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RV-7A Low, Slow and Less Fuel Burn

David-aviator

Well Known Member
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)
 
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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
 
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.
 
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?
 
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.
 
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.
 
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, 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.
 
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.
 
Gasoline with ethanol

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).
 
As I have posted in the past, I fly evening local flights as often as I can (fog permitting). My hangar is within 1000 feet of the end of the runway. And the first turn off is at 700 feet and is at my hangar row. And no tower.........
I fly an RV6A with an 0320 carb with one LSI and a Sensenich prop. The airport is 90 feet asl. When I start the motor, I lean and start to taxi. My gps is set to start the trip at 50 mph and end it at 50 mph. I will depart the airport and climb to 2500' and then set the the rpm to 2100 and manifold comes in at 21" also. As I change alt, I maintain that same number of 42 (total of rpm/10 and mp) as I climb and mp drops 1", I add 100 rpm to keep 42. I lean to rpm drop then enrich just to recover.

My point is that after six plus hours of flight on a tank of fuel, my gps average speed for over 50 mph is 140+ mph. And my fuel burn has been a consistent 4.8 gph.

Now, I understand that there is taxi time in there, that's why I gave you distances. But remember there is also at least 6 departures. So I decided to try another approach. I would get established at 2500' leaned in cruse at 42 (see above) and then switch tanks and fly for 30 minutes then switch back, land and top off that tank.

The results of five different flights doing the same pattern was in gallons used for the 30 minute flight..... 2.4, 2.3, 2.4, 2.4, and 2.5. This has been consistent for the past three years.
 
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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.

keep your power in on the descent while leaned, make all of that performance back with your potential energy
 
keep your power in on the descent while leaned, make all of that performance back with your potential energy

Except that you don't get it all back because you're in the climb for longer than you're in the descent. Mathematically, it's the same reason why "every wind is a headwind" on a round trip... you spend more time flying against it than you do with it.
 
Except that you don't get it all back because you're in the climb for longer than you're in the descent. Mathematically, it's the same reason why "every wind is a headwind" on a round trip... you spend more time flying against it than you do with it.

It takes me 9 minutes to get to 11,000 feet at gross; it takes me 22 minutes to get back down.

As for wind your right, a head wind hurts you more then a tail wind helps you because you?re in the head wind longer.
 
Yep.

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

Neither of those airplanes have fuel pumps....they're gravity fed.

Best,
 
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).

Craig,

I picked this up off the internet after a search of "ethanol and aluminum".

The Negative Affects of Ethanol on Recreational Boat Fuel Systems

The National Marine Manufacturers Association (NMMA) opposes the use of marine fuels that contain greater than 10% ethanol content by volume. This position is based on safety and durability concerns and supported by many well documented studies.

According to state boating registrations, there are over 12 million recreational boats in the United States. Boat builders utilize five types of materials to fabricate fuel tanks. These are aluminum, steel, cross-link polyethylene, high density polyethylene and fiberglass. For a rough estimate of today?s numbers, there are approximately four million boats that have aluminum fuel tanks; approximately seven million have steel or polyethylene tanks, and less than one million have fiberglass tanks. The data clearly indicates that the increased use of ethanol in gasoline has raised safety and durability issues for aluminum and fiberglass fuel tanks.

Aluminum Fuel Tanks

In the case of aluminum tanks, aluminum is a highly conductive metal that relies on an oxide layer for its corrosion protection properties. Low levels of ethanol, such as E10 (10%), are usually not a problem in aluminum tanks because the oxide layer provides a good measure of protection. The problem occurs when the ethanol content is increased.


I used E10 fuel about 90% of the time when messing with Subaru engines. There was a film in the tank inside, you could take a stick with a piece of cloth and remove it. That could have been the oxidation they mention. I believe that film is present in all RV tanks, no matter the fuel.

I spoke with a Tempest rep this morning. They do not recommend ethanol fuel with their pumps. That being said, the diaphragm is of a material that probably will be ok with E10, but there is one o-ring that is not ok. The problem, these pumps are certified and as such the materials are what they have been for a long time.
 
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

Neither of those airplanes have fuel pumps....they're gravity fed.

Best,

They don't have a mechanical pump even?

As stated, no pump of any kind, gravity takes care of fuel flow.
 
This is a pretty helpful discussion and I want to thank the participants for sharing their data.

I can't offer a direct comparison since I have a Cessna 180 with a Continental O-470, carb and mags, no fuel pump. Still, for what it's worth:

165 mph at 7,500 and full throttle and 2,300 rpm and 10.8 or 11 gph.

155 mph at 9,500 and full throttle and 2,300 rpm and about 9.5 or 9.8 gph

115 mph at 6,500 and 15" manifold with 2,100 rpm for 6.5 gph. This is too rich and the valves pick up carbon.

105 mph at 6,500 and 15" and 2,100 rpm for 5.5 gph. This is my evening local flight setting.

