R

Rocketscntst

I'm New Here
New RV-7A owner trying to solve my fuel gauge issues and wondered if any of you have experienced any similar problems that might help shortcut my troubleshooting.

I emptied the left tank then filled it in 2-gallon increments. The gauge was very accurate up to 12 gallons but then it all went to heck. Above 12 gallons the gauge was pegged at full. I realize that wing dihedral, tank shape, etc. all factor in but for the sensor to report accurate values for just over 1/2 tank and then go immediately to full-scale does not make sense.

Any explanations and/or suggestions from the experts out there?

Thanks.
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RV-7A Bought (while I build the next one)
N92BF
 
Common for the breed ...

Pretty much all two seater RVs do that. The combination of tank shape, angle, dihedral, etc mean you can't even see gas in the tank until about 5 gal, and most of 'em have the float at it's max point at 12-14 gal. That's why having a fuel flo sensor and totalizer as a backup is so handy. My tanks both show full until there's less than 14gal in each... only then do the float sensors start to read accurately. Any overall consumption less than that and I use the flo totalizer.
 
Do you have floats or capacitance senders?

I have capacitance senders in my -9 and they read all the way from full to empty. They are simply the most accurate fuel gauges I have ever had. Well, other than the float and wire in my '41 T-Craft.
 
What kind of sensors do you have?

What are you using for indicators? EFIS or Steam or ???

Do you have any sort of converter between your sensors and the indicator?
 
I have float senders and Van's steam gauges, no converters that I know of. I measured a resistance between the sender terminal and airframe ground of 71 ohms at 12 gallons and 130 ohms at 5 gallons. I initially thought these values were low but the gauge did display the correct fuel level.
 
Rocketscntst said:
I have float senders and Van's steam gauges, no converters that I know of. I measured a resistance between the sender terminal and airframe ground of 71 ohms at 12 gallons and 130 ohms at 5 gallons. I initially thought these values were low but the gauge did display the correct fuel level.
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Just saw that you are new to VAF. Welcome to the good ship VAF John. I hope you learn here and provide us with learning as well, professor.

Added food for thought. I have the same setup. Van's senders are more accurate and reliable with a separate ground wire all the way to the ground tree (not airframe grounded). That seems to be the consensus. I find that to be true on my -6 and the accuracy is indeed in the lower end of the scale. I even ran the tanks dry in flight to see exactly how much run time I had when the needle hit the empty peg. (about 7-8 1/2 minutes depending on the tanks)
 
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Regardless of what the gauges say, my suggestion is you make sure you have an accurate fuel flow and fuel totalizer as either a separate instrument or part of your EMS.

Make sure you get the K factor correct and accurate.

Then one at a time (doing both at once will save time but may attract undue attention) run a tank dry in flight?..not on the ground. Come back and fill it up. Then you know your usable fuel per side. Remember do both.

Use the fuel totalizer for fuel management and the gauges as a backup or cross check of the fuel computer.

Last of all on a long flight where fuel range is is fully utilised, run one tank dry in the cruise and finish the flight on one that has ALL the remaining fuel in it.

Better looking at it than looking for it! ;)
 
Everything above.

On my RV-8 with float senders, they peg at about 15 gallons. There was no difference between the original Vans gauges and when I install an EFIS and did the incremental tank fill to calibrate the unit.

The very first upgrade I did when I bought my RV-8 was a fuel flow monitor. It was the best thing I did that first year. Getting the k-factor set is important. I gradually refined my setting over several tanks of fuel.
 
The airplane does have a seemingly very accurate fuel flow meter/totalizer which is very useful. However, the engineer in me has a hard time believing that the gauge system is designed to only be accurate when less than 2/3 full. It would seem that if you ever developed a fuel leak in flight having accurate gauges would be the only way to know how much fuel is left in the tanks.
 
Welcome to VAF!

John, welcome to VAF.

Good to have you aboard.

Put on your engineer cap and draw a simple line drawing of the wings (front view) and be sure to include dihedral. Fuel float is at the bottom of the V.

It is easy to see that the range of motion of the float arm is less than what the fuel level can attain at the far end of the V.

Personally I use my dip stick to check fuel as my #1 source of fuel quantity information, my fuel flow totalizer as my second, and the float gauge as the last.

As I recall, the FAA requirement for fuel gauge accuracy is that it should be reading empty correctly.
 
Mike, Gary - Thanks for the welcome and the suggestions and commentary. It is wonderful resource having a forum devoted to Van's aircraft. I currently use the same 3-step process for monitoring my fuel quantity and I'll continue to hope I never develop a catastrophic leak in flight. I wonder if anyone has ever designed a system using a sender on each end of the tank to report an accurate quantity based on the two inputs?
 
Rocketscntst said:
I currently use the same 3-step process for monitoring my fuel quantity and I'll continue to hope I never develop a catastrophic leak in flight. I wonder if anyone has ever designed a system using a sender on each end of the tank to report an accurate quantity based on the two inputs?
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Welcome aboard John!

As was mentioned above, the capacitive sending system will give you more accurate readings throughout the full range of the tank - but they are more finicky, can be affected by the type of fuel, and modifying a tank in a flying airplane would take considerable effort. Everything is a trade - I guess that I have never seen a fully trustworthy fuel gauge in forty years of flying, so I am biased towards fuel flow and totalizers (modern technology is wonderful!), plus using my watch and a knowledge of that the engine burns.

