Lycosaurus

Lycosaurus

Well Known Member
I have searched this topic, however I am not clear on a couple of points.

As an example:

First fill the left tank with say 2 gallons of fuel. Use electric pump and meter out the fuel from carburetor hose. Fuel remaining is unusable fuel in tank, and in gascolator and fuel lines.

Fill the right tank with 2 gallons, use electric pump and meter fuel out of carburetor hose. Fuel remaining is unusable fuel in right tank plus some fuel left over in the right tank line to fuel valve. The common fuel lines and gascolator will have already been filled.

These two numbers will not be the same even if the pickup position is in the same spot for left and right. Which is correct? All we want is the left over fuel in the tank, no?

If the tank had some fuel in it, how would you go about emptying the tank completely so you could do the unusable fuel measurement again. Looks like a one shot deal unless you can find a way to safely suck out the remaining fuel via the fuel drain somehow.
 
I think you are close on what you want to accomplish, but consider this:

Put exactly 2gal in a largish container. The container should ideally "neck down" at the top. You need to mark the container with exactly the two gal you will be starting with.

Now, pour all of the two gal of fuel into the Left tank, and turn on the pump. Catch all of the fuel that comes out in the same container that you originally marked. What ever does not get put back in the container is your "unusable" for that tank plus the fuel line distribution network.

Your container should now hold less fuel than you started with. Use a calibrated syringe (medical supply) and fill the syringe with fuel (not from any of the fuel you started with...go get some more from the pump), and meter the fuel from the syringe back into the original calibrated container.

Stop when the fuel level in the container reaches the original level that you started with and using the marks on the syringe, you can compute the amount of fuel you just added. This is your unusable fuel amount for that tank.

Now, without adding any more fuel to the container (and it should be 2 gal in this example) pour all of that fuel into the Right tank, turn the fuel selector to select the right tank, and proceed to pump all of the fuel back into the container. Whatever does not come out will be the unusable fuel for that tank. You do not need to account for the fuel distribution lines because they were done in the previous tank.

Once all the fuel has been pumped into the original container, again use the syringe and fill the container back up to the original starting level. This will be the unusable amount for that tank. Add this Right tank value to the previous Left tank value, and you now have the total amount of unusable fuel.

Next step would be to verify fuel flow volumes for your ac and engine type.

The last step will be to fill the tanks and record each tank capacity.
 
noelf said:
Add this Right tank value to the previous Left tank value, and you now have the total amount of unusable fuel.
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Thanks for your posting, Noel. Agreed, you can calculated total unusable fuel for both tanks and lines.

I was looking for unusable fuel for each tank. For that, you would need to fully empty the left tank. I guess you could use a syringe with a small hose to try and get the left tank as empty as possible, and repeat the test for that tank.

Did others go through this as well, or did you just average out the unusable fuel for the two tanks?
 
I may be missing what you are asking, but...

From my previous post, the amount of fuel you add back to each tank / calibrated container represents the amount of fuel that is unusable for that tank, plus the fuel in the fuel line distribution network (and gascolator if installed). Adding these two numbers represents total amount unusable.

Are you also trying to determine how much fuel is in the fuel distribution network that is related to each tank?? If so, I guess that you could measure the length of fuel line for each segment and deduct the amount of fuel that each contains. I believe the easiest way to prove this is to take the measured amount of fuel line and construct an equivalent length of 3/8" line (if that is what you used), fill it up and measure how much comes out. At this point, I do not know how accurate this will be (what decimal point precision are we shooting for ?). The electric fuel pump will be pumping fuel, and ultimately the tank pick-up port will become uncovered. At what point the electric pump actually stops pumping fuel / how much fuel is in the line / how much volume of the lines becomes filled with air is a guess.

Oh, I guess you also need to drain the gascolator and measure that amount.

Is this what you are asking??
 
noelf said:
At what point the electric pump actually stops pumping fuel / how much fuel is in the line / how much volume of the lines becomes filled with air is a guess.

Oh, I guess you also need to drain the gascolator and measure that amount.

