[GETST8FRM]

I recently purchased an RV-9A with standard 18 Gal. tanks. I have been getting usable fuel info. all over the map..ie: 1/2 gal. un-usable per tank, up to 2 gal. un-usable per tank.
Is there a standard answer for true usable & un-usable fuel per tank??
By the way, I could not be more happy with my purchase. Great speed & I can't get it to burn any fuel <----Joke. I appreciate your help.

 

[scard]

It is generally accepted (but best proven by testing your own setup) that un-usable is just a few ounces. As it is in my -9A tanks.

 

[flyboy1963]

Feeling fuelish

I also bought a 9a and wondered about the fuel, but am too chicken to run a tank dry!
I just drained a tank to seal leaky fuel sender screws, and looking into the tank with a mirror ( too find out why gas was still squirting out the holes!) i see it's fairly open thru the nose ribs.
If you look at the drawings, the tank lets fuel run down to the pickup, but in a prolonged turn ( or bank, such as an approach into a strong x-wind) toward the low tank, it appears the pickup will be sucking air if less than half a tank is present!
I recall seeing a drawing with a little one-way flapper door which is common in wing tanks, so maybe some builders can help us out here!

My bigger boggle is how to accurately 'dip' the tanks, when only the top half can be accessed thru the filler cap! I'm thinking a plastic ( glass?) sight gauge on the cabin wall would be a welcome addition, if all the fittings were in the wing root to preclude leaks, fire hazard etc.

enjoy!

 

[9Driver]

We pumped our tanks dry using fuel pump (tri gear) and wound up with 4 ounces unuseable in each tank. However I decided to use 3 gal as a more realistic figure because of the posibility of fuel sloshing away from the pickup in rough air. We made our own calibrated dipstick by adding 2 measured gallons at a time and marking the tube at that point. Incidentaly at four gallons fuel is just visible directly below the gas cap.

 

[Kevin Horton]

If you look at the drawings, the tank lets fuel run down to the pickup, but in a prolonged turn ( or bank, such as an approach into a strong x-wind) toward the low tank, it appears the pickup will be sucking air if less than half a tank is present!
I recall seeing a drawing with a little one-way flapper door which is common in wing tanks, so maybe some builders can help us out here!

If you fly in a prolonged turn, where is the ball on the turn coordinator? I'm betting it is pretty close to the middle (the lowest point of the curved tube it rolls in). The same force that keeps the ball in the middle will keep the fuel in the inboard end of the tanks.

During a crosswind approach, why not simply crab with the wings level during the approach, and switch to the wing down method late in the game? If you want to use the wing down method for the whole approach, I'd put the fuel selector on the high tank.

 

[kentb]

And the answer is.....

18.1 gal.

I have run my tanks dry in flight. I was a little supprised that the tank took 18.1 to fill to the bottom of the filler neck. I usually like to leave a 1/4 inch below to the bottom to allow for expansion, but wanted to see how much I could add.

Kent

 

[rvbuilder2002]

It is generally accepted (but best proven by testing your own setup) that un-usable is just a few ounces. As it is in my -9A tanks.

It is usually a very small amount but as Scott already mentioned...It is important that you verify what the unusable fuel is for your airplane. There have been accidents related to the variables that are possible. It is easiest to do it in flight but the thought is un-nerving to some people. Using the electric boost pump to drain the tank in to fuel jugs works also.

 

[GETST8FRM]

So the answer is???

I like to land with a minimim 45 minuite fuel reserve. I have a hard time getting my 0-320 to burn 8 gal / hr. If there is 1/2 gal. un-usable in each tank, that means I need to have 3.5 gal. in each tank at my minimums ( I can accept this number). If there is 2 gal. un-usable in each tank that would mean I need 5 gal. in each tank at my minimums. (5 gal. in each tank =10 gal. total. Seem like a lot of un-usable fuel)
What are the minimum's other RV-9A pilot's use???
What is the safest way to to determine un-usable fuel per tank???
By the way, I am seeing Van's published performance numbers easily. I am surprised / pleased. I picked up my 9A 3 weeks ago in Oshkosh, flew it home to an airport just outside of Jackson, MS & averaged 7.1 GPH. Love my 9A!!!

 

[N941WR]

...
What are the minimum's other RV-9A pilot's use???

30 minutes for DVFR, 45 for NVFR translates to three & 4.5 gallons.