Interestingly enough, I did have a full-on fuel leak in my old Yankee one time - a fuel hose rotted out and I dumped a whole side of fuel on a cross-country. The roll imbalance gave it away. In an RV, you'd be hard pressed to find a way for a catastrophic fuel leak to develop, since it is a hard-line system. You'd notice if your fuel cap came off.


Oh, and I can't believe no one has said this yet, but....it ain't rocket science! :) (Said by a former rocket engineer....)
 
Try looking into capacitive fuel sensors. IIRC, they will read total fuel range, no moving parts to run out of travel as the tank fills above the floats range of motion.

Should be some good info here, get familiar with the "Search" function. A hint when searching, be sure to reset the time frame, it defaults to last year of input.
 
Ironflight said:
Welcome aboard John!

As was mentioned above, the capacitive sending system will give you more accurate readings throughout the full range of the tank - but they are more finicky, can be affected by the type of fuel, and modifying a tank in a flying airplane would take considerable effort. Everything is a trade - I guess that I have never seen a fully trustworthy fuel gauge in forty years of flying, so I am biased towards fuel flow and totalizers (modern technology is wonderful!), plus using my watch and a knowledge of that the engine burns.

Interestingly enough, I did have a full-on fuel leak in my old Yankee one time - a fuel hose rotted out and I dumped a whole side of fuel on a cross-country. The roll imbalance gave it away. In an RV, you'd be hard pressed to find a way for a catastrophic fuel leak to develop, since it is a hard-line system. You'd notice if your fuel cap came off.


Oh, and I can't believe no one has said this yet, but....it ain't rocket science! :) (Said by a former rocket engineer....)
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Well said Paul!
 
I've been flying my RV8 for 2 years now and I think I've finally figured out some of the fuel gauge tricks that I'd like to share for everyone's benefit:

1. Ground problems: ProSeal is a great insulator, and both tanks gave me grounding problems (over time). Solved that by using a single star lock-washer mounting the fuel sender to the plate, and one when mounting the plate to the tank. I originally had a grounding wire connecting them, but removed it later and the single washers are enough to give me a good ground. I highly recommend this technique ... had a problem with both sides before it finally got it right, and others have reported the same problem.

2. Unequal fuel quantity reading between sides. Installed per plans, my right side tank float did not drop as far down (as the left side) when empty. With 12 gallons in each side, the opposite effect was seen (right side float was higher). I could figure this out my viewing the ADC readings from my MGL Xtreme EMS. Calibrating the tanks gave me better readings, but they weren't the same at different fuel levels, which caused me to pull the floats last night (is everyone else as anal as me?). Upon inspection, I saw that the plans called for the rubber float to be installed pointing forward on both sides (symmetrically, which seems to make sense). Problem is, the wire arms are not symmetric, and the net effect was that the right rubber float was offset forward by about 3 inches when compared to the left,which caused it to bottom out early on the belly of the tank (that rises going forward), and not match the left side at other levels. Reversed the right rubber float (points aft now) and problem solved. Now both tank readings match at similar fuel quantities.

Additional notes others might find useful:

1. I experimented with various pitch angles in order to find one that matches the flight envelop the best. Found that raising the tail so that the rudder stops were 36" above hangar floor worked best. Use this and you'll only have to calibrate one time (instead of using my trial-and-test technique).

2. You will find that fuel level reading max out at about 13 gals./side with the tail down because of the dihedral of the wings. Values in the air will be more accurate (once the tail is up).

Hope this is useful info for people ... fly safe!
 
Mike,

Thanks for the insight on #2 above. My senders read slightly differently on the L vs. R, and I never thought to check the direction of the floats. Discrepancy explained! Doubt that I'll break the seal to change them around, though. So far no leaks in the tank end plate, and I'd rather keep it that way than fix the minor discrepancy.

Greg
 
I am going to try pressure transducers for fuel level reading, along with fuel flow which I have in my current airplane and LOVE! I don't know it the transducers will work, but I don't think they will be worse than a float system. All fuel gauges suck in my experience and are basically there to meet a regulatory requirement. The only reliable one I have ever seen was a friend's Champ that had a cork attached to a wire sticking up through the cap.

So I am going to try to be smarter, but that doesn't always work the way you hope. Stay tuned.
 
Rocketscntst said:
The airplane does have a seemingly very accurate fuel flow meter/totalizer which is very useful. However, the engineer in me has a hard time believing that the gauge system is designed to only be accurate when less than 2/3 full. It would seem that if you ever developed a fuel leak in flight having accurate gauges would be the only way to know how much fuel is left in the tanks.
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Those gauges ARE accurate, at that end of the tank. When mine read full, there are no worries. But if my tanks are reading 2-3 gal each, then I am looking to land. Plenty accurate at the end of the scale that matters.

However, the pilot in me was always drilled never to rely on fuel gauges. Forget accuracy; what do you do if they fail? I visually verify the fuel before I fly and monitor my performance and usage accordingly. As for the leak concern, a minor leak won't affect my calculations much (I had a dripping drain valve that would probably empty my tank ... in a couple of weeks). If there is a catastrophic leak, then there is probably damage that is going to prompt me to land immediately, not stretch out my flight to the extent of the fuel. In either case, the gauge is not part of the equation. IMHO, fuel gauges are there to make a pilot feel better if he forgot to check the tanks in his pre-flight. ;)
 
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