The bottom of the green arcs are the 15" and 2,100 rpm figures so I don't go below that except in the pattern. Best rate of climb is about 85 mph IAS.

Dave
 
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)

Just remove the mechanical fuel pump and run two electric fuel pumps. At under $50 each, run as many as it will take to feel good. If you are worried about your current dual electrical system, then another $20 will buy you a back-up battery (3x3x4" and 3.5#) that will run the pump for over four hours.

That fuel pump will go the way of the vacuum pump...........no longer needed.
 
There are several RV's out there that run on 100% ethonal. The Vanguard Squadron based out of Tea, South Dakota have four RV-3's. I met a gentleman this past weekend that also runs 100% ethonal in his RV-6A. He told me he has 600 hrs with out any issues. The Vanguards have been running it for I believe real close to twenty years. I spoke with the A&P that built some of the Vanguards planes and he believed that the seals and hoses produced in the last 20 years are all compatible with ethonal. You do need high compression pistons from my understanding which limits you to ethonal or 100LL. My .02 for what it is worth.
 
David,
I enjoyed your post as well as the replies. I thought you might like this link to gas stations who sell gas without Ethanol.

http://pure-gas.org

Keith

Keith,

Thanks. I've been there and its like a huge black out over eastern Missouri. Go west to Kingdom City and ethanol is available. In Springfield many stations proudly show 93 mogas "no ethanol" but not around here.
 
I find the ethanol debate very interesting.

Not airplane related per se, but I have been running for 2 years now e85 in my automotive "toy". So far I love it. Have not seen any issues I can blame on e85 yet, fuel injected fuel rails are aluminum, weldon fuel pump, rubber lines, etc. I did see some black buildup where the injector spray pattern hits the intake manifold. Not sure its e85 related though.

The nice thing is the 110 octane which I would think the planes would also enjoy.

In my process of running it I have seen a LOT of speculation by people that have no first hand experience. Granted if your engine stops in a plane its a bit more concern than if it should stop in my car.
 
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?

John,

Here are the rpm numbers added plus a 3 GPH line.

7 GPH = 130 KTAS = 149.6 MPH = 21.40 MPG = 2180 RPM
6 GPH = 124 KTAS = 142.7 MPH = 23.80 MPG = 2090 RPM
5 GPH = 117 KTAS = 134.6 MPH = 26.90 MPG = 1930 RPM
4 GPH = 100 KTAS = 115.0 MPH = 28.70 MPG = 1740 RPM
3 GPH = 78 KTAS = 89.7 MPH = 29.9 MPG = 1590 RPM

One of the guys in our geezer lunch group is a retired Navy flight test pilot and we talked about min L/D which is max endurance and from that point on a graph, determining best range. He showed how they would extend a line from the left bottom corner of the graph to where it touched the curve coming down as speed is reduced to the bottom of curve (min L/D). The point where the line touches the curve coming down represents best range. In most cases it is just a bit above min L/D.

At 3 GPH, this airplane is very near the bottom of the curve. At one point speed bled off and I thought I would have to add power but it came back, it probably was caused by the unsettled air. I should have tried 2.5 GPH but it was getting time to talk to SUS tower. Next time I will see if this thing will fly at 2.5 GPH.

My opinion on max endurance and max range is that they are a function of angle of attack. When the weight of any aircraft changes as fuel is burned off, these speeds are less than at the starting weight. Fuel flow and speed are the variables, AOA is the constant.
 
49clipper

David,
I took my rv-6 to Paducah on Teusday for some wiring issues and followed my neighbor down in his C-177B. To follow, I had to throttle back to 1900rpm and was only burning between 4.4 and 4.8gph. We were doing about 124mph at that power setting. I leaned it moderatly at 5500'. Nice and cheap. Took forever to get there though. Your numbers are close to mine. I have a O-320-D1A though w/Sensenich.
Jim
N189EM
 
Temperatures?

I wonder what effect (if any) these slower airspeeds have on engine temperatures (EGT, CHT, Oil).

I've read reports of slower airspeeds and steeper climbs causing high temperature concerns, but nothing about slow speed cruise effect on temperature.
 
John,

Here are the rpm numbers added plus a 3 GPH line.

7 GPH = 130 KTAS = 149.6 MPH = 21.40 MPG = 2180 RPM
6 GPH = 124 KTAS = 142.7 MPH = 23.80 MPG = 2090 RPM
5 GPH = 117 KTAS = 134.6 MPH = 26.90 MPG = 1930 RPM
4 GPH = 100 KTAS = 115.0 MPH = 28.70 MPG = 1740 RPM
3 GPH = 78 KTAS = 89.7 MPH = 29.9 MPG = 1590 RPM

Thanks for getting those rpm numbers for us David. That?s some good stuff to look over. I kind of like that 2,090 rpm and 6.0 gph figure the best since you are still going 142.7 mph and getting close to 24 mpg. Heck, that's what my Ford Ranger pick-up gets.
 
I wonder what effect (if any) these slower airspeeds have on engine temperatures (EGT, CHT, Oil).

