Orifice size for fuel sight gauge restriction

[rodsmith]

I'm having to retrofit these. Decided I want a restriction to the gauges in case of a leak, and also to damp the fluctuations. I can't find what size orifice builders have used. My aux fuel pump discharge is plumbed into the upper sight gauge lines. Wondering if with restrictions in place if you can still tell when the aux pump is pumping fuel and not air.

[alaskabearhawk]

Hey Rod...the info that I have archived says a #59 drill/.040 orifice for the restrictor size. Also from what I gather the aux pump "bubbles" still do their thing.

[kestrel]

From another thread:

I used a 1mm (0.040 ish") drill. I'm not sure it would be possible to cut a slit much smaller than that. But Nev is right, the fuel does still slosh around, but much slower than with no restriction. My last plane had no restriction and the gauges were almost useless in any kind of turbulence.

Also, I didn't build my Bearhawk and haven't disassembled the wing root enough to know for sure, but I would guess that my aux tanks are plumbed into the lower end of the fuel sight gauge. My sight gauge fill with fuel when I run the transfer pumps until the transfer pump is pumping air. At that point I get air bubbles rising in the sight tube. I'm not sure how it might behave if plumbed into the top of the sight tube system.

[AZBearhawk272]

Rod,

Aux tanks transfer better if plumbed into the bottom of the sight glass location at the tank boss. Then bring the line off to the bottom of the sight gauge.
Place an orifice of .040 at the inlet of the sight glass and an orifice of .030 at the top of the sight glass going back into the tank. The two orifices in series will damp the sight glass much more effectively.

Plumbing the Aux tanks into the upper sight glass location is good for keeping the fuel in the main tank from siphoning back to the Aux tank when on slope or if pump check valve leaks.
If you route the line from the Aux tank to the Main tank high with an anti siphon bend above the upper sight glass port that will usually stop back flow.

Bubbles will go up the sight glass when transfering fuel and be easily visible that transfer is occuring or stopped.

Kevin D
# 272
KCHD

[rodsmith]

Thanks for the info. Wish I had known the bottom port was the better location to pump into. As I recall the wing instruction manual had a picture of it plumbed to the upper port. Won't be changing that, would probably have to remove the wings and fuel tanks and still would be difficult.

[jaredyates]

Maybe we can fix the manual. Was this at bearhawk.tips or somewhere else?

[rodsmith]

The manual is still on bearhawkaircraft.com. Part Five, page 39, photo and description of plumbing aux tank pump outlet to T into upper sight gauge line.

[jaredyates]

Thanks Rod, the manual there is the one that we updated when creating the KBM at bearhawk.tips. But there wasn't any mention at bearhawk.tips about the merits of routing to the top vs the bottom. I have updated that article to add Kevin's input. If any of you would like to take a look at the language here please let me know if it can be better: https://bearhawk.tips/3406
If you don't have easy access I can email you a pdf.

[Richard Nicholson]

Rodsmith why would it be better to pump into the bottom?

[rodsmith]

See Kevin's reply below.

[AZBearhawk272]

When pumping into the bottom, the transfer is pushing into the fuel pressure head of fuel in the main tank. This helps the typical FACET type pump seal better internally and is therefore a more efficient transfer. Less back pressure when dumping into the empty atmosphere in the tank.

Creates less aeration of the fuel.

When the aux tank goes empty there is a noticeable stream of bubbles in the site glass indicating evacuation of the aux tank. This is far less prevalent or noticeable when plumbed into the top port.

That said, it is not the perfect solution.

Kevin D
#272
KCHD