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11-11-2011, 10:55 AM
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Join Date: Apr 2011
Location: Linden, TN
Posts: 79
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Max Endurance Fuel Burn?
I understand from the RV-12 POH that the max glide ratio speed is 85 kts. I assume that this is also the max endurance speed. For those who have tested it, what have you found the fuel burn rate 85 kts to be?
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11-11-2011, 12:31 PM
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Join Date: Aug 2008
Location: Texas, Fort Worth
Posts: 1,237
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85kt fuel burn
Probably around 3.8 to 4 GPH. Why do you ask? 
__________________
John
RV12 N1212K
Flying Since June 2010
1020 Hours as of 9/30/2017
Johnrv12@icloud.com
RV14 Wing, arrived and building at Rdog's new Hanger at 16X
S/N 140014
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11-11-2011, 12:49 PM
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Join Date: Oct 2010
Location: La Feria Texas
Posts: 3,822
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I think he was looking for max endurance speed. I wish there were a table that showed this. Is it better to loaf along at 4gph at 85 mph (21.25 mpg), or to open it up a bit and go for 5gph at 100 (20 mpg) for instance. Easy to do the math for those two, but what about the rest of the range.
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11-11-2011, 01:13 PM
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Join Date: Jan 2008
Location: KTCY
Posts: 643
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Quote:
Originally Posted by DonFromTX
Is it better to loaf along at 4gph at 85 mph
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85 kts = 98 mph .... So that would be 4gph at 98 mph...
But the most efficient way to spend money is at the Carson's speed which is ~1.32*L/Dmax. (L/Dmax~= Vy)
Above the Compton speed fuel burn increases significantly with a disproportional small increase in speed.
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Dave & Trina
RV-9A Flying  - 330 Hrs. Painted  Finishing the interior.
Last edited by MauiLvrs : 11-13-2011 at 10:20 AM.
Reason: it's Carson's speed, not Compton - hevansrv7a
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11-11-2011, 01:47 PM
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Join Date: Oct 2010
Location: La Feria Texas
Posts: 3,822
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I confused the knots with mph, my error. Never heard of the "compton" stuff, will have to investigate that.
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11-11-2011, 02:44 PM
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Join Date: Feb 2008
Location: Taylorsville, GA
Posts: 748
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Max endurance should be at best glide or max L/D. That's where total drag is the least. Faster and parisite drag kills effeciency. Slower, and induced (AOA) drag kills it.
__________________
Jeff Rhodes - Taylorsville, GA
RV-9, 7 - going fast
BC-12D - going slow
jrhodes@v1salesmgt.com
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11-11-2011, 03:25 PM
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Join Date: Oct 2010
Location: La Feria Texas
Posts: 3,822
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OK, that is helpful. With that knowledge then, if we cruise at 98 mph we should get the best fuel mileage then?
So when I add wheel pants, intersection fairings and every speed mod I can find, will that INCREASE that most miserly speed since there is less drag? By how much?
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11-11-2011, 03:28 PM
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Join Date: Oct 2011
Location: Manchester, UK
Posts: 9
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Max Endurance Fuel Burn
Max endurance, i.e. max time in air for given fuel quantity, is surely the speed obtained when flying at minimum fuel flow.
So providing T's & P's keep in limits, simply fly at lowest RPM in green RPM arc that allows level flight.
You will use the least fuel per hour and so stay airbourne for the most hours.
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11-11-2011, 04:01 PM
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Join Date: Apr 2011
Location: Linden, TN
Posts: 79
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confusion
I think perhaps I asked the question the wrong way. One source says that Max Endurance Speed (how to stay in the air the longest) is Best Glide Speed/1.316 (65 knots in this case)
It also defines V optimum cruise as the lowest fuel consumption per knot. that is (Vbg) best glide speed * 1.316, 112 kts in the case of the RV-12. That is referred to as Carson's Speed in the source I am reading.
What I was really looking for is the Maximum Range (Vbr) fuel consumption. According to the specs on Van's web site the Max Range is 614 miles.
They use 5000 RPM for the Max Range. My suspicion is that you would be traveling faster than 85 kts (98 mph) at 5000 RPM. If 5000 RPM does produce 98 mph and the fuel capacity (usable) is 19.8 gal then the fuel consumption is about 3.2 gal/hr., which is about 31 miles per gallon. (If it is true, that would be very good.)
Since 85 kts. appears to be the best L/D ratio, it would likely also be the most efficient cruise speed. I was just curious if anyone had verified the fuel consumption at 85 kts. in flight.
I am considering an RV-12 and trying to understand it better. Sorry for the engineer talk. :-)
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11-11-2011, 06:21 PM
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Join Date: Apr 2008
Location: Riley TWP MI
Posts: 3,070
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Wind is a factor too. In a head wind, going faster is better because the wind will not be pushing the plane backwards for as long. In a tail wind, going slower will increase the time that the wind has to push the plane forward. In my case, there is almost always a head wind. 
None of the above theory matters because my tank does not last as long as the aircraft tank, if you know what I mean. 
Joe Gores
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