Figure 7.8. Three possible wheel positions

the aircraft engineers. See the references for more details on undercarriage retraction kinematics.

7.5.1 Stowage Space Clearances

A tire expands as the fabric stretches during service. It also expands on account of heat generated during ground operations. It keeps spinning (further enlargement occurs due to centrifugal force of spinning) within the stowage space immediately after retraction. Stowage space within an aircraft should be of the minimum volume occupied by the retracted undercarriage with some clearance to avoid any interference that may occur. Enough cavity space should be inside the aircraft structure to accommodate tire expansions. Stowage space is dictated by the articulated mechanism for retraction from its unloaded free position. Semi-empirical relations govern the clearance gap to accommodate retraction. As mentioned previously, this book assumes that aircraft designers are in a position to offer proper stowage space with adequate clearances. This book does not discuss stowage-space computation. For thin-wing combat aircraft, stowage must be within the tightly packed fuselage, where space is limited.

Unless there is a breakthrough innovation (typically associated with unconventional new designs beyond the scope of this book) on retraction kinematics, the state-of-the-art undercarriage design has been established to maximize compactness. This book addresses articulation in its simplest form. The author recommends using CAD animation to check retraction kinematics and storage space during the second-term coursework.

7.6 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.

Figure 7.9. Positioning of main wheels and strut length

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 γ , 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 β is the angle between the vertical and the line joining the wheel contact point with the ground and the aircraft CG. Ensure that β is greater than γ ; otherwise, the CG position will go behind the wheel contact point. Keep β greater than or equal to 15 deg. The fuselage clearance angle, γ , 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.

Figure 7.10. Aircraft turn

7.7 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 θ is breeched.