VARIABLE GEOMETRY SAILPLANES MINISIGMA

Abstract

Variable Geometry for sailplanes refers to the use of some mechanical method of changing the effective wing area to provide a better compromise between the requirements for circling flight and cruising flight. Some of the earliest examples of this conceptl were the BJ-3, BJ-4 series of aircraft built by Beatty and Johl in South Africa. These aircraft featured a Fowler flap that allowed high wing loading for cruising flight and high lift for circling. In addition to the disadvantage of complex mechanisms, the BJ-4 had too much drag at high speed due to fixed slots on the ailerons. Too much drag at high speed proved to be a recurring problem with other Variable Geometry sailplanes. The slotted flap is the best solution for variable geometry. It provides nearly two to one increase in effective wing area, while maintaining effective aileron control power at low speeds. It has higher drag than the unslotted Fowler flap, but its higher operating lift coefficient more than makes up for the difference. Variable Geometry offers substantial performance gains for a 15 meter class aircraft without excessive weight. Safety does not have to be compromised for heavy wing loading and in fact the slotted flap provides impressive gains in safe handling even in the case of full water ballast.  Variable Geometry can provide performance comparable to open class with the advantages of light weight and convenient handling typical of 15 meter dass.