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777drvr

Well Known Member
Hi Guys,

I need to do a fuel flow test on my recently imported RV-4 with a Lycoming IO-320 B1A . It has a Bendix fuel servo . Where do I disconnect the fuel hose to get the sample from ? Do I need to do anything with the power lever and mixture to do the test ?

Thanks,
Marc
 
Disconnect the fuel line at the servo body. Get a gallon container. Turn on the fuel pump and time how long it take for the pump to deliver 1 gallon. Do the math for GPH. Then do this test again with the nose raised in a reasonable climb angle. I think I had 20 degrees up measured along the longeron. I did this by having someone hold the tail down over a ditch while I did the test up front. You should have a minimum quantity of fuel in the tanks for this test. I would suggest 3 or 4 gal. You are proving that you have adequate fuel flow both in flight and take off configuration even if the tanks are low.
 
I think I was thrown off track because the MD-RA inspector told me , over the phone , to set the power full and the mixture rich while doing the test . Therefore , it seemed to make sense to take the sample from the fuel distribution manifold at the top of the motor . That test yielded a miserable dribble of fuel that was nowhere close to satisfying the requirements.

I checked the fuel flow at the firewall bulkhead fitting and was getting close to 350 pph . More than enough !

So.... I should disconnect the hose from the engine driven pump that goes in to the servo body ?

Marc
 
If you remove it from the engine driven pump you should remove it from the output side, that way if the mechanical pump is adding any restriction to the line it will be included in your measurement. I disconnected that line where it attached to the throttle body. This way I could angle this hose into my container for easier sample collection. It also means that I am including all restrictions up to the throttle body in my measurements.
 
Thanks Andrew ! That all makes sense . I guess there would only be a small reduction in potential flow at the distribution manifold due to the difference in height ?

Marc
 
The fuel arrives at your throttle body at pump pressure, lets say 30 psi then the throttle body will adjust the pressure and flow according to engine needs. The fuel flow test really only proves that your electric pump can deliver the fuel to your fuel injection system, it does not actually test if the fuel injection system works. A number of planes have gone down on first flights because people did not test the fuel flow with the plane in a take off configuration then the fuel stopped as the pick up tube in the tank sucked air!
 
777drvr said:
I think I was thrown off track because the MD-RA inspector told me , over the phone , to set the power full and the mixture rich while doing the test . Therefore , it seemed to make sense to take the sample from the fuel distribution manifold at the top of the motor . That test yielded a miserable dribble of fuel that was nowhere close to satisfying the requirements.

I checked the fuel flow at the firewall bulkhead fitting and was getting close to 350 pph . More than enough !

So.... I should disconnect the hose from the engine driven pump that goes in to the servo body ?
Click to expand...
The fuel injection servo meters the fuel flow in proportion to the air flow. There is no air flow with the engine stopped, so it doesn't send much fuel to the distribution manifold.

The point of the test is to determine if your fuel system can deliver enough fuel to the fuel injection servo (or carb). You should disconnect the fuel line at the fuel injection servo (or carb) and measure the fuel flow that you get there. Doing the test at this point checks the whole fuel system upstream of that point, including the lines from the firewall to the fuel injection servo.
 
Fuel Injection Flow testing

As a field engineer, and having just done this on my RV-9A project, this my suggestion for fuel injection system flow testing:

Once the fuel system is intact, it needs to be leak checked and verification of proper operation.

The following items should be checked before first engine start:
• Fuel fitting leaks (both pressurized and static head pressure with pump turned off. The static leak check is important on the suction side of the electric pump, because with the pump running, the suction side is less than atmospheric pressure and a leak may not appear until the pump is turned off; trust me, I know this first hand),
• Fuel pp discharge pressure as expected,
• Fuel flow check, both, open end free flow and flow verification through the FI servo.
• Functional verification of the Throttle and Mixture controls.

Open end flow test:

Disconnect the fuel line at the FI servo inlet and run an extension tube to a gas can.
Turn on the electric fuel pp and flush several gallons through the system. Discharge pressure may only be 1-2 psi, but flow rate should be in excess of 35 gph. Determine flow rate by known fuel quantity over measured time. This should agree closely with your fuel flow transmitter if installed.

FI Servo flow test:
Because fuel flow through the FI servo is restricted with no airflow to regulate the fuel, you can not actually test full rated flow through the servo. However, a minimum expected flow can be verified.
• Disconnect the fuel line going into the FI distributor, as there is no need to actually charge fuel to the cylinders. Run extra tubing for the fuel to a gas can.
• Push the Mixture control full rich position and Throttle full in.
• Start the electric fuel pp and verify the following:
o Fuel flow present, pump primes and pressure is stable between 25 to 30 psi. Check fuel flow is about 4.5 gph. (This is the max amount without airflow through the servo).
o Pull Throttle out to Idle position and verify fuel flow reduces to minimal amount (about 1.5 gph). Fuel pressure should remain stable between 25-30 psi.
o Push Throttle full in, verify about 4.5 gph returns.
o Pull Mixture to Idle Cut Off position and verify fuel flow stops and fuel pressure remains stable.
 
fuel flow test

I'm nowhere near this stage yet but curious to know how to do this test with maximum safety. You obviously don't want lots of gas splashing around.
 
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