Mach 20 (oz8870)
About this Plan
Mach 20 A 42in span aerobatic model for 0.20 size motors and 2-4 functions RC systems.
Quote: "BEING AN AVID Club 20 pylon race fanatic, I was most interested when the Club 20 Association inaugurated an aerobatic class. The rules at that time were extremely simple; no limit on size, weight, (under eleven pounds of course!), plus the fact that the restricted engine class of the pylon race rules did not apply. However the original was powered with an HB2O fitted with a Perry carb, which though not an exotic Schnuerle ported motor, has proved morethan adequate for this design.
I had always fancied the Dirty Birdy (oz4915) shape, so bearing this in mind, armed with the tools of my trade I set to and Mach 20 was born. The design parameters I set myself were as follows:
1. Wing area around 350 sq in.
2. Maximum weight of 2.75lb.
3. Clean functional lines.
4. Upright engine for ease of starting and maintenance.
5. Thinner wing sections than usual to reduce frontal area and thereby give better vertical manoeuvre performance.
Other armchair aerodynaicists may scoff at the wing sections used as outlined in (5), thinking that the LE is too sharp and the sections too thin, thinking that such sections can only lead to sudden wing dropping and tip stalling. However, after flying 'Mach 20' now for two years this problem has not been encountered.
To obtain the lower weight desired, all the balsa to be used is medium soft to soft. Where maximum strength is required then spruce, ply and hard balsa have been specified. So taking balsa, ply, knives, glasspaper and cassette deck in hand to the workshop I commenced with:
Wings: I always build these first, reasoning that interest tends to wane (with me anyway) as the project progresses, and the accurate nice free flying model that you require to make the schedule easier is totally dependent on an accurate warp-free wing.
Prick through each rib shape from plan. Some experienced modellers might shudder at this and think maybe the sandwich method ought to be used. I have never agreed with this, as it tends to give incorrect forms at certain stations which must be doctored with the sanding block afterwards.
Pin down the 1/8 x 1/4 in spruce spar and add ribs using scrap balsa to pack up rear end, ensure ribs are upright..."
Mach 20, RCM&E, January 1980.
Direct submission to Outerzone.
Supplementary file notes
Article pages, thanks to RFJ.
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