RV-7A Low, Slow and Less Fuel Burn

David-aviator · #1 08-23-2011, 12:58 PM

No question, these airplanes are most efficient above 8000'. But 8000'+ isn't always practical so I gathered some numbers today at 2500', the intent to show how to fly and burn less fuel.

(Local 100LL is $5.25 today.)

It was not a great day to do this as it was bumpy but the data is in the ball park. Engine at about 40 LOP. Numbers are still air.

7 GPH = 130 KTAS = 149.6 MPH = 21.40 MPG
6 GPH = 124 KTAS = 142.7 MPH = 23.80 MPG
5 GPH = 117 KTAS = 134.6 MPH = 26.90 MPG
4 GPH = 100 KTAS = 115.0 MPH = 28.70 MPG

So, if one has time to burn (as opposed to fuel) the reward for flying at 100 vrs. 130 is almost 25%. It does feel a bit strange limping along at 4 GPH, but thats the beauty of the engine, it doesn't care and the airplane flies just fine at 100 knots. It's a simulated LSA flight with a lot of HP in reserve.

I'd like to cut fuel costs more yet by getting away from 100LL and going with 93 mogas - but the fly in the ointment is ethanol. The St. Louis area has dirty air so no fuel sold locally is ethanol free. Go west about 80 miles and it is available.

An alternative is to find a Lycoming mechanical fuel pump with upgraded seals and baffles to withstand alcohol. They are available. Greg Poe, the late air show performer flew on 100% ethanol. I am following that lead, his engine came from Ly-Con in California. If I can get such a pump at a reasonable cost, it would pay for itself quickly in lower fuel costs. (NOTE: Greg Poe died of a heart attack, not from using ethanol)

VETE76 · #2 08-24-2011, 07:22 AM

I am confused. ethenol is bad, yet my 2 friends have been flying their piper cub and champ close to 2 yrs now , the cub 95% of the time on car gas/eth, the champ 60-70% of the time. my question is when will they fall out of the sky? oh yeah, the cub flys .5-.75 hrs every day and the pilot is 82 yrs old, the champ, flys every other day about the same time as the piper. he's 70 yrs old and live right on the edge of the grass strip and the other lives 1.75 miles down the road. they are always flying
fred

lostpilot28 · #3 08-24-2011, 07:54 AM

Quote:

Originally Posted by VETE76

I am confused. ethenol is bad, yet my 2 friends have been flying their piper cub and champ close to 2 yrs now , the cub 95% of the time on car gas/eth, the champ 60-70% of the time. my question is when will they fall out of the sky? oh yeah, the cub flys .5-.75 hrs every day and the pilot is 82 yrs old, the champ, flys every other day about the same time as the piper. he's 70 yrs old and live right on the edge of the grass strip and the other lives 1.75 miles down the road. they are always flying
fred

That's a great question that many RV'ers want to know. The problem is that (I've heard) the ethanol degrades the seals and diaphragm in the mechanical fuel pump. Obviously it doesn't do it overnight...maybe it takes a few years. Maybe your 2 friend's fuel pumps are going the distance. Who knows...but I wouldn't want to be the guy to find out while flying along.

Other than the mechanical fuel pump, are there other parts of the fuel system at risk of breaking down from ethanol? I've got a good friend who wants to use auto-fuel, but it's all ethanol where he lives.

wrongway john · #4 08-24-2011, 08:04 AM

David, thanks for getting those numbers for us. I?m always interested in efficiency. Do you remember the rpms?s for any of those settings?

tmbg · #5 08-24-2011, 08:05 AM

Quote:

Originally Posted by David-aviator

No question, these airplanes are most efficient above 8000'. But 8000'+ isn't always practical so I gathered some numbers today at 2500', the intent to show how to fly and burn less fuel.

(Local 100LL is $5.25 today.)

It was not a great day to do this as it was bumpy but the data is in the ball park. Engine at about 40 LOP. Numbers are still air.

7 GPH = 130 KTAS = 149.6 MPH = 21.40 MPG
6 GPH = 124 KTAS = 142.7 MPH = 23.80 MPG
5 GPH = 117 KTAS = 134.6 MPH = 26.90 MPG
4 GPH = 100 KTAS = 115.0 MPH = 28.70 MPG

So, if one has time to burn (as opposed to fuel) the reward for flying at 100 vrs. 130 is almost 25%. It does feel a bit strange limping along at 4 GPH, but thats the beauty of the engine, it doesn't care and the airplane flies just fine at 100 knots. It's a simulated LSA flight with a lot of HP in reserve.

I'd like to cut fuel costs more yet by getting away from 100LL and going with 93 mogas - but the fly in the ointment is ethanol. The St. Louis area has dirty air so no fuel sold locally is ethanol free. Go west about 80 miles and it is available.

An alternative is to find a Lycoming mechanical fuel pump with upgraded seals and baffles to withstand alcohol. They are available. Greg Poe, the late air show performer flew on 100% ethanol. I am following that lead, his engine came from Ly-Con in California. If I can get such a pump at a reasonable cost, it would pay for itself quickly in lower fuel costs. (NOTE: Greg Poe died of a heart attack, not from using ethanol)

I spoke with Tempest at Oshkosh this year, and they assured me that their new mechanical fuel pump diaphragms are ethanol-compatible. That was the last sticky wicket in the ethanol planning for us.

prkaye · #6 08-24-2011, 08:26 AM

I'm a bit of a fuel miser myself, and sometimes just piddle around at under 5gph. On x-country, I've found I can get good speed at reasonably good fuel burns (around 6gph) if I climb quite high. But with the required long climb (with high fuel burn/slow speed) I always wonder how long of a trip I have to make before the trade-off makes the fuel burned during the climb to over 8K worth while.

wrongway john · #7 08-24-2011, 09:03 AM

I think there is enough interest in this, to where maybe RV?s can start competing in a friendly gas miser contest. Maybe a few already are doing this. Dave Anders had some remarkable numbers up high at 14,000 feet in his 4. At 190 mph ground speed, he burned 4.5 gph, which came out to 42 mpg.

For us mere mortals, I?d be thrilled to death to get 30 mpg @ 150 mph at lower altitudes, say 8,000 feet, and perhaps that is a realistic goal that could be obtained without doing too many extensive modifications.

David-aviator · #8 08-24-2011, 10:18 AM

Quote:

Originally Posted by wrongway john

David, thanks for getting those numbers for us. I?m always interested in efficiency. Do you remember the rpms?s for any of those settings?

I did not record the rpm but should have. I'll do this again tomorrow and will report the findings. I also want to find out how it will do on 3 gph.

The information on the Tempest fuel pump will be verified. I hope it is true.

Flyfalcons · #9 08-24-2011, 10:25 AM

Quote:

Originally Posted by David-aviator

I did not record the rpm but should have. I'll do this again tomorrow and will report the findings. I also want to find out how it will do on 3 gph.

The information on the Tempest fuel pump will be verified. I hope it is true.

It will do best when you're at Vglide.

gear1 · #10 08-24-2011, 11:51 AM

I have been told that the ethanol, being very hygroscopic likes to absorb water, creating a combination which is very corrossive to aluminum. Most of the boating industry strongly recommends that no alcohol be used in the aluminum tanks for boats. it appears that this would also apply to aluminum aircraft tanks, fuel lines and any other aluminum component (like the carburator or FI servo).