Hot Box (oz14708)

About this Plan

Hot Box. Formula P.30 rubber model.

Quote: "Hot Box P-30, by John Oldenkamp. Are you ready for the 1977 Nats free flight competition? Not until you put together a couple of P-30 Class models and come out to join the unofficial fun!

P-30's are coming out of the woodwork faster than lovesick termites: my proposal of two years ago for a threshold competition rubber event has suddenly taken on momentum - particularly in the areas where sites are diminishing in size. P-30 will be flown as an unofficial event at the USFF Championships this year, as well as at the Riverside AMA Nats. In addition, there will be an International Proxy Postal meet, to be flown off at Lake Elsinore by the San Diego Orbiteers on September 10th and Ilth. Latter is sponsored by MAN and Dave Whatshistrum.

The rules are very simple; no dimension of the airplane may exceed 30 inches, including propeller and D/T hardware. It must have an unaltered 9-1/2 inch freewheeling commercial plastic propeller, and is allowed ten grams rubber total. Very interesting mix of ingredients. Until you see one zooming up, or, better still, build one and witness their extraordinary performance, you simply won't know. Those plastic props are very efficient, and if matched to the correct rubber stranding, really do go places.

Hot Box is about the thirtieth P-30 I have designed, and although I haven't built this one yet, the prototype (in bright lilac and pink) flew more or less off Cynthia Sabransky's workbench, needing only very slight decalage increase. We haven't found the perfect pattern yet, but she expects to put up a strong challenge with it for the Ladies Cup at Taft and the Nats. Hot Box is straight-forward, robust, and easy to build, owing to generous stock dimensions. A Hot-Stuffed version should require no more than a day-and-a-half or so to complete.

The Airfoil: Besides wanting to help create an event for all to enjoy, I tried several different ways to make the whole project, in building terms, a snap. Hence, the cracked ribs. My first few P-30 models were rather small, underpowered, super-light, but each had the cracked rib thing all the way. As I gradually increased areas and went to six and eight-strand motors, I found that the diamond-sectioned mode provided a fantastic climb, but no glide to speak of. I had reasoned that the Driftwould OHLG (Geraghty) was getting by very nicely on the diamond shape, so why wouldn't a rubber ship of roughly three times the area and no more than twice the weight, etc? Several very pretty airplanes now sit in my 'museum' - terrific climbers, but gliders they ain't.

Curt Stevens, old line HLG theorist, suggested laying a turbulating spar somewhere on top of the diamond, run the tissue over it and see. It works. I've since built a couple of Coupe d'hivers (also plastic propped), a Payload, and an A/1 - all with same general layout, some undercambered. Success!

The airfoil break is at the spar, located 40 percent of the chord, while the turbulator spar is half that, giving an approximate 30% high point for good climb, plus moderate damping in the glide. It is quite evident at this point, that the glide of a rubber ship with freewheeling prop must be a whole lot slower than one equipped with a folder. The section then, must be at least 10% thick. Take it from me, the Cracked Rib works. Or go back a bit and use the RAF 32, if you dislike the idea of gluing all those little sticks together. George Perryman pointed out once, though, that you do it a piece at a time, and I agree.

To make this gorgeous little bird come alive, you will need to observe only a few hints from me. First off, order up a set of plans from your favorite publishing house, then knee it down to El Hobby for a miser's share of wood. The only hard stuff necessary will be four 3/32nd square pieces for the fuselage longerons (1.1 grams each). Everything else should be medium-weight, stringy sticks and very light quarter-grained sheet. Your target weight for the finished plane should be about 35 grams - eight of which are accounted for by the propeller and noseblock/shall assembly.

In General: Double glue everything with Titebond thinned about 30% with plain water. Use a straightedge to establish pin lines around all major shapes. Cut and fit everything finger-tight. Work on a flat surface. Breathe deeply while the critter takes shape.

Specifically: The wing tips should have at least 3/32 inch washout built in. A shim is indicated on the plan. It is also not such a bad idea to add about 1/16th inch washout to the stabilizer as well. On the wing and stabilizer, I pin down all the major outlines, then add the gusseting, perpendicular rib sticks, the diagonals..."

Direct submission to Outerzone.

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