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  #11  
Old 08-15-2016, 10:50 AM
aturner's Avatar
aturner aturner is offline
 
Join Date: Jan 2005
Location: Clarion, Pennsylvania
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Default Altitude and RPM

You guys with -9's are posting impressive numbers, you might have to form your own club. :-) As to the question as to what is going on with my -10, it might be a few knots faster than average, but I suspect that the most important things to note are the operational parameters.

In the post above, I am at a density altitude of 17,000 ft., and it seems that the unique airfoil of the RV-10 loves altitude. I am guessing that this has something to do with air density, Reynolds numbers, and laminar flow, but someone smarter than me can confirm this. Here is a plot showing segments of different cross-country flights, with cruise altitude plotted against fuel efficiency for each one:



These are flights at different configurations of power, RPM, fuel flow, etc., but even so, it is altitude that has the overwhelming effect on efficiency. And higher is better, at the rate of one mile per gallon for each 3000 ft.

The other operational aspect to note is the low prop RPM. I am running old fashioned Slicks at 25 deg. fixed timing, and 30-40 degrees lean of peak. As several excellent threads here have explained, the flame front needs time to do its work at high altitude and lean mixtures. Here is a plot from a test in which I did mixture sweeps at RPM's ranging from 2400 to 2000. Superimposed on the plot are isopleths for nautical miles per gallon. Test conditions were 9200 density altitude, so maximum efficiency is only about 18 nmpg.



There is a lot to be learned from this graph, but the simple point is that the maximum efficiency was at the slowest RPM tested, 2000. Here is another recent graph, showing the results of a test in which I simply reduced RPM in a stepwise manner while on a cross country flight (14,000' DA), without touching anything else.



X-axis is RPM, y-axis is true airspeed divided by fuel flow (nmpg).

So, fly high, and for those of us with magnetos, keep that prop slow.
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Last edited by aturner : 08-15-2016 at 04:30 PM.
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  #12  
Old 08-15-2016, 11:56 AM
togaflyer togaflyer is offline
 
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Location: Southeast
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Im not an engine expert, but if your running your rpms that low under a load, aren't you putting significant load on the engine.
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  #13  
Old 08-15-2016, 06:53 PM
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tfoster100 tfoster100 is offline
 
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Location: Southlake, TX
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I can't wait to get an RV-10 or 9A. Those numbers are awesome. I'm building experience and hours in a 182.
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  #14  
Old 08-15-2016, 07:35 PM
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Weasel Weasel is offline
 
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Our -10 will go 177knots on 10.7 GPH at 12,000ft Density Alt which works out to little over 19 statute MPG. I can get well over 20 MPH by slowing down just a little by leaning slightly. That is the only hard data I have handy.

will try to collect some efficency numbers soon.
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  #15  
Old 08-15-2016, 09:14 PM
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aturner aturner is offline
 
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Quote:
Originally Posted by Weasel View Post
Our -10 will go 177knots on 10.7 GPH at 12,000ft Density Alt which works out to little over 19 statute MPG.
Your data point falls right on the curves I posted above for the mixture sweeps. I routinely plan on, and see, 170 knots on 10.0 GPH at 12K, which also seems in line with your report.
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  #16  
Old 08-16-2016, 07:27 AM
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bobmarkert bobmarkert is offline
 
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Default 24.7 MPG

On the way to Oshkosh:
9,500?
-100 LOP
FF 6.6 GPH
163 KTAS
MPG 24.7
RV-8 with an Aerosport IO-375 high compression motor



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  #17  
Old 08-16-2016, 09:20 AM
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NovaBandit NovaBandit is offline
 
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Location: Hastings, MN
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Quote:
Originally Posted by rleffler View Post
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
Here are two screenshots from my last flight from Minnesota to Chicago. First one is LOP, second is ROP. The ROP settings were just quick and dirty to get a rough comparison, and were by no means tweaked for max performance.





Quote:
Originally Posted by aturner View Post
In the post above, I am at a density altitude of 17,000 ft., and it seems that the unique airfoil of the RV-10 loves altitude.
Totally agree. In my limited time flying my -10 (around 100 hours) I've found that I don't like cruising much below 9500. Better speed and efficiency!

Quote:
Originally Posted by aturner View Post
There is a lot to be learned from this graph, but the simple point is that the maximum efficiency was at the slowest RPM tested, 2000.
..............
So, fly high, and for those of us with magnetos, keep that prop slow.
I have dual electronic, and I've been normally cruising between 2360 and 2400, WOT. I'll have to play around with pulling RPMs lower.
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  #18  
Old 08-16-2016, 09:21 AM
jchang10 jchang10 is offline
 
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It seems like the -8's are the winners in "total" performance (ie. speed combined with efficiency). I wonder what an -8 with -9 or -10 wings would be like. Anyone else curious?
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  #19  
Old 08-16-2016, 09:27 AM
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NovaBandit NovaBandit is offline
 
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Quote:
Originally Posted by jchang10 View Post
It seems like the -8's are the winners in "total" performance (ie. speed combined with efficiency).
Not if the performance number you're looking at is "passenger mile per gallon"!
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RV-10 N829EC - Flying
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KSGS - Fleming Field, South Saint Paul, Minnesota

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  #20  
Old 08-16-2016, 10:43 AM
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rleffler rleffler is offline
 
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The missing data point for me was DA. When you take DA into consideration, I'm getting similar numbers.
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