Fuel Flow Testing

Time: 4 Hours

An item that needs to be tested and verified is the electric fuel boost pump had enough power or flow to supply the engine with enough fuel in the case the engine driven mechanical pump were to fail. There is a lot of guidance from the EAA and FAA on how to determine this through a process of steps. In the steps you use a formula to determine how much pounds per hour fuel you would need for your specific engines maximum horsepower. You then can convert this to various numbers such as gallons per hour and gallons per minute.

When I built my IO-375-M1S engine at Aerosport power they were re-building their Dino machine so we didn’t get to determine the exact horsepower of my engine, however we think it’s somewhere between 195 and 200 hp. show for my calculations I went with worst case scenario of 200 hp. BSFC(Brake Specific Fuel Consumption) is a factor that some engine manufacturers and it’s a factor that is used to multiply horse power to enter into the formula. If your Engine manufacturer doesn’t provide one you can use .55 as a conservative number.

So my math is 200HP x .55 = 110lbs/hour. That’s how many pounds of fuel my engine needs at its maximum horse power. The FAA says you need to factor in a margin of error, in my case with a fuel injected engine it’s 25%. So I take the 110 lbs/hour x 1.25 = 137.5 lbs/hour. Now 100LL fuel that my engine burns weighs 6 lbs per gal. I can divid the 137.5lbs/hour by 6 to get gals/hour. 137.5 lbs/hr / 6 lbs per gal = 22.92 or round up to 23 gal/hour. We can convert that number to gal/minute by dividing by 60 minutes. 23 gal/hour / 60 minutes = .383 gal/min. So my plan was to use 4 gal of fuel pumped to determine my fuel flows. That way I have a gallon of space in my 5 gallon gas can for spillage and such. So I needed to know how long I have to fill 4 gallons of fuel into the gas can using my boost pump. So I need to pump .383 gal/min, so I took 4 gal / .383 gal/min = 10.44 min. Therefore that’s the bingo number I need, if my pump transfers 4 gal in 10.44 min it is providing just enough fuel, with the 25% factor, to supply my engine at its maximum horse power.

Now that I know what minimum time I have to fill 4 gallons in the fuel can I can test my boost pump to see if it’s up to the challenge. The RV-8 draws fuel from either the right fuel tank or the left fuel tank based on the fuel selection valves position. So I will need to test each tank separately for flow rates. I also need to test the flow in three different aircraft positions, level, 25° pitch up and 10° pitch down. The process is pretty simple, I disconnected the fuel supply line that connects to the engines fuel servo. I added a AN fitting to the end of the line. Attached to the other end was a short piece of aluminum tube that I could slip a clear plastic hose to. This will allow the fuel to flow into a gas can on the ground. So I just needed to put more than four gallons of fuel in a tank, turn the boost pump on and time the fuel as it fills to four gallons. I would repeat this step for the left and right wing in the three different flight positions.

One cool thing about the Vertical Power system I use for virtual circuit breakers is that you can test and control everything on a laptop via an ethernet cable. So I could turn the boost pump on and off at the laptop while standing next to the gas can rather than running around the wing to flip the pump switch in and off.

One other test I did during all of this was to determine my unusable fuel in each tank. The fuel line in the tanks can only pick up so much fuel based on their positions. So the fuel that it can’t pick for the engine is called the unusable fuel. To know this helps in flight planning so that if you put in ten gallons you know that a certain amount of the can’t be used and not figured into the fuel you want to use. To do this test I emptied the tank completely using the fuel drain on the bottom of the wing. Then I added exactly two gallons of fuel to empty wing followed by turning the boost pump on. When the boost pump stopped or almost stopped pumping I turned it off and documented how much came out which told me how much stayed in.

The pump put out double what I needed in all three positions. The results are listed here along with the unusable fuel data. The results are below:

  • Pitch down = Left wing 47gal/hr
  • Pitch down = Right wing 37 gal/hr
  • Level = Left wing 51 gal/hr
  • Level = Right wing 50 gal/hr
  • Pitch up = Left wing 50 gal/hr
  • Pitch up = Right wing 51 gal/hr

Unusable fuel was a little over .25 gal in both tanks. I think I will use 1 gallon as a conservative number for flight planning.

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