Undercarriage Design Drivers and Considerations

There are three wheel positions, as shown in Figure 7.8. The application logic for the various types of aircraft is the same. The three positions are as follows:

1. Normal Position. This is when the aircraft is on the ground and the under­carriage carries the aircraft weight with tires deflected and the spring com­pressed.

2. Free Position. When an aircraft is airborne, the undercarriage spring is then relieved of aircraft weight and extends to its free position at its maximum length. Stowage space is based on the undercarriage in a free but articulated position.

3. Failed/Collapsed Position. This is the abnormal case when the spring/oleo col­lapsed as a result of structural failure, as well as tires deflated with loss of air pressure. This is the minimum undercarriage length.

The failed position of the aircraft on the ground is the most critical design driver in determining the normal length of the undercarriage strut. Following are design considerations for the failed positions:

1. Nose Wheel Failed. The nose will drop down and the length of the collapsed nose wheel should still prevent the propeller from hitting the ground with adequate clearance.

2. Main Wheel Failed. There are two scenarios:

(a) When one side fails, the wing tilts to one side and it must not touch the ground.

(b) If both sides collapse (the most critical situation is when the aircraft rotates for liftoff at the end of the takeoff ground run), it must be ensured that the fully extended flap trailing edges have adequate ground clearance.

Figure 7.9 depicts an important design consideration for fuselage clearance angle y , at aircraft rotation for liftoff, when the CG should not go behind the wheel contact point. Both civil and military aircraft types are shown in the figure. The angle в is the angle between the vertical and the line joining the wheel contact point with the ground and the aircraft CG. Ensure that в is greater than y; otherwise, the CG position will go behind the wheel contact point. Keep в greater than or equal to 15 deg. The fuselage clearance angle, y, must be between 12 and 16 deg to reach CLmax at aircraft rotation. The fuselage upsweep angle for clearance is discussed in Section 4.7.3 and it is revised here after the undercarriage layout is completed. Figure 7.9 corresponds to the worked-out examples.