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Old 02-01-2019, 08:12 PM
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I hammer at 1500 feet
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pulling ability

Question for the aerodynamic engineers...


Lets say given all things the same,

what pitch on a prop would give the best pull on an up-line?


so lets say it is a 40% extra aircraft
and the motor is 170 cc twin cylinder two stroke

would a higher pitch prop pull better on an upline than a lower pitch prop?

Lets say

30 x13 prop 2 blade
versus a 32 x 10 prop 2 blade
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Old 02-01-2019, 09:03 PM
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I fly 12 months a year
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This is very interesting. I am building the PAU Edge 540T and I have a DA 170. I purchased a Falcon 31x10W. I am not a precision flyer, just a stick banger at 3D.
I hope I purchased the correct prop, I wait the results.
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Old 02-01-2019, 10:00 PM
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Ok, this may not be exactly what you're after, but I ran some numbers through the late, great, Pe Reviers' prop calculator. I made the assumptions that a 170cc engine delivers 17hp, and is swinging Mejzlik props with the same 1.18 prop factor. I adjusted the rpm for each prop to deliver 17.00hp.

To deliver 17hp, these are the rpm's

30x13 @ 6305 rpm 85.71 lbs of thrust, 78.85 lbs static thrust, 67.4mph
32x10 @ 6236 rpm 89.48 lbs of thrust, 82.32 lbs static thrust, 51.3mph

So, which delivers more "pull" depends upon one's meaning of "pull", Ortho, right? The 32x10 will deliver greater static thrust, possibly pull like a tractor at full rpm, but fly upward at a slower speed than the 30x13. But this assumes steady state of the engine spinning the prop AT that rpm. Dynamically, the smaller prop will likely accelerate (spin up) more quickly to reach the rpm, respond to throttle changes more quickly, and might "feel" like it's pulling better. I truly don't know how it would feel.

It does seem that with the larger prop, the 4% increase in thrust may not be worth the 24% loss in velocity.

I should also note that according to the prop calculator, the props should rip nicely at those rpms.
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Last edited by RCAddiction; 02-01-2019 at 10:11 PM.
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Old 02-01-2019, 11:31 PM
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They're get'n lower mate.....
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How was prop efficiency at altitude calculated & also correction for humidity
Prop loading will most certainly be affected by these two factors
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Old 02-01-2019, 11:59 PM
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They were not used as variables for this exercise. The information should only be considered relative between the two props and not absolute value. Altitude, humidity and prop factor were assumed to be the same between the props.
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Old 02-02-2019, 05:57 AM
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RC Addict, yes, you are basically informing me of what I wanted to know. I do not have that knowledge to calculate this. SO this would be interesting to see. IS pull on an up-line improved with more speed or more thrust?
that is the question!

I can tell you, if I use the 30 x 13, on up lines, it does not rip.
If i use a 32 x 10, it will rip like crazy going up or horizontal flight.

But what I do not know is which will pull better and help me climb higher and faster.

Thank you so much.
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Old 02-02-2019, 07:31 AM
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Ortho, I cannot answer which will be better for faster and higher vertical. Generally, I view the this as a trade-off in thrust vs speed. One will give you the ability to get to a higher altitude because it has more thrust and the other will get you there faster, but poop out at a lower altitude.

Pe Reviers, who created the attached Excel-based calculator several years ago, was a rather brilliant contributor to our hobby. You can plug in the various prop options into the calculator and play with it. He refined it over the years with actual data from various props, and many consider it the most accurate thrust calculator for our toy planes. He offered it to our community for free, as a download on the MVVS engine site, in the Netherlands, I believe.

There is an rpm rev limit for noise, based on 0.6 mach tip speed. Both props will exceed 0.6 mach at less than 5k rpm. I don't know if that means that they will rip, just perhaps that they may be getting noisy. Rooman, there's also the opportunity to enter altitude, air pressure, and temperature. There's a carb calculator (bottom of first tab), airfield noise calculator, fuel mixture calc, etc as you click through the tabs. Amazing piece of work that Pe did for us.

You input your options in the light green cells. light blue cells are calculated. I have unlocked one of these spreadsheets to see some of the formulae, and to add some Falcon and Xoar prop constants that he had posted shortly before we lost him.

On the unlocked version, even though you can enter into any cell, please only enter data into the green cells. Otherwise, you might screw up the calcs. To avoid problems, many will be better served using the standard workbook and simply use the prop constants that I've highlighted in yellow in the unlocked sheet.

They are:
Falcon CF = 1.18 (same as standard Mejzlik, and might not be 100% correct)
Xoar Laminated = 1.4
Xoar CF = 0.9

The (2) files are .zipped together because FG does not support uploading .xls files.
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Old 02-02-2019, 07:38 AM
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Note: the Prop Thrust Calculator files have been scanned with Bitdefender, Norton Internet Security and MalwareBytes. To the best of my knowledge these are, and have always been, squeaky clean.


These might make a nice sticky at the top of the Engine Forum.
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Old 02-02-2019, 07:59 AM
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wrongwayfeldman
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I can't open file
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Old 02-02-2019, 08:28 AM
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As I said it's a zip file because FG does not support uploading Excel files. You need to extract the .zip file. Once downloaded onto your PC, right click and choose extract files or unzip, depending on your computer and operating system. Download 7Zip if you don't have an unzipping utility integrated into your OS.

If this sounds like a foreign language, no worries. Just shoot me a pm with your email address and I'll send the files to you.
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Old 02-02-2019, 08:30 AM
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Try this link http://godolloairport.hu/calc/strc_eng/index.htm
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Old 02-06-2019, 05:59 PM
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aircraft weight and drag play into this as well..... Larger diameter with less pitch = more thrust at lower velocities... generally....
Larger diameter has higher tip speeds at same RPM... so that is a limitation....
Broad brush statement... 32 x 10 will pull from a hover better than a 30 x 13 at same power
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Old 05-10-2019, 06:16 PM
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I hammer at 1500 feet
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Quote:
Originally Posted by Mithrandir View Post
aircraft weight and drag play into this as well..... Larger diameter with less pitch = more thrust at lower velocities... generally....
Larger diameter has higher tip speeds at same RPM... so that is a limitation....
Broad brush statement... 32 x 10 will pull from a hover better than a 30 x 13 at same power

Makes sense, it is like 32 x 10 is 2nd gear on a Mustang 5.0
and a 30 x 13 is 5th gear on a mustang 5.0
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Old 05-10-2019, 08:38 PM
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wrongwayfeldman
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What you guys are saying is exactly why I like a Vess prop with the "A" pitch. It's the least pitch they offer and you can go up an inch on prop length. The results are less top speed but a lotta pull and the throttle response is awesome.
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Last edited by ponyboy082662; 05-10-2019 at 09:43 PM.
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