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04-26-2011, 02:01 PM
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Join Date: Dec 2004
Location: Gold Hill, NC25
Posts: 2,400
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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
6A, S8 ,
Gold Hill, NC25
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04-26-2011, 02:03 PM
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Join Date: Jul 2009
Location: Bonney Lake, WA
Posts: 295
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Quote:
Originally Posted by Kahuna
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.
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This is true in calm wind conditions, but needs to be taken a step further in accounting for headwind and tailwind.
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Ryan Winslow
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04-26-2011, 02:14 PM
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Join Date: Jan 2005
Location: Broomfield, CO
Posts: 639
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Quote:
Originally Posted by Kahuna
What that means is that there really is no best mpg.
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Quote:
Originally Posted by Kahuna
Best MPG will always be max L/D.
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Huh? Kahuna, aren't these two statements contradictory?
__________________
RV-7 Flying since 2004
1,100 hrs+
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04-26-2011, 02:16 PM
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Senior Curmudgeon
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Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 15,420
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Mystery solved.
Quote:
Originally Posted by Ironflight
Mike - it is down in the lower left corner of the "Engine" display page, along with Density Altitude.
Paul
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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
I really wish they hadn't put in section 4.12 (page 4.5).
__________________
Mike Starkey
VAF 909
Rv-10, N210LM.
Flying as of 12/4/2010
Phase 1 done, 2/4/2011 
Sold after 240+ wonderful hours of flight.
"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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04-26-2011, 02:17 PM
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Join Date: Nov 2006
Location: Calgary, Canada
Posts: 408
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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.
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JV
Calgary, Alberta, Canada
RV7 QB - Airframe largely complete, sans canopy and glass... unfortunately sold
RV6 - O-360-A1A, Hartzell CS, dual G3X VFR... purchased
Dues paid 2015
"Being defeated is only a temporary condition; giving up is what makes it permanent."
-- Marilyn vos Savant
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04-26-2011, 02:24 PM
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Join Date: Aug 2010
Location: Thunder Bay Ontario
Posts: 335
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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.
__________________
RV-8
Empennage Passed Pre-close Inspection
Wings mostly done
Fuselage is "in the mail"
83126
Dash 8 day job is financing the RV-8
Donation till September 2021
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04-26-2011, 02:25 PM
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Join Date: Dec 2004
Location: Gold Hill, NC25
Posts: 2,400
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Quote:
Originally Posted by tobinbasford
Huh? Kahuna, aren't these two statements contradictory?
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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
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Kahuna
6A, S8 ,
Gold Hill, NC25
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04-26-2011, 02:33 PM
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Forum Peruser
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Join Date: Jan 2005
Location: Austinville, Alabama
Posts: 2,458
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Quote:
Originally Posted by Kahuna
And if someone puts winds into another answer Im gonna scream.
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Get ready to scream!!! 
__________________
Don Hull
RV-7 Wings
KDCU Pryor Field
Pilots'n Paws Pilot
N79599/ADS-B In and Out...and I like it!
?Certainly, travel is more than the seeing of sights;
it is a change that goes on, deep and permanent, in the ideas of living." Miriam Beard
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04-26-2011, 02:40 PM
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Join Date: Jan 2005
Location: Louisville, Ga
Posts: 7,840
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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:  . 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.
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,
__________________
Pierre Smith
RV-10, 510 TT
RV6A (Sojourner) 180 HP, Catto 3 Bl (502Hrs), gone...and already missed
Air Tractor AT 502B PT 6-15 Sold
Air Tractor 402 PT-6-20 Sold
EAA Flight Advisor/CFI/Tech Counselor
Louisville, Ga
It's never skill or craftsmanship that completes airplanes, it's the will to do so,
Patrick Kenny, EAA 275132
Dues gladly paid!
Last edited by pierre smith : 04-26-2011 at 02:54 PM.
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04-26-2011, 02:41 PM
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Senior Curmudgeon
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Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 15,420
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Time to start screaming
Quote:
Originally Posted by Kahuna
Plotting your TAS against FF is the way to get it.
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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.
__________________
Mike Starkey
VAF 909
Rv-10, N210LM.
Flying as of 12/4/2010
Phase 1 done, 2/4/2011 
Sold after 240+ wonderful hours of flight.
"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
Last edited by Mike S : 04-26-2011 at 02:51 PM.
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