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High MPG: a.k.a. Thrifty Flying

DeltaRomeo

DeltaRomeo

doug reeves: unfluencer
Staff member
From a short hop yesterday.... Turned on the 'ECO' (economy) data field on the top of my G3X (divides fuel flow into GPS ground speed). After about five minutes with the wind at a 90 degree angle I turned into the wind - mpg dropped to about 10mpg :(. Sorry for the blurred iPhone photo - it was bumpy.

As gas gets more expensive I'm thinking the occasional lunch trip down to see Mom will get 'best mpg power setting' instead of smallest fuel flow (which due to wind direction might not mean cheapest flying).

Question #1: Does prolonged flight at 16.9" (or lower) of MP hurt my engine if the CHTs and oil temps are in the green?

Question #2: What MP/RPM (not leaning) settings gets others the highest MPG?


1. 24.2 mpg economy
2. 138 kts ground speed
3. 126 kts indicated air speed
4. 27 kt winds from 190°true
5. 16.9" of manifold pressure
6. 51% power
7. 6.6 gallons per hour burning
8. 2490 rpm

30b0b2s.jpg
 
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Very nice, Doug...however...

...try dialing your RPM's back to around 2000-2100. Don Rivera at Airflow showed me that in my -10 after we'd balanced the injectors. My range in the -10 went from 850 miles while ROP to over 1,000 running LOP and around 2100 RPM.

I have a short flight today, so I'll play with it and report back.

Best,
 
I'm not sure why, but don't the lycoming manuals state that continual flight at 2000 rpm's or lower is not recommended? I have wondered this for a while as I have seen some posts where guys state they fly around locally with the power pulled way back and leaned out to save fuel.
 
bkilby said:
I'm not sure why, but don't the lycoming manuals state that continual flight at 2000 rpm's or lower is not recommended? I have wondered this for a while as I have seen some posts where guys state they fly around locally with the power pulled way back and leaned out to save fuel.
Click to expand...

Years ago I bought a first run 0360-A4M off a Cherokee that had been flown like that, plus it had 25 hour oil changes. I flew the thing for about 80 hours as it was and had it overhauled.

The crank was still in new limits. The overhaul guys said it could easily have continued operation with the cylinders and valves cleaned up.

Lycoming could have recommended low power is no good, but I have not seen it. I do a lot of local cruisin' at 5-6 gph and don't think about it much.
 
If 17" manifold pressure is bad, then I should never fly above 13,000' MSL even at full throttle.
 
Was it Lindbergh or Hoover

A different time, and different engines, but didn?t Lindbergh use the low RPM method to stretch the range while crossing the Atlantic? I could be miss-remembering, but it seems like I?ve heard that, I also seem to remember something about WW II pilots using it to get back home when they had over extended. I?ll be following this thread, as it?s something I have a keen interest in?..you know, I might be thinking about Bob Hoover.
 
RPMS

I think that Lycoming engines will last forever if run between 2000 and 2500RPMS and leaned properly with scheduled oil changes. The life of the engine is clean oil and normal engine temperature. The lead in our gasoline is the culprit when turning the engine slow. I believe that if you lean it to about 1375 degrees EGT, keep the CHTs below 400 degrees and keep clean oil in it, it will run indefinitely. My 2 cents and that's all it's really worth.
 
Glad that you highlighted this Doug - I need to go in and configure ours to show MPG - I hadn't read that far into the manual to make sure it had it. I use the MPG feature in the GRT EFIS in the -8 all the time - it really does help stretch range, and is another example of how simple computations in an EFIS can really help you out when picking altitudes for winds, etc.

I generally cruise the Val at 2350 RPM and full throttle (above 8,000') - well, full minus just a tidge because it is smoother, shows no decrease in % HP, and I can lean a bit more. I am looking forward to seeing how Junior will do with the injected engine!

Remember the dark ages when everyone set power by RPM and MAP - and maybe a badly calibrated analog fuel flow meter? Now I use RPM to set the noise level, percent power to get the power I want, and digital fuel flow, MPG and graphical EGT to lean. A brave new world....

