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  #1  
Old 04-26-2011, 05:13 AM
plehrke's Avatar
plehrke plehrke is offline
 
Join Date: Sep 2006
Location: Defiance, MO
Posts: 1,970
Default Cruise Performance Plots; RV-6A, 180 hp, Fixed Pitch

A few months ago in a previous thread, I had promised I would post my performance curves I have been generating from my recent flight test. I was waiting to get points at 12,000-14,000 ft and get a better understanding of where the wide open throttle (WOT) boundary might be but unfortunately I have had a medical set back and will not be flying for 6-9 months. I thought I would post what I do have and update later this year when I get back to flying.
My RV has an Aerosport Power IO-360-B2B (180 hp) engine with a fixed pitch Sensenich 85” prop. Figure 1 below shows all 61 data points that I collected during 5 flights and 9 flight hours. The numbers next to the points are the corresponding RPM at that flight condition. Each point actually represents >30 data points taken over a minimum of 60 seconds of steady flight and have been averaged to get one point on the plot. I can do that as I have a flight data recorder (FDR) in my RV-6A that records all flight performance perimeters every 1-2 seconds. True Air Speed (TAS) was obtained using a calibration curve, which I generated from earlier flight testing, to get from indicated airspeed (IAS) to calibrated airspeed (CAS) and equations from Kevin Horton’s webpage to get TAS from CAS.

Figure 1. TAS vs. Density Altitude at various RPM settings
One thing to point out is there are several data points close to one another on the plot. I ran most of the flight points at several mixture settings. I took data at full rich, at economy cruise (peak EGT), and at lean for max power (150º F rich of peak EGT). Figure 1 shows all the points, regardless of mixture setting.
From these points I wanted to be able to plot percent power lines so I converted each point using RPM, MP, OAT, and pressure altitude. It actually was not too hard as my flight data recorder automatically calculates percent power for my engine. I then used regression analysis to draw lines of constant percent power through the data. The plot in Figure 2 is for the data points where I leaned to max power (150º F rich of peak EGT). Also plotted on the chart are the two cruise points that Van’s quotes for the RV-6A. My data exactly matches Van’s for the 75% power 8,000 ft point but is off by about 6 kts on the 55% power point. (This could be due to me collecting only a few points down to 45% power which may have skewed the regression analysis to pull left the percent power curves. I found during testing that the lower the power setting the harder it was to get a constant leaning process.)

Figure 2. Leaning for Max Power Cruise Performance
As mentioned before, I took data at several mixture settings so the next plot, Figure 3, shows how the power varied with mixture setting. The blue curves are the max power curves (same as the previous plot). The green curves are when I leaned to peak EGT for economy cruise. The magenta curves are when I ran full rich, which I only did for 75% power and above where leaning is not recommended per Lycoming guidance.
In my data RPM tracked linear with TAS for my fixed pitch prop (see graph in previous posting) so the RPM limit is just a vertical line. I plotted the 2700 RPM limit line in Figure 3. I also sketched in a wide open throttle (WOT) curve but I am not sure how accurate it is as I was unable to get a good curve to go through the 6 points I had for WOT. As indicated in the opening paragraph of this post, I would really like to get some more data points to determine if it is linear or has some break to it.

Figure 3. Cruise performance for various mixtures settings
Looking at a max power cruise at 8,000 ft, my plot shows 75% power when leaning to peak EGT is about equal to 74% power when leaning to max power. This is approximately a 1.5% reduction in best power (74%/75%). The plot shows 55% power when leaning to peak EGT is about equal to 53.5% power when leaning to max power. This is approximately a 3% reduction in best power (53.5%/55%). A plot in the Lycoming engine operating handbook, shown in Figure 4, shows a 4% reduction in power for leaning from max power to max EGT. Since the peak EGT and max power curves slightly diverge, extrapolation shows that up at 12,000 to 14,000 ft the power loss will be 2-3% which in turn will calculate out to a 3-4% reduction in best power just like the Lycoming chart shows. The Lycoming curve is a generic plot so not sure if it is supposed to represent cruise at 8,000 ft or 14,000 ft. One other thing to note is that my data shows a fall off in performance if you run full rich just as the Lycoming curve shows. I lost about 1.5% of best power when running rich over running at max power (see magenta curves).

Figure 4. Representative effect of leaning on CHT, % Power, EGT, and Specific Fuel consumption
As shown in Figure 4, for each mixture setting there is a different specific fuel consumption (fuel flow). Figure 5 shows how my fuel flows varied with mixture settings. There is quite a bit of scatter in the data since measured fuel flow accuracy tends to be pretty rough. In my collected data, the fuel flow can jump around pretty good (±5%) even when sampling the data ever 1-2 seconds.

Figure 5. Fuel Flows for varying mixture settings
Overall conclusions:
Need more WOT data and flight data above 10,000 ft to see complete flight envelop.
At 75% power and 8,000 ft I got a 21% reduction in fuel flow for only a 1.5% reduction in best power.
At 55% power and 8,000 ft I got a 23% reduction in fuel flow for only a 2.5% reduction in best power.
It would be nice to see where leaning to 50º F lean of peak EGT would fall on the plots.
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RV-6A - flying 1000+ hours
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There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact. óMARK TWAIN

Last edited by plehrke : 04-26-2011 at 06:05 AM.
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  #2  
Old 04-26-2011, 07:12 AM
David-aviator David-aviator is offline
 
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Location: Chesterfield, Missouri
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Default

Great work, Phil.

