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04-26-2011, 02:58 PM
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Join Date: Jul 2005
Location: Detroit, MI
Posts: 1,587
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Nope
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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Assuming mpg refers to miles on the ground, this cannot be true. Please see here for details.
The issue is the drag curve. As you get more HW or TW, you must speed up or slow down somewhat. The usual rules of thumb over-estimate this required change, but it does exist.
Also as I stated above, it's not linear.
__________________
H. Evan's RV-7A N17HH 240+ hours
"We can lift ourselves out of ignorance, we can find ourselves as creatures of excellence and intelligence and skill. We can be free! We can learn to fly!" -J.L. Seagull
Paid $25.00 "dues" net of PayPal cost for 2015, 2016, 2017 and 2018 (December).
This airplane is for sale: see website. my website
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04-26-2011, 03:24 PM
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Join Date: Jan 2008
Location: Ashland, OR
Posts: 2,574
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Sorry, Kahuna, you have to scream louder
Kahuna, It doesn't mean anything if the FF vs TAS line is a straight line in terms of the question of this thread, best Miles Per Gallon.
I made a hypothetical plot: 100 mph at 5 gph, to 200 mph at 15 gph, STRAIGHT LINE. Along that line, the mpg is 20 at 100 mph, 15 at 150 mph, and 13.33 mpg at 200 mph. Again, this is just a hypothetical example, but it has a purpose: Just because the lines are straight doesn't mean there isn't a "best MPG" point. You might argue that since it is a straight line, then it is always better to fly slower, and mpg will keep increasing. BUT -Eventually if you extend the curves slow enough, they won't be straight anymore, right, because eventually you will get down in the back-side of the power curves. The hypothetical curve I described above, if you extend it as a straight line down to 50 mph, it would claim 0.0 gph fuel flow. So obviously the line must curve somwhere.
AND OF COURSE, THE WIND HAS AN IMPORTANT EFFECT ON MPG!!
Quote:
Originally Posted by Kahuna
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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__________________
Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet"
Hobbs 625
LS6-15/18W sailplane SOLD
bought my old LS6-A back!! 
VAF donation Jan 2020
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04-26-2011, 04:21 PM
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Join Date: Oct 2007
Location: Belvidere, IL
Posts: 169
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In sailplanes, we have glide computers to help us determine the most efficient speed to fly. They take into account a number of variables, including the aircraft glide polar, wind, water ballast (wing loading), expected climb rate at the next thermal, etc. Most computers even take into account a "bug factor"... decreased performance from dirty wings. I'd love to see a similar computer for powered aircraft. Perhaps XM weather information could be used to help plot an optimum flight profile.
An aircraft, built today, will burn around $250,000 in fuel during its life. Reducing fuel consumption by just 5 or 10% will save $12,500 to $25,000 in fuel costs.
__________________
David Shelton, Aerospace Engineer and Soaring Nut.
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04-26-2011, 05:07 PM
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Join Date: Nov 2010
Location: lake charles, La.
Posts: 699
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RPM'S
About the question of low rpm's on the lycoming. The engine has no problems flying with lower rpms, however, if you are running a hartzell c/s propeller, as far as I know they do have a restriction on flying extended times with rpms below 2250. If this has changed recently, please let me know. I wish they didnt have that restriction because I am of the opinion the lower rpms definately provides better fuel economy. Just my observations
bird
__________________
Bird
rv8 entire airframe at airport now, painting done, intersection and gear upper and lower fairings done, maybe order engine around first of year or before the next rate increase.
"to fly is a privilege that I am so thankful to God for"
http://www.mykitlog.com/tcb328/
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04-26-2011, 05:54 PM
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Join Date: Apr 2005
Location: KSLC
Posts: 4,021
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Quote:
Originally Posted by bird
About the question of low rpm's on the lycoming. The engine has no problems flying with lower rpms, however, if you are running a hartzell c/s propeller, as far as I know they do have a restriction on flying extended times with rpms below 2250. If this has changed recently, please let me know. I wish they didnt have that restriction because I am of the opinion the lower rpms definately provides better fuel economy. Just my observations
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For my 0360 & Hartzell C/S (not blended tip)............I have a restriction of 2000 to 2250 for extended periods of time. I don't know about 2000 and below, as I prefer not to run below 2000 rpm.
L.Adamson --- RV6A
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04-26-2011, 06:13 PM
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Join Date: Jan 2008
Location: Ashland, OR
Posts: 2,574
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Depends on engine/prop combination
Hi Bird,
different combinations of engine and prop (B/A or old style) have different operating speed limitations. Many combinations have no restrictions.