Paul
 
Excerpts from the Lycoming Key Operations flyer:

"There are some other considerations of low-power cruise operation. 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."

"Quite frequently, someone will ask if the engine will last longer if it is run at a slower RPM setting. The answer must be qualified. Operation at the recommended cruise RPM settings should allow the engine to reach TBO if it has regular oil changes, is operated within normal temperature ranges and is well cared for by pilots and maintenance personnel. Longer engine life may be expected from most engines when the operator is willing to sacrifice maximum performance for conservative cruise operation in the 60% to 65% power range. For many engines, these power settings are achieved at 2100 or 2200 RPM rather than the 1800 or 1900 RPM mentioned earlier in this discussion.

In summary, it is possible to run an engine at cruise using 1800 or 1900 RPM. A curve from the Engine Operator?s Manual should be consulted to ensure that manifold pressure limits are not exceeded. In reality, the recommendations of the Pilot?s Operating Handbook provide the best guidance for operation of an
aircraft/engine combination, and therefore the recommendations and limitations of the POH should be observed."
 
Experimental Chaos said:
A different time, and different engines, but didn?t Lindbergh use the low RPM method to stretch the range while crossing the Atlantic? I could be miss-remembering, but it seems like I?ve heard that, I also seem to remember something about WW II pilots using it to get back home when they had over extended. I?ll be following this thread, as it?s something I have a keen interest in?..you know, I might be thinking about Bob Hoover.
Click to expand...

It was Lindbergh who developed the technique of WOT, low rpm, LOP with the P-38's and the Allyson engine. That is how they intercepted and shot down Adm. Yamamoto. Plus they had broken the Japanese code so it was known where the admiral was that day.

Not a good day for the Japanese navy.
 
Aces High

Just read a great book, Aces High, about the great P38 ace of the Pacific, Richard Bong. The author mentions Lindbergh's contribution to the range problem with the leaning techniques.
 
DeltaRomeo said:
Question #2: What MP/RPM (not leaning) settings gets others the highest MPG?
Click to expand...


Why not lean it?

Dave

[ed. I should have qualified that better. What I should have said is "After leaning and everything else, what MP/RPM gave you the best MPG?" dr]
 
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Not EAA

Experimental Chaos said:
It was Lindberg, sometimes my memory surprises me, check out this article from EAA.

http://www.experimentalaircraft.info/flight-planning/aircraft-engine.php
Click to expand...

To set the record straight and while the web site you cite is full of useful information, it IS NOT the EAA web site.

This is an interesting thread. I've been interested in this ever since I noticed Dan Checkoway's RV-7 AFS display with "MPG" many years ago. Fuel flow doesn't necessarily get you the best fuel economy across the ground.
 
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I have the C/S prop and I am stuck on flying at a lower setting when doing a flight of just looking around mind you. My settings are 2400rpm, maf of 20, I lean to peak at 50 and burn 5.8 gph. 133kts.
 
Here is what I use for my local evening flights................

RV6A O320, FP, LSI.

At 2000 ft ASL, I set RPM to 2100 and MP comes in around 21. Add the first 2 numbers of the tach and the MP and here you get 42. With the FP prop, I just keep the 42 by increasing RPM as I climb, or conditions change. A local flight at 2000 ft with #s set at 42 and leaned, I get 150 MPH and a burn of 4.8 to 5 gph.
 
years ago i did a lot of flying at low power settings while teaching a friend the intricacies of flight. the engine definitely increased its oil consumption and oil in some cylinders. i regularly fly in the evenings at low power settings with aggressive leaning with no problems.
 
??????????

Ironflight said:
-----I use the MPG feature in the GRT EFIS -----

Paul
Click to expand...

O.K., so I read this, and went "WOW" another goodie I have yet to discover.

Immediately went to the GRT manuals, and while it is mentioned, there is nothing I can find detailing how to activate this option.

