Fuel Gauge

[Bcone1381]

I wanted to update this thread about the Fuel Probe Jim and others have mentioned.

I talked the folks at Radiant Technology who offer the fuel probe. They do NOT recommend the Belite Fuel Probe sensor in our application. I believe the variation in tank pressure from a forward facing tank vent effects fuel level readings when using the Fuel Probe. Radiant Capacitance type sender is no longer offered. They recommended using site gages, so that is what I will be using.

[robcaldwell]

So what if the tank vents are modified? I have not seen this detail from Belite about their fuel probe. Isn't Radiant a competitor with Belite?

[Bcone1381]

The probes seem to be a very sensitive pressure sensor. So if your tanks internal pressure varies, the I fear the sensor will pick that up and indicate it on your fuel gage. Ram air pressure from a forward facing tank vent may drive the indicator to always read full.......
Radiant is a division of Belite. I agree that this detail is not covered in Belite's fuel probe resources.
I contributed in this thread to thinking the fuel probe might be a good product.......
After reading a 2015 vansairforce.com thread I followed up with a phone call to Radiant, and felt that our community should be informed.

[JimParker256]

Now I'm wondering if we could use the Belite "bingo fuel detector" at the lower sight gauge fitting to annunciate "almost empty" levels. Anyone know roughly how much fuel is left in the tank when that lower port goes dry? (Might not be enough usable to matter, is my fear... But if there's 5 gal or so, it would be viable.) It would at least give a warning of low fuel by flashing a light on the panel.

If I was using a header tank, I would definitely want one of these to let me know the header stopped re-filling from the main tank(s).

[Bcone1381]

In the old days we ran tanks dry. I'd keep an eagle eye on the old fuel flow gage (the FF gage sensed fuel pressure in the FI flow distributor and gave us accurate FF reading in pounds per hour.) and when it twitched, I'd throw on the pumps and switch tanks real quick. That was SOP back then. I think things might be different now. I never had passengers.....and the canceled checks didn't mind. The engine never skipped a beat in that scenario. No one ever forgot either. If you forget Jim, you'll be the first.

Back to todays FF indications....why do we use a Red Box FF sensor. The pressure feed off the flow distributer seems pretty simple and no moving parts. But I really have no experience and dont know what I'm talking about. I'll probably install one on mine some day soon too.

But the Bingo Detector seem like a good product to help keep one from a self inflicted moment of silence.

[whee]

In the old days we ran tanks dry. ...

This was my SOP for managing fuel in my Luscombe and I fully intend to manage the fuel in my BH the same way. I won't know how the fuel injected engine likes it till I try it and won't be trying it till later in flight testing. However, with modern fuel flow gauges I think there are proably better methods for managing fuel. Once I get my red cube calibrated and confident in its function it is possible I'll change the way I do things.

I don't know anything about bendix fuel injection so can't speak to that. Continental uses constant flow fuel injectors and the flow is increased or decreased via more or less pressure thus making pressure a viable and accurate way to measure fuel flow...unless your fuel system gets out of calibration. I used a red cube because it was plug and play with my EFIS.

[zkelley2]

It was sop to run the tanks dry on the TIO540 on the pa31.

[JimParker256]

I ran one tank dry in my IO-540 powered Commander with a CFI on board, and while over an airport. We did it for two reasons: 1) so we would know how much usable fuel was REALLY in the tank, and 2) so I could experience the process with a CFI who had "been there, done that" before. Yes, the fuel flow (EI CGR-30P) flickered for a few moments, then the engine just quit. Switched tanks, turned on the electric boost pump, and the engine restarted within 5 seconds. It was really no big deal at all. Not even an adrenaline rush, since I knew it was coming... Might be different for a passenger not familiar with airplanes...

On the plus side, the "flickering" of the fuel flow was more than adequate indication that the tank was becoming empty. If I was ever in a "range extension" situation, I would just switch tanks when the flickering started.

