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Author Topic: 3D Printer and Sketchup prop design  (Read 2441 times)
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Crabby
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« Reply #25 on: December 02, 2017, 06:22:03 AM »

That idea reminds me of the prop blades used by Dick Baxter on his ultra light P-51 design. I thought the shape was unique at the time, but it has a subtle shape change kinda like a boomerang, not as extreme. It flys the plane really well.
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« Reply #26 on: December 02, 2017, 09:42:25 AM »

Hi All,
Paul I am intrigued by your 3D printed props and would be keen to try printing one. I can load the STL file no problem, the stumbling block for me is the support structure. How do you set this up?

thanks
Bernard
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« Reply #27 on: December 02, 2017, 10:46:15 AM »

If you try experimenting with the bucket solution, be mindful of left and right.  The solution I drew yields a prop that turns clockwise when viewed from the front.  This is not typical.  to build a typical prop, take a mirror image of the bucket picture.  Make sure the lower angle leans towards the left.  Also the lower angle is important but I'm not sure what it should be.  DPutt, in his Bristol Brigand build suggests 15 degrees.

Marlin
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PaulBrad
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« Reply #28 on: December 02, 2017, 11:44:29 AM »

BG- The support structure is created by the software used to slice the 3D model (the .stl file). I use Slic3R, but there are others. I orient the stl file with the front of the prop facing down toward the print bed before starting the slicing process. The rest is automatic. The supports are created by the slicing software. The supports are very thin and are easy to peel off the prop once printed. By orienting the prop with the forward face (the concave side of the blades) down, it is easier to clean off the residue left when the supports are removed.

I did add a section to my web site, parmodels.com, in the Free Flight models section that contains two and three blade prop stl files for sizes from 5 1/2" to 7 1/2" in diameter. Spinners are also include for those props.

Paul Bradley
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Hepcat
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« Reply #29 on: January 01, 2018, 07:05:44 PM »

Hypotwisted Propellers

I crossed (s)words with Bruce Holbrook some years ago on this subject. I thought the idea was rubbish but as several people regarded him as a friend I gave up arguing.  Sadly he died shortly afterwards. However the idea has come up again and is taking peoples time so I think some assessment of his assertions must be made. I must admit that I dislike his arrogance in dismissing the earlier work of experts and after ‘one and two hundred hours of analytical consideration’ considers himself better able to design a propeller than experts.  Another annoying thing is his tendency to ‘invent’ words which look technical but are actually meaningless. In this connexion his postscript ‘apology’ saying: ‘realization that literary play has no place and humor does not ease the presentation of what for most is a new and difficult topic’ sounded to me like an accusation that we had no sense of humour and also lacked the intelligence to understand him.

I have attached below a diagram which I shall refer to from time to time.  Something like this is almost always found in the first page or two of any technical work on propellers.

I AM SORRY BUT COORECT SKETCH IS IN NEXT POSTING


In an early paragraph Bruce says: ‘Using realistic estimates of my planes’ speeds and known rotational speeds of their props at different radial sections, I diagrammed such propellers’ interactions with the invisible medium surrounding them and calculated relative thrust values of radial sections.’
{Note: round about here Bruce is using ‘washout’ for a reduction in blade angle and also for the twist on a helical blade. He also invents static angle of attack and dynamic angle of attack but none are defined.}  
  
The next paragraph is the most important so far because it contains the kernel of his hypotwist theory and what is more I understand it and have modified my drawing to incorporate it.  I have increased the length of the rotational speed vector(into the print I’m afraid)  combined it with the forward speed at D and low and behold the closing vector is such that the wing section is at a negative angle of attack and could work at a higher angle of attack without stalling.  However, although this seems to support a tiny part of Bruce’s idea the nonsense of it all is obvious.  If we increase the rotational speed of any propeller it will obviously give more thrust.  We may not want to have a higher blade angle, we want the blade angle with the best L/D ratio and, overriding that aspect, rubber motors have less torque as they run down and so the rotational speed will be less.