How about a hint???;)
 
Mike S said:
O.K., so I read this, and went "WOW" another goodie I have yet to discover.

Immediately went to the GRT manuals, and while it is mentioned, there is nothing I can find detailing how to activate this option.

How about a hint???;)
Click to expand...

I don't rememebr that there was anything to set up Mike - it is down in the lower left corner of the "Engine" display page, along with Density Altitude. It is possible that it won't display if you don't have fuel flow information, but other than that, I think everything else would be part of the basic software (I'd think it only needs fuel flow and groundspeed).

check page 1-7 of the User manual, Rev C for a picture.

Paul
 
Yep, found that, and later in 4.1 it says you can display different data there, but I cant figure out how to select ..........

For all I know, the MPG has been there all along, but I have never noticed it.

I need to go sit in the plane, and look:rolleyes:
 
DeltaRomeo said:
Question #2: What MP/RPM (not leaning) settings gets others the highest MPG?
Click to expand...

The RV is VERY linear in speed and fuel flow. Its not a bell curve.
What that means is that there really is no best mpg. The ratio of FF to speed is pretty constant.
 
MPG in GRT

Mike look at the very bottom of page 5.3 in the setup guide. Looks like it is called performance/fuel box 2. You can select MPG there.
 
4_26_11 PIREP: 27.7mpg

.3hr lunch flight. Just in the door.

OAT 58°F. 1,300 MSL.
15.6" MP 2290 RPM 4.9 gph fuel flow
118.5 kts IAS (TAS and GS within 1kt of each other)
124 kts ground speed. Agressively leaned.
Hottest CHT 335°F. Hottest EGT 1410°F.
14kt wind from 90 degrees right. Autopilot on holding altitude and heading.
Image below photoshoped to get screens closer together.

27.7 MPG was the best I could get varying MP and RPM. Fun to play with.

23i6n15.jpg

Over $130 to fill up my RV last time.....I might be doing the 'Best ECO' settings a lot more in the future!
 
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Nice bunch of numbers, really shows the ability to conserve if need be.

It also reminds me of something my Bonanza friend said when I ask him about fuel burn while running LOP: I didn't buy a fast airplane to fly it slow. :D
 
BillFear said:
Mike look at the very bottom of page 5.3 in the setup guide. Looks like it is called performance/fuel box 2. You can select MPG there.
Click to expand...

Thanks Bill, I will try that-------it does state it is a "upper left" display box, but then it also says MPG:confused:
 
If you have a tailwind, best MPG, or range, will be to pull the power back.

If you have a headwind, it's best to fly a higher power setting.


Some of you guys should try it and see what happens. I've really on heard about this, but have never actually seen the numbers.

Steve
 
How can this be?

Kahuna said:
The RV is VERY linear in speed and fuel flow. Its not a bell curve.
What that means is that there really is no best mpg. The ratio of FF to speed is pretty constant.
Click to expand...

The drag curve is the same shape for RV's as any other GA aircraft (not laminar flow). Prop efficiency varies, but not a huge amount. Engine efficiency likewise. If you fly at your best L/D (for example, around 106-110 for a 6 or 7) you will get (no wind) best mpg. And if you fly at max velocity you will get poorer mpg.

I have never seen any evidence that an RV has a different shape drag curve than other airplanes. Do you have any?

This is an idealized illustration of the point, not specific to RV's. The green line for THP should be roughly proportional to fuel flow. Velocity is the x axis. It is not a bell curve but it is curved, not straight.

ClimbGraph.jpg
 
steve91t said:
If you have a tailwind, best MPG, or range, will be to pull the power back.

If you have a headwind, it's best to fly a higher power setting.


Some of you guys should try it and see what happens. I've really on heard about this, but have never actually seen the numbers.

Steve
Click to expand...

huh?
Headwind and tail wind has nothing to do with the effeciency of the plane.
It also has no place in discussions of trying to determine power settings and MPG discussions.
Plotting your TAS against FF is the way to get it. It wont matter if your readings are accurate as long as they are relative to themselves.
We have a gazillion of these plots running around.
 
steve91t said:
If you have a tailwind, best MPG, or range, will be to pull the power back.