There is a lot of data here and it will take a while for my brain to absorb it. I may steal one or two of the charts as it appears quite close to RV-7A numbers.

I've been using manifold pressure (23") as the 65% indicator to set LOP, looks like 8 gph would work also.
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  #3  
Old 04-26-2011, 10:56 AM
Bob Axsom Bob Axsom is offline
 
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Default Just a top speed comment

Your speed looks very competitive for the SARL RV Blue Class. I fly an RV-6A with several speed mods and a Hartzell B/A prop with F7496 blades and a Lycoming O-360-A1A (stock with 8.5:1 pistons). The maximum RPM I can get is 2720 and at 6000 ft dalt my top TAS is just over 184 kts.

Good luck with your health issue, we are dealing with one in the family that has stopped my racing activities for a while as well. Some things are much more important.

Bob Axsom

Last edited by Bob Axsom : 04-26-2011 at 10:59 AM. Reason: Added engine info
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  #4  
Old 04-27-2011, 06:35 PM
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Kevin Horton Kevin Horton is offline
 
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Philip,

You've obviously done a lot of work here, and have some interesting results. Congrats. I've barely started to look at this, but have a few comments and questions.

Which flight data recorder are you using? Do you know how it calculates engine power? Does it account for mixture setting somehow? Or, is it simply a calculation of power as a function of rpm, MP, altitude and temperature (which likely assumes the mixture is set for best power)?


Quote:
Originally Posted by plehrke View Post
I took data at several mixture settings so the next plot, Figure 3, shows how the power varied with mixture setting.
Given that speed vs power should not vary with mixture setting, it looks to me that what we are really seeing is inaccuracies in the calculated power. I.e, the curve for max power is to the right of the one for peak EGT because the calculated power for peak EGT is too high. If power calculations at all mixture settings were accurate, the magenta, blue and green lines for each power should sit on top of one another.
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  #5  
Old 04-27-2011, 07:23 PM
plehrke's Avatar
plehrke plehrke is offline
 
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Location: Defiance, MO
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Quote:
Originally Posted by Kevin Horton View Post
Which flight data recorder are you using? Do you know how it calculates engine power? Does it account for mixture setting somehow? Or, is it simply a calculation of power as a function of rpm, MP, altitude and temperature (which likely assumes the mixture is set for best power)?
The FDR I am using is a Pegasus Technology unit that was only marketed for a couple of years back in 2001-2002. If I remember correctly, they were RV-4 guys that created the box to help Sensenich gather data when they where trying to design/test the 180hp prop and have no rpm restriction. I saw them at OSH one year in a booth but never again. Since there was an RV connection, I bought one of the units. Do not know how many units they sold.

I do not exactly know how the unit is calculating % power but when I bought it they ask for my engine model. The unit has a sticker on the side with my engine called out. In the post processing software they provided me, it allows me to change the engine model number. My assumption is that they are using a table look-up from lycoming operator's manual which does not use mixture in calculations as it assumes best power mixture. I will check some calculations to try and verify.

Quote:
Originally Posted by Kevin Horton View Post
Given that speed vs power should not vary with mixture setting, it looks to me that what we are really seeing is inaccuracies in the calculated power. I.e, the curve for max power is to the right of the one for peak EGT because the calculated power for peak EGT is too high. If power calculations at all mixture settings were accurate, the magenta, blue and green lines for each power should sit on top of one another.
I think I agree with you since percent power is percent power. Do you know how to adjust percent power for various mixtures?

Another item I wish I had more insight into is how it is calculating TAS from CAS. It seems to be a 1-2 kts lower then what I get when I use the equations I got from your website. Unfortunately the company and email address are gone and I cannot contact them to get answers. I have spent many hours trying to "check" the calculations from the box to try and figure out how it is doing it.
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There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact. óMARK TWAIN
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  #6  
Old 04-27-2011, 07:50 PM
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Kevin Horton Kevin Horton is offline
 
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Quote:
Originally Posted by plehrke View Post
Do you know how to adjust percent power for various mixtures?
I've got an old Lycoming doc that provides a method to calculate power that is claimed to be valid at all mixtures. I've done a fair bit of flight testing with this method, and it seems to work. I.e. the speed vs calculated power is consistent even at different mixtures.

For more info, see here and here. I've got an OpenOffice spreadsheet somewhere that uses that method, with a hypothetical modification to support fixed pitch props. The method needs inputs of displacement, compression ratio, rpm and fuel flow at peak EGT and rpm and fuel flow at the test point condition. Essentially, before each test point you must find peak EGT and stabilize long enough to get an accurate fuel flow reading. Then, without changing throttle or altitude, set the mixture as desired for the test point then record fuel flow and rpm again.
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  #7  
Old 07-29-2022, 05:07 PM
Sl2trike Sl2trike is offline
 
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Location: Charlotte NC
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Default RV6A fuel burn

I flew my RV6A IO-360A1E CS, converted to carb long ago, to OSH and back averaging 6.7 gal/hr. @ 145 knots @ 8K. 2350 rpm @ 20 inches leaned to just ROP kept the oil temp at 185, head temps at 315, and the fuel flow below 7.
I haven't done any speed runs yet so I can't comment on WOT fuel burn.

These are amazing airplanes!

Joe
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