Wing Layout

Having decided on initial estimates for wing area, sweep, aspect ratio, and taper, an initial specification of the wing planform is possible. Three additional considerations are important:

High and Low Wings
High wing aircraft have the following advantages: The gear may be quite short without engine clearance problems. This lowers the floor and simplifies loading, especially important for small aircraft or cargo aircraft that must operate without jet-ways. High wing designs may also be appropriate for STOL aircraft that make use of favorable engine-flap interactions and for aircraft with struts. Low wing aircraft are usually favored for passenger aircraft based on considerations of ditching (water landing) safety, reduced interference of the wing carry-through structure with the cabin, and convenient landing gear attachment.

Wing Location on the Fuselage
The wing position on the fuselage is set by stability and control considerations and requires a detailed weight breakdown and c.g. estimation. At the early stages of the design process one may locate the aerodynamic of the wing at the center of constant section or, for aircraft with aft-fuselage-mounted engines, at 60% of constant section. (As a first estimate, one may take the aerodynamic center to be at the quarter chord of the wing at the location for which the local chord is equal to the mean aerodynamic chord.)

For low-wing aircraft, the main landing gear is generally attached to the wing structure. This is done to provide a sufficiently large wheel track. The lateral position of the landing gear is determined based on roll-over requirements: one must be able to withstand certain lateral accelerations without falling over.

The detailed computation requires knowledge of landing gear length, fuselage mass distribution, and ground maneuver requirements. For our purposes, it is sufficient to assume that the main gear wheel track is about 1.6 fuselage diameters. For general aviation aircraft or commuters with gear attached to turbo-prop nacelles, the value is usually much larger.

Airplane	ytrack / fuse dia. (approx)
737-200	1.39
747-200	1.67
757-200	1.85
767-300	1.67
E-3 Sentry	1.62
Citation III	1.49
Lear 55	1.25
Gulfstream III	1.70
MD-80	1.37
DC-10-30	1.76
Sweringen Metro III	2.61

It is desirable to mount the main landing gear struts on the wing spar (usually an aft spar) where the structure is substantial. However, the gear must be mounted so that at aft c.g. there is sufficient weight on the nose wheel for good steering. This generally means gear near the 50% point of the M.A.C. . For wings with high sweep, high aspect ratio, or high taper ratio, the aft spar may occur forward of this point. In this case a chord extension must be added. The drawing here shows the gear mounted on a secondary spar attached to the rear spar and the addition of a chord extension to accommodate it.