Warbler

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

imamac96

Active Member
Joined
May 10, 2018
Messages
28
Location
North Texas
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
28
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.
 
Top