Stand at the end of a runway and watch an airliner come in to land, and you will see its wings rise gently from the fuselage into a shallow V. Now watch a US Marine Corps Harrier hover over the same runway. Its wings do the opposite: they droop from the top of the fuselage towards the ground, like a bird about to fold them away.

Neither shape is styling. The upward tilt is called dihedral, the downward tilt anhedral, and both are a designer’s way of tuning one of the subtlest qualities an aircraft has: how it behaves when a gust tips one wing down and nobody touches the controls.

Getting it right makes an aircraft pleasant and safe to fly. Getting it wrong produces either a machine that slowly slides into a spiral dive, or one that wags its tail through the sky in a Dutch roll. Here is how dihedral and anhedral work, and why some of the most famous aircraft ever built wear their wings upside down.

The V you see from the front

Dihedral is the upward angle of the wings, or the horizontal tail, from the horizontal when an aircraft is seen head-on. Anhedral is simply negative dihedral: a downward angle. Wings with more than one bend along their span are called polyhedral, and a wing that bends sharply near the root is a gull wing.

The idea is older than powered flight. Sir George Cayley, the English pioneer who identified the four forces acting on an aircraft, recognised the importance of the dihedral angle for lateral stability. His landmark treatise On Aerial Navigation appeared in 1809 and 1810, almost a century before the Wright brothers flew.

The US Federal Aviation Administration uses a rear view of a Boeing 737 to illustrate the term in its Pilot’s Handbook of Aeronautical Knowledge, the textbook every American student pilot learns from. Look at almost any low-wing airliner on approach and the V is obvious.

How a tilted wing levels itself

Here is the clever part. When a gust rolls an aircraft, it does not just tilt. It starts to slide sideways towards the lower wing, so the air now arrives slightly from the side. That sideways slide is called sideslip.

On a wing with dihedral, sideslip raises the angle of attack of the wing slipping into the wind, which is the lower one, and reduces it on the raised wing. The lower wing makes more lift, the higher wing less, and the difference produces a rolling moment that pushes the wings back towards level. Engineers call this the dihedral effect: roll produced by sideslip.

Real Engineering explains why wing dihedral makes aircraft stable, and how sideslip turns into a restoring roll.

It is not a magic self-levelling device. The fin is busy at the same time, turning the nose into the sideways airflow like a weathervane and reducing the very sideslip that dihedral needs to work. The balance between the two decides whether an aircraft drifts gently back to wings-level, or slowly tightens into a spiral.

Wing position and sweep do the same job

Dihedral angle is only one ingredient. Anything that changes the rolling moment in a sideslip counts, and two of the biggest are where the wing is attached and how far it is swept back. The FAA’s rule of thumb: every 10 degrees of sweepback gives about one degree of effective dihedral, and mounting the wing high on the fuselage is worth about five degrees compared with mounting it low.

That is why high-wing and low-wing aircraft look so different from the front. A high-wing aircraft hangs its fuselage beneath the wing, and the FAA describes the fuselage acting like a keel, with its weight swinging back like a pendulum when a wing dips. It already has plenty of built-in roll stability, so it needs less dihedral angle. A low-wing aircraft lacks that help and makes up for it with a more visible V.

Now combine both effects, a high wing and a swept wing, on one aircraft. The result can be too much stability: a machine that resists rolling so strongly that it becomes sluggish and prone to oscillation. The cure is to tilt the wings down.

Why the giants droop their wings

Look at the Lockheed C-5 Galaxy oder die Antonov An-124 Ruslan from the front and the wings visibly sag from the top of the fuselage. Both mount a swept wing high above their centre of gravity, which gives them so much dihedral effect that anhedral is added to cancel some of it, so the aircraft can be manoeuvred more easily.

The Harrier follows the same pattern. Its shoulder-mounted swept wing has prominent anhedral, and the family carries anhedral on the tailplane too, a feature introduced on the fifth Hawker P.1127 prototype and kept for the production Harrier.

Fighters lean the same way for a different reason. Combat aircraft often have near-zero dihedral or outright anhedral because less dihedral effect means less roll stability, and less roll stability means a jet that rolls more eagerly. Many airliners show a modest V instead: a stable spiral mode matters more for a passenger jet than a fast roll rate.

The Starfighter and the Phantom: two famous fixes

Der Lockheed F-104 Starfighter is the textbook case. Its vertical fin was only slightly shorter than each of its tiny wings and almost as aerodynamically effective, so it behaved like a wing standing on end. A rudder input could roll the aircraft in the opposite direction. Kelly Johnson’s team answered by canting the wings down at 10 degrees of anhedral.

Der McDonnell Douglas F-4 Phantom II wears both shapes at once. Wind tunnel tests showed it needed about five degrees of dihedral, but redesigning the titanium centre section was out of the question. McDonnell’s engineers angled up only the outer wing panels, by 12 degrees, which averaged out to the required five across the span. The all-moving tailplane, meanwhile, was given 23 degrees of anhedral to improve control at high angles of attack and keep it clear of the engine exhaust.

Sometimes the bend has nothing to do with stability at all. The Vought F4U Corsair got its inverted gull wing because a straight wing would have needed impossibly long landing gear legs to keep its huge propeller off the deck. Bending the inner wing down shortened the struts considerably.

Flight-club’s short explainer on what dihedral is and what it does for an aircraft.

Too much, too little

Dihedral is a balancing act. Too little dihedral effect against a strong fin, and the aircraft has spiral instability: after a disturbance the bank slowly steepens and the nose drops into a tightening spiral. The divergence is usually gradual enough for a pilot to catch, but it is the reason no aircraft can be flown hands-off forever.

Too much dihedral effect against a weak fin, and the aircraft Dutch rolls, its nose tracing a figure of eight on the horizon as roll and yaw chase each other. Swept-wing jets are especially prone to it, which is why airliners carry yaw dampers. Designers generally prefer to accept a mild spiral tendency rather than live with the wobble.

The Wired Flyer walks through the four sources of lateral stability: dihedral, keel effect, sweepback and weight distribution.

Even a docile jet trainer such as the Aero L-39 Albatros shows the principle: its low-set, straight wing rises gently from the roots, giving a stable, forgiving platform for students. So next time you are at an airport or an airshow, look at the wings head-on. Up or down, every one of those angles is an engineer’s answer to the same question: how much should this aircraft want to roll?

Sources: FAA Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25, ch. 5), NASA SP-367 via US Centennial of Flight Commission, This Day in Aviation, Warbirds Resource Group, Wikipedia (Dihedral (aeronautics); George Cayley; Lockheed F-104 Starfighter; McDonnell Douglas F-4 Phantom II; McDonnell Douglas AV-8B Harrier II; Hawker Siddeley P.1127; Vought F4U Corsair)