A forum about all aspects of the construction and maintenance of the Billy Dawson inspired modified Hatz Classic biplane. Here is the place to ask your questions and get the answers from the real experts.
I need some help finding a source for a fuel cap that will fit my Makelan constructed tank. Makelan used a flange with an internal thread measuring 2-1/4" x 18 threads/in. See image below. I found a fuel cap in the Wicks catalog (http://aircraftproducts.wicksaircraft.c ... p/c36s-sl?) that may work but it's a bit pricy at $160. Does anyone know a source for a cap that will fit my flange that is priced a little lower?
Ok, I think I have it figured out and I feel dumb. I need a filler neck. This neck screws into the flange and then a non-vented cap twists onto the neck. The cap is made vented by brazing a ? shape tube.
I originally had a cap that I'd brazed a forward facing vent line too. A few weeks ago, just before heading to Junction City with the plane, I decided I needed to do something with the cap. It looked kind of ratty from the brazing where it had burned off the cad plating etc. I also wasn't crazy about the looks of it. That vent tube is always in the line of sight.
Anyway, I bought a standard vented cap that fits the same filler neck. It has a tiny hole in the top, and also a notch cut out of the spring disc that lets it vent under the cap flange.
I got ready to install the new cap, but then started second guessing myself, worrying about the suction on the top of the center section. What if the new cap restricted flow, etc.
I called one of my engineering buddies and he went through the calculations. What's the wing loading, vent size, etc.
Bottom line, the amount of negative pressure on the top of the airfoil is the equivalent of lowering the center section 2 inches. The weight and head pressure of the fuel far exceeds the negative pressure of the airflow.
I installed the cap, went on my trip and it never missed a beat, even when run down to minimum reserve.
I welded a ? on top of my cap. I did it to pressurize (guess that's not necessary after reading Mark's post ) and to prevent rain or wash water from going into the tank through the vent hole. I also kind of like the "antique" look. After welding, it looked pretty crappy so I spray painted it with some bright red rustoleum I had on hand. I think it really looks cool now with that bright red cap. Sort of like painting your front door red, it makes a statement. Spend 14 years building a biplane and I'm impressed with the red fuel cap...go figure!
One advantage of the ? vent is that if you forget to reinstall your cap after fueling and leave it on your cowling, it'll ride all the way from Ottumwa to Iowa City with no problem.
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