PS - turns out the Commander tanks held EXACTLY their rated "usable fuel" capacity. Good to know.

[nborer]

I lurk more on these threads than I post, but I learn something new everyday. Didn't know about the Belite probes. When I took my first fluid mechanics course (now over two decades ago!), we learned about the hydrostatic equation, and I wondered then why we didn't use pressure sensors for fuel gauges. After drawing one up (this was for a motorcycle), I largely concluded that it was due to cost of the sensors and any programmable logic devices vs. the analog systems (floats, etc.) in use at the time. Needless to say, prices for such systems have changed a lot since 1998.

Thinking back to any early trade from the 1998 musing, one thing I calculated was whether one or two sensors were needed. It seemed logical that just one was needed at the bottom for the motorcycle application. However, the altitude variation of airplanes, and the pressurized fuel systems (never mind g-loads), makes me think that a second sensor at the top, specifically calibrated to a finer resolution (say 100-1000x smaller than the bottom sensor), could help take out the difference due to changing air pressure at the top of the tank. The fine resolution would give up sensor range, but I figure if you put it at the top of the tank and the sensor is saturated (full-scale), well, you know you have full fuel, and you don't need to measure pressure. If the top of the tank had air (pressurized or otherwise), this could be picked up by the upper sensor, and differenced from the lower sensor to give better fuel readings, regardless of fuel venting.

I'd think such a system could work well with any sight-gauge design - there are already lugs in the top (sensitive sensor) and bottom (main sensor) positions. It also alleviates some of my concern (shared by others in other threads) about the leakage/crashworthiness concerns with sight gauges that go through the wing root and into the main cabin, but leverages the current tank design.

What do you think? Should I get back to hammering on ribs instead of musing about systems I won't install for years?

[rodsmith]

That sounds like a system that would work. It is discouraging to hear that the Belite system may not work accurately in the Bearhawk. I also thought it was a good alternative to sight gauges, even though personally I had decided to stick with them.

[OhioLSA]

Some wise aviator long ago impressed upon me that the most pessimistic indication of fuel quantity is the one you should believe. Only believe a fuel gauge if it shows LESS fuel than you've determined from your burn calculations. If it shows that you have MORE fuel than you thought you had, consider the gauge a liar. If it shows LESS, it may be indicating a leak or siphoning out of an unsecured filler cap, in which case I would heed its warning (it may still be a liar, but I can determine that after I'm safely on the ground.)

I'm installing sight gauges (I like Battson's design), and I'm considering Brooks' suggestion that the sight gauge should be made to stay with the wing in that worst case scenario.

But I'm also going to make a simple dipstick, which I'll mark with notches while fueling up the first time. One of the most critical parts of fuel management is knowing how much fuel we have in each tank before we start the flight. Fortunately that's easy to do on the ground. Looking forward to someday having a flying aircraft, and doing the testing to start to zero in on fuel burn at various power settings and conditions, and checking the sight gauge from time to time just to make sure.

[svyolo]

I am not sure having the sight gauge "stay" with the wing would help anything. You still have 2 fuel lines per tank that won't stay connected if the wing separates. Fuel is going to go get out, rapidly.

I did come across some 12 fuel shutoff valves that are "normally" closed. That means no electricity, no fuel comes out. Either switched off manually or a catastrophe. Pretty small and light also. Too late for them in this build, but I might try to think about incorporating them later.

Then again, that is one more thing that can fail, and make the engine go silent.

[Bcone1381]

I am re-evaluating things and may be installing sight gages in contrary with my concerns. I have yet to see an installation that avoids the wing-fuselage issue, and don't have a good fix for it so far. I once heard "airplanes were made to fly not crash." and "Perfection is an enemy to completion."

[robcaldwell]

For concerns about plastic or PVC tubes for fuel sight gauges, CubCrafters sells these glass sight gauges: http://store.cubcrafters.com/Fuel-Si...el_p_1213.html