Sight gauge connections.

[rodsmith]

Getting the last details completed on the wings before painting. I realized I don't have a good plan for connecting the sight gauges to the tank. I have excess length of 1/4" tubing sticking out of the wing root. I don't think I can measure accurately enough where to cut and flare the tubing to mate with the sight glass without reinstalling the wing. Also wondering if a rigid connection is OK. Would it be okay to use a short piece of hose for the connection? Would like to hear what others have done.

[svyolo]

Not a lot of room for any kind of transition or to work on between the root rib and fuselage. I had the hard line come straight out through 1 rib, cut it, and have a soft line through the root rib to the sight gauge. Sight gauge not installed yet.

[James]

Hi Rod,


I went with Jono Battson's approach, and used schedule 40 clear tube. I built my wings first, then waited till hanging the wings to do the sight glasses.
I had aux tanks installed, so there was a -4 bulkhead tee at the top of No1 rib.

The sight gauge is a lump of schedule 40 PVC, with AN822 fittings each end. The top flared tube is about 3" long, long enough inboard to clear the fuselage. The bottom flared tube runs from the tank through both ribs. You need to drill a 11/16" hole in ribs 1 & 2, large enough to pass the AN818 flare nut through to the tank. Then at No1 Rib I used a grommet with a 3/8" hole in the centre. Use a bit of 3/8" fuel tube about 1" long, and then pass the 1/4" fuel tube through the middle of the 3/8" tube before flaring off the other end. This allows the sight glass line to slide inboard/outboard as needed with fuselage flex, and is the best chafe protection you can hope for.

This set-up is plenty strong enough as-is. But because I also wanted a backing plate of 0.020" aluminium behind the gauge glass to put the lines on for calibration, I anchored this backing plate at the top with another 3/8" grommet squished onto the AN822 fitting, and near the bottom, sandwiched between two p-clamps: one onto the glass and one picking up the diagonal tube of the fuselage. Be advised that I haven't finished my aircraft, and this set-up has not been tested in the real world. Or in my version of it.

Can't go wrong!

James

[Bcone1381]

Rod, could you put a horizontal U-shaped bend in that 1/4 sight gage tube between ribs 1 and 2 to allow some flex and remove or lower the tolerances for an accurate length of the line? Seems like it would accommodate what James spoke of ....the flex between the Fuse and wing.

[rodsmith]

I had thought of that, It is a bit tight on space to make that work on the upper line with the T for the feed from the aux tank, but maybe. Doesn't help that the skin is riveted so have to work through the lightening holes. I guess if the bend is horizontal it shouldn't affect the reading.

[alaskabearhawk]

Planning on using a short piece of hose: http://www.mykitlog.com/users/displa...=281854&row=15http://www.mykitlog.com/users/displa...=281854&row=15

I did weld the AL tube ends closed and drilled a small hole as a safety measure/flow restrictor. Some put in an AN rivet in the end of the AN fitting and drill a small hole. I am going to cut the barb off the end of the blue AN fitting to make it easier to slide on/off.

[Nev]

Rod, could you put a horizontal U-shaped bend in that 1/4 sight gage tube between ribs 1 and 2 to allow some flex and remove or lower the tolerances for an accurate length of the line? Seems like it would accommodate what James spoke of ....the flex between the Fuse and wing.

Just pondering this idea myself, to allow some lateral movement. Then I can install the wing once, with the hookup in place, then connect it. Otherwise I cannot see how to fit it after the wings are mounted, and to get an accurate enough length.

Heres a rough drawing - essentially the same as Brooks suggested above. Although I've drawn the U bends vertically, they'll actually sit horizontally.

Can anyone think if having these bends will cause an inaccuracy in the sight gauge readings ?

002FFDD7-9187-493B-AF64-3731841B5ECE.jpg

[Mark Goldberg]

I do not know how those U bends would work any better than a good AeroQuip or Staroflex rubber fuel line between the wings and the fuselage. Not sure either is needed. Mark

[Battson]

Reflecting on the above drawing and for the benefit of other forum users - in my view, U-bends should not be installed vertically as drawn above. This presents a safety risk, if an air bubble enters the fuel sight gauge line (very likely with low tanks), then it is likely the U-bend will trap fuel in the sight gauge and the sight gauge will therefore over-read the fuel quantity when tank fuel level is below the height of the U-bend.

[kestrel]

Battson, you are correct, but it is more certain than you state. There are no "if's". The region of line between the lower U and the sight gauge will trap fuel to the height of the high point of the U. Except that turbulence and uncoordinated flight will slosh fuel out of that low region, it will never read lower than the high point.

