Drag force? Well, since you opened that can of worms...
Drag force is a real b**ch when it comes to analytically predicting it. Most people will probably just spit out the drag force equation with a v^2 in it, not realizing it's not always accurate.
The flow of gas or liquid around a body differs with shape and speed. For example:
F-4 flying at subsonic speeds has a coefficient of 0.021 (according to Wiki) and supersonic coefficient of 0.044. Granted, the flow of air will change drastically once you're approaching supersonic speeds and just above them, but it's still a huge change.
The flow of air, being smooth or turbulent, can be estimated with Reynolds number. You can calculate it like this:
Re = (Density * Velocity * "Diameter" ) / Viscosity
If the number is large... > 500 or so, we can use the drag equation that you've seen before, with v^2 in it. But if the number is 1 or below, that equation no longer applies and will give you a result that's far off... you need another equation, which increases linearly with velocity, not squarely.
But if the Reynolds number is between 500 and 1, we don't really have any equation for it. In that case, the flow of the liquid or gas around an object is unpredictable, sometimes being smooth and sometimes turbulent... sometimes it changes from one state to the other when conditions are right... whatever the flow, we can't predict it with an equation. We still have the option of building the object and measuring the drag force at different velocities or building a computer model to simulate the airflow around an object.
As for the stresses on the body... When it comes to aircraft, designers need to be very careful about every detail. Passenger jets usually store some of the fuel in their wings. That means that at the start of the flight, wings are heavy and bend down. But they still have to be large enough to lift a filly loaded plane into the sky. At the end of the flight, wings are empty and light and will usually bend upwards. Because the plane has burnt all the fuel and wings stay large, there's a lot of lift left... designers need to pick materials that will survive stresses such as bending.
You probably remember the Concorde... it was limited to Mach 2.02 because when it was first created, designers used Aluminum to build it. Aluminum melts at some 600°C and loses structural integrity before that... so they had to limit the top speed to prevent failure due to heating. Even with the limit, the nose heated up to 160°C and the airplane stretched by a foot. It went through 2 thermal cycles:
Cooling after takeoff when it gained altitude, heating up when reaching cruising speed, cooling down when breaking and warming back up during descent.
An airplane will sometimes encounter turbulence in mid air. That can produce a lot of vibrations and if the turbulence induces vibrations with the same frequency as the natural vibrating frequency of the wings... well, it can tare the off.
You're probably familiar with g forces... while passenger jets and small piston powered planes don't usually get high g loads, you still need to make sure the plane will survive if it needs to pull a tight turn.
These are just some of the forces acting on planes... though if you're looking to shoot a projectile at high velocity, I think an elongated projectile such as a bullet would be best. Just better make it out of something that won't melt. If fired from the ground at say... 10 km/s, it would probably shine brighter then the Sun...
