Fuel Flow Tests

[bret]

Working on documenting the flow #s for the fuel system, nose up and flat is at 45 GPH and .2 unusable, but what nose down angle did you use, and what method did you use to raise the tail on an A model?

[Robert Anglin]

When we contacted FSDO to get an inspection, we found that the local office was asking for a fuel flow test to be posted in a Log as one of the check off items on their list. We simply put it in the flight level attitude and did the test for both tanks. Ours is injected so it came out to a little over one gallon a minute. I call FSDO back and asked if that was OK and how they wanted it put in the Log. The answer was, they were looking for 150% of the flow rate at the maximum fuel flow at full power for the engine in that airframe. I rounded it off to one gallon per minute and was told just write it as being 60 gallons per hour in the log. I have heard that from a safety point and that of the requirements across the pound that just like in part 23 you should add the pitch angles to the test, but FSDO told us we where find in just the fight level set point. You may wish to check with your DAR or FSDO office and see what they like to see as well as what you are comfortable with. It is a very good idea to check you whole fuel system very well before your first flight. you will not regret being careful on this point. Hope this helps. Yours, R.E.A. III #80888

[bret]

So for 200 HP thats 16.6 GPH X 150% ya? I was measuring after the regulator so im sure free flowing is a lot more.

[rvbuilder2002]

If FAR 23 requirements are used as a guide......

Gravity feed systems (Cessna) require 150% expected flow at max. power.
Pump systems (like we have in RV's) require 125% of expected flow at max power.
The test has to be done with the aircraft in the attitude that would be the most adverse for the fuel system. In an RV that would be tail low at the maximum sustainable climb angle.

[BillL]

Bret, suggest you get a 1/4" ball valve and use it to restrict the flow down to the 125% value, then read the pressure and run your test. We added a clear section of tubing before the valve to see if the system was pulling air somewhere.

A pressure leak down test of the system from wing root to servo had already been run but used the clear section as more joints were now included. For completeness.

We weighed the fuel removed (pumped out) and kept a running log of how much fuel was in the tanks, we only used a few gallons, but over and over. Fuel was filtered each time it was poured back in the tanks.

It was a good test for unusable fuel. We shut off each time the main pump cavitated -it is the best indicator. The faster it is shut off the quicker it purges air for the next test.

This was fun and a huge confidence builder for first flight.

[bret]

Ya, this is the return line to the tank, after the pressure regulator, return to tank after the 35 psi regulator.

[az_gila]

If you are going to use the new "second pilot" option during flight test then some specific tests - including fuel flow - must be performed and documented.

b. Powerplant Testing. Powerplant testing is required prior to the initial flight and any time warranted thereafter to help ensure the reliability of the powerplant. Based on the recommendations of AC 90-89 and the industry on testing of the powerplant and fuel system, the following tests, if applicable, are required:
? Mixture and Idle Speed Check,
? Magneto Check,
? Cold Cylinder Check,
? Carburetor Heat Check,
? Fuel Flow Check,
? Unusable Fuel Check, and
? Compression Check.
c. Documented Testing. Documented testing similar to that of the build is required as proof of compliance. This includes appropriate logbook entries with test results. Photographs and diagrams should also be provided for tests where the applicant deems them beneficial or necessary.
d. Changes to Fuel System. Any change to the fuel system after conducting the fuel flow tests, except for normal fuel system/filter maintenance, requires a repeat of the tests prior to additional flight. All tests, including repeated tests, require documentation.

The main body of the document references AC 90-89a, which gives a good hint how to do the fuel tests -

e. Fuel Flow and Unusable Fuel Check: This
is a field test to ensure the aircraft engine will get
enough fuel to run properly, even if the aircraft is
in a steep climb or stall attitude.
(1) First, place the aircraft?s nose at an
angle 5 degrees above the highest anticipated climb
angle. The easiest and safest way to do this with
a conventional gear aircraft is to dig a hole and place
the aircraft?s tail in it. For a nose gear aircraft, build
a ramp to raise the nose gear to the proper angle.
(2) Make sure the aircraft is tied-down and
chocked. With minimum fuel in the tanks, disconnect
the fuel line to carburetor. The fuel flow with a gravity
flow system should be 150 percent of the fuel
consumption of the engine at full throttle. With a
fuel system that is pressurized, the fuel flow should
be at least 125 percent. When the fuel stops flowing,
the remaining fuel is the ??unusable fuel?? quantity.
(3) Since the fuel consumption of most
modern engines is approximately .55 pounds per
brake horsepower per hour for a 100 horsepower
engine, the test fuel flow should be 82.5 pounds (13.7
gallons) per hour for gravity feed, or 68.75 pounds
(11.5 gallons) per hour for a pressurized system. The
pounds per hour divided by 60 equals 1.4 pounds
and 1.15 pounds per minute fuel rate respectively.
NOTE: Formula for fuel flow rate gravity
feed is .55 x engine horsepower x 1.50 =
pounds of fuel per hour divided by 60 to
get pounds per minute, divided by 6 to get
gallons per minute. For a pressurized system,
substitute 1.25 for 1.50 to determine
fuel flow rate.

All of the above sound like wise items to check.

[xblueh2o]

Originally Posted by rvbuilder2002

The test has to be done with the aircraft in the attitude that would be the most adverse for the fuel system. In an RV that would be tail low at the maximum sustainable climb angle.

Which brings the obvious questions...
1. What is that angle (aware that this will vary with aircraft, engine, prop etc)?
2. How are people doing that?

I have a long way to go before I need to do this but just curious.

[rmartingt]

Originally Posted by BillL

Bret, suggest you get a 1/4" ball valve and use it to restrict the flow down to the 125% value, then read the pressure and run your test.

Why restrict the flow? Bret has an electronic injection system so you expect to see a substantial return flow to the tanks even at full power.


I'm asking because I'm basically going to be using the same thing in a few years...

[bret]

I put the tail on the ground of this A model 7 during one of the fuel flow tests, in my opinion, these tanks do not have a problem with a nose high attitude, the pickup tube slopes down and to the rear bottom of the tank, I measured less than one cup of unusable fuel, I have not done the nose low fuel flow test but I am thinking this will show a greater value of unusable, in any case, all this is being documented in the build log.