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Join the 20 nmpg club

aturner

Well Known Member
There have been a number of excellent threads on VAF regarding optimization of aerodynamic efficiency, and also some valuable threads on measuring true airspeed and static system errors. I think we need to start a thread for those of us interested in efficiency, and accurate measurements of performance....the 20 nautical miles per gallon club. Here is my entry.

Yesterday I flew my RV-10 home to Pennsylvania from a fantastic trip to Leadville Colorado with my two sons (more on that in another post). We climbed high for the return flight, and about four hours into a five hour leg, and with lots of time to study the numbers on the EFIS screens, noted that we were doing 155 knots on 7.5 gallons per hour, which works out to better than 20 nautical miles per gallon:



The other screen would have showed 2100 RPM and 17.3". Being the quantitative sort, I wanted to verify these data, and directed the autopilot to fly four legs 90 degrees from each other, letting the G3X do its once per second data collection thing. Today I entered the GPS ground speed and ground track into Doug Gray's spreadsheet, available from the National Test Pilot School http://www.ntps.edu/information/downloads.html, and got this:



So my true airspeed is 157.1 knots. Mean fuel flow during the test was 7.48 gph (the totalizer consistently agrees with actual fuel burn with 1-2% error, and the error is conservative - actual burn is slightly less than calculated). So, I have a solid 21 nautical miles per gallon, or 24 statute miles per gallon. And this is on a cross-country trip with the family and camping gear, no special preparations. Here is the best part: this is a dead stock setup, with a used Lycoming, Slick mags, and stock Bendix fuel injection. I have tried to pay attention to aerodynamic details, and perhaps that has paid off.

My RV-10 is a work in progress, and I have a list of things that need fine-tuned, so these numbers and just a starting point. In the mean time, it is a testament to the power of simplicity, and it sure is a great family hauling machine.

 
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My RV-12 is pretty efficient even at low altitude. 125 TAS @ 4.9 GPH = 25.5 SMPG (22 NMPG). Not bad if you’re not in a big hurry.

Plus it burns auto fuel… :D

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So my true airspeed is 157.1 knots. Mean fuel flow during the test was 7.48 gph...So, I have a solid 21 nautical miles per gallon, or 24 statute miles per gallon. And this is on a cross-country trip with the family and camping gear, no special preparations. Here is the best part: this is a dead stock setup, with a used Lycoming, Slick mags, and stock Bendix fuel injection. I have tried to pay attention to aerodynamic details, and perhaps that has paid off.

Well, I'm impressed. I had no idea an RV-10 could be that efficient. Is it typical, or is Andy's better than average?

FWIW, the best "efficiency club" yardstick would factor in payload and speed. Taken alone, MPG mostly rewards going slow. I'm sure the smart guys have a well-considered competition equation out there somewhere. Moot point, I suppose. Very few RV owners bother to actually weight what we throw into the airplanes.

I posted this photo last week; returning from OSH a few years ago, hauling camping gear, clothes, and a few half cases of Spotted Cow. Works out to be 22.8 MPG at 176 KTAS. Cowl door is shut, so total cooling exit area is about 30 sq in, with good oil temp and CHT. Mixture is LOP of course, with all cylinders peaking within 5 degrees.

 
Please help me understand how you were able to get 7.9g and 155kts IAS.

What ignition are you using? Did you do anything special with the timing? Do you know how far LOP you were?

Most RV-10s that I'm aware of are getting 10-11g at 155kts.

In my RV-10, anything below 11gph and the engine starts to run rough.

bob
 
155KTAS, LOP yields around 8.7gph at around 9k DA and 2500 rpm WOT Hartzell BA, 260hp engine. If I remember right.
 
Most RV-10s that I'm aware of are getting 10-11g at 155kts.

In my RV-10, anything below 11gph and the engine starts to run rough.

bob

I have been seeing a nice efficient cruise sweet spot at around 170kts TAS or so. I have a stock IO-540 (still running these #$!* mags) but I balanced the injectors. This gives me a smooth engine very deep into LOP. I typically do not run more than 20-30 degrees LOP as power starts to quickly fall off. I'm happy with the results, 170-173 kts TAS and fuel flow 10.2gph or so at 14,000 feet. Lower is more, higher is less. Down at 7,000 feet fuel flow is around 11.7gph for the same speed.

Most cross country is medium load - two people and a bunch of baggage.

Carl
 
Below are screenshots of a flight last Monday, Aug 8, 2016 in my 9A. This is a typical flight that is repeatable all day long. Just to point out a few parameters:
The true airspeed can be seen on the top left corner of the primary flight display (PFD) picture. All speeds are in Statute Miles Per Hour. Fuel flow labeled FLOW near bottom right on PFD. Statute Miles Per Gallon can be seen on the second screenshot of the Engine monitor screen in the second picture.

This is running LOP as can be denoted by the negative Exhaust Gas Temperatures (EGT) displayed on both the PFD and the Engine Monitor. So whether one looks at Gallons Per Hour or Miles Per Gallon, either way you look at it, this is an indicator of the efficiency of the RV9A. It is much more fuel efficient than any of the cars I own. There are many RVs that are faster than the 9A but not so sure they are as efficient.

Of course Dan knows I am always hoping to improve on that efficiency. ;)

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Live Long and Prosper!
 
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This seems too easy to hit - here is a recent cruise shot where I'm running 22.6, and that's without a tailwind.

screenshot-N16GN-SN04047-14.2.1.3698-20160612-182807-947-en_US_zpsdkohfteq.png
 
Altitude and RPM

You guys with -9's are posting impressive numbers, you might have to form your own club. :) As to the question as to what is going on with my -10, it might be a few knots faster than average, but I suspect that the most important things to note are the operational parameters.