A classic one for the IO-360 is something like no continuous operation between 2000-2250. Nothing says you can't run below 2000. Another common one is 2350-2550 above 25" MAP. Again, no low-RPM restriction.
Quote:
Originally Posted by bird
About the question of low rpm's on the lycoming. The engine has no problems flying with lower rpms, however, if you are running a hartzell c/s propeller, as far as I know they do have a restriction on flying extended times with rpms below 2250. If this has changed recently, please let me know. I wish they didnt have that restriction because I am of the opinion the lower rpms definately provides better fuel economy. Just my observations
bird
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__________________
Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet"
Hobbs 625
LS6-15/18W sailplane SOLD
bought my old LS6-A back!! 
VAF donation Jan 2020
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04-26-2011, 08:42 PM
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Join Date: Sep 2005
Location: Maple Grove, MN
Posts: 2,334
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Data
I took this data a few years ago, using two throttle settings (see chart for specifics), and simply dialed mixture across a range. I want to repeat this, only continue leaning until the mpg actually starts rising again. The speeds are not based on ground, only TAS.
The crossing of the TAS lines is interesting, and it is a coincidence that the crossing of the mpg lines happens to be directly at the same point (lucky scaling of the left and right vertical axes). Also noteworthy is that for the same fuel burn, the higher MAP delivers a better performance, presumably owing to less pumping losses, even though I arbitrarily chose a higher rpm.
Oh, and yes, the TAS (and associated engine settings) delivering the best mpg across the ground WILL vary with what the winds are aloft. Seems crazy, but the glider guys (thanks Larry P.) busted me a while back on a similar phenomenon regarding optimum gliding distance (also not a constant - depends on winds).
__________________
Alex Peterson
RV6A N66AP 1700+ hours
KADC, Wadena, MN
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04-26-2011, 10:25 PM
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Join Date: Jul 2008
Location: NorCal
Posts: 565
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Quote:
Originally Posted by Kahuna
huh?
Headwind and tail wind has nothing to do with the effeciency of the plane.
It also has no place in discussions of trying to determine power settings and MPG discussions.
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Head/tail winds most certainly belong in a discussion about ground miles per gallon. A simple thought experiment illustrates.
Your plane cruises at 100 smph at 65% power which is your best no-wind MPG speed. You take off, and encounter a headwind of 100 smph. If you remain at your best MPG speed you get ZERO smpg. You MUST add power to get forward ground speed. In this extreme example, the more power, the better your ground MPG. In a more realistic example you could work it out from those 1000 graphs you have, or, let your glass panel calculate it for you.
So you decide to head the opposite direction. Now you have a 100 smph tailwind. Your best ground MPG is to throttle back to just above stall speed while maintaining altitude.
In the real world, each flight, with a different head/tail wind component, is going to have a slightly different throttle setting for best ground MPG.
__________________
Ralph Finch
RV-9A QB-SA
Davis, CA
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04-27-2011, 07:24 AM
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Join Date: Jul 2005
Location: Detroit, MI
Posts: 1,587
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Close, but not correct
Quote:
Originally Posted by Buggsy2
Head/tail winds most certainly belong in a discussion about ground miles per gallon. A simple thought experiment illustrates.
Your plane cruises at 100 smph at 65% power which is your best no-wind MPG speed. You take off, and encounter a headwind of 100 smph. If you remain at your best MPG speed you get ZERO smpg. You MUST add power to get forward ground speed. In this extreme example, the more power, the better your ground MPG. In a more realistic example you could work it out from those 1000 graphs you have, or, let your glass panel calculate it for you.
So you decide to head the opposite direction. Now you have a 100 smph tailwind. Your best ground MPG is to throttle back to just above stall speed while maintaining altitude.
In the real world, each flight, with a different head/tail wind component, is going to have a slightly different throttle setting for best ground MPG.
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There is much here that is correct. However, the best speed for a tail wind depends on the tail wind, but using a 100 mph tail wind, the best speed would be about 76% of L/D which is your best endurance speed, not just above a stall. Please refer to the chart I posted above.
__________________
H. Evan's RV-7A N17HH 240+ hours
"We can lift ourselves out of ignorance, we can find ourselves as creatures of excellence and intelligence and skill. We can be free! We can learn to fly!" -J.L. Seagull
Paid $25.00 "dues" net of PayPal cost for 2015, 2016, 2017 and 2018 (December).
This airplane is for sale: see website. my website
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