Free Spirit (oz15685)

About this Plan

Free Spirit. Radio control sailplane model. Wingspan 2m.

Direct submission to Outerzone.

Update 27/11/2024: Added article, thanks to RFJ.

Quote: "1978 Nationals-winning Standard Class sailplane is designed for maximum duration and good penetration. Free Spirit, by Leon C Kincaid Jr.

If you are looking for a Standard Class sailplane with outstanding duration qualities, this is the one. The nose is rather short to give quick stall recovery, and the fuselage rather long to add stability. There are several excellent light Standard Class sailplanes on the market with nice wide wing chords. But I am convinced that mine, with its 14:1 aspect ratio and slightly undercambered wing, will stay up longer than any of them.

If the thought of building an undercambered wing scares you, don't let it. This wing is flat for the first third of the bottom and can be constructed on a flat board. The wing tips, or outer panels, can also be constructed on a flat board and no twisting for washout is required. Starting at the poly break, the undercambered airfoil goes to a flat-bottom airfoil at the tip. Consequently, the chord line at the tip is about two degrees less than the chord line of the root on the center panels, resulting in a two-degree washout already designed in the airfoil. If an NACA numerical designation were to be assigned to the basic airfoil, it would be around 5310.

The overall construction is exceptionally strong and there's no need to worry about saving weight. In four Free Spirits constructed so far, I have had to add weight to the CG to obtain the best performance. There is one exception to this rule - keep the extremities such as the wing tips and the tail sections as light as possible. The lighter the wing tips, the flatter the glide in thermals: the lighter the tail feathers, the less strength required in the fuselage.

The frontal area has been reduced to a minimum. A Kraft brick fits perfectly and leaves enough room for a third small servo, such as a KPS-12, if desired. Just remember, a third control will put the model in Class B, or Modified Standard. For one model in which I used a larger radio, rather than extending the nose or using a short battery, I cut a square hole in the third bulkhead and slid the receiver in the weight box area.

To win contests, you need three things: good luck in finding thermals, flying skill to land on the spot, and a well-designed sailplane to give you the advantage. I can't help you when the air is all 'down' and I can't give you the skill required to hit those spots, as I lack that myself. But I can give you the sailplane that will give you the advantage. Particularly in duration.

CONSTRUCTION: There is always more than one way to construct a model, and each method has its advantages and disadvantages. The following is just one way.

FIRST STEP: On the Free Spirit it really doesn't matter if you build the wing or the fuselage first, but I normally make the fuselage first. However, the first thing to do is cut out the master wing template and the plywood ribs W1, W2 and W3 for each wing..."

Update 28/11/2024: Added (second) article thanks to RFJ. This is from FM, April 1981.

Supplementary file notes

Article.
Article (FM 81).

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