Wings angled upward keep an aircraft level by themselves

Dihedral, and how a shape does the job of a control system.

Tilting an aircraft's wings upward from the fuselage in a shallow V, a shape called dihedral, makes the lower wing generate more lift than the upper one the instant the aircraft rolls off level, and that lift imbalance rolls it straight back without any control input at all.

What is actually happening

An aircraft that rolls slightly, dropping one wingtip, does not simply stay banked, it also begins to slide sideways through the air toward the low wing, a motion called sideslip, because the tilted lift is no longer pointing straight up and gravity pulls the aircraft toward whichever side is lower. That sideways-moving air does not arrive at the two wings identically. Because each wing panel is already tilted upward from the fuselage, the low wing's surface is angled more directly into the sideways component of the airflow, meeting it at a greater effective angle, while the high wing is tilted away from that same sideways flow and meets it at a smaller one. A greater angle to the airflow produces more lift, so the low wing, already at a disadvantage from being banked, actually generates more lift than the high wing during the sideslip, and that extra lift rolls the aircraft back toward level. The correction happens automatically, driven entirely by the geometry of the wings, with no sensor, no linkage and no pilot required to notice the disturbance and respond to it.

Where you have already felt this

Anyone who has stood with arms stretched out to the sides, tilted slightly upward like the wings of a paper dart, and leaned their shoulders over to one side has felt the geometry involved, even without any real wind to push against. Leaning to the left rotates the left arm so its flat underside faces more directly toward whatever is approaching from below and to the left, while the right arm rotates the opposite way, tilting its underside away from that same direction. A real wing undergoes the same rotation relative to the oncoming sideways air during a sideslip, and the extra lift that results from the lower wing facing more squarely into the airflow is precisely what a dihedral wing is shaped to produce, at a scale where the effect is strong enough to move the whole aircraft rather than just a pair of arms.

What follows from this

Because the correction comes from the shape of the wings rather than from anything the pilot or an onboard system does, dihedral works even on the simplest hand-launched or free-flight models that carry no radio equipment at all, which is why so many of them are built with visibly upturned wingtips. It also explains why the same aircraft with its wings mounted flat, with no dihedral angle at all, tends to hold whatever bank it is nudged into rather than rolling itself back level, since flat wings meet the sideways airflow from a sideslip at the same angle on both sides and produce no imbalance to correct anything. Too much dihedral brings its own problem, since an aircraft that corrects a roll too aggressively can start rolling back and forth in a slow, uncomfortable oscillation instead of settling quietly to level, so the angle chosen in a working design is usually a deliberate compromise between an aircraft with no self-correction at all and one that overreacts to every small disturbance.

Where this stops being true

Not every stable aircraft relies on dihedral to get this effect, and not every high-winged aircraft needs much of it. A wing mounted on top of the fuselage already hangs the bulk of the aircraft's weight below the wing itself, and that weight distribution provides some self-levelling tendency of its own, driven by gravity acting on the fuselage rather than by airflow acting on the wings, so a high-wing design often needs only a shallow dihedral angle to fly steadily. A low-wing aircraft has none of that advantage, since its fuselage weight sits above the wing rather than below it, and typically needs a noticeably greater dihedral angle to achieve the same self-correcting behaviour from geometry alone. Sweeping the wings back can produce a similar righting effect through a related mechanism even with the wings mounted flat, which is one reason a swept-wing aircraft can sometimes get away with less dihedral, or none at all, and still recover from a roll on its own.