Turning of an Aircraft
Aircraft designers must ensure that an aircraft can turn in the specified radius within the runway width (Figure 7.10). Turning is achieved by steering the nose wheel (i. e., the maximum nose wheel turn is « 78 deg) activated by the pilot’s foot pedal. There is a slip angle and the effective turn would be approximately 75 deg. Pressing the left pedal would steer the nose wheel to the left and vice versa. The tightest turn is achieved when asymmetric braking and thrust (for a multiengine aircraft) are applied. The braked wheel remains nearly stationary. The center of the turn is slightly away from the braked wheel (see Figure 7.10) and the steered nose wheel guides the turn. The radius of the turn is the distance between the nose wheel and the center of the turn. Checks must be made to verify that the aircraft nose, outer wing tip, and outer H-tail tip are cleared from any obstruction. If the inner wheel were not braked, the turning radius would be higher. Turning is associated with the centrifugal force at the CG and side force at the turning wheels.
A tail wheel aircraft turning poses a special problem for “ground looping,” particularly when the aircraft is still at speed after landing. If the tail of the aircraft swings out more than necessary in an attempt to keep the aircraft straight using pedal-induced turns, then the centrifugal force of the turn could throw the aircraft rear end outward to the point where the forward-momentum component could move outside the wheel track. This results in instability with an uncontrolled ground loop, which can tilt the aircraft to the point of tipping if the over-turn angle в is breeched.
Figure 7.11. Wheel arrangements
As an aircraft weight increases, the runway must bear the reaction and retain integrity to keep the vehicle’s field performance safe. Heavy commercial transport aircraft are intended to operate from a prepared runway (i. e., Types 2 and 3; see Section 7.10) to stay within the pavement strength; the load per wheel is restricted by distributing the total over several wheels. Various arrangements for more than one wheel per strut style are shown in Figure 7.11. Aircraft and undercarriage designers must plan for the number of struts, number of wheels per strut, and tire spacing and pressure (which determine the size) to distribute the load. As the aircraft MTOM increases, so does the number of wheels required, as well as considerations for stowing and articulation for retraction.
The fundamental wheel arrangements are single, twin, triple, and quadruple on a bogey. Wheel arrangements higher than a quadruple are not seen. The next level is their placement in a dual row as a single tandem, twin tandem (i. e., four wheels), triple tandem (i. e., six wheels), and so forth. The A380 wheel-arrangement model is shown in Figure 7.11. Figure 7.1 shows the wheel bogey of the world’s largest aircraft (i. e., the Antanov 225) with twin wheels per strut, for a total of seven struts.