Dual Fuel Tank Flapper Doors

[Daniel S.]

Are two flappers better than one? Okay guys, I'm finally working on the fuel tank guts. I have one standard fuel pick up and one flop tube pick up. We are supposed to the flapper door on the larger aft hole. My choice is to put flappers in both tanks.... But how many folks put flappers on the aft hole AND the hole in the middle of the lightening hole?

 

[Sparky]

I have flop tubes in both tanks, and permanently blocked off the hole in the middle of the 2nd rib lightening hole on each tank. So I have a single flapper door on the aft holes only. I suppose you could use a flapper door in place of blocking the lightening hole, but I couldn't think of a good reason to do so. Also, a flapper door on the lightening hole could possibly interfere with the flop tube.

Edit: Added picture:

 

[David Z]

I blocked off that hole too. My biggest influence was this build page:
http://www.mykitlog.com/users/display_log.php?user=jbtrellis&project=220&category=2961&log=52420&row=23

 

[tyvaf01]

shouldn`t the center hole of the rib left open to allow air evacuating the inner bay?The fuel only entering this bay as fast as air is able to leave!When you block the big center hole,there are just the small tooling holes left for venting.

 

[Daniel S.]

Awesome info... Thanks guys! I'll close up that hole. While I'm at it, I may just add an additional small hole near the top of that rib to provide a little more venting action in that bay.

 

[Sparky]

shouldn`t the center hole of the rib left open to allow air evacuating the inner bay?The fuel only entering this bay as fast as air is able to leave!When you block the big center hole,there are just the small tooling holes left for venting.

There are smaller vent holes at the top of the rib (you must be careful to keep these free of proseal when sealing the tank). They are sufficient to allow air to escape from the inner bay when filling the tank. Also, with the dihedral of the wings, if you leave the lightening hole open it will be under the fuel level anyway before the tanks are half full.

 

[tyvaf01]

actually I am more concerned about fuel entering the inner bay not fast enough during flight,not during refueling!

 

[Sparky]

actually I am more concerned about fuel entering the inner bay not fast enough during flight,not during refueling!

That's even less of an issue. During flight you're looking at about 10-20 gph max. During refueling you're looking at more like 10-20 gpm.

Edit: I just noticed you are in Austria, so I guess it would be lph and lpm

 

[tyvaf01]

Paul you are right,but that was not the issue I am concerned about!One excample,let?s say you fly with quarter full tanks prolonged wings not level
(aeros,slip)fuel will escape the inner bay even with the flapper door installed.After the wings are leveled again the fuel should be able entering
the inner bay as quick as possible.To obtain this you need a venting hole equal sized as the hole on the bottom af the rib,that`s why VAN`s punched
them in!

 

[Mel]

It really bothers me to see so many people modifying fuel systems without complete engineering analysis.

This is not an area to use TLAR engineering!

 

[Sparky]

It really bothers me to see so many people modifying fuel systems without complete engineering analysis.

This is not an area to use TLAR engineering!

Mel,

I agree, completely. Blocking the lightening hole and adding the flapper door in the 2nd rib are recommended by Van's when using the flop tube fuel pickup.