STRUCTURES

Attie Jonker, JS Chief Engineer, explains:

The structural design of the JS1 Revelation was done using normal engineering methods. Conventional structures (such as the controls and landing gear) were designed using traditional calculation methods, with hand calculations for simple geometries and laminate analysis for simple composite structures. The more complex structures were designed using modern FEM analysis techniques.

A combination of glass-fibre, aramid (Kevlar) and carbon-fibre is used in the load bearing structure of the JS1 Revelation. Each material is used where it is most effective. The very thin wing section (12.7%) posed quite a challenge for the designers, especially in the wing root area where the maximum thickness is only 100mm.

All structural design was according to the certification standards set in CS-22 and with a general safety factor of 1.725.

Finite Element Modeling (FEM) allows for the whole structure to be represented as a wire frame of cells or elements called “the mesh”. Loads can then be applied to the mesh and the stresses calculated at each point. The result is a colorful representation of the stress condition throughout the structure. This shows exactly where the structure needs strengthening and where weight can be saved. The figure below shows the stress condition in the front and aft fuselage due to a load on the tail, and a high-g pullout maneuver.

One of the advantages of using a full FEM model of the sailplane for design purposes is that the natural frequencies of the structure can be calculated for avoiding flutter. This is especially important with JS1 Revelation’s high VNE of 290km/h.

However advanced modeling and calculation techniques do not substitute for structural testing. The results of any model must be supported by experimental data. The tail plane of the JS1 Revelation was first modeled using the finite element method, then structurally tested, and the results compared. A comparison between the predicted and the actual measured deflection of the tail plane showed a correspondence of within 0.5%, which confirmed the validity of the FEM models and methods.

FEM methods, together with structural testing, have resulted in the JS1 Revelation having a very light but extremely strong structure, at least equal but probably better than any other sailplane in its class.

Left: The symmetrical half-model CAD geometry of the tail plane
Middle: FEM mesh of the tail plane
Right: Structural static deflection test of the tail plane

Structural verification of the wing structural stiffness : 6G wing bending test