Two Drags, Opposite Habits
The first kind is the one everybody pictures. Push anything through air and the air resists, and the faster you push the harder it shoves back. This is parasite drag, the drag of simply having a shape, and it comes from the skin of the aeroplane rubbing through the air and the wake left tumbling behind it. It grows with the square of speed, which is a brutal law: double the speed and this drag quadruples. It is what stops an aeroplane accelerating forever and sets the top speed.
The second kind is sneakier and runs the other way. Every time a wing makes lift it also sheds a swirl of air off each tip, a spinning vortex that is pure waste, and the slower the aeroplane flies the harder the wing must work for its lift and the larger those swirls grow. This is induced drag, the drag that comes bundled with making lift, and it is worst when the aeroplane is slow. Fly near the stall and induced drag dominates everything.
So the aeroplane lives between two tyrants. Go fast and parasite drag punishes you, climbing with the square of speed. Go slow and induced drag punishes you instead, rising as the speed falls away. One hates speed and the other hates the lack of it, and there is nowhere the aeroplane can sit where both fall silent. There is only the place where their sum is smallest.
The Bottom of the U
Add the two together and plot them against speed and you get a curve shaped like a broad letter U. Over on the right, at high speed, the total is high because parasite drag has run away. On the left, at low speed, the total climbs again because induced drag has taken over. In between, at one particular speed, the two are balanced and the total drag sits at the lowest value the aeroplane can achieve. That speed is one of the most useful numbers in flying.
At the bottom of that U the aeroplane is at its most efficient. It is making all the lift it needs for the least total drag, which means the least thrust to hold it up and the least fuel to keep it there. Fly faster or slower than that speed and drag rises either way, so the same engine carries you less far. Airliners cruise close to it. Gliders are flown to it when the job is simply to stay aloft as long as possible.
There is a subtlety worth carrying with you. The speed for the greatest range, the furthest distance over the ground, sits a little faster than the very bottom of the U, while the speed for the greatest endurance, the longest time in the air, sits right at the bottom. Whether you want to go far or to linger changes which speed you choose, and that single decision is the heart of gliding well in Glide.
Long Wings, Short Wings
Induced drag is the reason wings are shaped the way they are. Because the waste happens out at the tips, a long thin wing, which has less tip for the amount of lift it makes, wastes less than a short stubby one. Engineers measure this with aspect ratio, the ratio of the span of a wing to its width, and a high aspect ratio wing is a low induced-drag wing.
Nature worked this out long ago. The albatross, which must stay aloft over the ocean for hours on almost no effort, wears enormously long, thin wings, and it glides for a living. A glider built by human hands has the same silhouette for the same reason. At the other extreme, a fighter that has to fly fast and roll quickly carries short stubby wings, because at high speed parasite drag rules and long wings are just more skin to drag through the air, and their bending under load becomes a structural headache.
Winglets, those little upturned fins on the tips of airliners, are a trick played on the same problem. The tip vortex is where the waste leaks out, and a winglet gets in its way, recovering a slice of the energy that would otherwise have spun off into the wake. It is a small change that pays for itself over millions of miles, which is why they spread to nearly every airliner within a couple of decades.
Drag You Ask For
Not all drag is an accident. Sometimes drag is exactly what a pilot wants, and the aeroplane is built to make it on demand. Coming down from height or slowing for a landing, the problem is not too much drag but too little, and the aeroplane carries devices whose only job is to add some back.
Airbrakes and spoilers are flat surfaces that swing up into the airflow to make deliberate parasite drag, letting an aeroplane descend steeply or slow quickly without picking up speed. A glider has no engine to throttle back, so its airbrakes are how the pilot manages the approach, opening them to steepen the glide onto the exact spot chosen. Flaps, which hang down from the trailing edge of the wing, do a double job: they add lift for slow flight and, at large settings, a great deal of drag to help the aeroplane down.
Even the propeller and the wheels are part of the story. A stopped propeller on a dead engine acts like a disc held broadside to the wind, so pilots feather it, twisting the blades edge-on to cut its drag, when they want to glide. Lowering the undercarriage throws out a fistful of parasite drag, which is why it stays tucked away in the cruise and comes down only when it is time to slow for landing. Managing drag, adding it when you want down and hiding it when you want on, is half of what the hands of a pilot are doing.
Why Drag Makes the Game
Take the engine out of the picture and drag is the only thing left removing energy, which is why it is the silent opponent in every gliding game here. In Glide there is nothing to replace the energy that drag takes away, so every control input that adds drag is a withdrawal from a balance that only ever shrinks, and flying well means spending it in the right places.
A paper aeroplane is the purest case of all. It has no engine, so its whole flight is a slow surrender to drag, and its shape is a bet about which kind of drag matters most. A dart with swept-back wings is built for speed and low parasite drag. A wide gentle glider is built to keep induced drag down and hang in the air. Folding one in Paper Plane is really choosing which of the two tyrants you would rather fight.
Even with an engine, drag never stops sending the bill. A cargo aircraft flown heavy has to sit at a higher angle to carry the load, which means more induced drag, which means more fuel for the same distance. The route planner in Cargo is quietly a drag calculation dressed up as a delivery, because the air charges for every mile, and the size of the charge depends entirely on how, and how fast, you choose to fly.