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RV-3A Fuel Tank Dip Stick

V

VCMaine

Active Member
I weighed my recently acquired 1992-era RV-3A and decided that I needed to be at not more than 8 gallons to make the 1050 pound limit for fun flying. So I calibrated a 1/4 inch wood dowel as a dip stick for my nominal 24 gallon fuselage tank one gallon at a time. It was in doing it that I discovered the tubular inset or can build into the fuel tank behind the panel obviously intended to accomodate a "long" instrument in the lower middle of the panel. I don't have a long instrument in the panel now so had no idea it was there. Wonder if its a common feature of 3 and 3A fuel tanks or is it unique to this bird?

Anyway, I drained the tank, inserted the stick diagonally from the fill port to the bottom of the tank, and added one gallon at a time. Due to the can, the 1 gallon increment spacings are not what you might expect in the affected range. Recorded the measurements and made a backup stick as well.

Its very resurring now to know before and after each flight exactly how much fuel I have and used, for weight and fuel burn calculations.
 
I checked.............. and there currently is no patent on a RV-3A fuel tank dip stick.
 
3A fuel tank

I have an old (sn 785) 3A kit and the fuel tank, which is sitting on my workbench, has no tubular cavity in the face of it, only the opening for the mechanical, float-type fuel gauge. I like your idea of the calibrated dip stick.

Can you tell me where the fuel tank vent exits the fuselage and which direction it is pointing? fore or aft? Thanks
 
Plans, vent fuselage fuel tank

I have a set of plans from Vans for the fuselage fuel tank. Plans show a right angle AN fitting, not sure the number, on the top of the forward tank bulkhead and you attach a piece of tubing and exhaust the tank out the bottom of the aircraft. I belive the vent is close to the filler neck.

Could email a picture of the plans if you need it or you could buy the sheet from Vans.

Mark
 
fuel tank vent

My tank has the right angle AN fitting at the top you described. My question is where does the vent exit the bottom of the fuselage and does it point fore or aft? It is apparently important that you maintain positive pressure through the vent into the tank and I am assuming that you point the exit fitting so it points forward into the airstream but I haven't found anyone who can confirm that for me.
 
RV-3A Fuel Tank

I have an RV-3A with the fuselage fuel tank. It's a welded aluminum tank that sits on the supporting straps, not the original sheet metal riveted and sloshed integral fuel tank like the original RV-3 had. It has no cut-out provision for long instruments like you are describing, only an opening in the rear for a fuel gauge. I got the tank through Van's. The vent is in the top of the front end of the tank, and vents to the bottom of the fuselage through the .040 floorboard. Our engine is an 0-320 H2AD from a Cessna 172, and had no fuel pump. We thought it might work with just gravity feed, but we had problems with not getting enough fuel pressure with only the gravity head of pressure. We tried bending the vent at 90 deg. forward, so it would add some ram air pressure to the fuel tank, but it did no good. We still had problems with getting enough fuel flow, especially in climb. We added an electric Facet pump, and later added a mechanical pump as well, and now it is fine. We also made
a calibrated dipstick like you have, and it's nice, since the mechanical fuel gauge on the back of the tank is not accurate when the plane is sitting on the ground. On the ground, the gauge reads considerably lower than the amount of fuel you actually have. In level flight, the gauge is more accurate.
 
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Fuselage Tank Welding

Alex,

Great post regarding 3A fuel tank. I'm fabricating a tank and was wondering what method of welding Vans used to weld the aluminum panels. Mig, Tig or was it brazed? Any help appreciated.

Mark
 
Great POST!!

Keep a backup fuel stick!!!! Rule number 1...... don't trust the gauge! A fuel flow device is the best thing I can think of for keeping track of what you burn but if you get a leak.... a fuel flow only tells you what you have burned... not what is left. Go flying and lose a gas cap and you'll quickly realize how important a real number is that you can trust.
Best
Brian Wallis
 
RV-3 fuselage fuel tank

mmcdonald said:
Alex,

Great post regarding 3A fuel tank. I'm fabricating a tank and was wondering what method of welding Vans used to weld the aluminum panels. Mig, Tig or was it brazed? Any help appreciated.

Mark
Click to expand...

It's welded - very beautifully done by whomever Van was using for the work back in the early 1990s when we got it. Not sure if you could tell if it was done with MIG or TIG, but definitely not brazed. I imagine brazing can be good for some applications, but I'm not sure I'd want to have a brazed tank with me in the fuselage like that. I'll even admit to being a bit nervous about the fuselage tank, even the welded one. It's simplicity is the best thing about it. It made building the wings back in the days before the quick build wing kits much easier. The routing of the fuel line from the fuselage tank is very simple, and perhaps has less possible points of failure compared to bringing the lines in from the wings. Van's still has the RV-3 fuselage tank listed in their catalogue for $1050 .
 
RV3 fuselage tank fuel vent

To answer the original post's question: place the tank vent on the floorboard forward, a few inches aft from the firewall edge. Make a smooth 90deg angle bend, and aim the vent forward. Vertical distance: a few inches below the edge of the lower cowl, so it is directly in the slip-stream. Have it exit center fuselage, between the exhaust stacks. (all this from the plans, an my 650hr RV3)

- Steven
 
Fuel tank saddles and vents..

I will post pics later of my tank system, the vent ALWAYS point forward, you want 2 to 4 lbs PSI to pressurize the tank. Crucial when your below 10 gals and your at unusual attitudes....

Jeff Maynard
T-18,RV3A, HR II ( under construction)
 
Fus fuel tank vent

Thanks for the pic Jeff, it is helpful.

Is the outside end of the vent just the 90 degree bent tubing facing forward? Or is there a crimp or something on the end of the tube to get the proper 2 to 4 psi positive pressure?

Gordon
 
its just bent forward. don't crimp it... I would solder fine mesh over it to keep bugs out while flying.. and cover it with modified pitot cover when your parked or in hangar, make sure you have REMOVE BEFORE FLIGHT flag on it....
 
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