Fuel flow GPM/PSI on gravity fed system?

How is this calculated?
Two tanks, t'd together into one supply line using 3/8 fuel line.

Thanks
Ken

 

The easy way is to measure it...
There are some online calculators for pipe flow, but you'll need to find the viscosity of your fuel. That isn't specified for avgas, so could be tedious. Fittings can do nasty things to flow rate, so you should model those too...

 

The following is my non-professional suggestion, NOT based on any engineering knowledge whatsoever.

Look at the engine data, TCDS, manufacturer's fuel consumption, etc. and find the absolute maximum fuel consumption under the most extreme case.

Multiply that number by 1.333

Set up your fuel tanks and vents and whatever and measure the fuel flow with nothing more than atmospheric venting to the tanks, in ambient (ot dynamic) pressure. Do this at ALL angles of attack and ALL yaw angles. Raise the nose up to 25-30 degrees above the horizon and make **** sure that the fuel still flows as well as it did when the airplane was level.

If the fuel flow meets the 1.333 X of max fuel flow formr the engine mfg's data at all AoA and yaw angles, then verify that your fuel tank vents are facing into high pressure relative wind at ALL angles of attack and yaw angles.

Then go someplace where you have good safe landing options to test fly.

 

The Canadian requirements for that involve putting the airplane in the steepest attitude expected for a climb (like Vx). Dig a hole and put the tail in it, maybe. Put minimum fuel in the tank or tanks, disconnect the fuel hose at the carb, and run the fuel into a calibrated container. For a gravity-fed system, you must have 1.5 times the max fuel flow the engine will need. For a pumped system, it's 1.25 times.

But that all goes out the window if the vent system is badly designed.

 

Good reference here: https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_90-89B.pdf

Start on page 33.


BJC

Edit. If you will be at Oshkosh, you could attend a forum on the subject of fuel flow testing.

David Prizio has details in his Powering Your Plane book, available through EAA.

FWIW, 3/8” tube is fairly standard for engines up to 210 HP, with typical plumbing, pumps, and tank placement.

 

Lots of good info here guys, info that I will trake into account, thank you.

 

Two tanks together is no better than one. The pressure is only determined by the height of the fuel above the carburetor needle valve. I would check the fuel flow out of the carb with the bowl removed to open the needle valve. The needle valve may be the smallest restriction and a large fuel line won't help if the pressure (fuel head) isn't sufficient to flow past the restriction.
The head height in climb is very important. Maybe the head height required is listed in the engine manual or somewhere.

 

Good point on the flow check as it comes out of the valve. My question relates to a helicopter so climes are rather flat, none the less these are all valid points.This system seems to work just don't want to take things for granted.

 

Tanks over the engine can't get much better gravity wise. What you want to check is if one tank will feed. I know a couple of cases where one tank got clogged so only one was feeding the engine. They were helicopters. Also check with minimum fuel not full; the extra fuel weight helps flow, you want worse case. Depends on engine and its needs. The Enstrom helicopters I mess with have -8 lines I think.

 

Thanks,
What horse power on the Enstrom mentioned?

 

225 hp turbocharged. 205 not.