# Constraints

The type and number of constraints are highly problem-dependent. Typical constraints for com­mercial wing designs arc:

• Lift and angle of attack. Either the lift C{ or angle of attack a are constrained or defined as input values. For the horizontal tail plane, both can be simultaneously constrained.

• Minimum and maximum lift. Maximum lift at low speed (=» 50m/s) C{ for a given Rey­nolds number influences takeoff and landing performance and is therefore an important pa­rameter. For horizontal tails, the minimum lift at low speed C. is an important constraint.

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For slender supersonic aircraft the maximum lift is usually defined by a maximum angle of attack of 20°. according to the ‘Concorde’ SST regulations.

• Buffet onset at 1.3g relative to Ig cruise. Although a buffet onset is difficult to predict exactly, a conservative estimate can be made using the trailing edge separation criterion [340| based on trailing edge pressure coefficients. Buffet of this type is usually a problem for transonic wing designs.

• Pitching moment C. This is usually not particularly important for subsonic transports but is a major issue for supersonic transports The hinge moment ^ is an important constraint in the design of control surfaces.

• The aircraft thrust must be at least equal to the drag also at off-design points, of which the most critical are usually at the high lift and high Mach number comer of the envelope. The allowed drag creep as a function of Mach number or lift is also often constrained.

• Geometry. The airfoil thickness and thickness distribution are usually subject to structural and geometric constraints such as spar depths.

• Engine installation. The wing design should take into account the disturbances due to engine installation. This is done implicitly by including the engine in – and outflow with the nacelle in the drag balance. When a surface (such as a wing) is placed near a nacelle it will then as­sume a shape such that the drag is minimized (e g. interference minimized)