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Posted by ADB2 | Oct 27, 2008 @ 06:04 AM | 14,792 Views
I have decided to compile formulas for model airplane design in this thread. I have come up with these formulas by searching over the web, whenever the need for them arises.
I have noticed that you can design a model airplane without resorting to any formulas (i.e. by trial and error), but formulas come in handy to check things out before you buy a motor, test a prop, or simply want to compare one design with another. IOW they'll save you time and expenses.
  1. Stall speed.
  2. Pitch speed .
  3. MAC (Mean Aerodynamic Chord).

Posted by ADB2 | Oct 14, 2008 @ 03:42 PM | 16,614 Views
After thinking it over for a few days I decided to build a 1m WS Owl-RT, with either a flat sheet 6mm Depron wing with a 3mm CF tube spar, or a built-up thin symmetrical wing with a sharp LE using 3mm Depron skins and a wood or CF tube spar.
I scaled up my earlier Owl-RT plans drawn with QCad by 25% and modified slightly the shapes of the nose, stab and elevator, rudder and fin and ailerons.

Edit: I have updated the plans with the elevator cutout and a couple of notes and dimensions.
Posted by ADB2 | Oct 14, 2008 @ 03:54 AM | 15,365 Views
I bought an assortment of wood and CF at New Power Modélisme in Paris recently (recommended shop, excellent kits and relatively low prices for France) and wanted to compare their weight and strength, to use as spar materials in small and medium sized Depron models like my Owl-RT.
Balsa medium density strip, 3 x 10 x 1000 : 6.9 grams
Balsa light density strip, 3 x 10 x 1000 : 3.0 grams
Spruce strip, 3 x 10 x 1000 : 17 grams
Samba strip, 4 x 9 x 1000 : 14 grams
CF tube, 6 x 825 : 14 grams
CF tube, 5 x 1000 : 18.1 grams
CF tube, 3 x 1000 : 6.2 grams
CF rod, 2 x 1000 : 4.7 grams
CF strip, 1 x 3 x 1000 : 4.7 grams
CF strip, 0.5 x 3 x 1000 : 2.6 grams
Aluminum tube, 3 x 1000 : 9.6 grams
Posted by ADB2 | Sep 28, 2008 @ 03:46 PM | 15,369 Views
Today was the second Sunday afternoon I have spent with Louis Fourdan, a retired radar engineer and electronics teacher, on these photos with his lovely wife Françoise who is a great cook as I can attest after the fabulous lunch we had today.
After lunch Louis and I proceeded to test the Big Blue Wonder DT-750 motor on his fully-instrumented test stand.
Posted by ADB2 | Sep 13, 2008 @ 03:53 AM | 16,173 Views
This thread is about flying the Owl-RT, and how changes in the aerodynamics of the model affect its flying characteristics. The Owl-RT is a model that I designed, built and am flying with a purpose: it is intended as an easy-to-build, lightweight, inexpensive KF wing 3D trainer so that I can later upgrade to the larger, heavier and fully 3D-capable Regal Bipe (also a KF wing design, by Kaos2) that I am also building.
Because the Owl-RT is easy to build and inexpensive, I can build and fly many different versions and compare their flight characteristics as I test new ideas and develop my pilot skills.
I have documented the building process for the Owl-RT extensively in another thread. This thread is about flying the Owl-RT and, unfortunately, unbuilding it eventually.
On to the flight reports...
Edit: here is my first video of the Owl-RT taking off, in flight, and landing (unfortunately with a 2S LiPo and 30km/h wind).
Posted by ADB2 | Sep 08, 2008 @ 11:40 PM | 15,888 Views
I recently stumbled upon this post by Scott Stoops:
where he discusses cube loading for some big 3D models (8 to 9). So I was wondering what the cube loading for the Owl-RT is. At 240g AUW it's actually 3.27, which puts it squarely in the glider region...
The formula for WCL can be found here:
I wonder what the WCL for a typical 3D profile foamie is?
Is WCL only relevant for big 3D models or is it also applicable to lighter, smaller ones?
Posted by ADB2 | Aug 30, 2008 @ 10:02 AM | 17,222 Views
I have decided to build the wings for Owl-RTs no. 2 & 3 slightly differently from Owl-RT no. 1. So I am experimenting with Depron. First experiment is to simply fold two pieces of 3mm Depron - top & bottom skins - held together with packing tape along the leading edge.
As the two Depron skins get pressed together, the Depron cells along the LE compression line get crushed and the two skins are joined at 90 degrees.
But, as observed in the macro shot below, this results in a very pointy LE. Not as pointy as a 3mm flat sheet LE, but very pointy compared to e.g. a Clark-Y airfoil.
Is this good or bad for an aerobatic/3D trainer?
Posted by ADB2 | Aug 30, 2008 @ 03:18 AM | 15,596 Views
To add to my problems, yesterday my cheap HC LiPo charger decided to quit. Symptoms were that it would not charge one of the cells of every single one of my LiPo packs. A little later, after trying all my 2S LiPo packs again, it would not charge either cell.
Opening the charger requires removing four screws and takes a few seconds at most. Examining the solder joints with a lens, the problem was obvious: too small pads, too thin copper, not enough epoxy on the connector housings, i.e. too much corner cutting and cost saving and the result, too little quality in the design. That's why I call this charger cheap, and not inexpensive.
My time to resolder the bad joints and add some extra epoxy is worth more than what I paid for this charger.
Posted by ADB2 | Aug 30, 2008 @ 02:37 AM | 15,230 Views
Well, I decided to fly again yesterday afternoon, even though it was late, I was in a hurry to get back home and I was flying over unfamiliar terrain, with some wind.
I was practicing inverted flight and trying to repeat an inverted harrier which I had successfully flown earlier, but I guess I was too low. Conclusion, the Owl-RT nosed in at rather high speed!
Sunset was nice, though.
Posted by ADB2 | Aug 21, 2008 @ 10:24 AM | 16,984 Views
How do you make wheelpants from 3mm and 6mm Depron scraps? I was asking myself this question this morning and just sat down to draw the basic wheelpants shape on a sheet of paper.
These are for the Owl-RT 3D lightweight model which I describe in another thread.
You start with the wheel itself, of course, but as I am a newbie I just drew freehand a basic wheelpant shape, from memory.
First try was waaaay too big, so second try I took a real wheel and drew around it.
This gave me the basic shape which I cutout and laid on Depron scraps to cut the various layers.
I am using 5 layers arranged 3-6-3-6-3mm Depron. That gives me 15mm internal space which is fine for the wheel I am using (12mm wide).
The 5 layers are glued together using polyurethane glue (Gorilla Glue equivalent).
I then shaped the wheelpant roughly using a cutter.
Still needs sanding and painting, but looks OK to me!
Certainly gives the Owl-RT a more aerodynamic look. ...Continue Reading
Posted by ADB2 | Aug 03, 2008 @ 02:45 AM | 19,169 Views
When I look at a Troll in flight (see, I am amazed that the 3mm flat sheet Depron wings don't fold. That's because, of course, they are braced using 1mm carbon rods and the struts and cabane structure hold the wings together (the cabane is the part that mates the upper wing to the fuselage).
3mm Depron is very light, very thin and... very soft!
The first attachment is a picture of a Troll wing flexing under the weight of 20 euro cents coin at each end (approx. 5.7g each). The wing also shows very little torsional rigidity.
I did an experiment to determine how a similar wing (same wingspan and chord, also built using 3mm Depron), with a KF profile and a Depron spar, would flex.
The second attachment is a picture of the KF Depron wing with 76g weights at each end - that's 13~14 times the weight of the coin above.
The flat sheet wing weight is 15.2g, the KF wing weight is approx. 33g. The KF wing gets its strength from the spar and from the combination of the bottom and top skins. It is also, as expected, torsionally much stronger than the flat sheet wing. The extra weight comes from the Depron spar (I used a strip of 6mm Depron, but two 3mm strips could be used instead, or, optionally, a Depron + CF rod/tube combination), the top skin, a 3mm Depron strip along the LE to thicken it, some glue, and some packing tape around the LE.
Right now I am building two Trolls, one with the standard flat sheet Depron wings, with carbon bracing, and a...Continue Reading
Posted by ADB2 | Aug 02, 2008 @ 04:32 PM | 21,077 Views
The background is the Troll plan printed on A4 sheets taped together. The wing is finished and painted (the ailerons are not hinged yet). The fuselage sides are painted and I have started gluing its parts together. I haven't cut the elevator and rudder yet.
Why Owl-RT (pronounced "All Right")?
RT are the initials of Kaos2's real name, Rich Thompson.
Since the Owl-RT inherited its KF wing from the Regal/Regal Bipe model series designed by Rich, I thought I had to make reference to this fact.
And I like owls:
Plus the Owl-RT has short, stocky wings, so I thought it looked a little bit like an owl. I just hope it will fly "all right" (or like an owl)!
Finally, if you re-arrange the letters, you get "Trowl", which is a reference to the Troll lightweight aerobatic 3D bipe, designed by J.C. Spillmann. The Owl-RT inherited its general dimensions, fuselage and basic tail structure from the Troll.
The tail outline resembles that of a Yak, and the wings are more like an Edge, whereas the fuselage is vaguely like that of an Extra. IOW it's unlike any other aerobatic midwing but it borrows the looks from various well-known ones.

