I just tried to edit my post and I hope that all the comments didn't get deleted.

If so, I will have to go back through what I had, and try to add them in. oops sorry won't do that again.

There are fundamental principles such as:

Maximizing airflow, Minimizing weight, Frame Rigidity, Vibrations(mode, frequency, amplitude, localization), Torque.

In the arms specifically: There are torsional from the blades spinning, and lever forces, from the lift generated.

Compression is generally to be avoided, it can fail in a big way, so be careful with it. Oversize is better.

There are probably others so feel free to add. Those are common to all frames.

Most copters use a round boom or arm. This is bad in a wind tunnel. And if they get damaged, you have to replace the whole thing. My suggestion is to use vertical arms, allowing the downward force of the propeller to have the least resistance. Having vertical arms and using thin extruded aluminum not only make it easier to fabricate, it also makes it eco-friendly because you are not wasting fiberglass or carbon parts. You can bend the aluminum back into the proper shape effortlessly.

The torsion forces can be converted into bending/lever forces by appropriate locations of mounting points. This will increase stability because torsion is a pain to deal with. Tension is the best for most thin metals so trying to keep that in mind, putting some parts in tension will reduce needed material. Using a steel or similar wire around the arms will provide tension force against any torsion or bending because any individual arm's movement will be transferred to the other 3, causing them to act as a rigid body, it avoids sending vibrations down the arm.

By having the arms come to a central point, there is symmetry in the forces involved, makes them easier to design around.

If this is helpful at all, a thank you will do, and if you have anything to add, I will keep a record here AND in my heavy lifting design files, which will be used to make the next version of the ADrone. And I won't leave you out where credit is due. Collaboration is the future and we can do it together!

here is the work I am talking about, this is the first version, I know I am late to the drone field but that's no reason to quit.

http://www.instructables.com/id/Aluminum-sport-X-copter-frame-design-for-strength-/