I've read reports of slower airspeeds and steeper climbs causing high temperature concerns, but nothing about slow speed cruise effect on temperature.

I was wondering about some of that stuff too. Here’s Lycoming Flyer Operations that cover this and other aspects. Scrolling down about one-third it has one section entitled: Considerations for low-power low RPM cruise. It seems Lycoming has been getting plenty of requests of those asking for lower power cruise in an effort to save fuel.

It says the 60-65% power setting should be expected to also give us longer engine life, which it says is generally around 2,100-2,200 rpm’s for most of our engines. It does mention going lower and getting into the 1,800-1,900 range, but to be aware of not exceeding manifold limits. It states that low manifold pressures, below an arbitrary point of perhaps 18" for continuous cruise, may cause excessive oil usage and oil buildup in the valve guides which could lead to sticking valves.

Lots of other good stuff in that link that deal with saving fuel and proper leaning techniques.

Another interesting thing they took note of was rpm settings and what was required at various attitudes. According to them, 75% power is not achievable above 7,500 feet, although I understand Van’s specs for 75% cruise (WOT) were done 8,000 feet, and I'm sure not turbo or supercharged. Here’s some data on this:

Let’s look at the airframe manufacturer’s power chart for the O-360, 180 HP engine, and observe the gradual increase in RPM required with the increase in altitude, but maintaining 75% for cruise at each altitude. What the chart will not show here is that for flight above 7500 feet, it is not possible to achieve 75% power with a normally aspirated engine (meaning not turbocharged or supercharged).
POWER CHART
Percent Endurance on
Altitude RPM of H. P. 59 gals. fuel
2500 2550 75% 4.8 hours
3500 2575 75% 4.8 hours
4500 2600 75% 4.8 hours
5500 2625 75% 4.8 hours
6500 2650 75% 4.8 hours
7500 2675 75% 4.8 hours

If others experience similar results, I guess endurance remains the same during different altitudes even while 75% power is maintained at higher rpm’s. But I'm sure the real benefit to flying higher would have been the extra mph and distance we would have obtained. It would have been interesting to have seen that data as well.
 
Try a point at 3.5 gph

David, I think your 4 gph and 3 gph points are bracketing the best L/D point.
Try 3.5 gph, and I bet you'll get over 30 mpg.
 
Best range

by the way, what you described from your retired navy friend is not quite right, but is almost right for jets.
Jets and prop planes have different "best range" speeds in performance analysis because of the way trust varies with speed with constant fuel flow.

For jets, the range is proportional to V x (L/D), so if you go faster than best L/D speed, your range increases until L/D has decreased by the amount that V has increased. You want to maximize V x (L/D).

For prop planes, its easier. range is proportional to L/D. You want to fly at maximum L/D to maximize range, or mpg. For RV's, that is around 105-110 kts I think. In my phase I climb-rate testing, I got a very flat L/D curve from 100-120 kts, hard to get good enough data to tell. Others may have better data -- I'm sure Kevin H. can tell us his.

Yes, the speed for best L/D changes with weight too, as you observed. It is more or less a constant angle of attack. You can adjust your speed for weight change by multiplying the best L/D speed at a known weight by the square root of the ratio of the weights.
 
David, I think your 4 gph and 3 gph points are bracketing the best L/D point.
Try 3.5 gph, and I bet you'll get over 30 mpg.

7 GPH = 130 KTAS = 149.6 MPH = 21.40 MPG = 2180 RPM
6 GPH = 124 KTAS = 142.7 MPH = 23.80 MPG = 2090 RPM
5 GPH = 117 KTAS = 134.6 MPH = 26.90 MPG = 1930 RPM
4 GPH = 100 KTAS = 115.0 MPH = 28.70 MPG = 1740 RPM
3.5 GPH =88 KTAS = 101 MPH = 29.96 MPG = 1650 RPM
3 GPH = 78 KTAS = 89.7 MPH = 29.90 MPG = 1590 RPM
2.5 GPH = 65 KTAS = 74 MPH = 29.60 MPG = 1400 RPM

Not quite 30 mpg, but close.

The airplane will remain air borne at 2.5 gph but the AOA meter has come off the peg but is in the green. This thing stalls clean at about 56 knots. I wonder if 5? of flaps would have increased lift a bit and resulted in more speed. The reduce AOA might have offset the increased drag at 5.

This thread is perhaps totally mundane for the typical RV aviator, most are interested in how fast they can go, not how slow. :)
 
I wonder what effect (if any) these slower airspeeds have on engine temperatures (EGT, CHT, Oil).

I've read reports of slower airspeeds and steeper climbs causing high temperature concerns, but nothing about slow speed cruise effect on temperature.

This morning at 3.5 gph, the high cht and egt were 326/1326 respectively.

Oil temp was 129 right after take off.

I don't climb at low speeds. It is usually WOT and 110-120 knots to be able to see over the nose and then haul the throttle and mixture back at level off.
 
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