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Dec 23, 2017, 08:29 PM
Ted Luginbuhl
ukanduit's Avatar
Thread OP
New Product

BNF SolarBear, pure solar powered 32" wing + Free plans

I have been working on a solar wing for the last several months and wanted to start a thread on it. I have already published many details about it in the high performance forum but since this wing is not really high performance I thought I would start this thread. I have been working on solar powered rc aircraft in general since around 2009.

These are made by me one at a time. I plan to offer them in the classifieds from time to time if there is an interest and when time permits. Shoot me a pm to see if one is available.

What sets this wing apart from the other solar projects I have worked on is the solar cells and the maximum power point tracker (mppt).
The cells are Sunpower C50 cells and compared to regular solar cells they are extremely durable. They are rated 21.8% efficient (b grade) and will produce full power even when cracked. There is a limit to this, if you crumple one up it will degrade performance. but if there are only a few cracks you can expect full power.

I got my inspiration for this type of mppt from HolleL and as far as I know he was the first to use this approach on RC airplanes. I could never get his code to work so I decided to develop my own. It is actually a well know type of tracker called a "constant voltage" maximum power point tracker. What it does is decrease throttle when cell voltage drops and increase maximum allowable throttle when cell voltage increases thus maintaining a constant array voltage. In practice there is no need for a receiver battery and you can maintain full control of the servos even when the plane is upside down.

32" span
8.5" MAC
GWS 6x3
Cobra 2203/34 2300kv
Lemon RX DSM2 Feather Light rx
Attiny 85 mppt
Laser cut ribs
Carbon leading edge
12 watt array
AUW ~133g
BNF $575 with free CON US shipping

Here is a video of it flying on the day before winter solstice. Winter has the lowest irradiance so if it can fly in winter it can fly year round.

DSCN1796 (2 min 22 sec)
Last edited by ukanduit; Mar 15, 2019 at 06:25 AM.
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Dec 23, 2017, 09:13 PM
Registered User
garduino's Avatar
Very interesting project!

Would be nice to know more details about parts and construction.

You are only using the energy generated by the sun right? I mean, no batteries?

Dec 23, 2017, 09:27 PM
Ted Luginbuhl
ukanduit's Avatar
Thread OP
Yes, absolutely no batteries or capacitors. Technically this would be F5E legal.

What kind of details are you interested in?

The leading edge is actually the spar. I use carbon roving to fill the tube where it joins and then wick in thin ca... very sturdy way to join an otherwise difficult joint.

Covered in 1.5mil laminating film.

The control parts are chosen to work at low voltages to avoid unwanted resets. everything will work down to 3.3 volts.

The servos are the "3.7g" servos you find on eBay.

Span 32"
MAC 8.5"
AUW 133g
Solar array power 12 watts at 4.6 volts
MH 24 airfoil
272^2 in wing area
18 degree sweep
cg 3.5" to 4.25" back from LE
Flies year round

Uses 8 B grade Sunpower C50 cells that have been laser cut in half

~25mph top speed.

Mildly aerobatic. will perform loops and rolls.

Uses no batteries or capacitors, all power is derived from the solar cells. It is a solar plane in the purest sense.

Motor will automatically cut out when there is not enough power to run both the motor and the servos thus maintaining control of the model.The mppt mcu takes care of all this and it is fully automatic. You can fly with the throttle stick at wide open throttle and the mcu will make appropriate adjustments.

Cells are built into the wing and can be replaced if damaged beyond serviceability. Sunpower C50 cells are very durable compared to traditional cells. They will make full power even when cracked. In the case of a severe crash a cell might need to be replaced if it gets crumpled. The cells will handle less than perfect landings. Basically the cells are just about as durable as the airframe.

Here is a copy of the latest version of mppt code
/*MPPT v6.3 for F5E RC Solar Airplane
pin2 solar voltage coming off a divider. 1.1v max. 300k and pin2 across 47k to ground
pin3 rc input comeing from rx
pin4 rc output going to esc
December 2017
Sergey Vekli and Ted Luginbuhl 
              __  __
      Reset -|o \/  |- VCC              
       3 Rx -|ATTINY|- 2 Solarvoltage                
      4 ESC -| 45/85|- 1
        GND -|______|- 0
//#include <avr/wdt.h> // Watch Dog Timer
#define Vmpp 0.55  // good setting: 0.84 for 12 cell and .55 for 8 cell. If too low motor will be faster at less than full throttle.
#define VmppLOW 0.63 // low throttle mpp voltage. 0.97 seems to be good for 12 cell and .63 for 8 cell.Set so that motor cuts off when parallel with suns rays.
#define STEPdown 2 //default 2  If too high throttle will oscilate, if too low esc will reset
#define STEPup 1
#define iterations 15 //default 15. This is how many times the algo averages the array voltage. 
#define transition 150 // point at wich transition takes place from Vmpp to VmppLOW between 110 and 230. Default 150.If too high it will kick in too soon and mimick Vmpp set too low.
#define LOW  false
#define HIGH true
int x = 0;
int Vcell = 0;
int VMPP = 0.00;
int VMPPlow = 0.00;
boolean cur_level = LOW;
void setup()

 // wdt_enable(WDTO_500MS); //Enable Watch Dog Timer.
   // set pins mode
  pinMode(4, OUTPUT); //going to esc
  pinMode(3, INPUT); //coming from rx
  //convert Vmpp to adc levels
  VMPP = Vmpp * 925;
  VMPPlow = VmppLOW * 925;
  //set freq and interrupts registers
  GIMSK = B00100000;
  PCMSK = B00001000;
  TCCR0A = B00000010;
  TCCR0B = B00000010;
  TIMSK = B00010000;
  OCR0A = 110;
  //main control timer
  if (cur_level == HIGH)
  digitalWrite(4, LOW);
  cur_level = LOW;
  TCNT0 = 0;