If you have a headwind, it's best to fly a higher power setting.


Some of you guys should try it and see what happens. I've really on heard about this, but have never actually seen the numbers.

Steve
Click to expand...

Steve, I agree with you based on my non-RV experience. I'd like to see some real world data though.
P.S. to Mike. Slow down and think about this: We're talking about efficiency across the ground not efficiency through the air. The iPhone is slow right now so I'm out.
 
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Kahuna said:
huh?
Headwind and tail wind has nothing to do with the effeciency of the plane.
It also has no place in discussions of trying to determine power settings and MPG discussions.
Plotting your TAS against FF is the way to get it. It wont matter if your readings are accurate as long as they are relative to themselves.
We have a gazillion of these plots running around.
Click to expand...

that is not quite correct. A plane's best efficiency over the ground is dependant on the relative headwind or tailwind. The higher the headwind, the plane needs to fly faster for best MPG.
 
Allow me to re-iterate.
Best MPG will always be max L/D.
The rate of change of the MPG as you increase the speed is essentially linear.
 
Kahuna said:
Allow me to re-iterate.
Best MPG will always be max L/D.
The rate of change of the MPG as you increase the speed is essentially linear.
Click to expand...

This is true in calm wind conditions, but needs to be taken a step further in accounting for headwind and tailwind.
 
Mystery solved.

Ironflight said:
Mike - it is down in the lower left corner of the "Engine" display page, along with Density Altitude.

Paul
Click to expand...

Well, I now seem to remember you upgraded to the newer screens awhile ago-----

In the earlier screens like I have it is upper left, second one down,---- and if Density Altitude is available, well, just another goal for the continuous treasure hunt that is learning GRT:D

I really wish they hadn't put in section 4.12 (page 4.5).
 
Extreme example...

To illustrate using a non-realistic example, assume best L/D is 115kts. If you assume the best MPG will be at best L/D, then you should fly at 115 kts. If you are flying directly into a 115 kt headwind, however, your MPG will be exactly zero, so best L/D won't always be best MPG. If you push the throttle in and fly faster, your MPG will be poor, but it will be better than zero.

IIRC, best L/D will result in the best possible endurance (and accordingly the best GPH), but the wind will play a factor in how far you'll go in that time.
 
Examples

Lets take a Piper Cub flying at it's best L/D and ino a 60mph headwind. You'll be going backwards over the ground. Obviously not the best MPG. The best MPG in this senario will be found at a very high power setting for that little airplane.

If your flying with the 60mph tailwind, it's best to fly closer to the best endurance speed. Let the wind carry you to your destination while having a low airpseed and very low fuel flow.
 
tobinbasford said:
Huh? Kahuna, aren't these two statements contradictory?
Click to expand...

No. I assumed everyone already knew max L/D. thats what it its. Seemed an infantile discussion to discuss that.
Seemed the question was if plotting FF, RPM, MAP and speeds, was it a curve and is there a high point in the curve giving you best MPG? The answer is no. Or darn near no. Close enough to no for us.

I have a thousand graphs, data captures, props etc. This should put it to rest. And if someone puts winds into another answer Im gonna scream.
1. Plot FF and TAS from 5gph-15gph. Its a straight line
2. Plot RPM and TAS from 2700 to 1900. Its a straight line
 
Well, our -10 got 17.5 mpg

:.....and under 9 GPH at 51% LOP this afternoon: Jenny and I did a short XC and these are the numbers:
51LOP.jpg
. The MPG and the range are on the far right side. Note the range is over 6 hours at this power setting vs. 4 hours at 75%.

We were only at 3,000' and our TAS was over 150MPH and the range had climbed from less than 800 miles to over 1,000 as depicted by the D-120...lower right.

154TAS.jpg


On a short trip, the time saved running 75% is minimal...this one was 50 miles and took 22 minutes at 51%/8.9GPH and 17.5 MPG, whereas at 75% it took 20 minutes at 75% with a 50% greater fuel burn on my IO-540/260 HP Lyc. getting 12 MPG. We had a 90 deg crosswind both ways.