Is this what you are asking??
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Yes, sort of. The left tank will fill its line (not a big deal) but also the common lines, gascolators, fuel pumps etc. This fuel will be left behind in the lines, but not in the tank. So the actual unusable fuel in the tank will be less than measured.

As you suggest, I could drain the gascolator (which would probably be the largest single source of fuel containment in the fuel distribution system as it stands). The amount of fuel in the lines is probably not that much at this point. It would be interesting to know how much that is.

I could blow the lines out and find out how much more fuel is being stored in these lines, but I think I am just over-doing this. I'll just follow your method, since in the end, the RVs end up with such a small quantity of unusable fuel.
 
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Why not drain the tanks, then pour some fuel in each tank, and pump them dry. Measure the amount of fuel that you can drain from each tank, and that is the unusable fuel in that tank. This assumes that the tank drain is at the lowest point in the tank.

Note that the amount of unusable fuel will vary, depending on the aircraft's longitudinal and lateral acceleration, which will determine where the fuel will sit in the tank. E.g., if you are at high speed in an aircraft with a constant speed prop, and pull the power to idle, the negative longitudinal acceleration (i.e. the aircraft's deceleration) will push the fuel towards the front of the tank, temporarily increasing the unusable fuel. Similar story with sideslip, except the fuel will be pushed outboard on one of the tanks.
 
Kevin Horton said:
Why not drain the tanks, then pour some fuel in each tank, and pump them dry. Measure the amount of fuel that you can drain from each tank, and that is the unusable fuel in that tank. This assumes that the tank drain is at the lowest point in the tank.
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In the 9A, the tank drains don't seem to be at the lowest point (pre-punched kit). They are maybe an inch from the inner rib. That would leave some fuel behind. That's why I was thinking of using a syringe with a small hose to pull out the remaining fuel. We're probably talking ounces here, but I hear that this is the order of remaining fuel in the RVs.
 
Geico266 said:
Alfio, Can I ask a question? Why the obssesion with such a small amount of fuel?
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Don't know how much fuel is really left. Maybe ounces.... don't know. If I had placed the fuel drain myself (kit is prepunched) I would have placed the drain hole closest to the inner rib. That way you can get rid of more water and sediment.

Not an obsession, just asking what other people did, and why. It's a forum:)
 
I think knowing the amount of usable fuel is more important than knowing the amount of unusable fuel.

The worst case condition for unusable fuel is probably an idle power, full rudder sideslip with full flap at VFE. So, in flight, with adequate fuel in the tanks, note the pitch attitude and bank angle when established in a full rudder, constant heading, sideslip at idle power at VFE. Then, on the ground, put the aircraft at that bank and pitch angle, then pump all the fuel out with the boost pump. Then put the aircraft in an attitude so the drain was as close to the lowest point as possible, drain the remaining fuel and weigh it. This is the worst case unusable fuel (except for the extremely tiny bit that cannot be drained). Pour that fuel back in, then go to the gas pump and fill it up ? the amount of fuel it takes is the usable fuel.
 
Numbers and resolution...

Lycosaurus said:
Don't know how much fuel is really left. Maybe ounces.... don't know. If I had placed the fuel drain myself (kit is prepunched) I would have placed the drain hole closest to the inner rib. That way you can get rid of more water and sediment.

Not an obsession, just asking what other people did, and why. It's a forum:)
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...come into play here.

If you are working to an accuracy of 0.1 gallon - which is 12.8 ounces (aka 1.5 cups) then you have much more accuracy you need. Any more is down in the noise level in practical terms.

A tenth of a gallon is about equal to 45 seconds of flight at a consumption rate of 8 gallons per hour.

For all practical purposes, this is way more resolution that the other inaccuracies in the system, and to be more accurate that 2 cups or so creates more effort than any possible gains.....:)
 
In an ideal world

In an ideal world with a static attitude, it would be nice to know.
In a dynamic world (ie: flight), unusable fuel is only an estimate.

I assume you are looking for a good place to consider the tank empty for flight planning purposes and endurance. Personally, my comfort level is a 1 hour reserve.

