ

RV7Factory · #3 08-07-2006, 04:49 PM

CJ, I guess I will just have to remember to only fly knife-edge with the tank I am feeding from on top.

Seriously, my understanding is that the trap door is there to keep fuel from flowing out of the inner bay during certain flight attitudes (if I am misinformed, please correct me). That's great, but fuel is just going to flow through the other big hole, so what does the trap door really do other than maybe slowing that flow down somewhat? I suspect you are right, the trap door is probably good enough for the type of aerobatics the RV is capable of, but I would still like to know.

Captain_John · #4 08-07-2006, 05:10 PM

The excess WILL flow through the other hole, but it will always stay at a level high enough to keep from becoming unported because the floppie is drawing from the bottom (or top as the case may be).

CJ

Kenneth · #5 08-07-2006, 09:18 PM

I covered the hole. It makes no sense that it should be left open if we're making a trap door doohickey to prevent fuel from draining outboard. The way I figure it, even when filling the tank, I doubt that the trap door hole will restrict flow to that bay without the center hole.

Besides, I saw it on at least one builder's site somewhere that Van's said to cover it.

--Ken

szicree · #6 08-07-2006, 11:04 PM

Having built my wings several years ago (that's right, years) I had to go back to my plans to refresh my memory on this one. It seems to me that covering that big center hole is gonna slow down the filling process considerably. Furthermore, I don't see the center hole as a factor unless you're doing extended knife edge flying. Having said that, looking at my drawings did raise one question that I hope someone can answer. Suppose I have plain vanilla tanks (no flopper) and I'm flying in a slip. Isn't all my fuel running out through the lower hole? If this is the case, should part of my landing list be to switch tanks to avoid this issue?

RV6_flyer · #7 08-07-2006, 11:26 PM

Quote:

Originally Posted by szicree

Having built my wings several years ago (that's right, years) I had to go back to my plans to refresh my memory on this one. It seems to me that covering that big center hole is gonna slow down the filling process considerably. Furthermore, I don't see the center hole as a factor unless you're doing extended knife edge flying. Having said that, looking at my drawings did raise one question that I hope someone can answer. Suppose I have plain vanilla tanks (no flopper) and I'm flying in a slip. Isn't all my fuel running out through the lower hole? If this is the case, should part of my landing list be to switch tanks to avoid this issue?

Landing Checklist should switch the you to the FULLEST tank.

I built my tanks over 15 years ago. The airplane is not capable of extended KNIFE edge flight. A 180 HP RV-4 is decending (my guess is 200 to 300 FPM) during full power KNIFE edge fight. (My guess if from seeing it in flight.)

The RV wing does not fly that well upside down. Typically INVERTED flight is not sustained so all me need to do is slow the fuel loss down so that we get tired of being upside down before the engine quits.

RV7Factory · #8 08-08-2006, 11:25 AM

Guys, thanks for indulging my curiosity.

I think I was over-thinking this (as I often do). I was imagining fuel rushing out the center hole in extreme wing low (knife-edge) attitudes, but as it has been pointed out, extended flight in those attitudes in an RV is probably impractical, so the issue of fuel departing the inner bay is somewhat moot.