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  #31  
Old 04-26-2011, 02:01 PM
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Kahuna Kahuna is offline
 
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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.
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  #32  
Old 04-26-2011, 02:03 PM
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Flyfalcons Flyfalcons is offline
 
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Quote:
Originally Posted by Kahuna View Post
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.
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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  #33  
Old 04-26-2011, 02:14 PM
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Tandem46 Tandem46 is offline
 
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Quote:
Originally Posted by Kahuna View Post
What that means is that there really is no best mpg.
Quote:
Originally Posted by Kahuna View Post
Best MPG will always be max L/D.
Huh? Kahuna, aren't these two statements contradictory?
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  #34  
Old 04-26-2011, 02:16 PM
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Mike S Mike S is offline
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Default Mystery solved.

Quote:
Originally Posted by Ironflight View Post
Mike - it is down in the lower left corner of the "Engine" display page, along with Density Altitude.

Paul
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).
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  #35  
Old 04-26-2011, 02:17 PM
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Default 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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  #36  
Old 04-26-2011, 02:24 PM
David Z David Z is offline
 
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Default 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.
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  #37  
Old 04-26-2011, 02:25 PM
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Kahuna Kahuna is offline
 
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Quote:
Originally Posted by tobinbasford View Post
Huh? Kahuna, aren't these two statements contradictory?
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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  #38  
Old 04-26-2011, 02:33 PM
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Quote:
Originally Posted by Kahuna View Post
And if someone puts winds into another answer Im gonna scream.
Get ready to scream!!!
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  #39  
Old 04-26-2011, 02:40 PM
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pierre smith pierre smith is offline
 
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Default 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,
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Last edited by pierre smith : 04-26-2011 at 02:54 PM.
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  #40  
Old 04-26-2011, 02:41 PM
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Mike S Mike S is offline
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Wink Time to start screaming

Quote:
Originally Posted by Kahuna View Post
Plotting your TAS against FF is the way to get it.
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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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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