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Old 05-03-2021, 04:32 PM   #1
SrFalcon
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3-blade propeller equivalent


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Ok folks, looking for some advice on going from a 2 blade to a 3 blade prop in order to gain prop ground clearance. I currently have an 18x10 2 blade prop on a new NGH35. NGH recommends using 18x10, 19x8, 19x10 or 20x8, and I went with the smallest diameter due to a ground clearance issue.

Folks tell me the rule of thumb(ROT) is go down one inch and keep the same pitch to increase to a 3 blade prop. This would mean going with a 17x10 3 blade prop.

Elsewhere I found a prop load factor(LF) formula which is supposedly more accurate. I wont bore you with equations, but suffice to say, here's what it gives:

LF = 58,320 for 18x10 2 blade(baseline load)

LF = 69,470 for 17x10 3 blade(19% greater load than baseline)

LF = 55,576 for 17x8 3 blade(95% of baseline load)

LF = 57,917 for 16x10 3 blade(99% of baseline load)

My conclusion is to go with a 16x10 3 blade prop to keep performance as close as possible to the 18x10 2 blade. Added bonus is even better ground clearance.

What is your experience or practice with converting props??
Thanks!
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Old 05-03-2021, 05:12 PM   #2
Derek Hanna
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Re: 3-blade propeller equivalent

I'm using a home-brewed model program scoured off the internet that gives me a way of comparing prop performance.
Can you give me an idea of type of plane that you're fitting it to and what type of performance you're looking for? 3-D vs sport flying vs scale? Also, expected rpm (or a good guess) with the 18X10 prop combination, if available?
I'll get you some comparatives to look at. Then the debates can begin.... LOL.
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Old 05-03-2021, 06:07 PM   #3
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Re: 3-blade propeller equivalent

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Originally Posted by Derek Hanna View Post
I'm using a home-brewed model program scoured off the internet that gives me a way of comparing prop performance.
Can you give me an idea of type of plane that you're fitting it to and what type of performance you're looking for? 3-D vs sport flying vs scale? Also, expected rpm (or a good guess) with the 18X10 prop combination, if available?
I'll get you some comparatives to look at. Then the debates can begin.... LOL.
Hi Derek, sounds interesting. I've got the NGH35 and 18x10 2 blade prop mounted on a Skyward 120, weighing in at a slender 22.5 pounds . You can see pictures and coverage on a thread I'm participating in in the Fixed Wing forum. Its basically a big high wing trainer with a light wing loading, tricycle landing gear. Will probably convert it to a taildragger next winter, meantime would like to fly it soon in its present configuration.

I plan to fly it as a sport plane, no high G loads or 3D(I'm still on the flying learning curve). The engine RPM range per NGH is 1500-8000rpm with the recommended props. The NGH35 is on the big end of recommended engine sizes, though there are folks who fly it with something of similar size, with a tail wheel. I've already installed bigger wheels, but the nose wheel cant get any bigger without more major mods than I am prepared to make, and the ground clearance with the 18x10 prop is just ok for a hard surface runway, it will not be good on grass without more prop clearance.

Thanks for offering to share your analysis, interested to see what comments are generated. Cheers Doug
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Old 05-03-2021, 07:10 PM   #4
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Re: 3-blade propeller equivalent

Without a doubt and in most cases your 2 blade prop is more efficient than a 3 blade . There can be variables between the 2 blade vs a 3 blade and that can be the blade airfoil shape . If your airspeed is ok or if you could reduce it then maybe reduce the pitch by 1 inch to reduce the load ,otherwise try and keep the pitch the same .
So often you hear about someone saying that he runs this long prop and how great the torque is but to get that torque he has had to reduce the pitch to the point that he may only have sufficient airspeed at full throttle so it does take a good balance of prop length , pitch with available rpm to make a good working combination.
Reducing the prop diameter by 1 inch when going to a 3 blade is a start but you may have to experiment a bit. The prop charts/programs mentioned might also be a good start and would be interesting to work with.
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Old 05-03-2021, 09:02 PM   #5
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Re: 3-blade propeller equivalent

Doug,
For the weight and size of plane, it already seems that you're needing to change prop size and pitch to match the plane's expected performance.
Using the program, I picked some comparatives. i.e. similar HP absorbed. Not easy to match HP with this program, but I picked prop speed combinations that are generally in the same ball park.
No guarantees that your motor will give the performance noted, but the calculations serve as a guide for comparison when trying to match a prop to the airframe.
I've attached output graphs for several prop sizes considered plus a table, "NGH 35 prop suggestions" to compare the results.

