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  #1  
Old 02-05-2011, 09:22 AM
plehrke's Avatar
plehrke plehrke is offline
 
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Default Fixed Pitch Prop; Do I need more?

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?
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Last edited by plehrke : 11-25-2015 at 04:43 PM.
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  #2  
Old 02-05-2011, 10:35 AM
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N941WR N941WR is offline
 
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What's your MAP at those RPM settings?

If you have a "climb" prop, you will be able to spin it well past 2700 RPM but your MAP will be low because the prop isn't "loaded".
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  #3  
Old 02-05-2011, 10:56 AM
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plehrke plehrke is offline
 
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Quote:
Originally Posted by N941WR View Post
What's your MAP at those RPM settings?
At the points near 2800 rpm and 183 kts TAS I am getting 29-29.1" of MAP
At the 2750 rpm and 180 kts TAS I am getting 26.5"of MAP
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  #4  
Old 02-05-2011, 02:43 PM
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hevansrv7a hevansrv7a is offline
 
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Quote:
Originally Posted by plehrke View Post
...

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? ...
2700 rpm, 174 kts (my best effort at reading the chart and exactly what my 7A does).

174 kts = 200 mph or close enough for this excercise.

200 mph = 200/60 = 3.333 miles/minute = 3.333 x 5280 = 17,600 feet/minute x12 = 211,200 inches/minute.

211,200 inches/minute divided by 2700 revolutions per minute gives 78.22" inches per revolution. I call this the "effective pitch". I have seen other labels, too. This is what your prop is actually doing on your airplane. The 85" pitch specification is probably based on a geometric measurement at the 75% radius off the flat of the blade. That's good for comparing two Sensenich props and unlikely to be accurate between two props of different design and/or on two different airplanes.

As for prop efficiency, the short answer is "no". Your data do not tell you the efficiency. The prop efficiency is the thrust HP divided by the BHP. The thrust HP is best inferred from the drag curve in level flight. The BHP is very hard to know but there are several ways to estimate it. Please refer to the various articles and spreadsheets on my website (see signature). Contrary to a popular rule of thumb, you cannot even estimate relative BHP from RPM with much accuracy. The cube rule does not hold up well.

Thanks for the data. It seems very well done!
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  #5  
Old 02-05-2011, 02:48 PM
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Quote:
Originally Posted by plehrke View Post
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?
I can't help with your prop questions (black magic to me too) but assuming your R^2 comes from standard statistical calculations, what it indicates is that your "line of best fit" is 99.5% accurate in defining your TAS for a given RPM (or vice versa), within a valid range of course. In other words, it says nothing about the accuracy of your data points, per se, only that they fall nicely in line.

Cool exercise. I'll watch with interest.
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  #6  
Old 02-05-2011, 05:58 PM
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Kevin Horton Kevin Horton is offline
 
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Were you at full throttle for the 2800 rpm/182 kt TAS points? If you add more pitch so you hit 2700 rpm at full throttle, the engine power will be less due to the lower rpm. If the torque is contstant, the power will vary linearly with rpm. At the same conditions, the TAS will vary with the cube of the power, so your TAS at 2700 rpm and full throttle would be about 180 kt.

You can only optimize a fixed pitch prop for one point. Are you more interested in low altitude full throttle speed, or are you more interested in higher altitude cruise at some lower power? If you optimize the pitch for low altitude, high rpm speed, you may find that you lose speed at higher altitude as you hit full throttle at a lower rpm.
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  #7  
Old 02-05-2011, 08:47 PM
rubber314chicken rubber314chicken is offline
 
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Quote:
Originally Posted by plehrke View Post
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?
That means 99.5% of the variance between points can be explain with that line you generated. Or .05% is due to some other factor this graph doesn't cover. Basically, it means that that line fits the data really nicely. A r^2 value of more than 95% is considered good.
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  #8  
Old 04-01-2018, 02:51 PM
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az_gila az_gila is offline
 
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Smile 83 pitch Sensenich

I thought I would add my testing points to the OP original graphs. My -6A is 180 HP but has the Sensenich recommended "standard" prop of 83 inch pitch. The prop "screw theory" still seems to hold up and be accurate to a first order.

