Gas Tanks...Food For Thought! By Marvin V. Hoppenworth, EAA 2519 4975 Kessler Rd., NW., Cedar Rapids, Iowa

T SEEMS that in the efforts to create high performance Ivation. aircraft we have also created a problem of fuel starAll too often, we hear about an aircraft losing power on take-off, only to find that the fuel tank was not full and no apparent reason for the engine to cut. This unwelcome phenomenon is brought about in the gravity fuel systems by an element known as acceleration. In our efforts to build high performance aircraft, we take a small lightweight airplane and put in a little larger engine, hoping that the added power will make it a real spectacular performer. Let's take, for example, a 650 Ib. biplane and place 125 hp on the nose. Gross weight of this little machine might be about 900 Ibs. Then open the throttle on this 125 hp engine and we can get around 530 Ibs. of thrust. Thrust can be figured as approximately 4>/4 Ibs. of thrust per horsepower. At this point, we get into what is called "acceleration factor." In this case, we have an acceleration factor of 53 over 90 and, to show this on a graph, we draw a 90 on a horizontal plane and a 53 on a vertical plane. The line that this makes is the way the gas in the tank looks the minute the throttle is opened. Compare this to the common lightplane of 65 hp and 1,200 Ibs. gross weight. We get 276 Ibs. of thrust to 1,200 Ibs. of gross weight. An acceleration factor of approximately 28 to 120 . . . less than one-fourth compared to well over one-half before! So we can see a very slight incline on our gentle low-powered airplane. Yet, most homebuilts have a fuel system in them barely good for more than an acceleration factor of one-fourth. On a pull-up, it would be very easy to pull the carburetor up above the level of the fuel in the tank. When this happens, even Lyeoming and Continental will guarantee engine stoppage. It seems that the big problem is that the fuel has moved to the rear of the tank, either from acceleration or from excessive climb angle.

Now, for a solution to this problem, get the fuel at the front of the tank to stay there and help maintain a head of gravity pressure. Let's look at a system which was used very successfully on the wing tanks of the old Stinsons. They wanted to keep fuel in at the root because that is where the fuel pick-up was. In the event of a low tank and a skid, it is easy to see how the fuel could run outboard and starve the engine. Stinson took a bulkhead

the rear section. Then on acceleration, the fuel which was in the front area would stay there. We don't care what happens to the fuel in the rear area, so let it go back to the rear corner. As soon as you level off, it will come back and act as a one-cell tank again. Construction of this flapper valve is very simple. Cut a hole in the bulkhead and roll the edges forward just as they do on a lightening hole. Sand it off flat with No. 400 sand paper and a flat surface. Then make a flapper which may look like a toilet lid and fix it to hang down over the hole in front of the bulkhead. Make sure it fits your hole well and flat. That's all there is to it! The valve will work automatically and readily let fuel into the forward compartment but will not let it out. If you have a cough or sputter on take-off or climb, this may be worth thinking about. *

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close to the root end and made it tight except for about

a 1 in. hole near the center and at the bottom. Then, on the inboard side, they hinged a little aluminum flap to act as a check valve. This valve actually allowed the small inboard chamber to take fuel from the outboard cell but would not let it go back. In-flight motion could keep this small area almost full at all times.

This system worked very well and could be put to good use in our high powered homebuilts. Let's face it, this was successful. Copy it if it can help us. In the same principle, we can design our main fuel tanks so that we

have this chamber right at the front of the tank. Just make sure that we keep the fuel gauge in the other part of the tank and place a hole at the top venting back to 32

NOVEMBER 1965

"Now on final I figure we oughta hold in the neighborhood of ahhh . . . Oh, about four hundred and seventy-six or

seven knots!"