Advice on twin boom pusher design (20-30cc gas)

[Spekd'out]

I got my foam cutter MK 2 going and seemed to be inspired to design and build yet another project. I always thought multi-use criteria was a bad idea but it seems to have worked its way into this concept. The first was to be used as a trainer, also a endurance project (club fun), and camera/payload . Aero-tow would be nice but a pusher would require a harness. ( done it, works good.)I have calculated some weights estimates at 11 to 14 lbs. I was considering a slightly higher aspect ratio with a span up to 120 inches. Perhaps a root chord up to 15 ". As my friend says airfoil selection would be the first decision and not so sure what to use. A simple Clark Y was suggested as it has favourable stall characteristics and adequate thickness for structure which may be of traditional materials. I am looking for opinions on planform design and alternate airfoil choices.

Regards, Jan

[BMatthews]

If slow flying and landing speeds while loaded down is a criteria then there's some better options. But if you can keep the weight under control and don't plan on serious loads then there's not much to fault the good ol' Clark Y which works so well at "our" flying speeds.

For a slightly thicker and heavier model that is foam cored and veneer or balsa sheeted I'd suggest a few other options if you want to land even more slowly. Gotingen 500, Eppler 212, Selig 4233. All of these are as thick as the Clark Y. The Gotingen in particular has worked very well on an old timer of mine showing itself to hav a wider than expected speed range and efficiency even at medium speeds while still acting like it has the flaps out when you pull back and go for the slow flying descent. If you're after an airfoil that has excellent "loiter" capability for the camera work then that would be my own choice.

But for glider towing you actually need some speed. So if that is a requirement then the Clark Y will offer a more all 'round effectiveness and you can then add flaps to slow for landing.

[Spekd'out]

I will definitely be incorporating flaps into the design. I dont think I will end up with heavy payloads too often and think just cruising around will be the most common style of flying. I chose this layout for something interesting to design and build. Some of the foils I was considering were S8055,S7055,S3010,Clark Y,RG 8, HN 975,MH 18 etc.. I do like the looks of the S4233 however and believe I will end up happier with a liftier section. Next will be to layout some planforms. My initial thoughts were a straight leading edge with mildly tapered tips. I'm not sure about washout but am tempted to put in a little bit. I am also thinking of trying an inverted V-tail for the tail section. Structurally it should be quite rigid with the connection at the top. Time to start fumbling around in AutoCad, and thanks for the suggestions Bruce!