Mistral. Radio control slope soarer.
Quote: "Here's an elegant 150 in span 'low-tech' thermal soarer that's straightforward to build and won't break the bank. Mistral, by Jack Elliott.
Over the past couple of decades I have derived much pleasure in designing, building and flying radio controlled scale gliders, the Vintage Glider Club often supplying the inspiration for many of my designs. I count myself fortunate that, near at hand, I have some excellent hill sites that border the Black Mountains of Wales.
But inevitably there are those days when, having arrived at a favourite soaring site, I am faced with the situation that the sun is shining, the sky is blue and there is not a breath of wind coming up the slope -although from watching the buzzards I know that there are thermals about but weak and far between. It's at times like this that you'd need to be a braver man than me to launch a large scale glider over the edge of a 2,000 feet drop.
It's not that a scale glider can't make good use of even a weak thermal, it's the thought that, if I don't connect, it's a long way down to the bottom sliding on my backside on a steep ridge whilst still trying to fly the glider and at the same time trying to pick a suitable field in which to land! It is also the knowledge that the climb back will be even harder carrying a glider weighing 14 lbs in my arms, and the fact that I also carry extra ballast round my turn and am out of condition. (Thinks - if I lost a stone in weight would the glider appear to be lighter?) Having experienced lack of thermals a few times over the last twenty years I now have no wish to repeat the aforementioned slide down and climb up which might well cut short my retirement.
For this reason I decided it would be a good idea to have a lightly loaded thermal soarer at hand for such days. At worst an up the slope landing would be less daunting with a model which would be easier and less expensive to repair than a scale glider. Of course, one could go out and buy a suitable ARTF thermal soarer but as I enjoy building and my balsa stocks look healthy and my scrap box promising, I put pen to paper and designed a low-tech glider that is, by my standards, easy to build, and will not break the bank.
The design of my model conforms to the formula used for most thermal soarers of this type so they do all tend to look similar. In my case I don't like cutting bits of standard length sheets as, to my mind, this is wasteful, so each wing panel is 36 in long, or if longer wings are desired, a combination of 36 in and 48 in. As to the mean wing cord, Mistral uses one 4 in sheet and two 3 in sheets, edge glued together so giving a ten inch cord, so little wood is wasted and rib spacing of just under 2 in, so wood can be bought to size here.
And now to the model itself. My method for transferring plan parts to wood is to place a sheet of thin art card under the plan with a sheet or sheets of carbon transfer paper face down on the card. Then I trace over the parts on the plan so transferring them to the art card, which is then cut out using a modelling knife to make an accurate set of master templates.
My first job is to make a complete kit of parts so that once the construction actually gets under way all the bits are at hand and this really does speed up construction.
The fuselage. Using the template, cut out two 1/32 ply sides and two 1/8 balsa sides complete with all slots and holes, etc. Evo-Stick the ply sides to the balsa outerskins making sure you finish up with a left and right side. Glue on the 1/4 in sq longerons where shown on the plan. Evo-Stick in the 1/32 in ply doublers (they fit inside the 1/4 in sq. balsa longerons). Mark all former positions on the inside of each side. Lay one side flat on the bench and glue in formers 3, 4, and 5, making sure they are square..."
Mistral, Radio Modeller, April 1993.
Note this is a low resolution plan.
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