Don Rivera showed me how to really raise my MPG to the most efficient point if I didn't mind going slower...and here it is.

Best,
 
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Time to start screaming

Kahuna said:
Plotting your TAS against FF is the way to get it.
Click to expand...

Sorry, but for my way of thinking the above is just plane wrong.

"Miles" as used in the term MPG is a distance reference, not a rate reference.

Plotting your Ground Speed against FF will give you miles per gallon, as a snapshot of the rate at that moment.

Or, you could use the total distance flown and total gallons used. This will give total MPG for the entire flight.

As in the example of the cub above, winds absolutely do make a difference.

I am defining MPG as how many miles you travel over the ground, (in your direction of flight), not through the air, for each gallon of gas consumed.
 
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Nope

Kahuna said:
Allow me to re-iterate.
Best MPG will always be max L/D.
The rate of change of the MPG as you increase the speed is essentially linear.
Click to expand...

Assuming mpg refers to miles on the ground, this cannot be true. Please see here for details.

The issue is the drag curve. As you get more HW or TW, you must speed up or slow down somewhat. The usual rules of thumb over-estimate this required change, but it does exist.

Also as I stated above, it's not linear.
 
Sorry, Kahuna, you have to scream louder

Kahuna, It doesn't mean anything if the FF vs TAS line is a straight line in terms of the question of this thread, best Miles Per Gallon.

I made a hypothetical plot: 100 mph at 5 gph, to 200 mph at 15 gph, STRAIGHT LINE. Along that line, the mpg is 20 at 100 mph, 15 at 150 mph, and 13.33 mpg at 200 mph. Again, this is just a hypothetical example, but it has a purpose: Just because the lines are straight doesn't mean there isn't a "best MPG" point. You might argue that since it is a straight line, then it is always better to fly slower, and mpg will keep increasing. BUT -Eventually if you extend the curves slow enough, they won't be straight anymore, right, because eventually you will get down in the back-side of the power curves. The hypothetical curve I described above, if you extend it as a straight line down to 50 mph, it would claim 0.0 gph fuel flow. So obviously the line must curve somwhere.

AND OF COURSE, THE WIND HAS AN IMPORTANT EFFECT ON MPG!!



Kahuna said:
Seemed the question was if plotting FF, RPM, MAP and speeds, was it a curve and is there a high point in the curve giving you best MPG? The answer is no. Or darn near no. Close enough to no for us.

I have a thousand graphs, data captures, props etc. This should put it to rest. And if someone puts winds into another answer Im gonna scream.
1. Plot FF and TAS from 5gph-15gph. Its a straight line
2. Plot RPM and TAS from 2700 to 1900. Its a straight line
Click to expand...
 
In sailplanes, we have glide computers to help us determine the most efficient speed to fly. They take into account a number of variables, including the aircraft glide polar, wind, water ballast (wing loading), expected climb rate at the next thermal, etc. Most computers even take into account a "bug factor"... decreased performance from dirty wings. I'd love to see a similar computer for powered aircraft. Perhaps XM weather information could be used to help plot an optimum flight profile.


lx7007withshadows-s.jpg


An aircraft, built today, will burn around $250,000 in fuel during its life. Reducing fuel consumption by just 5 or 10% will save $12,500 to $25,000 in fuel costs.
 
RPM'S

About the question of low rpm's on the lycoming. The engine has no problems flying with lower rpms, however, if you are running a hartzell c/s propeller, as far as I know they do have a restriction on flying extended times with rpms below 2250. If this has changed recently, please let me know. I wish they didnt have that restriction because I am of the opinion the lower rpms definately provides better fuel economy. Just my observations


bird
 
bird said:
About the question of low rpm's on the lycoming. The engine has no problems flying with lower rpms, however, if you are running a hartzell c/s propeller, as far as I know they do have a restriction on flying extended times with rpms below 2250. If this has changed recently, please let me know. I wish they didnt have that restriction because I am of the opinion the lower rpms definately provides better fuel economy. Just my observations
Click to expand...