Edit: the build log for Owl-RTs no. 2 & 3 starts on post #21. It's a more detailed build log than for no. 1, and I build two models simultaneously.

Plans are now available. These plans are exactly what I used to build Owl-RTs no. 2 & 3 with small changes described in the thread.

Also I am writing a separate thread for flying the Owl-RT.
Posted by ADB2 | Aug 02, 2008 @ 03:51 PM | 18,438 Views
I built a few RC models when I was a teenager, kits assembled from balsa wood, covered with Monokote and powered by glow engines.
That was a long time ago. Then, about 10 years ago, I stumbled upon the web pages for a Depron flying wing, the slope soaring Ixir II, based on the original Ixir by Alexis Maréchal.
Flying wings are a fascinating subject by themselves, but building one out of Depron made the whole enterprise even more interesting. I built one and scratched it as it was completely warped, then built a second one that actually flew a couple of times. I was busy with many things at the time and interest subsided.
Fast forward to 2008: we have LiPo batteries, affordable brushless motors and R/C gear, carbon tubes and rods and most everything can be purchased online.
Depron is still as easily available nowadays as it was 10 years ago here in France. So recently I decided to go back to my old hobby and build and fly a few R/C models, built out of Depron if possible.

I recently completed an Ixir II, and have already flown it for a few hours. It is very stable in flight (once the CG has been correctly adjusted) but the flat bottom profile does not lend itself to any kind of aerobatic maneuver.

I also started building a Regal Bipe, designed by Kaos2 and discussed here on RC Groups. This is a very interesting model as it combines various features from different aircrafts and technologies to reach a surprising result.

Kaos2 designed the Regal Bipe for Bluecor...Continue Reading