Precertification Fuel Flow Testing Results

[Glenn Patterson]

Would any builders who used 3/8" fuel lines & did the fuel flow tests with please let me know what their flow results were.

We are installing a Continental IO-360 with a positive displacement engine driven pump that pushes 160 lb/hr at full throttle. FAR 23 requires the fuel supply to be 125% more than the circulating pump which requires a supply of 200 lb/hr. The engine actually burns 110 lb/hr at full throttle and returns 50 lb/hr. FAR 23 is trying to prevent pump starvation and using it as a companion to the AC90.89A for preflight requirements.

3/8 tubing feeds Lycoming 540's with no issues so I don't expect to have an issue with fuel flow. I am just trying to get a handle on the fuel system capacity as Barrow designed it.

Your help is appreciated. Thanks

Glenn
BH727

[Battson]

With fuel injection, you need more equipment than Bob designed the system for. A pump and pre-filter are required, and then you almost certainly want to fit a fuel transducer. Finding space for all this equipment can be tricky, especially if using a TCM engine which needs a return line. Each of these and the associated fittings and pipes (some with bends) has an associated drop in head of pressure, and therefore fuel flow. I have measured the fuel flow at every location in my fuel system except the tank outlets themselves, my numbers are at the hanger so I'll have to post them later. I can remember that the flow with no pumps running (measured at the outlet of the engine driven pump) was less than 40L/hr. Certainly each additional item in-line has a measurable drop in flow associated with it, but some are much worse than others. The worst is the electric pump's bypass system in my case. Long story short, you don't want a dual pump failure... the engine is going to starve to death unless you close the throttle fast... Sharp pipe bends, elbow fittings, and the fuel transducer are also pretty bad restrictions. Of course nothing upstream of the fuel selector makes any difference, as you basically have 4 lines into 1. Installing the fuel flow transducer downstream from the fuel servo is the only sensible way to install it if good fuel flow is your primary consideration. But, as with all things, there are also other considerations.... e.g. they say not to mount directly to the engine, 6" straight pipe after the outlet, etc. I've seen them plumbed in directly before the flow divider with a stainless fitting. Seemed to work fine? That was an IO-550 certified... So you can clearly do it that way! You do need to heat shield the transducer is it's mounted firewall forward.

[jaredyates]

I just did mine yesterday as it turns out. With the gold cube transducer in the line between the gascolator and the carb, I got around 23-25 gallons per hour. I didn't test without the transducer, but I suspect it was probably a reduction of about 5 gallons per hour, based on Eric's results. My fuel system is pretty much as Bob describes, except that I used threaded fittings at the wing roots instead of beaded tubes and hoses. That's with a Newton SPRL valve and Bob's gascolator.

[aerolite]

Glen,
The following is just to give you an example of the gravity feed fuel rate of Bob's standard fuel line layout on the Patrol I did for a customer.

In Canada we are required to do a fuel flow test with the aircraft in the climb attitude. If it's fuel injected they also want to see the rate of flow at the line before the servo. In the case of Bob's design it is all gravity feed to the gascolator so the results I got will give you a good indication of the flow rate. We have to meet or exceed 150% of max flow rate. I used a BSFC of .55 which is very liberal and the fuel flow rate still exceeded the max by a good margin.
Have a red cube transducer about 8" upstream from servo. Newton fuel selector, Bob's gascolator and Andair high pressure fuel pump.
As for what is being seen in the real world I cant say as I don't have the plane but fuel flow is not an issue! I can't remember exact number but when I ran the engine the fuel pressure was excellent.

Method:
The aircraft was set in a climb attitude with 1.5 gallons of fuel in each wing tank. Gravity test was done three times, with fuel selector changed to left, right, and both. Fuel was measured in ounces per minute and was measured at the fuel drain hole of the gascolator which is just before the high pressure fuel pump. With only 1.5 gals of fuel in each tank, and measuring from the gascolator drain, this represents the worst case scenario for flow rate. (Note that in the climb attitude with only 1.5 gal in the tanks, fuel will only be draining from rear drain).
The high pressure fuel pump rate was measured with the fuel line disconnected at the fuel injection servo and was measured in oz/min. with the selector on both position.

Results:
Brake Specific Fuel Consumption rate of .55 lbs/hr was used to determine BSFC for the 195 hp I0-375 engine installed in aircraft.
195 hp x .55 =107 lbs/hr
107/6(lbs/gal)=17.8 gal/hr max flow rate.
17.8 x 150% = 26.7 gal/hr
To measure in oz/min:
First we need gals/min= 26.7/60=.45 gal/min
Second we need ozs/min= .45x128 (oz in a gal)=57.6 oz/min at 150% flow rate.
Therefore: 26.7 gal/hr = 57.6 oz/min

Gravity feed test:
Right tank = 88 oz/min = 41.25 gal/hr
Left tank = 72 oz/min = 33.75 gal/hr
Both = 108 oz/min = 50.63 gal/hr

High pressure fuel pump test:
Both = 136 oz/min = 63.6 gal/hr

All the above are greater than the 150% rate of 26.7 gal/hr

[Glenn Patterson]

Steve,
Thank you for your write up as it is good food for thought. This is especially helpful as I live in Dryden, NW Ontario.

Those are good volumes. The Continental IO-360 fuel injection is an all mechanical system that is unsophisticated but effective. The engine has a turbine pump that circulates about 50% more fuel than the engine uses across its RPM range. The Continental engine manual flow charts show that at 2800 rpm the engine consumes 110lb/hr while the pump circulates 160lb/hr. The fuel pump was overhauled at Q1 and the bench test confirms the pump volumes at the tested RPMs. The Continental mechanical system returns the excess fuel to the tank or to header tank.

The FARs and CARs are usually very close. It was a concern to me that the engine driven pump circulates 50% more than it consumes across its rpm range, At full rpm the pump is circulating 150% max burn which is the test level for a carbureted engine. In my experience & gut there needed to be a safety factor for the pump as it is the governing flow which started my quest for that answer.

I found the FAR 23 which is the American std for aircraft worthiness that requires the test for a gravity system to be 150% of max burn & the test for pumped systems to be 125% of the max pump flow. Since I have found CAR 523.955 Fuel Flow gives the same test requirements. The pump delivers the 150% of the max burn and by ensuring the flow to the pump is 125% of max pump flow rate assures adequate flow. (160lb/hr x 125% = 200lb/hr =33.3 gallons) Reading your numbers & calculations shows a lot.

I have also just found that Summit Racing have aircraft like 3/8" AN "Y" fittings that I wish I had seen prior to plumbing the system. "Y"s give a better flow than a tee and would eliminate some of the resistance with the tight turn in a tee fitting. Replacing tees with Y's would make an improvement when feeding from the rear lines. We have tried our level best to make as smooth a system as possible with a minimum of fittings. Also a "Y" fitting in to the gascolator may help make the tank flows more equal.

Thank you again.
Glenn

[Battson]

Hi Glenn,

Promised I'd come back with a result - so I checked my log. The fuel flow was 160.5L/hr on average over the duration of the test, which equates to 42 US Gal/hr.
This is with a system as I described above, fuel injection with filters, pumps, gascolator, etc. which is probably why it's a little lower than the results Steve was able to post above.
Hope that's helpful,

Jonathan

[Glenn Patterson]

Jonathan,
Thank you. Seeing both Steve & your numbers shows that the system is capable of meeting the requirements of our engine. I ordered one Y to replace the tee in front of the gascolator to help reduce the resistance of turning such a nasty corner.
All the best,
Glenn