Got a spare .15? What are you waiting for!
October 2001. Shortening days warned me about the prospect of an imminent long winter, which would need to be filled with building projects, plus the odd day of cosy flying when good weather became available. Venturing into my chaotic workshop, I carefully laid a trail of wool so that I would be able to find my way back to the door. As you may suspect, my workshop is a little untidy, and crammed with models: half built, in boxes, castor-covered, yet to be rediscovered... all are in there.
Having had some experience of building plan models, various kits and even the occasional ARTF, I was looking for something different - something that would test the brain cells.
It was then that a little germ of an idea began to multiply. Perhaps I might be able to design, build and fly my own model! What form should it take? The design would have to be simple to construct, cheap, small (perhaps 15 / 25 powered), aerobatic and yet docile to fly, with good aerodynamic stability. Moving around the workshop, I started to make some approximate measurements of several existing models, working on the theory that, if these designs worked, using the same proportions would put me on the right track.
Time to make some design decisions. The model would be 15 powered, with a target weight of approx. 3 lb., a 50 span, a symmetrical wing section, four-channel radio, and a lightweight Rx with micro servos. Using an old 15-size trainer, I made a flat-bottom wing template, then, I simply reversed the top and made the section symmetrical. The design of this wing was closely adhered to throughout, with some minor adjustments to allow the inclusion of ailerons and accompanying servo. Hand-launching was chosen as a flight starting method, since an undercarriage would have added weight (and expense); according to my design, the wing would sit on top of the fuselage.
With these thoughts in mind, I put pencil and ruler to lining paper, marking out rib positions three inches apart. Then, spars were sketched in. Torn between micro or standard servos for aileron control, pure economics prompted me to site a central, standard unit connected to bell-cranks. Eventually, the wing came out at 54 plus 2 more for the tips.
Hand launching is easy
The need for aerobatic performance prompted my inclusion of spruce spars, and the exclusion of any dihedral. Hence, the wing went together very quickly, with the trailing edge simply made from standard t.e. stock. To add a little sophistication to this simple model, I top-hinged the ailerons and sanded the front edges appropriately. Fitting the aileron servo took a little thinking time, but with the aid of a fulcrum and some excellent bell-cranks, it worked perfectly in the end.
Having completed the wing, I sketched out the proportions for my fuselage, which would consist of balsa sides and 1/16 ply doublers extending to the rear of the radio compartment. Lying out the engine mount, 4oz. fuel tank, Rx NiCad pack, GWS Rx and Hitec servos, I began juggling things about to achieve the best (and neatest) fit - this gave me an approximate size for an equipment bay. As you can see from the plan, there are a lot of straight lines, because I found these were the easiest to draw - my own-design Spitfire is some years off! To facilitate wing fitting, a recess was added to the top of the fuselage by drawing around the bottom half of a rib.
The fuselage underside was kept as flat as possible, since this would provide a landing platform (possibly with a skid if required). Having chosen such an arrangement, I decided to make sure the tail control surfaces could not be snagged by any terrestrial projections and with this in mind, set the tailplane mid-fuselage, which keeps the rudder conveniently out of the way. Those previous model observations made in the workshop had highlighted the relatively small dimensions of many commercially designed control surfaces, so for improved aerobatic control, I decided to increase the size of those fitted to Hoehnia. Sample fins and rudders were cut out and test-fitted on the plan until a satisfactory compromise had been reached, whereupon I chose to utilise split elevators and a wire joiner. Rather than cut out formers, I made them (only two) using 3/16 x 1/2 balsa, with a 1/8 ply engine former.
With the rough design laid out, some of the structural parts could be cut, and I was able to spend time considering the possible snags that might be encountered. With nothing better to do and impatience niggling away, I began to assemble the fuselage. Two (left and right) sides were made up with ply doublers, and the two formers glued in place on one half, with care being taken to ensure a square fit. Once the glue had cured, I was able to add the second side and allow that to dry as well, cutting out a tailplane, fin and rudder in the meantime. These can also be sanded and hinged while the fuselage dries.
To accommodate the fuel tubing, throttle control and motor mount, I pre-drilled the engine former before epoxy-gluing it in place. Having done that, the tank floor could also be inserted and glued. I made a liteply servo mount for the fuselage, glued it in place, and then inserted the three micro servos.
PATIENCE, DEAR BOY
With the tank bay and engine compartment habitually painted and fuel-proofed at this early stage, another wait was necessary whilst everything dried. After a period of sustained patience it was time to add the engine mount, engine, and fuel tank. On went the tailplane, fin and rudder, followed by control snakes and a Bowden cable (bicycle brake cable in a snake outer) for the throttle control. Hoehnia was nearing completion, and so holes were cut to accommodate dowels for the wing bands.
With the job almost done, I decided to sit and ponder the design for a while. It looked like a flier, but in truth a little dull too; even with my proposed colour scheme of yellow and blue, Hoehnia really did need something to spice it up. Now, Id noticed that some of our club glider-guiders have models with cute little turned-down tips - I believe these are called Hoehner tips - which aid the non-stall properties of a wing. Maybe this was the aesthetic difference my design required! Two 1/2 pieces of balsa were glued together and sanded to shape using those extremely useful Perma-Grit bars; I was quite pleased with my first attempt, and so having spent the next two hours making another one the same, I was ready to glue them in place prior to final sanding. This added two inches to the span.
Blue and yellow Solartex adorn the wings, including those tips, which reminds me to warn you: Covering the concave underside was not for the faint hearted. The complex-curve problem can be partially eliminated by using three separate pieces of Solartex, and that way, there aren't too many wrinkles... honest!
TESTING THE NERVES
A crisp, cold and cloudless winters day beckoned me to the Kettle International Field for a test flight.
In truth, my experience of test flying is very limited, and I did have some worries for the as-yet unnamed experimental model, although these did ease a little as the O.S. 15LA started well, responding reliably to throttle control. With my personal photographer, Alex Whittaker, acting as launch supervisor, we managed to get her up - in fact, she leapt into the air and needed only two clicks of right aileron to initiate hands-free operation. On full throttle Hoehnia fairly whizzes about, scaring sheep and winged beasts of the air alike. It rolls well on full rates and loops beautifully into wind, but can sag downwind... well, its only 15 powered, after all.
Spins are sedate (almost spiral dives), and inverted flight no problem. On idle, Hoehnia will cruise on the lightest breeze, and at this setting, flights of 25 minutes have been achieved. If you get bored of glider guiding, the model will spin, but is sedate and descends quickly; landing is simplicity itself, because she comes in rock steady, and makes the most of any breeze. The flat fuselage bottom ensures a nice skid to stop, whilst those Hoehner tips, apart from making the model look good, add to the performance since the model is very difficult to stall.
Recently, I made a little presentation of the model to our club as a bit of light entertainment. One member of the audience informed me that those tips were called Hoehners, and ignoring the inevitable look at the tips on that type comments, I took on board one wags announcement that they should be called hernias - all I did was change the spelling, and thats how the model was christened Hoehnia.
Model type: Shoulder wing sports
Wingspan: 56" inc tips
Fuel tank: 4oz square
Engine: O.S. 15 LA
Functions: Throttle, elevator, rudder, ailerons.