Fuel Flow Testing

Hi all,

I attended an EAA Webinar on fuel flow testing. The primary point was that you really want to make sure you know what your flows and unusable amounts are. They said that if you were using a known design, built to plans, it would be adequate to test in flight level only. Being the obsessive/compulsive I am, I decided that I should do the how series.

Materials and Methods:
My plane is an RV8, not yet flown. IO360M1B. Airflow performance fuel pump and FI system.

I started by using a translucent white fuel can. I went to my local pump and filled it gallon by gallon, marking the levels on the outside to 4.5 gallons. Plastic and no ground made me a bit nervous. We had two people and a fire extinguisher at the ready during the test.

We poured the 4.5 gallons in the left tank. The fuel hose was disconnected from the engine pump and a hose fabricated to bring it back into the translucent fuel can. The plane was placed in flight level condition. I turned on the booster pump and then timed how long it took to pump each gallon of fuel. When it ran dry, I noted how much remained.

We then drained the remaining fuel back into the fuel can and repeated on the right side.

Next, we tilted the aircraft to 19 degrees down. That would be the angle at which it faced with my friend holding the tail wheel over his head. We repeated test. We reduced the angle to 10 degrees, noted the change in unusable fuel. The test was repeated at 11 degrees nose up (where it rests in a three point position) and at 19 degrees nose up, the best I could do with a ditch at the end of a ramp at our airport.

Results:
Timing the fuel flow and multiplying it out, here is what we found:

Flight Level
Left: 43.53 gal/hr about .3 gal unusable
Right: 29.4 gal/hr about .3 gal unusable

20 Degrees Nose Down
Left: 43.2 gal/hr about 3.5 gal unusable
Right: 29.1 gal/hr about 3 gal unusable

10 Degrees Nose Down
Left: Same flow, .9 gal unusable
Right: Same flow, 1.25 gal unusuable

11 Degrees Nose Up
Left: Same Flow, .2 gal unusuable
Right: Same Flow, .2 gal unusable

19 Degrees Nose Up
Left: Same Flow, .2 gal unusable
Right: Same Flow, .2 gal. unusable

Discussion:
I searched the archives, asked questions and even wrote Van's, trying to ascertain the correct angles to use for testing. 19 degrees nose down looked like a terrifying dive to me. 10 degrees will probably be an adequate benchmark for a normal approach. I just finished transition training with Mike Seager in Oregon. He used 10-12 degrees nose up as Vy. I hope someone will look at their EFIS in a Vx climb and verify that 20 degrees is reasonable.

The fuel flow was essentially identical at all angles. I think that is simply a function of the pump's capacity and the length and diameter of the fuel lines. The left tank has a considerably shorter length of fuel line between the tank and the selector. I believe that is the reason for the difference in the fuel flows left to right.

The interesting numbers are the unusable amounts. Flight level and nose up, they are really minimal. I wished I had had a quart measuring cup to get the numbers more exact, but the bottom line is that the tanks empty really well. The exception is nose down. The steeper the dive, the more likely to unport.

I felt a real sense of satisfaction after having completed this testing. The left tank flows at 2.7 times the maximal flow at full power (assuming 16 GPH, which seems to be the number I find most commonly here). The right flows at 1.8 times the maximum engine requirement. I think I saw the figure of 1.5 as being the desired minimum. Perhaps someone knows that figure? Assuming the above, fuel starvation should not be a problem on my first flight--assuming I remember to fill the tanks first.

Regards,

Michael Wynn
RV 8 Finishing
KLVK

Nice job in testing. I did put a bare wire from the aircraft into the fuel can.

I admit I did not perform all of the climbing and descending angles when I ran my tests. My thoughts were the design is proven so I need not verify that, only that my implementation of it was correct. The Andair boost pump primed from empty lines in approx .5 second and flowed right at 60gph from either tank. It's exciting when you get to the fuel testing stage. First flight is in sight by then. Enjoy it!

AC 90-89A

AC 90-89A gives some good guidance on fuel flow checks prior to first flight for both gravity and pressure systems.
But you should have at least 125% of maximum fuel flow IIRC.
Tim

On my -10, I came up with 42 gph using a 3/8" X 15' rubber fuel hose attached to end of hose that normally attaches to fuel servo inlet.

Did the testing but did not like it

I did the fuel flow testing in the level, dive and climb attitudes. For the climb attitude, I used about 25 degrees. For the dive attitude, I used about 14 degrees. The dive attitude I did not like at all. Having an "A" model, I had to get the mains elevated to get the attitude. This puts a lot of weight on the nose gear and looks like a dangerous position. To prevent rolling, I blocked the main wheels and heavily sand bagged the nose gear to prevent rolling and pivoting. I used a hoist to relieve the nose gear of some of the additional weight. I was relieved when the testing was over. My results were similar to those already mentioned with the nose down attitude having the greatest amount of unusable fuel.

I know this thread is a little aged, but studying Micheals report, I was wondering why ther is about a 13 gph difference in between the right and the left fuel flow?

Bird

Perhaps a flop tube in the LH side vs a rigid pickup in the RH side.

Perhaps a fuel valve located low on the LH side of the cockpit with more diastance and restriction to flow from the RH side?

What sayeth the builder and the A&P readers?

mlwynn said:

Hi all, <snip>

The fuel flow was essentially identical at all angles. I think that is simply a function of the pump's capacity and the length and diameter of the fuel lines. The left tank has a considerably shorter length of fuel line between the tank and the selector. I believe that is the reason for the difference in the fuel flows left to right. <snip>

Click to expand...

I think having a 30 psi pressure restriction on the pump would be a more accurate test and would probably negate any side to side differences.

I know on my 8 that left tank lines are longer in total length, I can't see that making that much of a difference in flow. I am yet to test mine.

Bird

Fuel flow test

Tested my fuel flow today, got over 50 GPH on each tank. Fuel exit was just before the Airflow Performance FM-200 into a gas can. I have an Andair PX375-TC fuel pump, with a big honkin fuel filter. Basically identical numbers on left and right tanks.



With everything in the loop, with the Airflow Performance purge valve open/flowing back to the tank, the fuel flow dropped to 4.3 GPH. If I close the throttle or mixture, this goes to zero. If I close the purge valve, throttle open, mixture full rich, eventually flow comes out the sniffle valve. Fuel pressure was about 35PSI.

I read in the Airflow Performance manual that I should do a test to the injectors, which I will do, and it said by blowing air in the intake it will increase the fuel flow. I have not yet tried this.

I guess this seems reasonable, but wanted to see if anyone else has done this kind of test with the purge valve, and gotten similar numbers.