I love AR Mini wing by Sonic Model. It’s a racing wing with a wingspan of 600mm. With 4S 850mAh battery it weights around 300g. But I was wondering, is it possible to make a smaller and lighter version of it. There was the same project somewhere in the internet las year. But I didn’t find it. So starting from scratch with own learning progress.
An idea is to leave a fuselage out and glue wings together. I cut away the bulges from the wings, so it would be nice to glue wing parts together. To check is it flat enough, put cut side against the table and look for the gaps. If there’s no one, it’s good.
Measured to cut 90mm off the spar. But when combined wings together with the spar. They weren’t parallel. Just a small angle between them. I hope that the glue will withstand that if I press them together.
I use UHU POR glue and tightly secured with fibreglass tape. Seems to work well. I’ll let it dry over the night.
While waiting for glue to dry, I glued winglets. After 10 hours tapes are removed. The wings are glued pretty well together. Nano wing feels very sturdy and impacts resistant just like its bigger brother Ar Mini.
The weight of the body with the servos is 80 grams! Pretty light!
As the body is kinda ready, we have to decide what electronic parts to use. Am I making a close-range naked, long-range naked or long-range iNav wing? All of them are sub 250g! Maybe I just start with a close-range naked setup just to test how it flies and learn its characteristics. So below is the table of the parts with their weights.
|Wings, winglets, spar and servos
|Motor Racestar BR1306 4000KV
|Cam+VTX random AIO
As I don’t have any proper 3S battery for this wing. I decided to try to remove one cell from the 4S 850mAh battery. Then it could weight around 75g. So total AUW could be 190g. Way below 250g magic limit! For the long-range setup I’m planning to use 2S Li-Ion VTC6 cells. But this is the next step.
Stock motor 1806 2400KV is too big for this Nano wing, in my opinion. I found a smaller motor in my random boxes. It’s RaceStar’s 1306 4000KV with a 5mm shaft. And I have plenty of random 5″ props. Another option is Emax RS1407 3800KV (15g) with T-mount and I have plenty of 3″ props. But I’m assumiming they are too small.
2020-03-09 – CG, Mass and motor mount
I actually decided to try 2S Li-Ion VTC6 battery with 1306 4000KV motor and 5×4.8×3 propeller. Attached all together and pushed the throttle. Whoa, there was a pretty good lift. I was lazy to use my test bench, as it was somewhere. But by hand feels, that it pulled more than 250g weight. At least there is more than 1:1 thrust/weight factor.
I realized, that 2S was pretty good to use also in naked setup for park flying. So I’m going to forget the surgery of 4S LIPO to 3S. Or maybe later if I’m getting power-hungry.
Next step is to calculate CG (centre of gravity) of the wing and mass distribution. There’s a pretty awesome tool for this at http://rcwingcog.a0001.net/
I’m going to cut a bay for RX and ESC at the bottom of the wing. The battery and AIO camera at the top of the wing. At this moment I’m thinking of using an AIO camera that weighs 5g. But I also have Triangle VTX-DVR, that could be also a nice option.
I hoped not to put the battery too close to the tip. But the mass centre mark (red cross) was way behind the CG mark (black and white mark). Actually it’s on 20% Allround position of CG. The “expert” position is 7mm farther. This could allow me to move battery for 15mm away from the tip. So I could make a battery bay 15mm longer than the battery itself and have better CG adjustment possibility.
Next thing is to design a motor mount. Measured and sketched the dimensions of the rear part of the wing. But I was lazy to find out an angle between the bottom surface and wing’s airfoil chord line. So actually I don’t know at what angle I have to put the motor. The solution is to design a parametrized mount with an adjustable angle.
For a start, I’ll use 94° angle. It probably will keep the motor in a slightly upward position. After trimming I’ll get to know should there be more or less angle. The mount will be secured by the bolt, that makes changing the mount possible.
A friend advised using WCL calculator to get a possible characteristic of the wing. WCL stands for Wing Cube Loading factor used to indicate possible flight characteristic of RC model aircraft. More info about it you can read in The Ampeer’s article.
AR Nano’s wing area is 72722.5mm2, that is around 113in2. The approximated weight is 214g, that is 7.5oz. Calculated WCL from these values is 9.6.
Here’s the list of suggested WCL values accordingly to characteristic classes:
- Gliders – under 4
- Trainers – 6 to 7
- Aerobatic – 9 to 10
- Scale – 12 to 13
- Racers – 15 and over
So Nano’s WCL settles exactly into aerobatic class. Pretty good!
2020-03-12 Motor mount, esc and rx
After printing and installing 94° mount, I saw that motor was still pointing too upward. Printed new mount with 96° angle. Now it’s looking kinda promising. But it could change again after flying tests.
The bays at the bottom of the wing for receiver and ESC are cut. I have to shorten servos’ and ESC’s signal wires and crimp new dupont plugs to them. Then they will be arranged nicely in the between. ESC’s power plug will be changed to smaller XT30 plug and guided to the top side of the wing. I’ll also add thinner wires for powering an AIO camera.
Receiver’s antennas will be taped on the bottom along with both wings.
2020-03-13 Battery, assembly and wiring
I made a compact 2S Li-Ion battery pack with short power plug and hidden dupont-plug as a balance plug. With an adapter I can charge this pack. Total weight of this battery pack is 98.5g.
Measured new weights and parts position in CoG calculator and got a position for the battery pack. Cut a bay for it at the top of the wing. I made it a bit tight. Shortened and guided power wire from ESC through the wing. I also guided thin power wire for the camera from receiver’s +5v and ground pins.
I’ve shortened servo’s wires and esc’s wire to fit between the receiver and esc. Receiver’s antennas are guided through a small cutting on the front edge of the bay and secured with a fibreglass tape under both wings.
In this setup, I use a camera without a battery voltage sensor, but I still want to know the voltage of the battery. Luckily RX6R receiver has an input port for analog data. So made a divider between the battery and the data port. It’s in blue heat shrink.
2S battery’s maximum voltage is 8.4v and data port’s maximum input is 3.3v. So 3.3v is representing battery maximum. By digging my box of the resistors, foud two sizes that make perfect dividing. 5100ohm and 3300ohm.
Dividing factor is 3300/(5100+3300) = 0.393. And divided voltage is 8.4v*0.393 = 3.3v! Exactly what we need 😄
By now I’m going to use small AIO camera with 25mW VTX and Cloverleaf antenna. As quick and simple setup as possible. Just to get to know how this wing flies. Then maybe I’ll replace with VTX with DVR and separate tiny camera. Or some HD split camera. Who knows.
And finally, the final result of AR Nano compared to its bigger brother AR Mini. Now I have to wait for good weather for maiden flight and trimming.