Why Do Airplanes Not Turn Like Cars?

Contrary to what some may think, it is possible for an airplane to turn without banking. By using the rudder (which controls yaw) and careful use of ailerons (which control roll), the airplane can change its direction in a semi-similar way to that of a car. However, it has some notable disadvantages:

•The overuse of control surfaces creates drag, slowing the airplane.

•Air hits the airplane at strange angles, causing excessive noise in the cabin.

•Passengers can very easily get airsick.

•You look like a dope.


But most of all, it is very difficult to generate enough force to actually cause an airplane to turn based on yaw alone; the airplane, like any other vehicle, turns much more efficiently when banked.

Banked Turns

Imagine the values one would use to solve a banked turn problem involving a car rounding a corner: the mass of the car, g, the angle of the incline, the force due to gravity, and the normal force.

In an airplane, some of this changes: while the mass and g still form the force due to gravity (weight), the angle of incline can be any value, and the normal force does not exist. It is instead replaced by the force of lift, which acts perpendicular to the wings and is separable into vertical and horizontal components:

Diagram of forces during a turn

Forces in turns with lift vectors broken into horizontal and vertical components
(Photo: Avstop: Turns During Flight)

Since simply banking the airplane would do nothing but reduce the vertical component of lift (and cause the airplane to do the equivalent of "sliding down the ramp" in a frictionless banked turn problem), the pilot uses the elevator (or "pulls up") to create an increased force in the direction of lift, increasing the centripetal acceleration of the airplane.Since F=ma and the mass of the airplane cannot change, this means that the load factor (acceleration in units of g) on the airplane increases. Because more of the lifting force is directed in the horizontal direction as the angle of bank increases, an increasing load factor is required to maintain altitude. In fact, this load factor increases exponentially, increasing to infinity at 90°:

Increasing Bank Causes Increasing Load Factor

(Source: Ascent Ground School)