The following paragraph is interesting in that he admits to only finding two references to propellers with P/D increasing from root to tip, one in a Chinese journal which he took to confirm his own heretical understanding and one professional discussion, more realistic than most, which confirmed his understanding but maintained that greater efficiency does not always result.  He went on to say that the professional discussion excluded some relevant variables such as the non predictable reduction of airspeed under rotational speed due to entrainment of air.  I think here he is talking about the slipstream and the rotary component around the fuselage. These have been part of propeller design for as long as I can remember
And appear as the a and b characters in my diagram .
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Re: 3D Printer and Sketchup prop design
« Last Edit: January 01, 2018, 07:19:27 PM by Hepcat » Logged

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« Reply #30 on: January 01, 2018, 07:16:48 PM »

Below is correct sketch for previous post.
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Re: 3D Printer and Sketchup prop design
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« Reply #31 on: January 02, 2018, 12:17:11 AM »

Over at Stick and Tissue they have a reprint of the multi-part FAC article.  It's convenient because it's all in one spot.  http://www.stickandtissue.com/yabbfiles/Attachments/PropsHypotwisted.pdf

After reading the part one section the author lays out the basis of the hypotwist design in three key and numbered paragraphs.  In paragraph 2) If I may paraphrase, we get the best efficiency, Lift over Drag (L/D), of an airfoil the closer we are to a stall. The thing is we DON'T get the best L/D when we are close to a stall.  We DO get the best sink rate when close to a stall but from efficiency standpoint the best L/D is achieved when the angle of attack is a bit lower and the airfoil is traveling a bit faster. Glider pilots succeed based on this bit of knowledge.  When they are in lift they slow the glider down to slowest rate of descent.  When the lift gives out they speed up to best L/D while they try to find a new source of lift.

So the problem with the hypotwist prop, as I see it is this: It is an attempt to try to get the prop tips to operate in this critical high angle of attack.  Yet this is not where the best L/D is and it's not where we would expect to find the best efficiency either.

Marlin
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« Reply #32 on: January 02, 2018, 07:39:49 AM »

Thanks for the post John. I was hoping that you would give your view.
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Tapio Linkosalo
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« Reply #33 on: January 03, 2018, 05:15:43 AM »

Already in 1964 there was a Finnish book on model airplanes, and in the Wakefield section the author suggested, that instead of a helical pitch distribution, one that is slightly less for the root and a bit over helical for the tips would be more efficient. The reasoning was the larger RE-numbers towards the prop tips. Sorry I do not have the book at hand now so cannot scan the figure.

Some issues also to consider:
- prop is a lifting surface with tips, so tip vortex needs to be accounted for. If prop roots are too thin, there is a root vortex also.
- the lift vector is perpendicular to the airfoil, which means the the closer to the root you get, the smaller portion of the lift pulls the plane forward, and the larger portion of the lift resist the prop from turning. But if you make the root of the blade narrower, or reduce the pitch there, you add to the root vortex....
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« Reply #34 on: January 03, 2018, 01:08:47 PM »

I appreciate the comments of John Barker regarding the technical problems associated with the paper by Bruce Holbrook. John's knowledge in the area of propeller design is well established and I respect his comments very much. I have to admit I hesitated making reference to the Holbrook paper in my posts to this thread because of the technical holes in the paper as pointed out by John and Marlin. My reason for mentioning the paper is that paper is what inspired me to take on the task of learning to develop 3D prop models for 3D printing. I was intrigued by the concepts presented by Holbrook and wanted to see if they might have merit by using an accurately created 3D printed prop.

In thinking about my results with the props I have created with my 3D printer based on the Holbrook paper, I realized my very simplistic approach to determining a baseline helical pitch prop for comparison was flawed. Being in a bit too much of a hurry I simply took the average P/D value between the root and tip. My root P/D was 1.2 and the tip 1.8. By doing that I neglected to account for the fact that 75% of the blade has the root P/D value and only 25% the increasing P/D values. If this latter factor is considered, my baseline helical prop P/D value should have been 1.34 rather than 1.5. I have the feeling that using the lower pitch helical prop would likely produce similar results to the Holbrook style prop with the root P/D of 1.2 and the tip P/D of 1.8. My test and flying observations results for the helical pitch prop with a 1.5 P/D and the Holbrook style prop showed the same run time but a better acceleration with the Holbrook prop. A slightly lower P/D for the helical prop should produce a better acceleration with similar run times to a 1.5 P/D prop.

