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Old Feb 07, 2014, 07:44 PM
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Piper PA-12 Super Cruiser Build

I hope the title doesn't become misleading; I do truly hope to make this project a reality one day, but it has to start somewhere.

In his youth, my grandfather was a recreational pilot, and a machinist by trade. He combined these skills and built his own Piper PA-12 Super Cruiser (I guess they sold kits back then?). He owned, flew, built, and bought a few different models in his life, and in his retirement, he continued to build and fly balsa models.

Last February, he passed away at the age of 99. Coincidentally, it was just a few months earlier that I got my first foamie and learned to fly RC. After he passed, I began to think that a fitting tribute would be to build a balsa scale model myself.

As his children and grandchildren inherited momentos from his life, somehow I ended up with the original propeller from his Piper PA-12. It is a wonderful momento of something he was passionate about, and a very neat piece of "art" on my wall.

Anyhow, this got me thinking that the PA-12 would be an appropriate plane to build in his memory..............
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Old Feb 07, 2014, 07:55 PM
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So, I found a great website that scans and hosts a large number of vintage aircraft plans. I am a Mechanical Engineer, and was educated in the digital age, so I figured I would start with 3D models.

My original plan was to simply reproduce all of the balsa parts digitally, then get a job shop to laser cut them for me. The problem is, I really have no place to fly balsa at the moment, and am kind of limited to park flyers.

Especially since I only have a year of experience, it would be a shame to put all of that work into something that can be be destroyed so easily, so I decided to approach the 3D model in a manner that would allow me to kit the parts, but also allow me to surface the plane in 3D so that it could be milled from foam, or a mold could be made to cast the foam.

Last year I started to work on the model in Pro/E, which is the software I know best. I worked for a few weeks strait and got this far:

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Old Feb 07, 2014, 08:00 PM
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One problem was that the parts didn't exactly assemble perfectly (probably since they were just traced from a scan). Also, though I have never done a balsa kit, I am assuming that some of the parts are meant to be bent and manipulated into place, which made the modeling cumbersome, and frankly kind of useless.

So, I switched to surfacing. I began to build a wireframe from points on the balsa pieces I had already modeled. It started to come together, but eventually I hit a wall, and just couldn't seem to generate a continuous surfaces.

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Old Feb 07, 2014, 08:23 PM
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About a year went by, and I came across Haiduk's WILDLY IMPRESSIVE work on an SBD Dauntless rendered in 3D and milled from foam.

http://www.rcgroups.com/forums/showthread.php?t=2037344

This was EXACTLY what I had in mind when I started this last year, only he was successful. So it inspired me to pick up the Piper project again. I decided to leave the fuselage alone and start fresh on the wings. All was going well, until I got to the tips:







As you can see, the software did a very poor job of blending the tips. The wings in the plans are built from flat strips, square stringers, and four different cross sectional ribs. I traced each rib, placed the sketches at the spaces called for in the top view, and blended the sections.
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Old Feb 07, 2014, 08:29 PM
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I was at work today, where we use Autodesk Inventor. I decided to try and sketch 4 really rough airfoil sections from memory, and blend them in the same manner. The result was many times better, though not perfect.

I decided to get a 30 day trial of Inventor for home use, and try again. I am beginning to think that the more basic functions of Pro/E aren't very good at complex 3D geometry. Perhaps there are more advanced functions that would help, and if someone knows Pro/E well, I would love some suggestions!

For now, I am going to give this all another in Inventor, and for now purely for the purpose of foam construction. Should I be successful, this thread will document any progress I happen to make.
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Old Feb 08, 2014, 05:31 AM
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Nice project. Subbed.
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Old Feb 09, 2014, 05:46 PM
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After a few more hours of work, I learned that Inventor was producing similar results on the wing tips. This leads me to be believe that the error is in my method, not so much the software.

For now, I am going to push on in Inventor, it is a little more user friendly. Not sure what will happen when the 30 day trial expires.

I also learned some other interesting things. First, maybe the problem is that when monokote is stretched over ribs and spars, the curves arent gentle, I assume the skin draws straight, taught lines. In 3D, I am going for a smooth surface, and maybe the balsa shapes I am working from just dont lend themselves to smooth curves.

I kept seeing the same dips, bulges, and kinks in the same places. Finally, I switched to a more wireframe type of construction. This did not eliminate the issues, but did allow me to drag individual points around until the resulting splines became smooth. Next is to fill in the wireframe and see what comes out:

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Old Feb 09, 2014, 06:31 PM
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Well, that didn't work, lol.

Looks like splines and boundary patches don't play too well together in inventor.
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Old Feb 09, 2014, 06:34 PM
All can fly...Launch stronger!
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Don't you want to try Sketchup?
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Old Feb 09, 2014, 07:41 PM
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Well, I dont know sketchup very well, believe it or not. Pro/E and Inventor are industry leading 3D design tools, they should be up to the task. I think it is a matter of me honing my skills with them.
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Old Feb 09, 2014, 10:29 PM
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Finally, some success! It still has a faceted look to it, but there are some settings for edge conditions between the sections that I can play with. Even if this is as good as it gets, I am pleased with the shape, and if it ever sees a CNC, the facets can surely be sanded out.







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Old Feb 11, 2014, 11:40 AM
Flyer, not a builder ;)
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That looks awesome, I look forward to watching how this progresses! I would love to have one!
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Old Feb 11, 2014, 11:52 AM
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Thanks! I sincerely hope I get to that point. The hours I have already spent in 3D have been taxing, haha.

I worked on a third redesign of the wing last night. My goal was to get the surface smoother. Inventor really doesn't want to let me make this shape with a loft, but I'm still trying to bend it to my will. Still got a few more things to try before settling on the patched surface quilt.

On the bright, after three refinements I think I am getting a better handle on the shape each time.
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Old Feb 11, 2014, 08:11 PM
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Finally, success with smooth lofts. This is perfect, but I still need to reconcile the tip, which I cannot do with another loft because the tip profile does not lie on a single plane. Back to it....

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Old Mar 03, 2014, 08:39 AM
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Boy, I just can't seem to make up my mind on this, or make any significant progress.

The geometry of the wing tips is proving to be extremely difficult to model as part of a single feature wing, which I was striving for due to rampant perfectionism.

I am realizing quickly that perfectionism will probably never result in a plane in the air. So, I am currently re-examining my options with this project such that I can build it in my garage, cheaply. If I can get something decent built the old fashioned way, perhaps I can translate it to higher tech and accuracy methods in the future.

Hope to have some progress this week.....
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