Hi Ali, people write books on the subject so there is a tremendous amount of information and theory that could be discussed here. There are actual design processes and strategies just for designing things (anything.) There are a lot of basic formulas and relationships that can help size an airplane for what you want to do, and also estimate important things like stall speed, efficiency, range. There are tools like ecalc (web site) that can help with planning the best motors/esc/battery combinations. There is much to discuss about autopilots, control theory, sensors, algorithms, and robustness. There is plenty to learn about electronics, wiring, connectors, RF signals.
An aerospace engineer doesn't need to know everything, but they need to know some of everything. (They might think they know everything.)
So I've done a little of this sort of thing ... enough to have quite a few strong preferences and simultaneously realize how little I know about how anything works.
I will also observe there are two main approaches taken to the overall design process. 1. you have people that know enough formulas and theory to do a design on paper and come up with a plan. 2. you have people who have been designing and building model airplanes as long as they can remember and have built up a tremendous amount of experience and intuition. Group #1 can fail by missing something obvious like making a hinge or linkage work right, or never considering where the CG should go. Group #2 is successfully building and flying great and fun designs, but it's unlikely anyone from that group will be working on the next space shuttle. The best person will have a healthy portion of both theory and experience. People that do this seriously ... no matter which group they start in, will will want to learn and absorb as much theory and subjective experience as possible.
So all that said, here is some simple advice. Keep your project as simple as possible. Use off the shelf airplanes and components as much as possible. Best case scenario, if you pick a ready to fly airplane and a ready to fly flight controller (pixhawk?) you will likely spend months putting things together and figuring out how the software works. Don't pile on insurmountable design/build challenges along with just getting a basic system flying. (I saying this with the assumption you are a student juggling a full class load.) If you are a DIY'er sinking in all your evenings and weekends and daydream time into the project for the fun of it, then you might be able to get things working a lot quicker.
Pick a plane that is very easy to fly. Find something that is slow and very cheap or very quick to [re]build. Plan on having multiple mishaps. Find a good RC pilot to help you out, or plan on investing a few weeks/months and a few crashes learning basic piloting skills yourself.
Be safe. The prop(s) are like sharp knives that can spin at 1000's of RPM. Respect the danger. Remove the props when you work on or around the aircraft. I put a big slice in my couch setting up a uav in my basement. Due to a switch on my transmitter being misconfigured and then accidentally bumped ... the throttle ran up to full and shot across the room. Thankfully I got out of the way, but just about everyone has a similar story when they let their guard down or tried to cut a corner.
Now, when you find yourself at the point where you have a basic airplane flying with a basic autopilot system, you are ready to work on whatever your next priority is. Do you want to integrate a camera? Do you want to run down that path of adding vtol capability? Do you want to design a purpose built airframe? I suggest you forget the solar cells, but maybe you want to run down that path next and investigate what you can do there. It is good to establish a solid baseline of what you are confident you can fly and then add to it.
If you are doing this as a student and need to knock a home run out of the park for your first try and you only have until late April to turn in your final report, then plan carefully and realize that everything takes way longer than you think and is way harder than you'd ever imagine. If you press through, you will learn way more than you ever though possible. You'll get stuck waiting for parts to arrive, you'll break things, or plug something in backwards and let the smoke out. You'll run into something that just doesn't work and you can't figure it out. And then time runs out on the project before you can complete it and fly. So keep things as simple as possible. Get something flying as quickly as possible, even if it's just a simple RC airplane. If you've never built/flown one before, it's not hard, but it's not easy, there are plenty of places to get tripped up with just the simple things.
These projects are tremendously rewarding when they come together and fly. Everyone here knows the feeling of a successful maiden flight with a new airplane you've just spent weeks or months putting together. Good luck!