Seems like I read that in a 7/7A that 2 gallons a side left was a good place to safely call tanks empty which lets you have 19 a side for usable fuel. Since I'm still in the paint shop, I cannot tell you real life experience.

For the folks out there - Is 2 gallons a side a good estimate of unusable for planing?
 
I did it the easy way - close enough for me.

Webb said:
For the folks out there - Is 2 gallons a side a good estimate of unusable for planing?
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I just did this in my 7 taildagger:

I went the simple route. I leveled the airplane and put 2 gallons (metered from a truck into a 2 gallon gas jug for verification). I then marked a line on the 2 gallon jug at the gas level.

I poured the gas into the right tank and then we started the electric pump and caught the fuel at the injection servo in the same jug. I shut the pump off when the sound indicated the pump was starting to cavitate and the flow meter indication began falling. I did not run the pump dry.

There was less than 8 oz. missing from the line marked on the jug. I know because an 8 o.z. cup of avgas refilled it to the line and a bit above.

Repeated on the other tank and got the same result. Bottom line is 1/2 gallon per side is being conservative on my bird. Do not take my test for granted though, its too easy to do your own and test for your own installation. Your fuel pick up may be in a different location!

BTW, Just for kicks, I put the cup under a wing drain and got another 4 o.z. out of it.
 
Pucker Factor

Joe - If I was even close to a 1/2 gallon, I would be at full "pucker factor".

Thanks for the info. It makes me wonder what others are using for "usable" fuel in their POH.
 
KISS

Webb said:
......It makes me wonder what others are using for "usable" fuel in their POH.
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I really don't see the "real world" point in attempting to account for every single drop of fuel. How would that knowledge help faced with an outright fuel emergency?

Today, I filled the -8 tanks with 100LL for the first time so I could calibrate the FL-2C capacitive fuel gauge. The calibration procedure is a precise multi step process. Van's specifies that each -8 fuel tank has a 21 gallon capacity. That number became my starting point.

Based upon the number 21 and to make the calibration process easier by using 5 gallon increments, I first poured one gallon of gas into each tank and mentally declared that amount to be "unusable." I then started the incremental tank filling and calibration process. When the tanks were filled and the fuel gauge calibrated to indicate 40 gallons total.....20 gallons in each tank.....there was STILL room in each tank for more fuel. I went on to add exactly one more gallon of fuel into each tank. With the tanks topped off now totalling 44 gallons of avgas on board....2 additional gallons over what Van's specifies, I will declare the unusable fuel in the yet to be written POH to be 2 gallons per tank. I have no desire to ever flirt with fuel exhaustion and when the fuel gauge indicates....say 2.5 gallons per tank, in my mind a nearly empty condition....I will consider the tanks nearly empty. What gallons actually remain to slosh about or what few ounces of fuel may happen to occupy the fuel lines into the engine compartment is just not that important to me....but an adequate margin of error always is. As always, YMMV.
 
I ran each tank dry (separate flights of course) then determined how much was usable by the amount added on the ground. I plan on 0.5 gallon unusable (37 gallons usable).
 
The ultimate reason I want to know the unusable fuel amount is to establish the "empty" weight of the ac (yes,I also need to add in the oil quantity weight) and this gets applied in the CG calculations of empty vs full and the crew plus baggage. I guess I do not need to know where this fuel is physically distributed within the fuel plumbing system, as long as I do an ac empty weight measurement with the unusable fuel accounted for.

Unusable fuel also allows me to calculate usable fuel (weight / time /range) by looking at what I add to each tank. Just my opinion...
 
Way back when...

I was doing my unusable fuel tests, the airplane was on boards in the hangar, leveled as in flight. I used the electric boost pump to get out all the fuel via the hose removed from the fuel pump on the back of the engine. I then used the fuel drains on each tank and got only a few ounces of fuel from each tank. For practical purposes, call it zero remaining fuel. Your mileage may vary depending on the shape and placement of your fuel pickup tubes inside your fuel tanks.
 
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