Gravity feed fuel flow rate

I would fit a level switch/sensor in the destination tank, with a cutoff valve, which is normally-closed.

It may not take a lot of flow control, depending on how you run it.

If the 12 gallon fuselage tank is run down to 5 gallon level (about one hour reserve for a C90), then it takes 19 US GPH or so aux tank flow (with the engine burning 5 GPH) to refill the fuselage tank to 100% over half an hour, at which point the aux tank will be empty. With any GPH less than that the fuselage tank will not overflow.

It may be that 19 GPH requires full or significant opening of a standard ball valve, or possibly it can’t flow that much wide open. For context, the certification requirement for the main fuel supply to the engine is only 150% of maximum fuel consumption, or 12-ish GPH in this case with a C90 engine, although two or three times more is typical.

Obviously the earlier you want to start transferring fuel, the more you have to restrict flow. If the ball valve and piping is suitable I might adopt a standard practice, forego flow measurement and set the ball valve handle to a certain ‘known flow’ position at a certain time after takeoff. Then monitor the level in the main tank but otherwise do nothing until turning off the transfer valve per pre-landing checklist.

For shorter flights I’d never open the transfer valve, although one could keep some wing tank fuel held in reserve.

I would be tempted to use a needle valve in line with the current valve.
This way you can set the flow using the needed valve through trial and error on the ground and then wire-lock it in place. Then maybe a test flight to check that flying speed/airflow and attitude does impact it. And an item in your annual maintenance schedule to retest the flow rate.

By example, http://idealvalve.com/flowcal.htm (and in particular the PDF on the page), give the formula and example of what flow rate is achievable with the orifice size.

DeltaP/SG will be the head (or closely linked to it, too late to do math) assuming both tanks are vented in a similar fashion.
The formula has a square root so not linear. So indeed need calculus to get the average flow rate in theory. But if the depth of the wing tank is small compared with the head it won’t really matter.

Graham wrote:

The Vagabond is on an LAA Permit, so anything is legal as long as the LAA approve the mod and your inspector signs it off.

I’d say go forward carefully regardless Make sure the design is LAA approved before committing to hardware, fuel system changes tend to get more attention. Also that your inspector likes how the design is implemented, e.g. non precision filing of a valve might not get the LAA’s attention because the dimensions are all called out, but won’t please the inspector because those dimensions are hard to achieve by the method planned (or not, as the case may be).

(Same as with one-time FAA field approval and IA sign-off on an N-registered certified plane in the US – the process including potential delays is very similar)

The parts won’t be expensive so avoiding pre-buying them is not as important as it might be. But I wouldn’t ‘cut metal’ before its time.

Graham wrote:

Do you have a link to anyone selling it?

Look for V-ball valve or V-port ball valve. Or just get a conventional one with a smallish hole-to-ball diameter ratio and take a square/rhombic file to it.

Ultranomad wrote:

There is a special type of ball valve just for this purpose: instead of a round hole in the ball, it has a teardrop-shaped one.

If you could get one rated for gasoline (which I imagine you can) that would be a neat solution, in combination with your suggestion of a flow meter. I’d size the tubing as large as that supplying the carb, meaning much oversized so that when the valve is fully open it dumps quickly into the the main tank.

The normal legalities apply. I once ran into a situation where a commercially available main fuel supply ball valve fitted under STC was later judged non-compliant because it had a ‘value engineered’ plastic handle (versus metal handle) fitted to the same valve part number in later production.

Is the wing tank larger than 50% of the fuselage tank? Generally that is not the case with an ‘auxiliary’ wing tank so you just open the valve at 50% or less and the main tank can’t overflow. I could imagine that a Vagabond might have a different system.