The tendency to wing‐tip stalling, especially in tailless aircraft, is reduced by providing slots 10 near the leading edges of the wing tips, the openings in the lower and upper wing surfaces being provided with doors 14, 15, which may be operated by the pilot or automatically in accordance with the lift coefficient at the wing tips. The doors 14, 15 for each wing are controlled by a pair of pressure switch mechanisms each consisting of bellows 37, 38 in a compartment 35, the bellows being subjected to the upper and lower wing surfaces pressures respectively, while the compartment communicates with a tube 36 extending below the boundary layer of the lower surface. The ratio of the pressures acting on the bellows is a function of the lift coefficient, movement of the bellows being used to actuate microswitches 43 of which one, the trip switch, closes at a coefficient of 06 and the other, the resetting switch, closes at a coefficient of 07. Normally the doors 14, 15 of both wings are closed, and the automatic control system is so arranged that when the coefficient reaches the higher value on cither wing the resetting switch operates to open the doors, which remain open so long as this coefficient is maintained on cither wing. When the coefficient falls below this value the resetting switch opens, but the trip switches are not operated to close the doors until the coefficient falls below the lower value on both wings. A landing gear switch is closed when the undercarriage is retracted and opened when it is extended, to ensure that the slot doors are always open when the undercarriage is extended.
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