I do apologize to pedwards the originator of this tread for bringing up an element in the discussion that has steered the comments away from the subject of using 3D modeling and 3D printing to create props for our rubber powered models. Those tools open the door to many propeller configuration possibilities. Hopefully some of the propeller design discussion will help make informed decisions when setting up a particular prop design for 3D printing.

Paul Bradley













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BG
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« Reply #35 on: January 03, 2018, 06:14:53 PM »

Back to 3D printing:
I have printed one of Paul's props using ABD plastic. Paul I found first that the quality of the print was less than what I would like. Do you have any tricks to ensure a higher quality surface?
Next: I found that the props were very flexy at the root ... this is good for preserving props but I worry that the blades must be flexing in flight. Paul do you think it is worth trying a thicker blade root?

BG
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« Reply #36 on: January 03, 2018, 06:48:50 PM »

BG - Did you mean ABS plastic ? I ask because I am not familiar with ABD plastic. I do use ABS for my props. I have found the strength to be more than adequate. The flexibility of the blades goes a long way in keeping them from breaking when the model lands or hits something. During flight the blades hold their shape just fine.

As far as surface finish goes, much depends on your printer and the settings being used. Parameters like layer height and feed speed can affect the print quality. That said, the props I print do require post print clean up. That is one of the reasons I use ABS plastic for my props. It sands easily. By sanding the prop after printing you can get a very good surface finish. Once you are finished sanding the prop, brush on a coat of acetone. The acetone will produce a very nice gloss finish.

Paul Bradley
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« Reply #37 on: January 03, 2018, 07:00:09 PM »

I was thinking that you might get better performance by leaving the prop as is straight out of the printer.  The rough surface should provide turbulation.

Marlin
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PaulBrad
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« Reply #38 on: January 03, 2018, 08:10:16 PM »

Interesting idea. I have not tried that as I just could not bring myself to put such a rough looking prop on my model. Probably would be worth trying.

Paul Bradley
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« Reply #39 on: January 04, 2018, 11:09:44 PM »

Boomerang-shaped blades

Crabby and strat-o visualized boomerang-shaped prop blades and Crabby mentioned Dick Baxter's P-51 with this blade shape. I searched for his P-51 plan but did not find it. However, gryffinaero.com had 4 or 5 of his plans. His Almost Wright 1903 had similar shaped blades from Carl's Jr coffee cups. Crabby and I are in Eastern US, so Carl's Jr translates to Hardee's for us. <grin>

Fred Rash
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pedwards2932
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« Reply #40 on: January 05, 2018, 11:00:20 AM »

One of the advantages that you would get using a 3d modeling program like Sketchup is if you had the dimensions of the Hardees's (I'm on the east coast) you can plug that into the drawing and duplicate the blade.
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« Reply #41 on: January 05, 2018, 03:25:40 PM »

pedwards2932

Looks like you have worked through most everything. You have a hub design, 2 (or more) slotted blade holders, a method for printing a conical form for molding balsa blades if desired, and a way to create a thin bucket and cut it into prop blades while it still exists only in Sketchup software. Impressive.

Fred Rash
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« Reply #42 on: November 05, 2019, 01:07:06 PM »

I don't know if anyone out there is still following this thread, but I would like to make a suggestion to those using 3D printing to make prop blades:  Instead of printing the blade in the horizontal position and using support material, try standing the blade on end and print it using the 'vase mode' setting of your slicer (or, in Cura, turn on the setting 'spiralize outer contour').  Your prop will probably not be as strong as it would be if it were printed horizontally with supports, but it will require very little post-processing and will probably have a much better airfoil shape.
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