[svyolo]

There is very little space to connect things up when you install the wings. I didn't come straight out from the tank fittings to the sight gauges, I offset them a few inches to give me more room to make the connection. Aeroquip fittings are pretty bulky, especially if you want to come straight out. I almost wish I made the sight gauges vertical, which would have given me a lot more room.

[svyolo]

To connect to the gauges, I made short stub fittings out of flared tube and a flare nut, soft rubber fuel line connected to that. Maybe I will give a solid line a try.

[Battson]

I used a solid, straight line from the tank to the sight gauge. You can bend them around a few times, without the work hardening becoming a problem, provided the holes line up about right.

In terms of length, it will depend how your sight gauge works. For the standard gauge per the Bearhawk book, you don't need to get it especially accurate. It's not difficult to add up all the measurements, and get the location to within a 1/4 inch or so. If you had a fixed gauge with AN fittings, maybe it's more of a problem??

[Nev]

I solved the issue - the local Supercar shop sold the fittings below, allowing me to use flexible hose rather than aluminium tube. This will allow me to adjust the length as necessary after the wings are fitted, without having to remove them again.

C7089D28-6DF9-4F93-80F6-3377932B4F11.jpg

The Supercar shop also gave me a free, very artistic calendar for the workshop. It really seems to compliment the Bearhawk pictures that I have on the walls

[Battson]

How does Sarah feel about this?

[Nev]

Surprisingly very happy - she even said I could sleep in the workshop. Not her exact works, but she seemed very enthusiastic about it

[Battson]

Nev - my sight gauges just poke through holes in the fabric, so needed to be self supporting. They rely on the support from the solid tubes through the wing ribs (in tight grommets) to hold the gauges in place and prevent vibration or wobbling.

How are your fuel sight gauges attached to the interior of the fuselage? Do they fit onto the Kydex?

[Nev]

How are your fuel sight gauges attached to the interior of the fuselage? Do they fit onto the Kydex?

Jon - I made up some .025 fittings for the sight gauges, and the headset jacks that are also in that area. Then the interior wing root panel is Kydex over the top. A bit more weight overall, also a bit more complicated in hindsight. But very secure.

17C5C38F-AA55-4716-80E7-14C34614F1B0.jpg

[spinningwrench]

I like your little flow restrictor, how did you do that?

[Nev]

Sorry, just saw this now. I squeezed a solid rivet into the oriface, then drilled it.

[DBeaulieu]

How does one squeeze a solid rivet into the end of that fitting without damaging the flare seating?

[gregc]

Another idea for implementing a site gauge flow restrictor - cut threads inside the AN fitting and use a hex screw with a hole drilled through the center. For AN4 the .172 ID is about right for 3/16 threads. This one has a .04 hole.

restriction.jpg

[DBeaulieu]

Gregc, I assume you drill and tap from the opposite side of the fitting which seats the flared tubing and not the side shown?

[gregc]

Good question. This is an AN919 reducer fitting that connects to my site gauge. I threaded it and inserted the hex screw from the side shown because I wanted the screw to be to be captured on the site gauge side if it ever worked itself out (unlikely but extra care with the fuel system seems prudent).

[DBeaulieu]

Okay got it!

[Nev]

Reflecting on the above drawing and for the benefit of other forum users - in my view, U-bends should not be installed vertically as drawn above. This presents a safety risk, if an air bubble enters the fuel sight gauge line (very likely with low tanks), then it is likely the U-bend will trap fuel in the sight gauge and the sight gauge will therefore over-read the fuel quantity when tank fuel level is below the height of the U-bend.

Jon thanks for highlighting this. The post above does mention that although the U bends are drawn vertically that the intent was to install them horizontally, but no problem clarifying further. The final version had a smaller horizontal bend for the fore and aft fuel pickups, and I used flexible hoses (Mark G's advice above) for the sight gauges with no U bends. So far it has worked very well.

03C84D3B-029C-4D71-B382-9A43192146B5.jpg

[svyolo]

This subject seems simple, but is much harder in actuality. How do you assemble plumbing in restricted areas? You have to engage threads without ruining them, then screw the threads in without putting undue stress on anything.

I will hopefully post some pics tomorrow.

[Nev]

How does one squeeze a solid rivet into the end of that fitting without damaging the flare seating?

From memory I think I read about this idea in a Kitplanes article. It was pretty easy - I used a Vice. If I was to do it again, I would try what someone else suggested and file a small groove down the side of the rivet. Even a small hole through the rivet allows fuel to slosh in the sight gauges. The hole really needs to be tiny.

[DBeaulieu]

Thanks Nev...I get it now! Cheers...