Fuel lines and 90 degree bends.

[rsr3]

Hi everyone.

I’ve read a few times about 90 degree bends in fuel lines and get the impression they are a bad idea. A recent post mentioning the avoidance of 90 degree bends prompted me to ask the question: why?!

I’m not yet at the stage of running fuel lines through the fuselage but will be shortly.

Thanks in advance.

[fixnflyguy]

There is no reason to believe they are a bad idea and many times cannot be avoided. maintaining the largest possible radius helps, and using a good quality bender made for the purpose will yield a perfectly smooth bend. I have made too many to count. Here is a pic in my -4 build from years ago.

[Desert Rat]

I believe that guidance comes from the well known principle that the sharper the change in direction, the bigger the pressure drop and potential for cavitation.

I remember from A&P school a few decades ago that two 45* AN fittings have less pressure drop than one 90* but I couldn't tell you if that was written down anywhere or if I just got it from general tribal knowledge. It's really the same principle as the idea that the bigger the bend radius the better.

Obviously, you're never going to be able to get rid of all 90* bends, and a few 180s as well. Just look at the Vans hi pressure boost pump. That thing looks like a french horn stuffed under the floor. But the principle seems valid to consider if space allows.

[Toobuilder]

The avoidance of 90's in the system is almost universally reserved for the single piece AN fitting, where the passages are drilled and the intersection is sharp. And in the fuel system, its the suction side that needs attention, the pressure side not so much. A drilled 90 creates localized turbulence and pressure loss - couple that with high temps and the lower pressure of the suction side and you will often find that you have a big section of vapor rather than a slug of fuel. Home bent tube and even the often very sharp bent tube hose ends essentially solve this problem.

[rsr3]

Thank you!

[MJarreau]

Bill,

The pic you posed is very helpful to us (not there, yet, but learning).

Michael,

The explanation is awesome.

Thank you both for your contribution to this discussion!

Cheers!
Mike

[TS Flightlines]

IN reference to the 90*AN fittings or multiple fittings, we had the same knowledge as everyone else that more 90*s were bad. In doing the concept for our wing root filter packages, 90* AN fittings just werent avoidable, so we wanted to know just what the flow drop was. So we bought a pump flow tester--no not like the one that Don Rivera has--but something we could afford!. The idea was to use a boost pump regulated at 30 psi and get a baseline of open flow with no fittings. Was 150 liters/hour. Then we started putting in AN 90* adapters starting on the suction side, testing, then moving to the discharge side and retesting. Eventually we ended up with multiple AN 90*s on BOTH suction and discharge side and got a result of 149.8 litres/hour. Thats something like .02 gallons/hour difference between open flow and a bunch of 90*s in the system.

I personally called it good, and moved on. But---YES there is a difference in a true AN fitting and an imposter fitting---so do your due diligence.

Tom

[Avanza]

If you have a straight pipe, a AN 90 deg fitting or a valve, it will cause a pressure drop if there is a liquid flow.
Sharp bends like a AN fitting create much higher pressure drop than in a
straight pipe. The pressure drop is dependent of the flow rate.
The pressure drop increases as the square of the volumetric flow rate.
If uncertain choose one size bigger fitting.

http://www.pressure-drop.com/Online-Calculator/

Good luck