Increase in Maximum Takeoff Weight
Many of the vortex generator kits available for light twin-engine airplanes bring with them the added benefit of an increase in maximum takeoff weight. This might seem paradoxical because installation of vortex generators does not increase the strength of the wing.
The maximum takeoff weight of a twin-engine airplane is determined by structural requirements and single-engine climb performance requirements (which are lower for a lower stall speed). For many light twin-engine airplanes, the single-engine climb performance requirements determine a lower maximum weight rather than the structural requirements. Consequently, anything that can be done to improve the single-engine-inoperative climb performance will bring about an increase in maximum takeoff weight.
In the USA from 1945 until 1991, the one-engine-inoperative climb requirement for multi-engine airplanes with a maximum takeoff weight of 6,000 lb (2,700 kg) or less was as follows:
All multiengine airplanes having a stalling speed greater than 70 miles per hour shall have a steady rate of climb of at least in feet per minute at an altitude of 5,000 feet with the critical engine inoperative and the remaining engines operating at not more than maximum continuous power, the inoperative propeller in the minimum drag position, landing gear retracted, wing flaps in the most favorable position …where is the stalling speed in the landing configuration in miles per hour.
Installation of vortex generators can usually bring about a slight reduction in stalling speed of an airplane and therefore reduce the required one-engine-inoperative climb performance. The reduced requirement for climb performance allows an increase in maximum takeoff weight, at least up to the maximum weight allowed by structural requirements.
An increase in maximum weight allowed by structural requirements can usually be achieved by specifying a maximum zero fuel weight or, if a maximum zero fuel weight is already specified as one of the airplane's limitations, by specifying a new higher maximum zero fuel weight.
For these reasons, vortex generator kits for many light twin-engine airplanes are accompanied by a reduction in maximum zero fuel weight and an increase in maximum takeoff weight.
The one-engine-inoperative rate-of-climb requirement does not apply to single-engine airplanes, so gains in the maximum takeoff weight (based on stall speed or structural considerations) are less significant compared to those for 1945–1991 twins.
After 1991, the airworthiness certification requirements in the USA specify the one-engine-inoperative climb requirement as a gradient independent of stalling speed, so there is less opportunity for vortex generators to increase the maximum takeoff weight of multi-engine airplanes whose certification basis is FAR 23 at amendment 23-42 or later.
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