What is the safest way to to determine un-usable fuel per tank??? ...

If you don't want to run a tank dry in flight, run both tanks down down low. Level the aircraft. Pull the quick drains out and empty the tanks. Re-install the quick drains, pour a 1/2 gallon in each side. Disconnect the fuel line at the carb and point it into a calibrated gallon jug. Turn on your electric pump and run each tank dry. Look at how much is in the jug after the electric pump starts sucking air. The difference is your "unusable".

 

[kentb]

Unusable or Usable.

My tanks are placard for 18 gal. After running the tank dry (engine starts to stumble), I switched to the other tank. When I got back to the airport I added 18.1 gals to that side. I repeated the process on the other tank and got the same answer.

We each build the tanks slightly different, so the quantity will very with your plane. I don't know how much is unusable in my tank, but I do know that the engine will stop after using 18.1 gal.

I would not normally run my tanks dry, but did this for testing purposes during phase I.

During normal flight I would switch off of a tank that got down to 3 gal. I would land somewhere before the second tank got to the 3 gal point.

Kent

 

[airguy]

I don't know how much is unusable in my tank, but I do know that the engine will stop after using 18.1 gal.

And this is all we really need to know. Fill your tanks to the same point every time, and you know how much fuel you have available. The requirement of knowing the unusable fuel left in the tank is only a backhanded way of finding the truly relevant parameter of how much USABLE FUEL you have available.

My 2 cents - install a fuel totalizer and fill it to the tabs every time, and be done with it.

 

[GAHco]

My Thoughts...

I thought that usable fuel was the amount that was available during normal flight attitudes.

We don't all fly the same, some fly at more extreme attitudes than others, intentionally.

Play it on the safe side. I don't want to hear my engine sputter from lack of fuel unless I am planning to do so for research. After that I would strongly consider a decent safety margin once determining the sputter point, and that is what I would consider unusable.

http://www.avweb.com/news/leadingedge/leading_edge_8_unusable_fuel_196061-1.html

Be safe, live to post another day

 

[L.Adamson]

My 2 cents - install a fuel totalizer and fill it to the tabs every time, and be done with it.

The fuel meter/totalizer is the best advice. I previously didn't have one, and had to make educated guesses, since the gauges are rather worthless. My totalizer is always within a 1/2 of a gallon per full tanks. That's close enough, that I don't even worry about fine tuning it. But it's sure much nicer to know exactly what's used, and what's left.

L.Adamson --- RV6A

 

[N941WR]

... the gauges are rather worthless. My totalizer is always within a 1/2 of a gallon per full tanks. That's close enough, that I don't even worry about fine tuning it. But it's sure much nicer to know exactly what's used, and what's left.

L.Adamson --- RV6A

Larry, I would agree with you except I have the capacitance fuel gauges and they are remarkably accurate, as long as I use 100LL.

With them I never look at the numbers my fuel flow give me.

Truth is, the combination is worth its weight in gold.

 

[24Golf]

Need Ride

Evan... this has been a very interesting thread to me - I just wanna know when you are going to take me up! You know, those of us building need a little carrot to keep us focused.

 

[Leland]

Larry, those are impressively accurate readings. Where and how did you mount your flow sensor?
Leland

 

[Mel]

My totalizer is always within a 1/2 of a gallon per full tanks. That's close enough, that I don't even worry about fine tuning it.

I have calibrated mine to be 1 gal conservative at the empty.
When I fill up, it always takes about 1 gallon less than the totallizer says.

 

[L.Adamson]

Larry, those are impressively accurate readings. Where and how did you mount your flow sensor?
Leland

The place that's not considered best by some.

I have a 6A, and the auxiliary fuel pump is on the left side of the cockpit floor, just past the gear tower. The flow sensor is between the aux pump & the firewall. Tucked as close to the side skin as possible. I also put a filter in each line as it comes from the wing to the fuel selector.

The first time I used it, it was 3/10ths of a gallon off, at 22 gallons. And that was 3/10ths less than what it said. I figured that was good enough, and didn't do any farther adjustments. I've set the full fuel at 36 gallons for a 38 gallon tank. Plus and minus adjustments can be entered into the menu if required.

L.Adamson

 

[Mel]

My transducer is mounted similarly to Larry's except mine is upstream of the boost pump just after the selector valve. I also have filters upstream of the selector valve. Been working very accurately there for 16 years.