Why a Flat Wing Still Lifts

You already know the answer your dart depends on: the wings push air downwards, and pushing air down is what holds the plane up.

Now look at the wing you actually folded. It is a flat sheet of paper. Hold a flat sheet flat and slide it through the air and it pushes nothing anywhere — it just slips along. So where does the downward push come from?

The wing does not fly flat

It flies tilted. Only slightly, but that slight tilt is the whole machine.

A tilted wing meets the oncoming air with its underside. The air cannot go through the paper, so it goes round — and it comes off the back travelling a little downwards. It arrived going straight ahead and it left going ahead-and-down. Something turned it.

That something was the wing. And whenever you push on air, the air pushes back on you, just as hard, the other way. Push a stream of air down and it holds you up.

Feel it in about four seconds

Hold the card flat and level, pinched at one edge, and sweep your arm quickly through the air. It slides along and barely tugs at your fingers.

Now tilt the leading edge up by a small amount — a few degrees, no more — and sweep again at the same speed. The card lifts. You can feel it climbing against your grip.

Nothing about the card changed. Only the angle it met the air at.

Now the fix list makes sense

The last article gave you four rules for a plane that misbehaves. They were true, and they were just rules. Every one of them is the same idea in disguise.

It dives, so bend the back edge of the wings up. Bending the trailing edge up makes that little flap push air upwards. The air shoves back down — on the tail end of the plane. The tail drops, the nose comes up, the whole wing meets the air at a bigger angle, and the plane climbs out of the dive.

It curls to one side, so check the wings are even. If one wing sits at a bigger angle than the other, it makes more lift than the other. One side rises, the other does not, and the plane rolls into a turn. Not a fold problem exactly — an angle problem you built with a fold.

It flies better when you throw it flatter. A steep throw does not just aim the plane upward. It swings the whole plane so the air arrives from further underneath, which is a bigger angle of attack than you meant to ask for.

The part that is not obvious

Bigger angle, more air turned, more lift. So the biggest angle should give the most lift.

It does not, and the way it fails is the surprising bit.

Air will bend round a corner, but only so far. Past a certain angle — somewhere near fifteen degrees for a flat wing — the air can no longer stay stuck to the top surface. It breaks away, tumbles into loose swirls, and the smooth downward turning stops happening.

That is a stall. It is not the plane running out of speed. It is the plane running out of angle.

And it does not fade in gently. Lift climbs, climbs, climbs, and then drops away almost at once. That is exactly what the last article called stalls and flutters: the nose pitches up, the angle passes the limit, the lift vanishes, the nose drops, the air reattaches, and it does the whole thing again a metre further on.

What to change next

Knowing the mechanism turns fiddling into aiming. Change one thing and predict what it will do before you throw.

• Wider wings, same plane. More wing meeting the air means more air turned at the same angle, so it can fly at a gentler tilt. Gentle tilts are further from the stall, so it should be steadier.
• A paperclip on the nose. Heavier at the front means the nose sits lower, which lowers the angle of attack. If your plane keeps stalling, this often cures it outright.
• Trailing edges bent up a lot. Push it too far and you will make it stall on purpose, every single throw. Worth doing once, just to watch the flutter arrive on demand.

A paper dart is a flat sheet with no engine, and it stays up because it is tilted just enough and not one bit more. Every crease you were told to make even was really a crease that sets an angle.