With tricycle undercarriage systems, the small nose or tail wheel is generally steerable and also provided with modest self-centering by using an arrangement with wheel caster, also seen in
Figure 7.3 Nose wheel and strut showing suspension elements, main bearings, control servo, and caster.
Figure 7.4 Tail wheel showing suspension spring.
Figure 7.5 Nose wheel mechanism with combined spring-coupled steering and vertical suspension spring.
Figure 7.3. This is typically connected to the steering servo via stiff coupling springs rather than the solid linkages used elsewhere, because wheels are subject to many impact loads during ground roll and it is desirable to protect the servo mechanism from such loads. Figure 7.5 shows the internal workings of a simple spring-coupled nose wheel steering system which also has a vertical suspension spring. When dealing with nose wheels, it is not uncommon to find the wheel strut to be quite long so as to provide the correct attitude during takeoff; this inevitably means the strut is subject to quite large bending loads and so it and its bearings in the fuselage must be sized accordingly. On the other hand, it is wise to avoid overly stiff nose wheel struts because during heavy landings it can be desirable for the strut to bend or even collapse if this reduces damage to the main fuselage or limits deceleration in any sensitive on-board systems such as cameras or other sensors.
As already mentioned earlier and in the previous chapter on wings, it is sometimes desirable to reduce drag by retracting the undercarriage, particularly the main wheels. We rarely do this
Figure 7.6 UAV with a pneumatic, fully retractable undercarriage system. Note also the nose camera that has been added to the aircraft shown in the image with undercarriage retracted.
Figure 7.7 Details of fully retractable undercarriage system.
because of concerns of cost, weight, and particularly reliability. If part of the undercarriage fails to deploy for landing, it is almost inevitable that serious airframe damage will result. However, Figure 7.6 shows one UAV that we built with fully retractable undercarriage, while Figure 7.7 shows the details of its wing-mounted gear. Significant reduction in drag can be achieved by such means, which can more than compensate for the increased aircraft weight. Such systems are typically pneumatically powered or use electric motors.