Since aircraft have to operate from the surface, land or water (or in special situations on snow), in some form their handling on the surface has to be incorporated into the design; wheels on land, floating on water and skidding on snow. There are many operational and design considerations. The mechanical arrangements are a subsystem known as the undercarriage (or the landing gear). Once airborne, the undercarriage becomes an appendage and needs to be dealt with in a manner so as to minimise the penalty associated with it; one way is to retract it to reduce drag and nose‐down moment. This is also true for birds as they reposition their legs in flight; some even retract them.
Chapter 8 illustrates how to progress with the concept definition to arrive at a preliminary aircraft configuration of a new aircraft project starting from scratch, to arrive at a tentative aircraft configuration without an undercarriage placed. To progress further, the next task is to lay out the undercarriage position relative to an estimated aircraft centre of gravity (CG) location. Some iteration of wing repositioning may be required to suit the CG location with adequate fuselage clearance angle, γ, and the tipping back angle, β (Figure 5.6). This complete aircraft configuration with undercarriage is still tentative and may have to be iterated again to a more appropriate configuration when the aircraft CG location is accurately estimated in Chapter 10.
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