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  #11  
Old 03-06-2018, 08:35 AM
rv7charlie rv7charlie is offline
 
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Add a couple of gallons, pump til no flow, pull the quick drain & measure what's left.

Then remove the flop tube.

:-)
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  #12  
Old 03-06-2018, 08:46 AM
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az_gila az_gila is online now
 
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Quote:
Originally Posted by JonJay View Post
I get your logic but when most of us can measure unusable fuel in ounces, it wouldn't matter what you used "out". It is going to be an obvious difference between a partial gallon and several gallons.
Not if you have a 10% error in the "full" mark on a 5 gallon container.

Some of mine were that far out.
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  #13  
Old 03-06-2018, 10:37 AM
BillL BillL is offline
 
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Quote:
Originally Posted by KatanaPilot View Post
Next time I will completely drain the tanks, weigh the airplane, fill the tanks and weigh again. Might be a little more accurate, but not convinced accuracy of scales is any better than that of the pump meter. Plus density of fuel changes with temperature, so weight/gallon comparisons might not work too well.
I discussed accuracy of the pumps with my airport manager. He told how they were calibrated and the legal standards. I would trust the pump above all for delivering a known volume. It less than a tenth of a percent. Probably changes as soon as it is dispensed, though.

3 gallons sounds like a lot.

I took a gallon jug and put in a measured gallon of distilled water (by weight), marked the jug and then used it as the measurement container for fuel. Using the mark and then weight used the density found from that for unusable. Poured in the gallon and pumped out what would come. Did each tank separately. Shut off pump when hearing cavitation, did not wait for depletion or pressure drop. Just to be consistent. The unusable was higher for the first tank-first test, just due to filling the system. There were just ounces left in each tank, not flight usable, but good for that condition. Tail low, tail high and level conditions all nearly the same result. The stock pickups are right on top of the drain. I can see mine.

If you do the tanks separately then the issue of the flop tube should be clear.

BTW - Although the gages are calibrated to 17 gal, I still don't know exactly how much fuel my tanks hold. The level on a dipstick that was extrapolated from 17 gal shows more than 21, high probability it is wrong. I am going to have to drain and tow the plane to the pump when weather permits. Even then, I am not sure what the definition of "full" is. Is a measured distance from the upper skin?

Let us know what you find.
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Last edited by BillL : 03-06-2018 at 06:03 PM.
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  #14  
Old 03-06-2018, 12:11 PM
KatanaPilot KatanaPilot is offline
 
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Quote:
Originally Posted by rv7charlie View Post
Add a couple of gallons, pump til no flow, pull the quick drain & measure what's left.

Then remove the flop tube.

:-)
Unfortunately, the tanks are now full from the calibration. At some point, I will again pump them out and then remove the quick drains.

I'm not overly worried about this right now - too busy getting ready for first flight. I will get it figured out though.

Flop tube is staying given the amount of effort made putting it in and a certain young pilot son of mine who actually likes negative G's. Or so he thinks...
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  #15  
Old 03-06-2018, 12:55 PM
Ralph Inkster Ralph Inkster is offline
 
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Location: Calgary, Alberta
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When I perform these tests, I use a calibrated container to pore fuel into the tank in 1 or 2 gallon units, this insure I have an accurate record of what went in. I also have my 5 gallon containers calibrated so I have a fairly accurate idea what is in or drained into that container during the test.
I have found in 7/7A tanks that they can actually hold slightly more than the stated 21 gallons per side, fuel filled to bottom of filler ring, overfilling during operations would not be desirable as the vent circuit would get filled.

I do the test on one tank at a time (level) as fuel flow could be affected by differences in the plumbing & pick-up & if there is a notable difference, I would go back & find out why. This portion of testing validates the plumbing.

For us in Canada, the required fuel flow test is performed not level, but at the anticipated most extreme angle of attack, to test the delivery (pump or gravity) system to insure engine fuel volume needs are meet.

