[rv8180]

I've done some research but would like to fortify that with your opinions / experience on how to determine useable fuel for my RV8QB.

Thanks!

 

[chrispratt]

Usable fuel

At the risk of sounding facetious, you fly the plane until you run out of gas in one tank and then land and fill it up. Since it's a 21 gallon tank, the math is easy. If you think I'm kidding, I've done this. I fly until one tank reads 2 gallons on my EI gauges (I'm near an airport naturally just in case the other tank has a problem). I then set the power to normal cruise and note the amount of time the aircraft will fly until the fuel pressure takes a precipitous drop and then switch tanks. This not only tells me the one tank is dry but also how accurate my fuel gauge is since I know my fuel burn.

The first time I did this I flew the right tank dry. I landed and filled the tank. It took exactly 21.0 gallons per the meter on the pump. On another occasion, I verified this by unscrewing the fuel drain plug and measured the amount of fuel that came out. It was less than one of those small dixie cups you use for mouthwash.

A secondary benefit of this check is that it told me (without having to take the fuel senders out per Van's SB) that my fuel pickup was in the right place and had not moved.

If you haven't flown your -8 yet and are asking how to mark your "Usable Fuel" placard that the DAR is sure to check (hi Mel ), I'd suggest using 20 gallons for the standard RV-8 as a reasonable number. You can always change it later when you've done you're own testing.

Just my thought. Hope it helps.

Chris

 

[hevansrv7a]

Lazy version

I've been accused of being lazy on these pages before, so here I go again.
I agree with everything above. I called Van's and got essentially the same information. I set my GRT EIS to read fuel level, empty, as -1 gallon on each side, just for safety. I did not fly my tank(s) dry, not really wanting the risk of bad consequences - unpredicted. My fuel flow meter, as far as I can tell from fillups, is accurate and it is usually within a gallon of the float-indicated levels.
I have noted that on my -7A (same wings as the -8/8A?) it is wise to fill no higher than the forward side of the filler lip.

 

[n5lp]

Unusable Fuel Definition

I certainly don't know that this language applies to Amateur Built aircraft but it sticks in my mind that unusable fuel is supposed to apply to the "most adverse" planned flight conditions, and that would seem to make sense.

If someone really wanted to determine this, shouldn't they, at the minimum, find out when fuel no longer feeds at the maximum normal nose up and nose down condition? Maybe not necessary to determine unusable fuel for a roll or reduced G maneuver, or maybe so, if that is normal.

At any rate, I certainly didn't go to those lengths, but to really do it right it would seem logical.

I have done the simple thing of running a tank dry in level flight, but that only tells me about that flight condition.

 

[AlexPeterson]

You probably need to fly high over an airport, run a tank very low, doing steep side slips with the tank wing low. I believe that will be the worst condition. Not too common, but it will be worst case. It is advisable to make sure that the other tank has fuel in it. Monitor fuel pressure, as that will be your first indication that the tank is sucking air.

 

[jbDC9]

I just checked the unusable fuel on my 8 a few days ago... well, sorta. It was just the left tank, I'll get to the right one in a day or two. I used the "run the tank dry" method and it worked fine. Orbited the Brenham, TX airport at 4500' for a bit with the left gauge reading empty. On the EIS I watched the fuel pressure start to drop, saw the Big Red Light come on, pressure went to 0, engine started to sputter. I guess it's empty? Switch tanks and boost pump on, the engine was purring again after a second or two. No big deal. Refueled at Brenham and was surprised to see the pump meter at 21.1 gals... works for me!

 

[Bob Martin]

unuseable fuel

Good Post, and right on time as I'm installing new fuel tanks and a new EI gauge in my -6.
on the ground.......
couldn't I level the plane to simulate cruise flight, then add a gallon of fuel.
Then unhook line to carb, or undo gascolator and turn on boost pump to drain all fuel from the tank, via the pickup.??
Seems to me that would be pretty close to flying a tank dry.
Then I can measure what I pour into the tanks to calibrate the gauges.
Wouldn't this work?

 

[Ron Lee]

That sounds good Bob but...

It does not give you the useful experience of running a tank dry then switching tanks to get the engine running again. I have run each tank dry twice in flight and for my -6A I use 0.5 gallons unuseable (level flight). The full value on my RMI engine monitor is then 37 gallons.

 

[gasman]

Ran the 6A out of gas (one tank) at 3k over O69 fuel pressure went to "0" motor quit, switched tanks counted to three with no use of electric fuel pump, the 0320 fired right back up. There was less than 10oz left in the tank. good to go.....

 

[Rick6a]

I've done some research but would like to fortify that with your opinions / experience on how to determine useable fuel for my RV8QB.

Thanks!

I suspect that just because the RV-8 fuel tanks are rated at 21 gallons does not necessarily make it so. Data point. The fuel tanks for the -6 series are rated at 19 gallons capacity. Upon the inital carefully controlled fuel filling...the procedure calls for a few gallons at a time to precisely calibrate the capacitive sending system, I discovered with the fuel selector in the OFF position.... each of the fuel tanks I built hold.....19.9 gallons each. That totals 1.8 gallons more than the placarding suggests....not an insignificant amount. I really don't know how much of that is useable though. My point is if you really want to get to the truth of the matter, start from square one.

 

[ao.frog]

Here's what we did...

Here's how we did it today:

We disconnected the fuelhose just before the Silverhawk Fuel injector and put the fuelhose into a Jerry can.

With approx 2 gls in each tank, and the plane in level flight attitude, we started the elec boost pump and ran it until it started to suck air, then turned it off.

Then we drained the rest of the fuel out of the tank with the drain valve and measured the drained fuel.

We found that each tank contained approx 0,13 gls of fuel (0,5 liter)

To compensate for flight attitude etc, I think I'll set the unusable fuel to 1 gls on the calibration menu of the AFS-3500.

What has other RV'ers with the AFS-3500 or 2500 done?

Thanks.