Hello All,
I have been working on A solar RC planes. the main objective was "To make a plane which can fly by using just solar energy" (no battery to store energy). So we can have unlimited flying time.

As per some resources, It takes around 40 watts of energy per a pound of weight to fly for a glider. (ref:http://www.rc-airplane-world.com/watts-per-pound.html )

The solar panels I was using SunPower C60 solar cells. This cell weighs 8g and give 0.5V at 5.5Amp supply. so we have we have 8 grams per 3 watts. This 3 watt is peaK power for cells. So when cells in inclined or during a cloudy day, power output reduced. (ref: http://www.sun-life.com.ua/doc/sunpower%20C60.pdf)

wing: to attach solar cell, flat top airfoil was used (refer: Ishii airfoil). Making exact profile was a bit difficult so I tried to make flat top airfoil so that easily solar cells can be attached.
motor and prop: I have used the coreless brushed gear motor and prop from a quadcopter which can provide the thrust of 40-50 grams max. I also tried bldc motor, for that, I connected the panel to DC-DC voltage booster. but power(I think voltage too, as sometimes it drops to 2.5V) was not sufficient to run a motor.

I have made two models.
The first prototype was a failure and also damaged 8-10 solar cells. it was designed to have 10 cells (so max 30 watts) and having the weight of 310 grams. this plane as 4 propellers/motor pairs for good efficiency. but the main problem was balsa wood was used for reinforcement, which was not enough for this plane. So the plane can not gain height. A photo is attached (videos also available if anyone needed).
While making the second prototype, the main objective was to reduce weight as much as possible. so I have replaced 4 propellers to 2 props. I also reduced cells fro 10 to 8. now this plane has the weight of 240 grams. still, power to weight ratio was not as good as mentioned in above reference. but still, it was flying good enough. I generally prefer to fly underpowered planes for better feel. and experience of it was useful while flying this plane.

video of flying:
video of instruction of making:

problems still need to be solved:
-some sort of covering for solar cells to avoid contamination and damage of cell surface
-speed control for motors

anyways, flying this plane is such a fun as we do not need to care about battery voltage. 

Beautiful effort there:)

Since the motors are powered directly from the solar cells' output and that solar cells' output depends on external factors, do you feel the motors getting hungry for power at times(maybe due to a roll? Or maybe the sun just got hidden by cloud?) while flying?

Just a suggestion, adding a capacitor can act as a buffer/reservoir of energy to handle those momentary drop in solar cell output. Again, nice work.

that seems a nice idea, I will try to implement that on my plane. thanks lot for the suggestion.

Can you tell me how much did it cost please

As I explained in video It takes around 3500 to 4000 INR excluding Tx-Rx
here is rough breakout:

solar cells=8*180;
coreless motor = approx 300*2;
lipo battery 300-500mah= 300;
3.7g servo= 3*300;
carbon rod =200

However, it cost me much more than that as few attemt was a failure which damaged many solar cells and even coreless motors.
hope this is helpful to you.