I decided a few months ago that, after flying for 4 years, I wanted to go back and get some really accurate performance points for my RV-6A. Therefore I have been busy the last few months (weather permitting) converting avgas into data points. My RV has an Aerosport Power IO-360-B2B (180 hp) engine with a fixed Sensenich 85” pitch prop.
The figure below shows all 61 data points that I collected. (I added the mph to the bottom of the chart for you folks that think knots are what you get in your kid’s shoe laces.) The numbers next to the points are the corresponding RPMs I was at during that flight condition. Method to obtain the data was to get speed, altitude, RPM, etc to stabilize for 5 minutes then take the data. 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 in my RV 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. The data I collected is going to be used to get a plot showing TAS vs density altitude for different throttle and mixture settings (yes, I guess you can tell I am an aeronautical engineer so stay tuned for a future post with that plot).
After looking at the data I thought I should draw a line through the data points representing the 2700 RPM limit. It appears from first look that the RPM tracks pretty much directly with TAS so I thought I would re-plot the data to just show RPM vs TAS. The plot below confirms this.
I have always thought propeller aerodynamics is pretty much black magic and have never spent the time to fully understand it but this plot caused me to ask several questions:
A prop is like a screw so what is the “threads per inch” so to speak of the propeller and how is that related to the pitch? Can I calculate the prop efficiency from this plot?
I hit engine red line of 2700 RPM before getting to Vne. The engine has the horsepower to get my RV to Vne in level flight but above recommended RPM limit. I read some where that 1” of pitch is worth about 40 RPMs. If I had a 88” pitch prop would I get Vne at 2700 RPM meaning the curve in the plot above would shift down? Or does the slope of the curve also change?
If the data should really be linear for a fixed pitch prop, does the R^2 term from the trend line indicate the accuracy of my flight test data? Does it mean that my data must be 99.5% accurate?
The figure below shows all 61 data points that I collected. (I added the mph to the bottom of the chart for you folks that think knots are what you get in your kid’s shoe laces.) The numbers next to the points are the corresponding RPMs I was at during that flight condition. Method to obtain the data was to get speed, altitude, RPM, etc to stabilize for 5 minutes then take the data. 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 in my RV 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. The data I collected is going to be used to get a plot showing TAS vs density altitude for different throttle and mixture settings (yes, I guess you can tell I am an aeronautical engineer so stay tuned for a future post with that plot).
After looking at the data I thought I should draw a line through the data points representing the 2700 RPM limit. It appears from first look that the RPM tracks pretty much directly with TAS so I thought I would re-plot the data to just show RPM vs TAS. The plot below confirms this.
I have always thought propeller aerodynamics is pretty much black magic and have never spent the time to fully understand it but this plot caused me to ask several questions:
A prop is like a screw so what is the “threads per inch” so to speak of the propeller and how is that related to the pitch? Can I calculate the prop efficiency from this plot?
I hit engine red line of 2700 RPM before getting to Vne. The engine has the horsepower to get my RV to Vne in level flight but above recommended RPM limit. I read some where that 1” of pitch is worth about 40 RPMs. If I had a 88” pitch prop would I get Vne at 2700 RPM meaning the curve in the plot above would shift down? Or does the slope of the curve also change?
If the data should really be linear for a fixed pitch prop, does the R^2 term from the trend line indicate the accuracy of my flight test data? Does it mean that my data must be 99.5% accurate?
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