---
For the OP's concern of the fuel left in the tank after level testing, I would be concerned that the pick-up tubes are set too high in the tank, check if the rigid pick-up was bent to the right position & if there is some reason the flop tube is being held off the bottom skin.
---

For fun I googled "unusable fuel" & got the following: (BillL basically stated the same)

In aviation, usable fuel is the fuel on board an aircraft that can actually be used by its engines. The opposite of usable fuel is unusable fuel.[1]

The unusable fuel figure is calculated for an aircraft fuel tank in "the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank".[2]

The figure usable fuel is used when calculating or defining other key figures of an aircraft such as MTOW, zero-fuel weight etc.[citation needed]

Usable fuel is the total amount of fuel in an aircraft minus the fuel that cannot be fed into the engine(s): fuel under the pump-intake, fuel behind ribs of a tank, fuel in lines between the tanks and the engines etc. As this figure is calculated/defined for a plane in level flight it is possible that the engines of an aircraft run dry (out of fuel) even when the amount of usable fuel is still above zero, such as if the wings are not level and/or the angle of attack is higher or lower than when cruising. The inverse is also possible; in some conditions, fuel can continue to be fed to the engines when the usable fuel is below zero


It would be informative to get a DAR's (& MD-RA's in my jurisdiction of Canada) official interpretation of 'Unusable Fuel' is.
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  #16  
Old 03-06-2018, 02:12 PM
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RV7A Flyer RV7A Flyer is offline
 
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Quote:
Originally Posted by KatanaPilot View Post
Unfortunately, the tanks are now full from the calibration. At some point, I will again pump them out and then remove the quick drains.

I'm not overly worried about this right now - too busy getting ready for first flight. I will get it figured out though.

Flop tube is staying given the amount of effort made putting it in and a certain young pilot son of mine who actually likes negative G's. Or so he thinks...
I think a few people have dropped clues here...is the flop tube properly installed, with anti-rotation and anti-hang-up brackets, and did you at least audibly confirm it was not hanging up when you built the tank by moving the tank in all axes and listening for the tube to move appropriately?

A fuel pickup problem on a new airplane sounds...bad. Make sure you don't have one.

BTW, my unusable fuel quantity was less than single fuel strainer on one side, and something like 2-1/2 fuel strainers' worth on the other. I just pumped out the fuel in the tank using the boost pump into containers, then used the strainer to sump whatever was left. Easy peasy, don't know why everyone is making it so hard with measuring/weighing fuel going in and out and up and down and all that.

ETA: BTW, you might try to simplify your procedures a bit...quit worrying about how much is being pumped IN to the tank for a while, save that for your fuel level calibration task. Just put a few, say 5, gallons into the tank, then pump it out with the boost pump until the pump "runs dry". Pull the quick drain and see what comes out...that's your unusable fuel.

If you're into the gallons range of unusable fuel per side, something is wrong.
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Last edited by RV7A Flyer : 03-06-2018 at 02:19 PM.
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  #17  
Old 03-06-2018, 02:29 PM
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Bob Martin Bob Martin is offline
 
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Quote:
Originally Posted by RV7A Flyer View Post
I think a few people have dropped clues here...is the flop tube properly installed, with anti-rotation and anti-hang-up brackets, and did you at least audibly confirm it was not hanging up when you built the tank by moving the tank in all axes and listening for the tube to move appropriately?
Post 5 above states he has QB tanks!
This makes it subject to workmanship error, especially since it is an internal part issue. The main reason I suggested using a borescope in the fuel drain to look around.
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  #18  
Old 03-06-2018, 02:49 PM
Aluminum Aluminum is offline
 
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Quote:
Originally Posted by az_gila View Post
I found that the "full" marks on my 5 gallon and 2 gallon containers were way off.
Yup. The best way to measure fuel quantity with high precision is by weight. Even a cheap bathroom scale eliminates several sources of systematic error that can foil visual estimation. To wit, gasoline will expand by about 3% from freezing to 80 degF; the container, who knows. A nice digital scale will have better than 0.1% accuracy over this temperature range.
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  #19  
Old 03-06-2018, 04:02 PM
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RV7A Flyer RV7A Flyer is offline
 
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Quote:
Originally Posted by Bob Martin View Post
Post 5 above states he has QB tanks!
This makes it subject to workmanship error, especially since it is an internal part issue. The main reason I suggested using a borescope in the fuel drain to look around.
Ah, righto...yep, borescope (will most of the common ones fit in that opening?)
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  #20  
Old 03-06-2018, 05:01 PM
KatanaPilot KatanaPilot is offline
 
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Yes, QB tanks. Flop tube installed at Synergy, anti-hangup brackets installed per the plans.

Easiest "fix" would be inaccurate meter on buddy's fuel trailer.

Doubtful there is a problem with the pickup in the QB right tank.

I'll get it figured out. Thanks for the opinions.
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