Sabre (oz10346)

About this Plan

Sabre. Radio control sailplane. A simple six foot thermal soarer.

Quote: "Sabre. Rattling good - Robert Cairn's simple six foot thermal soarer

My philosophy was that Sabre should be inexpensive and an easy model to build and fly. For this reason I selected a rectangular wing planform using standard 36in length spars and leading edge strips. The rectangular wing plan has excellent stall characteristics and will not tip stall (even if it doesn't have washout).

The wingspan was chosen at 72in (two 36in spars) and an aspect ratio of nearly ten, giving a chord length of 7.25in. I decided upon a flat-bottomed section for the aerofoil, the actual shape of which was arrived at by taking stock balsa leading and trailing edge pieces and joining the outline of them both with a smooth line using a french curve.

A tail moment arm of '3.5 times the wing chord length' was chosen as a good average value. This gives a wing-trailing-edge to tail-leading-edge distance of approximately 2.5 times the wing chord.

The tailplane and fin/rudder area were calculated using the formulae from Andy Lennon's book. The elevator area is 25 per cent of the area of the tailplane/elevator combination; the rudder was made as large as practical.

The tailplane was set at 'minus three degrees' with respect to the wing, The battery and servo bay was located in front of the wing and reinforced with epoxy impregnated nylon weave. Also, the built-up structure of the tail reduces the amount of ballast needed in the nose to produce the correct CG position (roughly one-third the chord length back from the leading edge).

The dihedral angle for each wing tip panel is 16 degrees. Hence, the vertical distance of the tip above the wing root would have been the same if a dihedral angle of eight degrees were incorporated at the centre of the wing instead. Armed with the above figures and ideas, I started building the Sabre, using my favourite construction techniques.

Wings. The ribs are cut out from 1/16in balsa and are fitted over the 3/16in x 1/4in rear spar and into the corresponding notches made in the trailing edges, both of which should be pinned over the plan to ensure correct alignment. The leading edge is added, followed by the top spar. Note that the end ribs on the centre section are angled inwards (using the template provided to achieve the correct angle) in order to accommodate the dihedral. Add the tip blocks and also the leading edge sheeting. After sanding down the whole structure, join the three panels using the ply dihedral braces. Attach the ply reinforcement to the trailing edge. The wing should balance at the centre. If it doesn't then add the necessary weight to the lighter tip..."

Sabre, RCM&E, July 1990.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

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