The physics of a gliding airplane are simple.
There is potential energy, proportional to height above the ground.
There is also kinetic energy, proportional to speed squared.
First, understand the speed.
If the plane isn't slightly nose-heavy, it will fly a scalloped up-down cycle.
If it does that, add a little weight to the nose, or distribute the wing area more toward the rear.
Assuming you've done that, you control the speed by turning up the trailing edges. The more they are turned up, increasing the angle of attack, the slower it flies.
(Up to a maximum angle of attack, at which the wings stop working, or "stall".)
Back to energy.
If there were no drag, the plane would never come down.
Since there is drag, the drag tends to slow the plane down, decreasing its kinetic energy.
Countering that is the plane's tendency to maintain constant speed and kinetic energy, so it descends, turning potential energy into kinetic energy, just like a ball rolling down a slope.
So the more drag, the more quickly it descends, the less drag, the more slowly it descends.
A way to minimize drag is to minimize speed, because drag force is proportional to speed squared.
(Therefore the sink rate is roughly proportional to speed squared.)
So the speed you trim it for depends on what you want to maximize:
To maximize gliding range, you trim for a speed which is slow enough to have low drag, but not so slow that you don't cover much ground.
To maximize time aloft, you trim for an even slower speed which has even lower drag, thus minimizing the sink rate. This speed is roughly half way between the speed for maximum range and the even slower stall speed $V_S$.
Check these links: V-speeds, and Gliding flight.
Landing - stress is highest on the pilot, because he/she can't afford to be too fast/too slow, or too high/too low.
Takeoff - stress is highest on the engines. They are at full power and have to hoist a heavy aircraft as high as possible in as short a time as possible.
Cruise - stress is highest on the airframe when encountering up/down drafts at cruising speed (bumpy air).
Look at it this way, every plane in level flight, for a specific weight and configuration (flaps, etc.) has a stall speed. That is the slowest speed at which it can support its own weight.
If it is going twice that speed, it can support four times its own weight (and the passengers will feel 4G, and it can go negative as well).
If it is going three times that speed, in principal its wings could support nine times its own weight if they were strong enough.
Typically they are not strong enough, and they will break instead, if they encounter a strong enough updraft or downdraft.
That is why every aircraft has a particular speed, a fairly low speed, called "maneuvering speed", that they slow down to if they stupidly stumble into a storm cell.
At that speed, there is no amount of up or down draft that can cause structural damage.
Technically, it's the speed at which maximum control deflections cannot cause structural damage. Remember the accident in Far Rockaway NY?
The pilot tore off the tail fin by stomping the rudder pedals too hard from one side to the other.
By contrast, military fighters and aerobatic stunt planes are built for high-G turns (12G is possible).
You can see, since available lift is proportional to velocity squared, it is not at all difficult to go fast enough to get that kind of lift.
Also, don't forget a jetliner is a pressurized air bottle, for high-altitude flight with passenger comfort.
It cycles from un-pressurized to pressurized every time it climbs to cruise altitude and back.
This has been known to cause metal fatigue cracks, resulting in some accidents.
Best Answer
There is no simple equation for how a paper airplane flies like there is for a simple projectile because the airplane can interact with the air in complicated ways.
The physics of a paper airplane is described by Newton's laws of motion. These laws apply to both the airplane and the air it travels through. The plane is acted on by a constant gravitational force and by contact forces with the air, especially drag and lift.
The nature of the force between the air and the plane can be quite complicated, and requires an extremely detailed analysis for accurate simulation. For example, by constructing the plane slightly differently, you can make it fly faster, slower, further, curve left or right, or bob up and down.
The basic physical ideas are those of fluid dynamics and the basic equation involved is the Navier-Stokes equation. Modeling something like an airplane accurately is mostly the domain of expertise of aeronautical engineers.
To make a simple model for a game, you might want to start with a simple constant gravity force, a drag force proportional to the square of the velocity, and a lift force also proportional to the square of velocity (which comes from here), and then play around with the parameters until you find something pleasing to your eye.