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imamac96

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33
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Tell us about your design project, and be fully prepared for candid critiquing, which you should appreciate. Of course, you need to learn to separate the BS from the valid comments.
OK, I'll post a little here. If you'd like to talk about it, shoot a message.

I'm trying to design a light sport that fills the role of the Sonex model A. I think it's insane we aren't cutting our own kits. People don't want to build from plans anymore, and I think that's really only because the old plans are terrible compared to our standards now.

The Sonex B is great, but that fancier kit made it go outside my grad student budget. Every Sonex owner I talk to (well, at least 90%) are trying to use it as a XC flyer, and they all put autopilot in it because they all complain it's neutrally stable in pitch. It's pretty clear people are attracted to the mild aero use case but are using it for the wrong thing. I am working my way through the Chris Heintz book and putting everything in spreadsheets. I'm cross-referencing the Sonex, the Zenith 601HD, and the Zenith 650. All I want is something that stalls at 40-44 mph Vso, cruises at 120 mph, and is easy to build with minimal tooling. That 8' brake needed for most builds is an absolute project killer for the average guy in a garage. I don't know about you, but I'd much rather use the $1,800 for a blown canopy on 100LL. That, with shipping, is about a year's worth of fuel for a typical LSA pilot. Common-size channel (just like the Sonex cross-tie channel) is what I'm hoping will bring the build time down.

Most everything in LSA has those speeds already, so now it's a matter of getting the build times down. The LSA is not fancy. It's beat to death. They all look relatively the same, which gives us lots of sample data points. I accept that I will never make money in aviation, so I'm planning to take the Sandlin approach by dumping everything in the public domain, which lets me get away with using non-commercial CAD licenses.

It's not finished by any means, but here's a peek at the elementary CAD work if you want to see:

1659745166424.png

Note that the rear spar design is based on the Thatchers, and it's just a placeholder. I prefer the channel like on the Sonexes/Zeniths. There's also very little internal structure because it's hidden in the shot. I know that anybody can just dump a CAD drawing of a plane and claim wonderful things, so yes, this all means absolutely nothing yet.

-C
 

imamac96

Active Member
Joined
May 10, 2018
Messages
33
Location
North Texas
I have asked several questions on here about Chris Heintz's book. This is because this is one of my primary references. I wanted to show off my (incomplete) spreadsheet that follows the example at the end of the book. I've found lots of typos, mistakes, and a general lack of references. One thing this does not consider is tail loading, so that's on the to-do list. I haven't even started it yet.



You'll probably notice that in general I try to highlight negative margins of safety with a red cell. You'll also notice that I try (but am only in the middle of) to backward-engineer the 601HD as well. This gives me a good baseline of measurements. Again, none of this is really finished. It takes time.

Also, Heintz's estimates are very very conservative. This is especially the case for the spar because he assumes a uniform wing loading. This is part of the reason I'm eyeing an 18% airfoil. The preliminary runs in XFOIL/XFLR5 are showing not much of a difference between a Riblett GA30A415 and GA30A418. I'd like to take the fatter airfoil for the spar strength.
 

imamac96

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Joined
May 10, 2018
Messages
33
Location
North Texas
I wanted to quickly write down that the EAA has done an absolutely disgraceful job at marketing SW 3DE. I have a MS in CS, and just now learned how to access it. It's not just an entirely online environment like they marketed. THERE IS AN OFFLINE COMPONENT AS WELL. There is practically no help for this. Since Dassault advertises the online component, I figured they intended for it to be a true competitor to OnShape. My online component did not have sheet metal tools. When I reached out to Dassault, they told me I had to pay extra for the sheet metal tools. At no point did anyone tell me that there is an offline component until a guy online said, "that's odd, I have sheet metal". He didn't tell me about the offline component, so I asked, and he noted there were extra steps. It does not help that there is practically no documentation on the online component. It's mostly all marketing hype without any real information.