  //interrupt for rx repeater mode
  if (digitalRead(3) > cur_level)
    digitalWrite(4, HIGH);
    cur_level = HIGH;
    TCNT0 = 0;
    if (cur_level == HIGH)
      digitalWrite(4, LOW);
      cur_level = LOW;


   //main measurement-set cycle
   void loop()

  //wdt_reset(); //Pat the dog
   if (OCR0A <= transition) // throttle level at wich higher Vmpp kicks in else statement.
   x = VMPPlow;
   x = VMPP;
   for (int XX = 0; XX < iterations; XX++) //iterations for average array voltage.
   Vcell = Vcell + analogRead(A1); 

   Vcell = Vcell / iterations;

  if (Vcell > x)
      if (OCR0A <= 230 ) //230
      OCR0A += STEPup;

   if (Vcell < x)
    if (OCR0A >= 110) { //110
      OCR0A -= STEPdown;
Last edited by ukanduit; Mar 17, 2019 at 10:33 AM.
Dec 24, 2017, 10:32 AM
Laughs at un-boxing videos...
basicguy's Avatar
Thank you for showing such an innovative design.
The code is for your voltage regulator.
Dec 24, 2017, 10:38 AM
Ted Luginbuhl
ukanduit's Avatar
Thread OP
Yes, this code is for the voltage regulator that is called a Maximum Power Point Tracker (mppt)
Here is a video demonstrating how it works:

F5E MPPT V6.0 (0 min 36 sec)
Last edited by ukanduit; Dec 24, 2017 at 10:48 AM.
Dec 26, 2017, 02:03 PM
Ted Luginbuhl
ukanduit's Avatar
Thread OP
Added plans and instructions to the first post so you can build your own.
Dec 29, 2017, 05:25 PM
Ted Luginbuhl
ukanduit's Avatar
Thread OP

New Flight Video

Here is a flight video from my first customer (AJWoods). I think he does a better job piloting it than I do!

Solar Bear First Flights (2 min 25 sec)
Last edited by ukanduit; Dec 30, 2017 at 02:56 PM.
Dec 29, 2017, 05:44 PM
Laughs at un-boxing videos...
basicguy's Avatar
Why land in 2 minutes.

Have you run any calculations on doing long range fpv flights?
Dec 29, 2017, 07:50 PM
Ted Luginbuhl
ukanduit's Avatar
Thread OP
No I haven't, But I would like to try the micro fpv from Lemon RX. Just going by the specs it looks like it should work.
Dec 30, 2017, 02:48 PM
Aircraft Designer Guy
AJWoods's Avatar
Originally Posted by basicguy View Post
Why land in 2 minutes.

Have you run any calculations on doing long range fpv flights?
This is a very fun little plane! Ukanduit has done an amazing job on the design, and it's a very good price for a true solar powered airplane. I was able to get it flying between 1030ish and 1pm at 37.8 deg north on the 29th, so you should be able to extrapolate from that. It would make for some very long flights in the summer

Latest blog entry: Star Wars A-Wing Submarine
Dec 31, 2017, 01:32 PM
Broke RC Pilot
could you post more info on the Attiny 85 mppt? Like where to get and how to program if needed? This is an awesome project and I would love to build one of these real soon.
Dec 31, 2017, 02:16 PM
Ted Luginbuhl
ukanduit's Avatar
Thread OP
First buy one of these:

Then download and install Arduino IDE

Then add "attiny" by David Mellis to the board manager.

You can get a ATTiny85 from ebay or any of the electronics supply houses.

Once you do the above it is just a matter of plugging in the attiny85 into the tinyavr programmer and then copy and past my code into Arduino IDE and then select ATTiny 85 from the board menu and select "internal 1MHZ" for the cpu speed then hit the upload button...

As simple as that.

1) Now it is time to solder up the MPPT. Locate all the parts. Attached is a schematic of the MPPT.
u1 is the ATTiny 85
u2 is the POR IC (optionaly a switch on Vcc could accomplish the same thing but then pin 8 connects to pin 1 thru R1)
Pin 1 is Vcc
Pin 8 is ground
Pin 6 goes to the Receiver signal
Pin7 goes to the ESC signal
Pin 3 has no connection
Pin 4 has no connection.
R4 is a solar cell with negative on the bottom.
R11 has positive on the bottom.
R2 is a 300kohm resistor
R3 is a 47kohm resistor
R1 is a 10kohm resistor

Please let me know if you have any more questions... I would like to see more solar planes flying
Last edited by ukanduit; Dec 31, 2017 at 02:58 PM.
Jan 04, 2018, 12:25 AM
Registered User
achiii's Avatar
this is amazing. Just found this thread through your ad (which I am sorely tempted by). I so want to try this. You think it would work with a light foam pusher wing with the sunpower c50s laminated on top?
Jan 04, 2018, 08:45 AM
Ted Luginbuhl
ukanduit's Avatar
Thread OP
Originally Posted by achiii View Post
this is amazing. Just found this thread through your ad (which I am sorely tempted by). I so want to try this. You think it would work with a light foam pusher wing with the sunpower c50s laminated on top?
It should be possible but you would need so much sweep to balance it you are better off with a tractor layout.

I've built a foam version of Solar Bear that is super quick and easy to build. The cells can be laminated to the wing no problem.
Jan 04, 2018, 10:50 AM
Registered User
achiii's Avatar
right. I was thinking about the problem of balancing the lack of a battery on a pusher after I had posted.

That tractor foam version looks neat! The motor pod is the same as the one posted in the solar bear plans, but where did you get the foam wings? Or did you cut them yourself? And do you have some cf buried in there? How did the AUW of the foam version compare to the wood/cf version?

This is awesome.

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