Undercarriage Layout, Nomenclature, and Definitions

The position of the aircraft CG is a most important consideration when laying out wheel locations relative to an aircraft. Basically, the undercarriage consists of wheels on struts attached to aircraft points. The geometric parameters in placing wheels relative to the aircraft CG position are shown in Figure 7.3, along with the basic nomenclature of related parameters. The geometric definitions are as follows:

Wheel Base: The distance between the front and rear wheel axles in the vertical plane of symmetry

Wheel Tread or Wheel Track: The distance between the main wheels in the lat­eral plane of the aircraft

The wheel base and wheel track determine the aircraft turning radius (see Sec­tion 7.7) on the ground. The forwardmost aircraft CG position relative to the wheel base and wheel track determines the aircraft over-turn characteristics. The over­turn angle, в, is the maximum angle for a tilted aircraft with the CG on top of a main wheel; beyond that, the aircraft would turn over on its side. Determination of the angle в is shown in Figure 7.3. Over-turn tipping is not exactly around the X-axis (i. e., sideways) when a low-wing aircraft could have a wing tip touching the ground before в is reached. The tipping occurs about the axis joining the nose-wheel and main-wheel ground contact point, when the wing LE is likely to hit the ground.

Figure 7.3. Aircraft CG position rela­tive to the undercarriage layout

It is better to maintain a lower angle в to avoid an aircraft turning over; the value depends on the airfield surface, and the tendency increases with higher sideways ground friction. For simplification yet still representative, typical values used in this book follow (see the references for more details). For a paved runway, keep the angle в less than 60 deg; for an unprepared field, it should be less than 50 deg. There are aircraft with в = 35. Most of the aircraft have a в between 40 and 50 deg.

An aircraft also can tip backwards if its rearmost CG goes behind the main wheel of a tricycle-type undercarriage; it can tip forward if its CG is in front of the main wheels of a tail-wheeled aircraft (Figure 7.4).

Definitions of the related parameters concerning wheel and strut provided in Figure 7.5 are more pertinent to the nose wheel ahead of the aircraft CG. These are not critical items at the conceptual design phase and can be omitted from the coursework. In the industry, these parameters are considered at an early stage.

1. Caster or Rake Angle. Angle between the spindle axis and the vertical line from the ground contact point of the swivel axis.

2. Caster Length. Perpendicular distance from wheel contact point to ground and spindle axis.

3. Trail. Distance from wheel contact point to ground and spindle-axis contact point to ground.

4. Offset. Perpendicular distance from wheel axis and spindle axis.

5. Loaded Radius. Distance from wheel axis to ground contact point under static loading.

6. Rolling Radius. Distance from wheel axis to ground contact point under dynamic loading.

Wheel alignment and wheel camber (i. e., the tilt from being vertical) are impor­tant issues for wheel positioning, which can be omitted from the coursework prelim­inary aircraft layout.