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  #1  
Old 08-14-2016, 09:47 PM
aturner's Avatar
aturner aturner is offline
 
Join Date: Jan 2005
Location: Clarion, Pennsylvania
Posts: 571
Default Join the 20 nmpg club

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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RV-10 N784JC

Last edited by aturner : 08-14-2016 at 10:35 PM.
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  #2  
Old 08-15-2016, 04:33 AM
Piper J3's Avatar
Piper J3 Piper J3 is offline
 
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Location: Hinckley, Ohio
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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…

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Jim Stricker - EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
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Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 750

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Last edited by Piper J3 : 08-15-2016 at 04:42 AM.
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  #3  
Old 08-15-2016, 08:12 AM
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DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 10,352
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Quote:
Originally Posted by aturner View Post
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.

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  #4  
Old 08-15-2016, 08:39 AM
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rleffler rleffler is offline
 
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Location: Delaware, OH (KDLZ)
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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
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  #5  
Old 08-15-2016, 09:41 AM
TS Flightlines TS Flightlines is offline
 
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Great job Andy, and I would also like to know how you do it.
Tom
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  #6  
Old 08-15-2016, 09:49 AM
aerhed aerhed is offline
 
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155KTAS, LOP yields around 8.7gph at around 9k DA and 2500 rpm WOT Hartzell BA, 260hp engine. If I remember right.
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  #7  
Old 08-15-2016, 09:50 AM
rv7charlie rv7charlie is offline
 
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Location: Pocahontas MS
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Bob, his altitude might explain it....
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  #8  
Old 08-15-2016, 09:53 AM
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Carl Froehlich Carl Froehlich is offline
 
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Location: Dogwood Airpark (VA42)
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Quote:
Originally Posted by rleffler View Post
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
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  #9  
Old 08-15-2016, 10:26 AM
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RVbySDI RVbySDI is offline
 
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Location: Tuttle, Oklahoma
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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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Last edited by RVbySDI : 08-15-2016 at 10:28 AM.
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  #10  
Old 08-15-2016, 10:33 AM
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airguy airguy is offline
 
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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.

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Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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