Hatz Biplane Association

How I designed my header tank. About 7 to 8 gallons

Michael
N838MM

What if the aux tank is plumbed inline with the wing tank and therefore has no filler neck? You'd only need to vent the wing tank. In flight, when the wing fuel gauge reads empty you know how much fuel/time you have left in the aux tank. You'd still have a lowered head pressure when the wing tank was empty, but I don't think that would be an issue considering the fuselage tank in a cub doesn't create a head pressure problem. I'd planned on a fuselage tank of 5 gal. I figured that I'd use this extra 5 gallons as my FAA required reserve. In other words, I'd never (or rarely) use it. Fly till the wing is empty then land. That tank would add an extra 35 or so pounds to the empty weight.

If you did use it, how are you going to fill it? If the aux tank isn't vented, it's going to take a long time to fill while the air burps it's way up the fuel lines.
Head pressure's not a big deal on a Cub with a little Continental, but it can be with bigger engines. A Luscombe with 65 HP is fine, but with 85 HP is placarded to use carb heat for take off. It will outclimb the fuel.
Again, I'm not opposed to modifications, just understand the ramifications.
Mark

Hi,
As far as venting the small tank, I have a similar setup in my Cygnet with the main tank (behind my head) feeds the small tank just aft of the firewall above my feet. Fuel goes from small (header) tank to gascolator with a valve at the discharge of the header tank. There is a vent line running from the top of the header tank to the main tank. Foolproof for 20 years.
Al

Hi Al
That vent line sounds doable. Can you expound on that design a bit. It is plumbed thru the smaller tank to the engine right? Vented from top of aux tank to top of main tank?

Another thought I had was why are all the tanks only 10 gals. or less? Seems to me to be more space than that under the top longerons in front. Is there a reason to keep them small other than weight?.....which you could control just by how much you put in. I do think if they were plumbed straight thru like that the lower tank would need a gage also of some sort.

My five gallons gives me an extra thirty minutes of flight... which puts me at two hours with reserve. Here in the auto industry we have a saying.... "the mind can ony process what the backside can endure". You're going to find out two hours is about the limit.
Jeff Moore