For my 0360 & Hartzell C/S (not blended tip)............I have a restriction of 2000 to 2250 for extended periods of time. I don't know about 2000 and below, as I prefer not to run below 2000 rpm.

L.Adamson --- RV6A
 
Depends on engine/prop combination

Hi Bird,

different combinations of engine and prop (B/A or old style) have different operating speed limitations. Many combinations have no restrictions.
A classic one for the IO-360 is something like no continuous operation between 2000-2250. Nothing says you can't run below 2000. Another common one is 2350-2550 above 25" MAP. Again, no low-RPM restriction.


bird said:
About the question of low rpm's on the lycoming. The engine has no problems flying with lower rpms, however, if you are running a hartzell c/s propeller, as far as I know they do have a restriction on flying extended times with rpms below 2250. If this has changed recently, please let me know. I wish they didnt have that restriction because I am of the opinion the lower rpms definately provides better fuel economy. Just my observations


bird
Click to expand...
 
Data

I took this data a few years ago, using two throttle settings (see chart for specifics), and simply dialed mixture across a range. I want to repeat this, only continue leaning until the mpg actually starts rising again. The speeds are not based on ground, only TAS.

N66AP%20fuel%20mileage%20August%2010%2C%202008.JPG


The crossing of the TAS lines is interesting, and it is a coincidence that the crossing of the mpg lines happens to be directly at the same point (lucky scaling of the left and right vertical axes). Also noteworthy is that for the same fuel burn, the higher MAP delivers a better performance, presumably owing to less pumping losses, even though I arbitrarily chose a higher rpm.

Oh, and yes, the TAS (and associated engine settings) delivering the best mpg across the ground WILL vary with what the winds are aloft. Seems crazy, but the glider guys (thanks Larry P.) busted me a while back on a similar phenomenon regarding optimum gliding distance (also not a constant - depends on winds).
 
Kahuna said:
huh?
Headwind and tail wind has nothing to do with the effeciency of the plane.
It also has no place in discussions of trying to determine power settings and MPG discussions.
Click to expand...

Head/tail winds most certainly belong in a discussion about ground miles per gallon. A simple thought experiment illustrates.

Your plane cruises at 100 smph at 65% power which is your best no-wind MPG speed. You take off, and encounter a headwind of 100 smph. If you remain at your best MPG speed you get ZERO smpg. You MUST add power to get forward ground speed. In this extreme example, the more power, the better your ground MPG. In a more realistic example you could work it out from those 1000 graphs you have, or, let your glass panel calculate it for you.

So you decide to head the opposite direction. Now you have a 100 smph tailwind. Your best ground MPG is to throttle back to just above stall speed while maintaining altitude.

In the real world, each flight, with a different head/tail wind component, is going to have a slightly different throttle setting for best ground MPG.
 
Close, but not correct

Buggsy2 said:
Head/tail winds most certainly belong in a discussion about ground miles per gallon. A simple thought experiment illustrates.

Your plane cruises at 100 smph at 65% power which is your best no-wind MPG speed. You take off, and encounter a headwind of 100 smph. If you remain at your best MPG speed you get ZERO smpg. You MUST add power to get forward ground speed. In this extreme example, the more power, the better your ground MPG. In a more realistic example you could work it out from those 1000 graphs you have, or, let your glass panel calculate it for you.

So you decide to head the opposite direction. Now you have a 100 smph tailwind. Your best ground MPG is to throttle back to just above stall speed while maintaining altitude.

In the real world, each flight, with a different head/tail wind component, is going to have a slightly different throttle setting for best ground MPG.
Click to expand...
There is much here that is correct. However, the best speed for a tail wind depends on the tail wind, but using a 100 mph tail wind, the best speed would be about 76% of L/D which is your best endurance speed, not just above a stall. Please refer to the chart I posted above.
 
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