I encourage everyone to read this FAQ sheet from the EAA:

I'll be redrawing in SW now. Jeez, I can't believe it took me a year to get to this point.
 

imamac96

Active Member
Joined
May 10, 2018
Messages
33
Location
North Texas
I've been working every night trying to get caught up in SW. This is pretty exhausting after a workday as a code monkey. Truthfully, now that I've found some good extensions for OnShape (and made some quick modifications of my own), I'm still in the OS world. This project serves two purposes: (1) understand a bit about how to calculate how strong the thing is I am flying to give me some confidence when I sit in someone's plane and (2) get better at CAD. Neither of those goals are to fly an airplane I designed.

The picture attached is an example of the "modular construction" that I'm interested in exploring. I really like the channel used by the Sonex, so it looks a lot like that. The Davis DA-2 does something very similar, but the fuselage/turtledeck sides are one continuous piece that requires a form block. The goal here is to remove as many unneeded form blocks as possible to keep build time (and material waste) down.

The big takeaway from this week has been the use of the "move face" feature. I had no idea why anybody would want to use it. I have always thought that you just extrude-remove something if you need to remove material. This produces weird, unanticipated behavior for things not in-line with the coordinate system. It takes the global coordinate system into account. Move-face can remove material in-line with the part you are working on.

I have a TODO list item I'd really like to get to: write a plugin to make tabs along a curved surface. I plan to experiment with the sheet metal form plugin that someone wrote to model the wing ribs. If that doesn't work, I'll go back to drawing tabs. There are many people who don't like the individually-tabbed ribs, but they make the most sense for all-drilled at-home-CNC'ed construction. An example of someone doing this is Mr. Haines: Thatcher CX4. Doing this by hand is not great. It's not the end of the world, but it would be swell to have this done automatically.
 

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imamac96

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Messages
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Location
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On the topic of simplicity, I'd like for the longerons to be 1" x 1/8" angle (like the Sonex). This is a much better way to do things than the Thatcher CX4 IMHO because it cuts out any bending of longerons. Bending angle generally seems like a bad idea to me. I'd like for bent plate to be used as the gussets to connect the tail cone section to the cabin section. You can see in the other picture how I've started adding gussets for the sake of having something there.
 

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imamac96

Active Member
Joined
May 10, 2018
Messages
33
Location
North Texas
A little bit of work this Saturday morning. There are some issues on Ubuntu with the graphics (thanks, NVIDIA), so I'm on Windows at the moment.

Back to the wing. Doodling some aileron stuff and cleaning up some bad references. One mistake I got used to very early on was referencing bodies instead of other sketches. This is probably the fourth time drawing a wing, and I'm finally getting the hang of the workflow. OnShape has a problem with the sheet metal tool of not always being able to move faces properly on sheet metal flanges. That's not exactly terrible, because that excess material can easily be trimmed by the builder with a set of snips. You can also see in the picture that I have the flanges not rendered so it doesn't take a minute to generate.

Screenshot (3).png

There's something of a real rear spar here now. The other chunk of what I did this morning was drawing piano hinge. Unfortunately, OnShape was not very good at this. I'll also note that the rib positions aren't finalized. I still have the numbers in crunch-limbo.

Until next time,
Connor
 

imamac96

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Joined
May 10, 2018
Messages
33
Location
North Texas
More doodling: Went in a new direction of modeling the skins as flat sections instead of a spline. This makes drilling holes easier with the added benefit of keeping the skin taught during drilling (straight lines are shorter from point to point). It just looks really ugly. It's pretty clear that although tabbed construction would be faster to build, there's really not a reason for it. Every other design has a solid flange, and there have been expressions of concern on this forum about straying away from the norm. That's fine. Moving on.

You can see how I doodled a baggage compartment in the wing towards the inboard section. This makes sense. Might as well use the space.

I will likely have to redraw the wing again, but I have gotten workflow down so much better than when I started. After all, getting better at CAD was actually one of the two missions of this project, so that's great. I say this because some of my references in the aileron and flap sketches are a little off, but they're still there for aesthetic reference.
 

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