Fuel system thoughts and questions
 

Since it is pretty easy to have issues with cavitation/air bubbles, etc. with long suction lines in our fuel systems, would it not be a good idea to locate the fuel pump close to or even in the tanks? This way the pump would push the fuel instead of pulling it. The advantage I can see doing it this way is the fuel would always be under positive pressure eliminating the cavitation/air bubble problem. Modern cars are plumbed this way with their fuel pumps located in the gas tank. Since I'm new to turbine-powered airplanes, I've been searching through old posts trying to get up to speed on these beasts, and it seems that the fuel systems cause their fair share of grief. I realize that an in-tank pump idea would require a redesign of the pump, but the close-to-the-tank idea seems plausable to me. What are your thoughts?
Thanks,
Luckyflyer

RE: Fuel system thoughts and questions
 

If you use 6mm O.D (4mm I.D) pvc tubing up to the pump and make sure all the tank fittings, clunks and tubing are also 4mm I.D you won't go far wrong. The only time I would be happy to have a bore less than 4mm is either when I use a small engine or I have two parallel tanks of the same size feeding a hopper which has a 4mm I.D feed to the UAT.

Jason

RE: Fuel system thoughts and questions
 

I don't see a sumberged pump as practical, and I don't think cavitation is a common issue with large fuel lines, but u may be on to something when u suggest putting the pump next to the tank, eliminating the UAT as a possible cavitator.

One would have to use something more substantial than a simple plug to cap the line used for fueling though !

Interesting idea.

Oops, just thought of a problem. One had best use a rigid UAT, lest the thing cause response lags as it expands/contracts under varied pump pressures.

RE: Fuel system thoughts and questions
 

A fill line could be ran directly to the tank.

RE: Fuel system thoughts and questions
 

I don't think so. u still want to fill at the UAT to make sure that it is "toppoed off" before each flight. Filling at the UAT is essential to remove any air introduced on previous flights thru aggressive manuvering or any other cause.

RE: Fuel system thoughts and questions
 

Most jets have a shut off valve to use in an emergency & there was some discussion in another thread on whether the valve should go on the suction or discharge side of the pump.

The main objection to the valve being on the discharge side of the pump was the valve & fuel lines would be subject to the considerable pressure fuel pumps are capable of generating & the risk of a line or fitting bursting spraying fuel everywhere.

With what you are proposing the entire fuel system would have to withstand this pressure.

As Jason said, a lot of work has gone into the current system & the bugs are pretty well ironed out, lessons learnt the hard way mostly.

But new ideas are always welcome! :D

Welcome to jets! - John.

RE: Fuel system thoughts and questions
 

The pressure problem could be solved by adding a by-pass to the shut-off valve, this would return the fuel to the main tank without stressing the system.

RE: Fuel system thoughts and questions
 

But the two scenarios are different, right? In one case u have a potentially "run away" pump going all out against a closed valve, and in the proposed case (even at full throttle) u have a "relief" in the form of a running engine taking fuel out of the other end of the supply. Again, I think this ONLY increases pressure (or decreases bubble potential) AT THE UAT, assuming the pump is not submerged in the tank. I think that would be totally impractical.

RE: Fuel system thoughts and questions
 

What we are using currently when done right works perfectly. It is easly to do PM on the system as well as the install is pretty straight forward. I have over 200 flights and never a hickup from fuel delivery. Don't need to re-invent the wheel here. A clean install really makes the differance.

RE: Fuel system thoughts and questions
 

They are different until you have that 'runaway pump' or other problem and you shut the valve. With the proposed idea the valve must go on the pressure side of the pump which is usually just before the engine as shown in Mike's (lovely) install.

Then there's no 'relief', the pump would be pushing against the second tank & the UAT if fitted. The first (main?) tank is, of course, vented. - John.

RE: Fuel system thoughts and questions
 

Lots of fuel.... Electricity.....

I keep the pump close to the engine. All electricity well away from the fuel. I rather spend more time dealing with cavitation then with a fire hazard.

RE: Fuel system thoughts and questions
 

Given that most of the cavitation issues mentioned in the recent thread on the topic occur at the UAT...I think the idea has merit, and that someone (else) should give it a try :-)

RE: Fuel system thoughts and questions
 

To many ideas on here to know which one you are refering to, I don't get cavitation at the UAT because its in the workshop ever since I took it out and just started using a decent clunk. :)

Mike

RE: Fuel system thoughts and questions
 

Cool. Which clunk are U using, the Sulliven Super?