It also shows I live at a higher DA, but even with that the current Vans recommendation of 85 inch pitch as a "standard" prop would probably be better for my plane.

My points added to the first chart. The top point is real, it was 2710 rpm at 14,500 ft DA and wide open throttle. I didn't exceed 2700 rpm at lower altitudes.



And on the second chart. As expected pitch essentially shifts the line left or right. Looks like 4-5 kts for the 2 inch pitch change.

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  #9  
Old 05-31-2018, 04:05 PM
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smokyray smokyray is offline
 
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Default Perfection...

Quote:
Originally Posted by plehrke View Post
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?
Phil,
Actually, by my standard you are a bit over-pitched.
First, the 2700 RPM "limit" on Lycomings was placed there for certified engines many years ago by the factory. Subsequently it was signed off as Lycoming gospel by the FAA and is arbitrary at best IMHO, but a good baseline for performance calculations.
"Rated" Horsepower is achieved at that RPM and notated appropriately by the manufacturer.

Testing...
In 25 years of testing and flying Nine different FP props on my 2 RV's, I've developed my own "not so scientific" field data.
Sea Level WOT in a properly faired airplane should produce VNE at 2800-2900 RPM on a properly pitched FP prop. This will allow rated RPM WOT above 10,000'. It will also closely match the Van's performance numbers for your airplane.

Why?
You desire a pitch to produce rated RPM at higher altitudes where TAS is higher and MP lower.
This will allow you to maximize low altitude performance and utilize the tried and true "48" rule of MP+RPM=48 for 75% power.

By the numbers:*
My best props allow me to run at WOT at 11,500 and achieve rated rpm(2700) @175KTAS. Below 5000' I should be able to set 24" MP and achieve 2400-2500 RPM at 155-160KTAS. WOT @ S.L. produces 2800-2900/28" at 175KIAS.
Static and takeoff RPM should be around 2250-2350. Use 120 Knots as a cruise climb at 2450 and you should achieve "tech order" (best rate) climb.

You should seek similar numbers with your setup for optimum performance....
V/R
Smokey

*My carb equipped 180HP RV"X" 945lb EW, 2 blade Catto Gen 2, homemade fairings, Van's PR wheel pants, 380 tires, TX mud stuck here and there.

Last edited by smokyray : 05-31-2018 at 04:15 PM.
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  #10  
Old 05-31-2018, 04:19 PM
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az_gila az_gila is offline
 
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Smile

Quote:
Originally Posted by smokyray View Post
.....

By the numbers:*
My best props allow me to run at WOT at 11,500 and achieve rated rpm(2700) @175KTAS. Below 5000' I should be able to set 24" MP and achieve 2400-2500 RPM at 155-160KTAS. WOT ay S.L. is 2800-2900/28" at 175KIAS.
Static and takeoff RPM should be around 2250-2350. Use 120 Knots as a cruise climb at 2450 and you should gain "tech order" (best rate) climb.

You should see similar numbers with your setup...
V/R
Smokey

*My carb equipped 180HP RV"X" 2 blade Catto Gen 2, homemade fairings, Van's PR wheel pants, 380 tires, TX mud stuck here and there.
Thanks for the numbers, as you can see my testing numbers above, my 2700/WOT limit was 168 kts at 14,500 DA

Sensenich just repitched my prop from 83 to 85 inches, so new tests should get pretty close to your numbers.

I'll update my orange diamonds above when tests are complete, but since my house is at a DA of 6,000 ft at the moment, sea level testing is out.

I'm expecting about 4 kts increase at most conditions, but don't know what the altitude for 2700 rpm WOT will be. I'm sure it won't be as high as 14,500 DA.

*My carb equipped 180HP RV6A, 2 blade Sensenich, homemade fairings, Sam James wheel pants, Std. kit tires, No AZ mud around.
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Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ

Last edited by az_gila : 05-31-2018 at 04:25 PM.
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