I don't expect that your Skyward will reach speeds in excess of 70 mph, so best to rule out the 18-10 prop due to its lower thrust. In order to absorb approx. 4 hp, while matching speed for the airframe, I'd suggest a larger diameter prop with less pitch, or alternatively a 3-BL prop, also with less pitch.

Have a look at the program results paying attention to THRUST as this equates to climb performance. Speed is relative, as its only achieved if the airframe drag is low enough. i.e. a high speed model vs large heavy.
From this perspective, the 17-8, 3 Blade may be a good mix of thrust and speed.

My own preference for ground clearance is switching to a tail dragger configuration first, as I hate those times when the nose wheel finds another low spot on our grass field..... plus I think conventional gear looks cooler.

Besides better ground clearance, a 3-blade will reduce prop noise; otherwise, a 2-Blade may give more total thrust / performance. (Higher prop tip speeds for a 2-blade prop vs 3-blade.)
This is a big consideration where I fly.

On that note, one of our club flyers had great success when he switched from a 2-Blade to a 3-Blade for his 100cc powered Extra to lower prop noise. He's not a guy for numbers but his comments were that it flies much better. (Higher thrust in this application gave a better match to the airframe.) Only drawback was higher cost of the prop and a new spinner to match.
Attached Files
File Type: pdf 18-10 7986 rpm.pdf (629.2 KB, 5 views)
File Type: pdf 17-10 3 BL 7448 rpm.pdf (583.8 KB, 3 views)
File Type: pdf 16-10 3BL 8110 rpm.pdf (586.4 KB, 3 views)
File Type: pdf 17-8 3 BL 8193 rpm.pdf (581.4 KB, 2 views)
File Type: pdf NGH 35 Prop Suggestions.pdf (219.2 KB, 3 views)
File Type: pdf 16-8 3BL 8358 rpm.pdf (580.8 KB, 0 views)
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Last edited by Derek Hanna; 05-04-2021 at 10:08 AM.
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Old 05-04-2021, 11:09 AM   #6
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Re: 3-blade propeller equivalent

Derek,
This is great info, thanks!! The 17x8 3 BL does look optimal, I've ordered a 16x10 yesterday and may get the 17x8 as well if I can find one. Since the plane is way overpowered already, I'm not so fussed about the thrust number. Great to have the analysis though and we'll see what it flies like, always good to have a few prop options in my back pocket. Hope you have a good flying season.
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Old 05-04-2021, 11:33 AM   #7
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Re: 3-blade propeller equivalent

Glad to contribute to your project Doug. I find this little program quite useful and the information stays on my computer spreadsheet for the next project or "what if" when selecting an engine combo.
Often we will fit a prop based on what's in the prop box, and if it flies good enough, we're happy. When a carbon fibre prop is costing $100US plus shipping, it's desireable to order the best one, especially when looking to reduce noise with alternative 3-blade sizing.
If you can't find a prop from a Canadian distributor, consider ordering one direct from China with Ali-Express.
Once you get the motor running (after break-in), I'd like to know max. rpm for any of the props that you try.
Cheers,
Derek
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Last edited by Derek Hanna; 05-04-2021 at 11:50 AM.
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Old 05-04-2021, 11:42 AM   #8
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Re: 3-blade propeller equivalent

Like you, I prefer to make a selection based on some informed rationale, and then see how the empirical results compare. I'll be happy to let you know how it goes. Thanks again.
Doug
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