Building review.

Building method of the Graal is a synthesis of several modern techniques. It use wood, aviation plywood, carbon fiber and epoxy resin. Choice of materials had been made with to goal in mind : build light and build cheaper.

Wood is the historical material of choice of plane making. It has been left over for composite plastics in the 70's. This move was made maybe more for productivity increase than from comparative study of materials. As a matter of facts if we compute the ratio of material performance by its density, it can be seen that for a beam (i.e. a wing spar) this ratio is linear (Rr.d-1, E.d-1). For a thin skin (fuselage skin, wing surfaces), density play for its square in strength ration and for its cube in stiffness ratio (Rr.d-2, E.d-3). The following table summarize some figures :

Material Density Rr (MPa) E (GPa) Rr.d-1 E.d-1 Rr.d-2 E.d-3
Spruce

0,45

70

10

160

22

350

110
UD glass E / epoxy

2

1100

40

550

20

280

5
carbon fiber / epoxy

1,5

1300

135

870

89

580

40

This show that a spar will be more solid if made from glass/epoxy than from wood and still much more for the carbon/epoxy. For a thin skin the wood stay the best material for relative stiffness and second behind carbon for strength. Moreover for the same weight a carbon skin should be very thin and this raise construction and maintenance problems.
On modern plastic glider weight is not a problem it give more speed and some L/D points (Reynolds number is higher). But you should remember that you pay for the weight, and it's high price when talking of carbon fiber.

The Graal use carbon fiber sparsely where it's really a must : wing spar, landing gear, control gear, fuselage frame. The low thickness of modern airfoils enjoin high-tech material. Despite Graal lightness, its spar would be unrealizable with wood construction.

Nevertheless wood stay the base material for the Graal, but with a technical method coming from boat building (you may have heard of "West System" by Gougeon brothers). Epoxy resin is used as well as glue and as insulating coating. Protected by this coating the wood can keep its optimal characteristics and is safe from moisture. Then it need no more care then plastic.

Wings are build on a foam kernel. Two carbon UD spar caps form the spar. Wing surfaces are plywood 12/10 mm.

Fuselage is a thin molded wood skin fitted to a frame of wood/carbon/epoxy.