If an airplane is flying sideways, is it in free fall? If a plane turns 90 degrees such that it is flying sideways, is it accelerating towards the earth at the usual 9.8 m/s^2?
My guess is that the plane must be in free fall because I don't see what force is counteracting gravity.
 A: The problem with such a maneuver is that you're kicking in some strong g-forces there. Once you you hit an angle of 60$^\circ$ (the common banking angle for a commercial airliner), you are hitting 2G's. Past that, it goes exponentially:

(source--load factor is the hypotenuse to gravitational & centrifugal forces. Note that the typical person can only handle about 5 G's, so less than 90$^\circ$ bank.)
The above chart assumes that the bank is at a constant altitude. What it shows is that the plane cannot actually fly at a 90$^\circ$ bank at a constant altitude, it must either incline or decline.
The barrel roll maneuver (source for image as well) turns the plane both longitudinally and laterally, while at two points the wings are banked at 90$^\circ$.

Since this maneuver can be done, it suggests that the plane would not experience free-fall while banked at 90$^\circ$, or else it would not be able to make the climb upwards.
A: Forget gravity for a little while, and put yourself in the pilot seat of an aerobatic airplane.
The plane has so-called "elevators" at the tail or "empenage", and those are connected to the stick that you hold.
What the elevators do is determine your angle of attack - the angle at which your wing meets the air stream.
If you pull back, the elevators move up, pushing the tail down, increasing your angle of attack.
This increases your lift, causing you to accelerate up in whatever direction "up" happens to be.
If you push the stick forward, that means less angle of attack, which means less lift.
If you push it far enough, you get zero lift - "free fall" if you want to call it that.
Push it even farther, and you get negative lift.
If you're flying inverted in a straight line, this is what you have to do.
Now put gravity back in the picture.
It is just another force acting on the airplane, that's all.
As I said, for continuous inverted flight, you need to push the stick forward to get -1g of vertical acceleration, to counter gravity.
You're hanging by your seat belt, which is why you keep it tight.
By the way, if you do a barrel roll, g-forces are positive throughout.
Even though the plane goes upside-down, it is never at 0 or negative G.
You can hold a cup of coffee through the whole maneuver.
