Undercarriage Layout Methodology
After obtaining the necessary information available on the undercarriage as a system, the next step is to systematically lay down the methodology to configure the undercarriage arrangement in order to integrate it with the aircraft conceived in Chapter 6. All aircraft designers benefit from existing designs by having guidelines – this is what is meant by “experience”: past designs provide a good databank.
First, the undercarriage commonality for variant designs is considered. In general, in civil aircraft design, the baseline aircraft is the middle of the three main sizes (other variants are possible). Therefore, the largest version is more critical to the undercarriage layout for carrying the heaviest load. The methodology described herein should be applied to the largest of the variants and then all other variants should be checked for commonality. At the conceptual design stage, all versions have an identical undercarriage layout except for the wheel base and wheel track. A production version has the scope to shave off metal, making it lighter for smaller variants; this requires only minor changes in the manufacturing setup. Following is the stepwise approach for the undercarriage layout geometry, load estimation, and tire sizing:
1. Determine the type of undercarriage: nose wheel or tail wheel. For the reason explained in Section 7.3, it is a nose wheel type. Low-speed smaller aircraft sensitive to weight and drag could have a tail wheel type.
2. If the aircraft operating speed exceeds 150 knots, the undercarriage should be retractable.
3. Estimate from statistics and experience the CG position; it is suggested 20 to 40% of the wing MAC at about the fuselage centerline, depending on the wing position. Ensure that the CG angle в with the vertical is about 15 deg.
4. The main-wheel strut length should allow full rotation that clears the fuselage aft end, solves oleo-collapse problems, allows for full flap-deflection clearance, and makes a trade-off with the wheel track to prevent the aircraft from turning over. The fuselage clearance angle y is between 12 and 16 deg, which should clear the fuselage aft end at the rotation when the oleo is fully extended at liftoff.
5. The nose wheel strut length should be relative to the main-wheel strut length in order to keep civil aircraft payload floorboards level (for a propeller-driven aircraft, ensure that there is propeller clearance at the nose-oleo collapse). Avoid making the strut length too long. The nose wheel attachment point ideally should be located ahead of the cockpit.
6. Determine that the wheel-track turn-over angle в is less than the recommended value and satisfies the desired turn radius. A trade-off may be required but, in general, runway widths are wide enough. A wider wheel track is better but it should not be too wide, which would create turning and structural problems on the wing attachment.
7. Determine the stowage space; less articulation is simpler and requires less maintenance and lower cost. In general, civil aircraft articulation is simpler than the military aircraft type. Ensure that the storage space has adequate tire clearance. At this point, it is assumed that stowage space is available.
8. Compute the loads on each point of support and determine the number of wheels (see Section 7.9). Establish the operational airfield LCN. If an aircraft is to use a Type 1 airfield, it is better for the wheel load to be less than 10,000 lb; then, the LCN also will be low. Use Table 7.6 to determine tire pressure; there are options: the higher the pressure for the LCN, the smaller is the tire size.
9. Use tire manufacturers’ catalogs to select a tire (see Appendix E and www. airmichelin. com and www. goodyearaviation. com).
Two worked-out examples follow. As discussed previously, the civil and military aircraft design methodologies are similar but differ considerably in their operational mission profile. There are various levels of options available to maintain component commonality, including the following:
1. Low-Cost Option. Maintain the same undercarriage for all variants even when there are performance penalties. In this situation, design the undercarriage for the biggest aircraft and then use it for other variants. The biggest aircraft may have tighter design criteria to sacrifice some margin in order to benefit smaller designs.
2. Medium-Cost Option. In this situation, design the undercarriage for the biggest aircraft and then make minor modifications to suit the smaller variants and to retrieve some of the performance loss associated with the low-cost option. These modifications maintain the external geometry but shave off metal to lighten the structure, to the extent possible. The smallest aircraft may require a shortening of the strut length without affecting the shock-absorber geometry, which may require a spring change. Wheel, brake, and tire size are kept the same. The smallest variant is nearly half the weight of the largest but may be unchanged; it may be possible to change a dual wheel to a single wheel.
3. High-Cost Option. Make major modifications to the undercarriage. In this situation, design three strut lengths for each variant, maintaining the maximum manufacturing-process commonality – this would reduce costs when NC machines are used. Maintain the other items with the maximum component commonality. The spring of the smallest variant may change without affecting the external geometry. Performance gain could be maximized by this option.
I ІІ-геаГ 25-26 ft (7.7 m)
In all cases, the nose wheel attachment point remains unchanged but the wheel base and wheel track change with new attachment points for the main wheels on the wing
The industry conducts a trade-off study to examine which options offer the maximum cost benefit to operators. This book uses the second option – that is, start with the biggest variant shown in Figure 7.9. Iteration is likely to occur after accurate sizing (see Chapter 11).