Military Aircraft: AJT

The worked-out example of an AJT aircraft, as developed in Chapter 6 (see Fig­ure 6.14), continues with sizing of the undercarriage and tires. The combat air­craft tire sizing for the variant designs is considerably simpler because the affecting geometries are not altered, only the weight changes. This military aircraft exam­ple has only two variants, the AJT and CAS versions. The CAS role of the variant aircraft has the same geometric size but is heavier.

Because the affecting geometries (i. e., the wheel base and wheel track) do not change, the cost option logic is less stringent for maintaining undercarriage and tire component commonality – especially for the worked-out example. In this case, it is designed for the heaviest variant with shaved-off metal for the baseline trainer version. Therefore, only the CAS version design is discussed; it is assumed that the AJT baseline will have lighter struts but the same undercarriage and tires.

The reference lines are constructed using Figure 7.17. The aircraft centerline is taken conveniently through the center of the engine exhaust duct. Although at this stage, when the exact CG is not known and placement of the undercarriage is based on a designer’s experience, the example of the AJT given here has been sized to avoid repetition. The exercise for readers is to start with their own layout and then iterate to size.

Even as a tandem seat arrangement, the aircraft CG travel in this aircraft class would be less than that of the civil aircraft example – for example, from 20 to 35% (aftmost) of the wing MAC. The CAS version of the CG variation is from 25 to 38% (fully loaded).

The nose wheel load is based on the forwardmost CG position. An armament payload is placed around the aircraft CG, and the CAS aircraft CG movement is insignificant between a clean configuration and a loaded configuration. Following is the relevant information for the two variants. Only the heaviest aircraft needs to be considered in this case, as explained previously.

The most critical situation for both the main wheel and the nose wheel and the tire is the CAS version with a fully loaded MTOM equal to 9,000 kg. The aftmost CG position from the reference plane is 6.8 m (22.32 ft). The AJT has an MTOM equal to 6,500 kg.

The following information is used to determine wheel loading, LCN, and tire sizing (use Figure 7.17 to compute wheel loading):

• Place the main-wheel ground contact point at 45% of the MAC. The aftmost CG position is considered first, placed at 35% (40% is a limiting situation) of the wing MAC. It will be revised when the CG is known by actual compu­tation.

• The fuselage clearance angle, y > 15 deg, is adequate for both variants.

• Then, the most critical CG angle в = tan-1(0.874/1.9); i. e., в = 24.7 deg.

• The main-wheel load is computed at the aftmost CG, which gives IReaR =

6.8 – 2.2 = 4.6 m (15.1 ft).

• Equation 7.2 gives Rmain = (Irear x MTOW)/Ibase

• Or Rmain = (4.6 x 9,000)/5.33 = 7,767 kg (17,127 lb).

The load per strut is 3,883.5 kg (8,563 lb). It is better to keep the wheel load below 10,000 lb in order to have a smaller wheel and tire. Appendix E provides the tire data from which to choose; there are many options.

The typical airfield LCN for this class of CAS aircraft is low. From Figure 7.13, the LCN is below 15 for the CAS variant and the AJT is still lower. This means there is good flotation and the aircraft can operate from semiprepared airfields. Several options are listed in Table 7.6. From the tire catalogs (see Appendix E), a suitable match is the New Tire Type (Type VII; although the equivalent inch code is available, the metric code is used to familiarize readers) with a designation of 450 x 190 – 5 (22-ply) and an inflation pressure of 15.5 bar (225 psi) that takes 4,030 kg (8,886 lb). The maximum speed capability is 190 mph (165 knots), which is a sufficient margin because this aircraft class does not exceed 130 knots during an approach. The AJT is much lighter with a MTOM of 6,500 kg and, therefore, the same tire at a reduced pressure can be used (or a suitable smaller tire can be used, if changing the hub is required).

Nose wheel and tire sizing are based on the forwardmost CG position; there­fore, the aft CG nose-wheel load is not computed. The nose-wheel load at the forward CG at 6.07 m (19.92 ft) from the zero reference plane gives IFORwARd = 7.53 – 6.07 = 1.46 m (4.8 ft).

Equation 7.3 gives Rnose = (Iforward x MTOW)/Ibase, or Rnose = (1.46 x 9,000)/5.33 = 2,465.3 kg (5,436 lb). A single nose wheel of a smaller size is chosen.

From the tire catalogs (see Appendix E), a suitable match is the New Tire Type (Type VII, inch code) with a designation of 17.5 x 4.4 – 8 (14-ply) and an infla­tion pressure of 220 psi (14.47 bar) that takes 6,000 lb (2,721 lb). The maximum speed capability is 210 mph (182 knots). The AJT is much lighter with the MTOM of 6,500 kg; therefore, the same tire with a reduced pressure can be used.

Deflection is estimated as in the civil aircraft case and therefore is not shown here. The high-wing configuration also shows sufficient clearance. The author sug­gests that readers undertake the computation.

Airplane	MTOM (lb)	Wheel (per/strut)	Type	Tire size	Tire pressure (psi)	Turn radius (ft)
Cessna 152	2,500	1	S	6.00-6
Beech 58	5,500	1	S	6.50-8	56
Beech 200	12,600	2	T	18 x 5.5	105
Learjet45	22,000	2	T	22 x 5.75-8	200
ATR42	41,000	2	T	32 x 8.8R16	126	57
CL600	48,300	2	T	H27 x 8.5-14	175	40
CR200	53,000	2	T	H29 x 9.0-15	162	75
BD700	95,000	2	T	H38 x 12.0-19	200	68
B737-700	140,000	2	T	H40 x 14.5-19	200	68
Airbus 320	170,000	2	T	49 x 19-20		75
B727-200	173,000	2	TT	49 x 17	168
B707-720	336,000	4	TT	46 x 16	180
DC8-63	358,000	4	TT	44 x 16	200
L1011	409,000	4	TT	50 x 20	175
B747B	775,000	4	DTT	46 x 16	210	159
C130A	124,000	2	ST	56 x 20	65	85
C17	586,000	3	TTT	50 x 21-20	138	90
Hawk	20,000	1	S	650-10	143
F14	74,300	1	S	37 x 11	245

Notes: Abbreviations: S – Single, T – tandem, ST – single tandem, TT – twin tandem, DTT – double twin, TTT – triple twin tandem