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What is the Useful Fuel Amount?

R

Rik

Member
Does anyone know the "Useful Fuel Amount" in the RV12?

In the POH it gives max capacity as 19.8 gallons and that there should not be a takeoff attempt with less than 4 gallons in the tank but what is the residual amount of fuel in the tank when the engine can longer run?

Thanks
 
Prior to removing the fuel tank from the airplane, I ran the electric pump to drain the fuel at the gascolator. When the pump started sucking air, I shut off the pump and waited for fuel to flow through the little holes in tank baffle, then ran the pump again. I repeated this process 3 or 4 times until no more fuel came out. The tank was completely empty. Only a few drops came out when the fuel lines were removed from the tank. My conclusion is that 99 percent of the fuel is usable, especially if the attitude of the plane is nose down while descending towards the destination airport. For many of us older pilots, the range of the bladder is less than the endurance of the fuel tank. :D
 
All the fuel is usable except what ever fills the gascolator, maybe a cup? I also changed the EFIS to allow for 20 gallons. I fill to just inside the neck. Works out with the fuel computer perfectly.
 
I find that my usual cruise setting (5100 RPM and 21") burns about 4.5 gal/hr. A three hour flight on a full tank has me landing with about 6 gallons left. With that kind of fuel burn why would you want to cut your margins and fly beyond 3 1/2 hours?
 
Is that fuel available in a climb or descent? Or a slip?
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During a climb, gravity pulls the fuel to the rear of the tank and away from the fuel outlet. Baffles inside of the fuel tank slow down the flow of fuel to the rear of the tank. But if the fuel level is low and the RV-12 is in a prolonged climb, then the outlet port could be uncovered. That is the reason for no takeoff with less than 4 gallons of fuel. If slipping the plane with very low fuel, step on the right rudder to make the fuel move to the left, over the outlet.
 
Just to get an idea, I took off with 5 gal left: it quickly gets pretty scary! As soon as you lift the nose the fuel level goes into the red. It gets back in the yellow when you level. If you happen to go around with this little fuel, it's good to know that you will still be able to fly the pattern despite the ominous warning. I did not try at 4 gal...:eek:
 
WingedFrog said:
Just to get an idea, I took off with 5 gal left: it quickly gets pretty scary! As soon as you lift the nose the fuel level goes into the red. It gets back in the yellow when you level. If you happen to go around with this little fuel, it's good to know that you will still be able to fly the pattern despite the ominous warning. I did not try at 4 gal...:eek:
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That is to be expected since the fuel level sensor is near the front of the main (un-baffled) part of the tank. In a steep climb the fuel moves aft away from the fuel level sensor but the baffled area where the fuel pickup is, is still full of fuel.
 
Taking off with only 5 gallons

I remember talking with a guy in Ohio last year standing at the gas pumps and mentioning to him that I was going to take off with the 5 gallons I had and hop over to a nearby airport that was suppose to have car gas (rather than just 100LL) He had that puzzled look on his face, a cross between "this guy is crazy" and maybe "I should call the FAA on him". Tried to explain to him that I still had about an hours worth of fuel for a 10 min flight, but I don't think it sunk in...;)
 
Brent,

I agree with your math. It's just that old Murphy can really screw us sometimes. I flew with my German cousin in a P92 with an 80 Hp 912. We took off on a 20 minute flight to his private strip with 4 gallons. We landed with about 2 gallons. He seemed very pleased, but I pointed out that if the farmer next door had decided to leave his tractor in the middle of the grass runway we would have been forced into an off airfield landing. Margins are a personal decision. I'm not criticizing you or the guy who was concerned, just saying things don't always go as planned.

Rich
 
If you were trying to land with only 4 or 5 gallons at a fairly short strip with tall trees on either end, would it not be quite foolish to try to land since you would be in the trees if you had to go around?
 
Fair points. It was the one and only time I took off with the small amount of fuel and it was driven by a slightly irrational fear of leaded fuel last year while getting use to the Rotax world. I've since become more comfortable to top up with 100LL rather than take any possible risk of fuel starvation. I change my oil more frequently than even Rotax suggests for my low amount of leaded fuel use so I'm probably just fine. Rarely fly with less than full tank now even for short trips.
 
PilotBrent said:
Fair points. It was the one and only time I took off with the small amount of fuel and it was driven by a slightly irrational fear of leaded fuel last year while getting use to the Rotax world. I've since become more comfortable to top up with 100LL rather than take any possible risk of fuel starvation. I change my oil more frequently than even Rotax suggests for my low amount of leaded fuel use so I'm probably just fine. Rarely fly with less than full tank now even for short trips.
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Just making an observation here. Your comment about "slightly irrational fear" caught my attention. I am glad you came to grips with your own thoughts to make that statement about your thought processes. The relatively low probability of having some undetermined amount of lead be so destructive to an engine that it would cause irreparable harm to your engine possibly could be a problem but really is not very probable. Even if it were to become a problem, to worry more about that expense/inconvenience of repairing such damage than you would over the higher chance of fuel starvation causing serious or fatal bodily injury is indeed a bit "irrational".

Live Long and Prosper!
 
DonFromTX said:
If you were trying to land with only 4 or 5 gallons at a fairly short strip with tall trees on either end, would it not be quite foolish to try to land since you would be in the trees if you had to go around?
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The RV-12 is approve red for take-off as long as there is 4 or more gallons in the tank, so I see no reason to assume the airplane would be in the trees.
 
I just did a 2:31 x country at 5500 RPM and we added 15 gals of gas to the tank to fill it up.

The fuel left had just turned red and was showing 4.0.....Made me nervous as **** landing.
 
The difference in fuel burn versus speed between 5100 RPM AND 5500 RPM hardly makes the extra 400 RPM worthwhile.
 
RFSchaller said:
The difference in fuel burn versus speed between 5100 RPM AND 5500 RPM hardly makes the extra 400 RPM worthwhile.
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The difference in fuel burn versus speed between 5100 RPM AND 5500 RPM hardly makes LOSING THE EXTRA SPEED WORTHWHILE. :D

Different strokes for different folks.
 
Just for comparison, what are you seeing for fuel flow and speed at 5100 and 5500. Looks like FF is 6.0 at 5500rpm, what do you typically see for airspeed. What do the FF and airspeed numbers drop to at 5100rpm?
 
If I run 5100 I get 4.5 GPH like clockwork. That seems to yield about 113 KTAS. I tried 5500 RPM once and got about 119 KTAS. But I've never maintained it long enough to get a reliable value for fuel burn.
 
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