Today I spent a lot of time making bread and pumpkin cheesecake for tomorrow’s Thanksgiving day feast, but in between I decided to build some more parts for a “So” twin boom foam flyer, using the KFm3 wing that I photographed yesterday. My goal wasn’t so much to build a flying plane today, but to just test the design out, and see if there were any difficulties or tweaks to be made. So, I made two twin triangular booms, the 13″x4″ stabilizer, tail fins, and the body. Here’s the result:

It’s a fairly nice looking design. The twin booms and stabilizer presented no serious issues, except for maybe alignment: making sure they are parallel and at right angles to the wings and stabilizers. I cut the internal bevels on the boom freehand, which made them look a little rough, but that presented no real problem once they are glued and folded. The fuselage body was a bit trickier: the instructions call for cutting a 36″ long strip, 1″ wide that separates the two sides. I cut a pair of 1″ wide strips, since the stock that I had is maximum of 30″ wide. I free handed the curve at the front of the fuselage, cutting both sides simultaneously. I then glued the strip along once side with hot glue, then spread hot glue on the other side and lowered it down. It isn’t perfect, but seems okay. I haven’t covered it with packing tape: my skills at covering flat sheets are pretty good, but irregular shapes seem a bit trickier. I’m pondering building a similar plane, but using the Experimental Airlines 2.5″ square fuselage.

But it looks pretty good. I’ll be able to weigh the model and work out the wing loading and some of the balance issues. I’ll also be able to think about how to place the servos, receiver and battery. I may just hang it from the ceiling of my office.

Okay, I haven’t had much time to work on RC airplanes. I’ve been meaning to try to find something a little tamer than my Nutball to fly (it seems to be a bit twitchy) but I haven’t really done too much other than watch videos on YouTube and scout around on the rcgroups forums for inspiration. But I have been thinking alot about wings, and wing fabrications. I ended up making three different wings from Dollar Tree foamboard, and thought it might be nice to show what they look like, and what I learned in building a few of them.

The first of these is the so-called Armin Wing, championed by Ed on the Experimental Airlines channel. I’ve made a few versions of these. The nice thing about these is that they have a nice airfoil shape. The version that I have here isn’t the fancy version, with the knife-edge trailing edge, but it’s pretty nice. I used a piece of 3/16″ dowel to stiffen it (carbon fiber would be better, but I was just experimenting).

Ed’s videos show in great detail how to make these, if they interest you, go review those. The one disadvantage is that they are a little bit involved, compared to the two designs that follow, but they aren’t really difficult, and I suspect that they might perform better than simple KFm airfoils, although mine have pretty blunt leading edges, which could make for higher drag than you might like, at least for high speed flights.

While watching flitetest’s video on Airfoils, they constructed a KFm3 style stepped airfoil to replace a damaged wing on their Cessna model. They didn’t go into any details on how they did it, but I thought it might be pretty easy. The airfoil has steps at 50% and 75% of the wing chord, so I cut a piece which was 12″x30″, and another which was 6″x30. I folded the 12″x30″ piece 4″ back from the leading edge, and clamped and glued it around the 6″ piece, resulting in a wing with an 8″ chord, and three thicknesses of foamboard. Because there was so much area to be glued, I didn’t use hot glue, I used Gorilla glue, and clamped it for a couple of hours. It worked, but also had a blunt leading edge. Not sure I liked it.

The “So” design that I linked in my last posting was also a KFm3 wing, but constructed a bit differently. This wing has a 6″ chord, so you begin by cutting a 9″x30″ of foamboard, and then score halfway through it 3″ back from one edge. You then cut a shallow bevel on either side, so you can fold it back. A piece 1.75″ wide is first glued 1.5″ back from the trailing edge, and then the bevelled part folded back and glued to the top edge of the 1.75″ piece. Okay, that’s a crappy description, but here’s the result. The bevel allows the leading edge to be sharper than the previous one.

All three seem credible designs. I probably will use the third technique if I go with the KFm3 style wing. I like the So design, so I might try putting one together shortly, although I’m considering the “trainer” mods (polyhedral wing, with rudder rather than aileron). But the Armin wings aren’t that much harder, and make for kind of a spiffy looking wing.

Stay tuned for more foamboard madness. The cat is enjoying playing with the scraps at least.

Addendum: The KFm3 wing can be scaled up. Check out this video of a 96″ version (almost 5x the size of the one I just put together):

I’m constantly on the lookout for interesting and simple airplane designs. This design from foamflyer is interesting in a couple of different ways. First of all, it’s made from cheap dollar tree foamboard, which is nice. It has an overall pleasing shape with a nice KFm3 style airfoil, constructed a bit differently than the one that I made as a test recently. It’s a pusher prop configuration (which might be nice for FPV/video) and most interestingly, it has just a single aileron. I like it!

The build log appears on this page on the rcgroups forums, and includes lots of build details. Really nice. He’s also got a terrific website chock full of two hundred planes that he’s constructed. Great stuff.

