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Old Mar 25, 2015, 05:35 PM
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Custom props (Attention Prop Gurus!)

Hey guys,

So although I love the highspeed world, I am actually coming here for a different reason. To add to my growing list of RC hobbies, 3d, speed freak, soaring, etc.. I have ventured off into very high efficiency fixed wing UAV's.

I am currently working on an airframe that is being held back by both electronics, as well as available prop selection.

It is currently ran on a 420kv motor 6s, and a 14x12 APC prop. The problem is that the airframe's most efficient cruise speed is fairly high. Our current problem is that to fly at cruise we need 45-50% throttle, simply due to the RPM required, to utilize the under-size (pitch-12") prop. The only way to get more pitch, is to increase the diameter, and we do not need the wasted energy coupled with a larger prop.

The plane is already flying with a larger motor than it needs to simply because it is beneficial to add the little extra weight, to gain the electrical-mechanical efficiency (torque) which allows us to fly lower rpm, with higher pitch. That is the theory anyways, and it seems to correlate with our results.

This being said, our option is to lose some electrical efficiency by increasing rpm (kv), or dabble with custom props (14x14 should be a good starting point).

Now, from what we are seeing, it seems as if we would be best off with a custom prop.

Problem: I have already contacted Xoar, and several other prop manufacturers who want tooling costs. This is not a problem in the long run, however my biggest dilema is that I need to prototype, and prove the physical numbers before investing in tooling. It simply would not make sense to invest in tooling, for 3 or 4 different prop variants, to just throw away all but one. Therefore I literally need to find a way to prototype 3 or 4 different pitch props, before investing in molds, etc....

Now I come to you guys because I am very aware, and own several of the somewhat custom hotliner and f5d props. (Even a few from DUP!)

Do any of you guys know of a more efficient way to prototype (I am thinking CNC'd beech, etc) props before investing in the tooling costs coupled with manufacturing?

Thanks a ton! This group has always been helpful in my adventures!

Sincerely,
Justin
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Old Mar 25, 2015, 09:42 PM
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Justin,

Interesting project - I will add my experiences and I'm sure others will chime in. As you will be aware designing a prop from scratch and then making a CNC mould can be quite expensive, especially if you want to compare 3 or 4 different versions.

One option might be to make a folding 14x14 and then adjust the pitch with the yoke - still not perfect but in the real world perhaps good enough.

I would say that copying an APC Sport prop will not yield best efficiency - I had to design my own 7x10 (it had origins in the 7.x10 APC Sport) but the results were much better, approx 10% lower current draw and slightly increased RPM.

Not sure what your budget is though.....
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Old Mar 25, 2015, 10:02 PM
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I know my buddy Mark F. of Dinogy Lipos has pitched "up" a few larger diameter APC electric props before when he ran into the same issue. You basically only need a pitch gauge, a heat gun (monocote heat gun, not an industrial strength gun), a pair of needle nose, and a slow & steady approach so you don't overheat the nylon. It's a technique thing mostly. Let me text Mark and find out what he did. I've done so many props this way I can't remember if he did it or "we" did it together.

Another option is to have some molds 3D printed for prototypes. That prop size is big enough to get decent resolution from most systems. Might have to make the mold in two halves length-wise.
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Old Mar 25, 2015, 10:27 PM
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Yes 3D printing can be an option - I have made a 9x19 folder and got the plug 3-D printed, it came out good enough and it was very reasonably priced.
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Old Mar 25, 2015, 10:38 PM
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Interesting. I do have a 200mm^3 3d printer, so could easily print blade molds.

I have already been printing spinners, (pressure recovery for pusher application) And also have a spinner mold designed, just not printed as of yet.

My personal composite skills are not much more than joining DLG wings and minor repairs and patch work. I wouldn't trust myself with a prop.

Also, my aero-design experience is restricted mostly to airframe design. Propulsion has too many what-is and contradictory papers. (Example, reading up on Paul Lipps props makes total sense, but contradicts most o what I have been taught through the years).

That big said, design at these Reynolds numbers seems to be more accurately tried and tested than predicted and documented (When compared to full scale).

This being said, I love the idea of molds, as I can do that in house. Not too confident with my knowledge on blade design (although I do agree I can at least do as well as APC, not sure if I could do much better), to design it myself. Not to mention my molding skills are lack thereof.

My original thinking was literally flying CNC'd beech one-offs, and determining which pitch is best suited for our cruise envelope. Then once said and done invest in tooling/molding and manufacturing.

Edit, don't mind the messy garage or beat up airframe... Its a workhorse prototype, so it's been through a lot.
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Old Mar 26, 2015, 01:38 AM
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Why not contact Fiala. I have found their e-props quite efficient and they can do just about any size you want at special request?
http://www.vrtule-fiala.cz/shop/

Gerben
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Old Mar 26, 2015, 05:50 AM
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You can try an APC14x14 pattern prop for testing.

Rene
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Old Mar 26, 2015, 10:18 AM
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Have you considered variable pitch props?

For your application where efficiency is important for duration, variable pitch gives you the best efficiency for both takeoff and cruising modes.
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Old Mar 26, 2015, 10:29 AM
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Quote:
Originally Posted by Jwmflying14 View Post
My original thinking was literally flying CNC'd beech one-offs, and determining which pitch is best suited for our cruise envelope.
If you are mostly looking at pitch selection can you use larger props cut down in diameter to avoid overloading your motor, then worry about finding an efficient prop design once you have the pitch range sorted?


