Fuel Line Sizing

I won't venture to provide any actual data here, but I would expect that since your smallest opening is 1/4", then 3/8 would be sufficient in reducing fuel flow resistance down to the engine.  If you want to try to cram more fuel through the 1/4" opening, some form of higher pressure would be needed, and at that point you can probably overcome any difference in fuel flow resistance between 3/8 and 1/2.

All of that said, I generally like to play things safe.  If you are concerned, and have the space anyway, run 1/2" line, and put a boost pump on it anyway.  It never hurts to have a backup method of forcing fuel into the engine.

I would think the big concern would be during takeoff when the engine is at full power and odd angles.

All of this is just my $0.02.

i would look at the engine driven fuel pump on the 540 and let the fuel inlet fitting size determine the feed line size.  For a 540 at 250HP I suspect that will be a 1/2 inch size.  Don't worry about one or two fittings that have to be smaller than 1/2 but the overall size should match.  At 250 HP you'll need to show that the fuel flow is adequate to feed the engine at the power on stall deck angle.  That's a steep angle and the full power fuel flow for a 540 is about 20 -24 GPH... (if you were using a Continental I could tell you offhand) .  A small fitting will be like adding a few feet of tube to your fuel system.

Good luck.

My basic rule of thumb is that the fuel system should flow 150% of full power fuel flow when set at a 15 degree angle on the ground. If it can't do that it needs a boost pump to ensure sufficient flow for take-off and possible go-around during landing.

To get your expected full power fuel flow you can use 0.45 gallons per horsepower hour (or a more conservative 0.5). Using .45 you would need 0.45 g/hp-h times 250 hp divided by 6 pounds/ gallon or 18.75 gallons/hour. At 0.5 you would get 20.8 g/h.  Thus your fuel system should gravity feed about 30 gallons per hour by my rule.

I have not seen a high wing plane with 3/8 fuel line that would flow 30 g/h, so based on my experience, I would say that you should use 1/2 fuel lines.

Here is how I'd approach it.

Given that you have a Lycoming O-540 and it is gravity fed I looked in the

and found a chart showing that maximum power is at 28” Hg of manifold pressure will yield a fuel burn of approximately 25.25 gal/hr.

Looking in the Federal Aviation Regulations (FAR) 23.955(b) we see that certificated aircraft (not your experimental) must ensure that a gravity system is capable of delivering 150 percent of the takeoff fuel consumption of the engine. FAR 23 gives good, safe, numbers if you want a starting point. So, if we assume you will want to use all of your engine's horsepower at takeoff we multiply 25.25 by 1.5 and get 37.875 gal/hr that the fuel system should be capable of delivering to the engine.

If it were my plane, I'd mount the fuel tank in the airframe and run the smallest lines and fitting that you think you want to use, 3/8” in your case. Ideally, you should tilt the airframe to the most adverse angle for the flow but that will likely be very difficult. Cap the end of the line and fill the tanks, then uncap the line let it run for 10 minutes and measure how much fluid passed through the system in gallons (or use pounds and convert to gallons). If the number is less than 38 gallons, you need bigger fittings and/or lines. Note that you can run this test with any line, it doesn't have to be aircraft quality since this is just a test. If you are not able to determine the most detrimental angle for fuel flow or you can't get the airframe into that position, I'd add a significant safety margin to compensate, perhaps use 200 percent instead of 150 percent and then confirm that all is well during the initial flight testing?

If you don't want to rig the test lines, there are ways to mathematically calculate the flow rate based on gravity but that can't take all the variables into account. As I said, I'd go with testing it on the airframe.

Based on the information you've provided and some quick calculations, I think you are going to have a problem with the 1/4” fittings being too restrictive. If you increase the size of those, I think you might find that you actually need 1/2” through the whole system, but I'll leave that for you to make the final determination.

Hope this helps. I'm not an engineer, so confirm everything I've said for yourself.

Keith

These are all good responses. 1/2" lines are the best solution in this case. If possible, avoid local restrictions from smaller sized fittings, but if it cannot be avoided, there should still be overall better flow through the large line than a smaller one. You can also find this kind of information in my book "Efficient Powerplant Installation" (web site www.caro-engineering.com).

Sonja

Also don't forget to consider the layout of the entire system, more specifically, the effect of elbows on your fuel flow capability.  In building our Lancair one of their engineers gave us some pointers and the cumulative effect of several 90 degree bends was quite astonishing.  Our original system had six 90 degree elbows!  Our revised system had only one!  Just another thought.