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Mar 15, 2017, 04:36 PM
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localfiend's Avatar
Discussion

If build time and complexity are no object, what's the best airfoil?


Like the title says, if you don't have to worry about building a rotor blade, what airfoil would have the best performance for your typically sized RC autogyro?

I'm experimenting with thin wall 3D printing and think that tech has come far enough to make printable blades viable. The excellent work by the guys at 3dlabprint.com take a lot of the speculation out of this kind of project.

I've got some 3D printed leading edges printed that work with a foam rectangle to make low weight quick building blades, but would like something with a complete airfoil.





I plan on doing several different versions of airfoil once I have the process all ironed out, but am left needing feedback on what profile to use.

My drawings have used the NACA 8-H-12 as it seems to be popular on full scale autogyros and it looked cool.









Also been playing with different spar methods and have come up against the limit of what I'm capable of in Sketchup. Time to switch design software to something better like Fusion 360.






Never flown an autogyro before. Hope to remedy that as soon as I finish designing a rotor head, but would like suggestions on what airfoils I should try.

Any profile out there that you would like to use or try, but is too much work to build? How about something that's well proven I can use as a benchmark?

I'll be releasing all my STL files when things are done so that anybody with access to a 3D printer can get cheap no hassle blades.
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Mar 15, 2017, 05:08 PM
Now I have to land that thing?
PGregory's Avatar
I saw your comment about Sketchup - I just spend three days battling a problem which I thought was my own issue from not understanding something critical about SU 2017 Pro. Turns out I got seriously bitten by a problem I hear from time to time that SU's legacy is with construction industry, not 3D printing. Turns out the issue I was having disappeared when I tried recreating my part at 1 meter scale instead of the actual 10mm'ish scale I was using. I was using the SolidInspector2 extension and it wouldn't pass thru until I made the part jinormous - then it passed thru. I am kinda licking my wounds right now after getting stymied for a ridiculous amount of time with this.

Thought I would mention. When I catch my wind, again, I think I am going to switch back over to the Fusio360 learning curve I was on and see what kinds of bugs lie within that software. Sometimes it comes out as a wash, at the end of the day.

This was the video that turned the light bulb on in my head about Sketchup: https://www.youtube.com/watch?v=vPTEUfONi7Mrcg
Mar 15, 2017, 07:20 PM
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localfiend's Avatar
Quote:
Originally Posted by PGregory
I saw your comment about Sketchup - I just spend three days battling a problem which I thought was my own issue from not understanding something critical about SU 2017 Pro. Turns out I got seriously bitten by a problem I hear from time to time that SU's legacy is with construction industry, not 3D printing. Turns out the issue I was having disappeared when I tried recreating my part at 1 meter scale instead of the actual 10mm'ish scale I was using. I was using the SolidInspector2 extension and it wouldn't pass thru until I made the part jinormous - then it passed thru. I am kinda licking my wounds right now after getting stymied for a ridiculous amount of time with this.

Thought I would mention. When I catch my wind, again, I think I am going to switch back over to the Fusio360 learning curve I was on and see what kinds of bugs lie within that software. Sometimes it comes out as a wash, at the end of the day.

This was the video that turned the light bulb on in my head about Sketchup: https://www.youtube.com/watch?v=vPTEUfONi7Mrcg

Yeah, I'm aware of the scale bug, that's the only way I've been able to get as far as I have with this stuff. I scale stuff up 100x, and occasionally 1000x to get intersected faces to work. Maybe the PRO version handles things better, but the free version still gives me trouble. You also run into issues with your camera when you scale parts back down. You can't zoom in without things clipping, so you have to scale back up. Gigantic pain.

Hadn't heard of SolidInspector2 though, I might give that a look and see what it can do for me.
Mar 15, 2017, 08:02 PM
Now I have to land that thing?
PGregory's Avatar
I'm surprised we haven't seen more warnings about this with SU. I have Pro and the only difference I know of is extra solid modeling controls which are OK but the Outer Shell command is the prime one you have access to.
Mar 15, 2017, 09:11 PM
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localfiend's Avatar
Quote:
Originally Posted by PGregory
I'm surprised we haven't seen more warnings about this with SU. I have Pro and the only difference I know of is extra solid modeling controls which are OK but the Outer Shell command is the prime one you have access to.
It seemed to me that there were some fancy intersection tools as well, if I can remember back to when I still had the trial edition of pro. They seemed to work better but still weren't perfect. Stray faces and funky angles everywhere. Been too long though.

I think I'm done with Sketchup. Took a look at solidinspector, and it couldn't really tell me anything I didn't already know, which is sketchup is obnoxious when you intersect complex faces. Been playing with Fusion 360 since my last post, and I think it's what I need to stay with. I'll probably be better off in the long run if that's where I focus my time.
Mar 16, 2017, 02:41 PM
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localfiend's Avatar
If no one has and ideas on airfoil, what about going further outside the box? I've been reading things about how leading edge and tip weight can be important. I think that's something that can be accounted for here. What about changing the shape of the rotor blade and having it wider out at the tip like a lot of props?
Mar 17, 2017, 05:54 AM
I'm not as bad as they say.
Quote:
Originally Posted by localfiend
If no one has and ideas on airfoil, what about going further outside the box? I've been reading things about how leading edge and tip weight can be important. I think that's something that can be accounted for here. What about changing the shape of the rotor blade and having it wider out at the tip like a lot of props?
There is not a lot of gain left in airfoils this small.
Tip weight pocket is a nice feature.
But if you want to make a big improvement, print something with the correct twist.
Latest blog entry: AIrcraft I've built.
Mar 17, 2017, 12:57 PM
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Finally learned enough about Fusion 360 to get a design completed and printed. It's a rather big leap from sketchup, but a lot of the tools are really nice.