Addendum: The lack of dihedral may make this a less than ideal trainer aircraft, but one of the commenters constructed a similar airplane with a polyhedral wing to add dihedral, and used a rudder-elevator setup instead of the single aileron (my guess is the single aileron doesn’t work as well with the dihedral in place). Also worth looking into!

I snapped awake at 5:30AM this morning, and couldn’t get back to sleep, so I started a loaf of bread to bake later tonight, and then settled in to thinking about airfoils some more. I had seen references to the “Clark Y” airfoil, but didn’t know how it was defined, so I set out to figure it out. A few minutes of googling revealed a fascinating resource, the UIUC Airfoil Data Site. It has data definitions for literally hundreds of airfoil profiles, including coordinates for the Clark Y. I then dusted off my dim knowledge of gnuplot to draw this (properly scaled) version of the airfoil.

There is a trick to getting gnuplot to work. Here’s the set of commands I used:

set terminal png size 1024,256
set size ratio -1
set ytic .1
set xlabel "Clark Y Airfoil"
plot "clarky.dat" with lines

(You could probably make it look prettier with a bit more work.) The important bit is to set the size ratio: without it gnuplot will change vertical and horizontal scales to fit the data in the window.

Okay, off to make breakfast.

As I was staring at the cross sectional diagram of the Armin wing I constructed yesterday, I began to think about airfoil shapes in general. I knew from reading done years ago that there were standardized ways of describing airfoil shapes, but didn’t recall any details. I didn’t have a copy of Abbot’s Theory of Wing Sections, and I loaned Mark H. my copy of Simon’s Model Aircraft Aerodynamics, so I had to turn to Google and Wikipedia to sate my desire for knowledge.

A few minutes of searching uncovered this Wikipedia entry on NACA airfoils. It’s pretty helpful, and includes formulas for generating the airfoil profiles of a wide variety of standard wing shapes. I hacked together a quick python program to generate the profile coordinates for 4 digit NACA airfoils, and as soon as I verify the results, I’ll post the code here. It won’t be hard to use this to generate a PostScript/PDF file which you could then directly print to make templates.

A bit more searching revealed this repository of Public Domain Aeronautical Software which included this implementation of a program to compute airfoil profiles for NACA wings. It’s written in “modern FORTRAN”, which isn’t my preferred language of choice, but I think it will be excellent to use it to verify the profiles produced by my Python code.

Just a tease, here’s some GNUplot output for a 6412 airfoil (the wing is fairly cambered with the maximum occuring at 40%).

The NACA 4 Digit 6412 airfoil. Not checked, and not properly to tscale

Ed over at the Experimental Airlines Youtube channel has a nifty way of constructing wings for RC airplanes out of cheap Dollar Tree foamboard. Last night, I was feeling kind of brainless, so I decided to practice my arts and crafts by constructing another, this time using a piece of 3/16″ dowel to stiffen it. When I was done, I thought it might be nice to figure out how it really ranks as an airfoil. For that, I needed two things: some accurate measurements, and a program to do some simulation on the wing. I haven’t got as far as the second, but I took out my cheapy Harbor Freight calipers and took some measurements, and created the following scale drawing. I didn’t bother computing nice spline curves for the top surface, but the leading edge is pretty close. The back doesn’t use Ed’s nifty tapered tape edges: I just put a bevel on the base, and then glued the top down, so it’s basically the thickness of the 1 layer of foam board at the back. Anyway, here’s a PNG, if you click on it, you should get a 300dpi version which is lifesized.

Click for full scale 300dpi PNG of my Armin Wing, 7″ Chord…

I’ll try to get this into an airfoil simulator, and compare it to other simple wings (plates and KFM2 wings, hopefully) to see how it compares. I also want to get a precise weight for the wing, so I can figure out how the lift and weight compare).

Anyway, that’s what passes for arts and crafts at my house. Stay tuned.

Addendum: Josh and Josh over at flitetest have a nice video I hadn’t seen before on airfoils. It wasn’t super technical, but it did show an interesting KFm style stepped airfoil made from what appears to be the same Dollar Tree foam I was using. I had made one for my glider prototype that was a KFm2 airfoil (two layers, with the step at 50%), but they use one that appears to be more like a KFm3 airfoil, with the bottom and top layers created from just one piece, folded over a middle piece. What’s especially cool is that they used it to replace the conventional (and complicated) wing for one of their Cessna models, and it seemed to work great. I’ll have to try making one of those too.

I’m pretty interested in autonomous vehicles, and because of the wide availability of cheap electronics and compute power, experimentation in this realm is increasingly within the grasp of amateurs. I like the idea of building small aerial drones which can not only follow a predetermined coarse, but can also work to avoid obstacles and the like.

Today I found a press release for a paper “Low-Power Parallel Algorithms for Single Image based Obstacle Avoidance in Aerial Robots”, which seemed right up my alley. A quick Google revealed this web page, containing links to their papers. The math involved is a little beyond the casual, but I think I’ll be able to sort it out. Very nifty stuff, bookmarked for later.