Mark
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Old Mar 26, 2015, 11:08 AM
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Justin, after I mentioned the 3D printed option I checked you blog and saw your machine. Happy coincidence I guess.

If you want to mess with some APCs of different pitches, I could re-pitch a few for you and just send them back. I would probably do 3-4 variations and that should get you in a good ballpark area to pursue custom props. Lots of guys are using the Javaprop program to design their own props and such. David Fee did this design and it is almost identical to some props the Swiss did on their own.

Nice design, btw.
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Old Mar 26, 2015, 02:08 PM
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3D printed plugs are a very cheap option do build props .

I printed a F5d folder plug at my sons self built 400 euro printer and the result is very promising.
I have the same prop built out of a proper cnc-milled mold to compare,static there is no noticable difference... inflight logs will be taken soon.

The plug takes less than 4 or 5 Euro of filament (PLA) per set and an hour or two to prepare for moulding.
The mould itself takes another 2 hours and probably 5 euros of resinstuff....

Prop itself in carbon would be less than an hour and probably less costs than a hobbyking 3x3 prop....
A professional aluminium prop mold would be arround 150 Euro or more whithout the security that the prop works well.

Yello
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Old Mar 26, 2015, 04:11 PM
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Gerben, Thanks for the contact! Email sent!

ESpeedBuff; I had previously thought of that, and used a 14x10 pattern vs electric as a constant to get an idea before actually trying the 14x14 pattern. Unfortunately, the inefficiency didn't even warrant trying the 14x14 pattern.

Dave Sawers, Variable pitch is definitely the best solution, and we already have it planned for the future. Unfortunately, our power system uses a 6mm motor shaft, which eliminates ANY COTS equipment (one idea was using 600-700+ size helicopter tail assemblies.) Unfortunately, this forces custom machining of a variable pitch system, as well as custom blades.

MMatock, Tried that as well, however once again the inefficiencies of the cut down prop start to weigh in once again. Our biggest problem is that we are dealing with already high efficiency numbers. This being said we still have huge gains that are accessible, however we are already past the point where many applications would benefit (cut down props, etc). Or at least that is the current train of thought.

Troy thanks for the re-pitching offer, that may be the most cost effective solution as of right now. I definitely would love to look into designing our own prop, but I don't personally have the time to read up on blade design (I know enough to most likely match APC or come close, but would definitely prefer a more in depth approach.) Unfortunately there is limited research on efficient prop design at this scale.

MrYellow, 3D printed plug is more than easy enough, even a mold is possible (at least for folding, can't fit a 14" prop on the printer, but I can in halves). Downfall of all of this is the learning curve. Again, my propulsion design expertise, well isn't quite up to par with the airframe, secondly I would be completely new to composite molding, let alone props. (not sure I would trust my props until I had some experience)

I really appreciate everyone's info!

This field of propeller design is rather interesting. Unlike fullscale where cruise is usually ~80%+ of VNe , and dealing with higher thrust requirements. For example, we cruise at about 50% or of max speed, which correlates to about 35% throttle. After Reading into Paul Lipps propellers and their benefits in full scale, it is interesting to consider what is not yet developed for the UAV world. For example, tip speed mach numbers are much slower, with a high lift to drag. Meanwhile trying to utilize electric power systems that are horribly inefficient. (most power plants reducing to ~50% efficiency or less at lower throttle, compared to ~85%+ at higher throttle, simply the nature of electric.) This being said, this is where I am hung up, it is kind of an open question and seemingly uncharted.
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Old Mar 26, 2015, 06:42 PM
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Okay, a bit off topic.... I am further diving into some of my Castle Logs (we all know they are only so accurate.)

But after selecting the proper KV and # of magnetic poles, it is telling me we are spinning approximately 4300 RPM... With a 12" pitch prop, which leaves us with a theoretical pitch speed (not taking into account prop inefficiency, and the fact that the CC log is inaccurate) of 48 mph, and we are cruising (average) at 44mph indicated. This can NOT be right, as that would indicate 91% of pitch speed at cruise... so how accurate is CC RPM logging? I know their current and voltage is so-so an reliable for averaging. This RPM seems way out of reason, when comparing the two.

This being said, due to the low KV and inadequate pitch, we are forced to operate at higher RPM than what would be ideal, hence why I am in a search for a higher pitch prop.

Still, the question remains, is CC that out-of-line?
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Old Mar 26, 2015, 07:36 PM
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Quote:
Originally Posted by Jwmflying14 View Post
... so how accurate is CC RPM logging?

In my experience their RPM is very accurate. Also heard from several people who think it is very accurate.

In the 40K-46K RPM range, I have seen it to be within 16 RPM to 200 RPM of Eagle Tree V4 logger's data (brushless RPM sensor).


Gryphon
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Old Mar 26, 2015, 07:54 PM
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Originally Posted by GryphonRCU View Post
In my experience their RPM is very accurate. Also heard from several people who think it is very accurate.

In the 40K-46K RPM range, I have seen it to be within 16 RPM to 200 RPM of Eagle Tree V4 logger's data (brushless RPM sensor).


Gryphon
Well something has got to explain the pitch speed difference then. I know this airframe is efficient, but there is no way it can be that close to actual pitch speed....

Anyways, regardless, the current prop is too low pitch, and our 2nd best (behind variable pitch) option would be higher pitch prop, followed by our worst option which is higher KV motor (have a 540kv, and 630 kv on hand just in case....)
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