After a lot of trial and error on the design side of things I'm very happy with how the last revision has printed out. Weight doesn't seem to bad either.







Quote:
Originally Posted by mnowell129
There is not a lot of gain left in airfoils this small.
Tip weight pocket is a nice feature.
But if you want to make a big improvement, print something with the correct twist.
Thanks for the reply man. Yeah, been seeing that Reynolds numbers really aren't your friend at this size. Of course now that I see things will be working, going bigger is an option. I have some .40 sized motors that could be fun.

I can certainly add a tip weight pocket, that's a good idea. Maybe a slot for a quarter or something? Or is that not enough weight? On a related note to that, any thoughts on what I should do with the tips? Are they simple left flat or open do to expediency? Wondering if a rounded tip for looks would have any negatives. I see some of the older full scale autogyros had various shaped rotor tips rather than the flat stuff on the more modern cheap autogyros.

Twist is also possible. I'll have to figure out how to do it, but if it will be an improvement I'll tackle it.

What twist direction? Like this or reversed?



Also curious on where it would be best for the twist to start. At the base? Some sort of exponential curve? Total twist degrees?
Mar 17, 2017, 02:36 PM
I'm not as bad as they say.
Quote:
Originally Posted by localfiend
Also curious on where it would be best for the twist to start. At the base? Some sort of exponential curve? Total twist degrees?
It helps to have a rough estimate on the aircraft performance, speed range and rotor speed mostly. My calculations on my models yielded about about +8 (tip at +8 from the root, that is the root is more negative than the tip, unlike a propeller twist, which is negative). Linear is probably good enough.
Latest blog entry: AIrcraft I've built.
Mar 17, 2017, 03:10 PM
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localfiend's Avatar
Quote:
Originally Posted by mnowell129
It helps to have a rough estimate on the aircraft performance, speed range and rotor speed mostly. My calculations on my models yielded about about +8 (tip at +8 from the root, that is the root is more negative than the tip, unlike a propeller twist, which is negative). Linear is probably good enough.
Cool man, thanks.

As for aircraft performance, to start with I'm just kinda thinking along the lines of the Hobbyking Auto-G. I need something simple and easy to fly to start out with. So no excessive power or or speed. Once I get the first one in the air and doing well I'd like to make things a bit more complicated. I don't even know what current rotor speed will need to be. More research.

The nice thing about Fusion 360 is it's timeline feature. I can go back and tweak something like blade twist, and it will update the finished project without my having to redo a bunch of work. Once I get the workflow all figured out for a twisted blade I'll be able to output blades in a range of sizes, twist rates, and even possibly airfoils.
Mar 18, 2017, 12:31 AM
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Got a preliminary blade hub worked up. I've got shims that will only go on in one direction due to the fillets. I've got shims in half degree increments starting at 1 and going to 4.5. Anyone think it would be worth it for even more variety? Maybe smaller increments closer to 4 degrees? Unsure how much a tenth of a degree or something like that could have.

I think a blade extension that slides into the hollow printed blades will be best. Might need to rethink the base of my blades, but it should work this way.




Gonna start one of these printing and a couple sets of shims. Think I'll start out with 3.5 and 4 degree shims. Next I need to do the rotor head assembly. Think I'm going to only make it for aileron/roll control to start, but am thinking about making it easily swappable for pitch. Maybe I'll make it do both...
Mar 18, 2017, 05:19 PM
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So, I finished drawing and printing the hub assembly last night. Made up blade tips and blade to hub attachment points this morning and finally got to test stuff out.

3d Printed Auto-Gyro Blade Test (0 min 39 sec)


Seems to be working pretty darned good to my untrained eye. I didn't realize it would spin so fast from the wind (8-10mph). Was kinda scary to have that close to my face lol.

Assuming my Hobbico Digital tach is correct, I'm getting around 700-750 rpm from the breeze. Final blade length was just a bit over 16" from the center of the hub to blade tip. Anyone have ballpark figures for what takeoff RPM should be for something this size assuming a decent wing loading build?
Mar 19, 2017, 03:10 PM
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Here goes nothing....



Yeah, it's ugly and there might be a few issues. One of which is the nose is probably too long. I just wanted to see if this works. The air frame is Flite Test's FT Explorer, and it's made for experiments. If I don't have enough control I'll chop the nose. That is if I can get airborne at all.

Specs as it's setup now:

3.5 oz per square inch (Airframe isn't really efficient for this, but 3.5 seems to be ok)
10 degree Mast angle
-4 degree spacers on the blades
2212 1400kv (probably somewhere between 170 and 200 watts)
Rotor height, tail sizes and spaceing are all pretty close to the Hobbyking Auto-G

I drew up and printed a steerable tail wheel, so hopefully I'll be able to just run around on the ground until I can get auto-rotation. Looks like there isn't much wind today so that will be needed.