Fuel Burn Estimating

Kyle Boatright · #1 08-01-2012, 07:19 PM

Historically, the power setting I use, combined with my leaning technique (good or bad), has resulted in a fuel flow of 8 GPH.

On the way back from OSH I wanted to verify the fuel burn with my new Pmag which replaced an Electroair.

Anyway, I got to altitude and flew on the right tank for an hour. Then switched to the left tank (which was absolutely full) and set the timer. I estimated 2:22 of fuel burn (19 gallons @ 8 GPH) until the tank ran dry. As I closed in on that time, two things happened.

First, the airplane became far enough out of trim (one hour of fuel burn on one side, 2:15 or thereabouts on the other), that the autopilot lost track. A little help from the spring trim on the ailerons and the autopilot was happy again.

Second, at 2:21 on the left tank, the fuel pressure indication began to drop (I already had the electric pump on). So I switched back to the right tank and completed the flight home.

I am pretty impressed at how consistant my 8 GPH cruise fuel burn is. From a practical standpoint, that means I have 4 hours of fuel plus a 40 something minute reserve.

Sid Lambert · #2 08-01-2012, 07:40 PM

Before I put the fancy gauges in (GRT EFIS & EIS with FF) I would do about the same.

Fly 30, switch tanks at 2 hours or when empty then switch back.My thought process was that unless I made wholesale changes in throttle or mixture I had at least 1 hour in the right tank if I burned 8 an hour on the other side.

Now I just do 30 on right, fly until I have 4 gallons in left and then switch back to land on right. That's about 2:45 total and that's time for me to land.

RV10inOz · #3 08-01-2012, 10:25 PM

Kyle, good test and congratulations on doing what most people are afraid of doing. Running a tank dry.

You should know EXACTLY what fuel you have, and where it is. In a long range flight having all your available fuel in the one tank is a very smart thing to do.

Better than flying something with 4 tanks and a few gallons in each

Assuming a 180HP engine 8 GPH is about the sweet spot, if you have the 160HP you could be a little leaner still. Around 7.2 - 7.5 GPH off the top of my head.

Bavafa · #4 08-01-2012, 11:50 PM

I rigorously keep a track of my fuel burn at each fill up and compare the actual usage with what my FF and GRT report. It has always been less than .5 G discrepancy. On longer flight, I burn around 8 GPH, this includes the takeoff and landing. My trip planning however is based on 10 GPH but with less reserve, knowing I burn less than 10.

Snowflake · #5 08-01-2012, 11:57 PM

Interesting. On long flights I switch tanks every 30 minutes, to keep the plane balanced. It reduces trim drag from holding the stick off to one side, which should be more efficient overall (although possibly negligibly so...).

Sig600 · #6 08-02-2012, 12:33 AM

Quote:

Originally Posted by Snowflake

Interesting. On long flights I switch tanks every 30 minutes, to keep the plane balanced. It reduces trim drag from holding the stick off to one side, which should be more efficient overall (although possibly negligibly so...).

True statement. One of the biggest drags on Mooneys are controls out of rig (or trim as the case may be). I've flown identical M20J's that varied upwards of 5 knots.

If you're saving yourself even .1 GPH it adds up.

RV10inOz · #7 08-02-2012, 04:29 AM

Indeed!

Manage your fuel to suit your mission.

Flying involves careful thought and execution. Half the fun really

Geico266 · #8 08-02-2012, 05:10 AM

Quote:

Originally Posted by Snowflake

Interesting. On long flights I switch tanks every 30 minutes, to keep the plane balanced. It reduces trim drag from holding the stick off to one side, which should be more efficient overall (although possibly negligibly so...).

I have my Garmin 496 set to automatically tell me to switch tanks after an hour of flight. Works well.

I agree about running the tank dry. On a long haul you know there is no reason to switch back to the tank again. However, my EFIS is set to give warnings of 5 gallons or less in the tank. The red lights become a distraction. I could set the warning to zero gallons I guess.

Kyle Boatright · #9 08-02-2012, 05:26 AM

Quote:

Originally Posted by Snowflake

Interesting. On long flights I switch tanks every 30 minutes, to keep the plane balanced. It reduces trim drag from holding the stick off to one side, which should be more efficient overall (although possibly negligibly so...).

I usually do the same thing. Actually, I go 30 minutes, then switch tanks every hour after that. The airplane is never more than 30 minutes out of balance.

But the idea here was to run on one tank continuously to eliminate the possibility of either forgetting to switch tanks or by having a brain fart and forgetting to track the time on the "cruise" tank down to the exact minute.

rv9av8tr · #10 08-02-2012, 12:34 PM

30 min difference between tanks is only 4 gal, pretty insignificant. Swapping back and forth between tanks just makes it more cumbersome to keep track of exactly how much fuel is where, especially if the flight turns out to be "tight".

My practice for years has been to burn the right tank until exactly 10 gal remains (per EFIS Flow). Swap tanks and burn the left until time to land or it goes empty. Since most all traffic patterns are left hand turns, that gives me 10 gallons in the right tank to work with in the pattern and I don't have to worry about fuel running away from the pickup line while low and making left turns.

My 2 cents.....