In the post above, I am at a density altitude of 17,000 ft., and it seems that the unique airfoil of the RV-10 loves altitude. I am guessing that this has something to do with air density, Reynolds numbers, and laminar flow, but someone smarter than me can confirm this. Here is a plot showing segments of different cross-country flights, with cruise altitude plotted against fuel efficiency for each one:



These are flights at different configurations of power, RPM, fuel flow, etc., but even so, it is altitude that has the overwhelming effect on efficiency. And higher is better, at the rate of one mile per gallon for each 3000 ft.

The other operational aspect to note is the low prop RPM. I am running old fashioned Slicks at 25 deg. fixed timing, and 30-40 degrees lean of peak. As several excellent threads here have explained, the flame front needs time to do its work at high altitude and lean mixtures. Here is a plot from a test in which I did mixture sweeps at RPM's ranging from 2400 to 2000. Superimposed on the plot are isopleths for nautical miles per gallon. Test conditions were 9200 density altitude, so maximum efficiency is only about 18 nmpg.



There is a lot to be learned from this graph, but the simple point is that the maximum efficiency was at the slowest RPM tested, 2000. Here is another recent graph, showing the results of a test in which I simply reduced RPM in a stepwise manner while on a cross country flight (14,000' DA), without touching anything else.



X-axis is RPM, y-axis is true airspeed divided by fuel flow (nmpg).

So, fly high, and for those of us with magnetos, keep that prop slow.
 
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Im not an engine expert, but if your running your rpms that low under a load, aren't you putting significant load on the engine.
 
I can't wait to get an RV-10 or 9A. Those numbers are awesome. I'm building experience and hours in a 182.
 
Our -10 will go 177knots on 10.7 GPH at 12,000ft Density Alt which works out to little over 19 statute MPG. I can get well over 20 MPH by slowing down just a little by leaning slightly. That is the only hard data I have handy.

will try to collect some efficency numbers soon. :)
 
Our -10 will go 177knots on 10.7 GPH at 12,000ft Density Alt which works out to little over 19 statute MPG.

Your data point falls right on the curves I posted above for the mixture sweeps. I routinely plan on, and see, 170 knots on 10.0 GPH at 12K, which also seems in line with your report.
 
24.7 MPG

On the way to Oshkosh:
9,500?
-100 LOP
FF 6.6 GPH
163 KTAS
MPG 24.7
RV-8 with an Aerosport IO-375 high compression motor



 
Please help me understand how you were able to get 7.9g and 155kts IAS.

What ignition are you using? Did you do anything special with the timing? Do you know how far LOP you were?

Most RV-10s that I'm aware of are getting 10-11g at 155kts.

In my RV-10, anything below 11gph and the engine starts to run rough.

bob

Here are two screenshots from my last flight from Minnesota to Chicago. First one is LOP, second is ROP. The ROP settings were just quick and dirty to get a rough comparison, and were by no means tweaked for max performance.

RDbq6FSh.png


wXdMNYAh.png


In the post above, I am at a density altitude of 17,000 ft., and it seems that the unique airfoil of the RV-10 loves altitude.

Totally agree. In my limited time flying my -10 (around 100 hours) I've found that I don't like cruising much below 9500. Better speed and efficiency!

There is a lot to be learned from this graph, but the simple point is that the maximum efficiency was at the slowest RPM tested, 2000.
..............
So, fly high, and for those of us with magnetos, keep that prop slow.

I have dual electronic, and I've been normally cruising between 2360 and 2400, WOT. I'll have to play around with pulling RPMs lower.
 
It seems like the -8's are the winners in "total" performance (ie. speed combined with efficiency). I wonder what an -8 with -9 or -10 wings would be like. Anyone else curious? :D
 
The missing data point for me was DA. When you take DA into consideration, I'm getting similar numbers.
 
It seems like the -8's are the winners in "total" performance (ie. speed combined with efficiency). I wonder what an -8 with -9 or -10 wings would be like. Anyone else curious? :D

Not if the performance number you're looking at is "passenger mile per gallon"!
Aww! That pesky word: Efficiency!

Each of us thinks we KNOW what the word means. And we indeed do. That is, we know what it means in terms of what our internal thoughts are concerning efficiency. However, we tend to think of it in specific terms only as it relates to what WE envision it to mean. If we think of speed as efficient, that is what we concentrate on. If we think of energy expenditure, well we think of that. Neither is completely appropriate to use without the other in the equation. Those are just two components. In most instances, there are many other variables that need defining in order to understand what constitutes efficiency. In order to fully discuss the term, all of them have to be defined.

To really comprehend efficiency for any given analysis the thing(s) being measured for efficiency have to be defined up front. Such is the result of the above contributions to this conversation. The term has different meanings for different people.

What makes an efficient Reno racer is most certainly going to be very different to what makes an efficient cross country glider.
 
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Coming' home from Reno

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RVbySDI says:
"What makes an efficient Reno racer is most certainly going to be very different to what makes an efficient cross country glider."

That is a fairly fast glider! If I push it up a bit (not quite 20NMPG, but movin' right along):

2llbqsi.jpg


Next time I'm up there I'll see what it does ROP.

Carry on!
Mark
 
L/D Max!

Great numbers Andy/Mark!
Having owned a Rocket (GAMI Injectors dual F.I.) and 2 RV's I'm looking at new efficiency numbers albeit on a lower scale in my Sonerai 2. I flew from Amarillo TX to Dallas (254nm) on 5.9 gallons of fuel. Roughly 40MPG @129KTAS!
The Ellison TBI on the GP2180 engine cranking out some amazing numbers, a bit slower but impressive nonetheless.
Thanks for sharing.
V/R
Smokey
 
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26.6 NMPG
RV7A, IO360 M1B direct from Lycoming (via Vans)
Dual P-Mags
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