The properly executed turn

Unlike a car, an aircraft cannot be turned satisfactorily by means of the yaw control alone. This is because there is no road to provide a reaction to produce the cornering forces. In an aircraft, the cornering (centripetal) force must be provided by aerodynamic means. When the rudder is deflected so as to yaw the aircraft, the force that it produces is actually outwards; the opposite direction to that required.

As illustrated in Fig. 10.12, to execute a level turn properly, the aircraft must be banked, and the lift increased so that the horizontal component of lift is exactly the right size to provide the centripetal force required for the turn, and the vertical component exactly balances the weight. Normally a certain amount of rudder control is necessary in order to keep the aircraft pointing in the intended direction. Excessive use of the rudder, however, produces a skidding turn, with an uncomfortable sideways acceleration, and a potentially danger­ous sideslip.

vertical component of lift

Horizontal component of lift

Weight

Fig. 10.12 Turning flight

For a correctly banked turn, the lift force must be increased so that its vertical component exactly balances the weight. The horizontal component can then provide the required centripetal acceleration

The precise coupling between roll and yaw varies from one aircraft design to another. In general, a combination of aileron and rudder movement is required, but most aircraft can be turned smoothly using ailerons alone. Some early Farman aircraft had no rudder at all. The balance between rudder and aileron control also depends on whether the aircraft is climbing, descending, or flying level. A more detailed description will be found in practical flying manuals such as Birch and Bramson (1981).

Note, that once a properly executed turn has been initiated, the control stick or handlebars are returned to somewhere near the neutral or mid-position, and the aircraft keeps turning. Holding the stick over would cause the aircraft to continue rolling. This is quite different from steering a car, where the steering wheel must be held in the turned position.

One very special case where flat turns were necessary was in the man – powered Gossamer Albatross shown in Fig. 10.13. Because of its exceptionally low power, this aircraft required a high-aspect-ratio wing with a span similar to that of a large airliner, and could only fly close to the ground. In a banked turn the wing tip would be likely to hit the ground. The aircraft was therefore turned by means of the canard foreplane, which could be canted over so as to

produce a sideforce component to pull the nose round. Note that no fin or rud­der was provided.