About this Plan
HFJ-16. Radio control sport twin, for electric power. Wingspan 1370 mm.
Quote (google-translated from the German) : "Twin-engine aerobatic model HFJ-16. Construction: Ing H Junghardt.
The HFJ - 16 is the 16th member of an electric model family, which includes, among other things: a biplane, a delta, some twin-engine and a four-engine electric flight model. The model is designed for aerobatics, the wing has a semi-symmetrical profile, the elevator and ailerons are controlled, the rudder can optionally be steered. It is powered by two normal, cheap motors that are powered by two 8-cell batteries connected in parallel. The standing thrust with 8 x 5 or 8 x 6 propellers is approx. 1,100 g. The speed is around 8,200 or 8,000 rpm.
The current consumption per battery is 12.5 or 15 A. In flight, the current consumption drops to approx. 10 or 11 A, which results in a motor running time of just under 6 minutes. This results in gentle operation, particularly of the battery, which in turn means a long service life and a low failure rate for the cells. Loads in flight of 15 A and more - this can be said from our own experience - make sense for competition use at best.
The motors are arranged on the surface and this arrangement has proven itself very well in several models over the course of several years: so far there has not been a bent motor shaft and not a single broken propeller.
For weight reasons and because of the better aerodynamic properties, a chassis was omitted. The hand start is completely problem-free, the model climbs away well after starting. Looping up and down, Cuban eight, Immelmann etc. are easy to fly.
The two engines do not cause any problems at all in terms of flight technology, on the contrary: the stall behavior is very uncritical - probably an effect of the wings being blown wide by the two propellers. Equipped with 2 x 10 cells and controlled rudder, successful competition use is certainly possible. But now to the construction.
Wing: First the ribs are cut out and then the 4 ribs between which the motors are attached are reinforced with 0.6 mm plywood. The auxiliary strips part 73 must also be glued to these ribs. The ribs between the motors also have a 5 mm hole for the supply lines to the motors. The area is in one piece and can be built directly on the construction plan:
First attach the lower spar and the auxiliary bar to the building board and glue all ribs to the spar. Then glue on the webs 75, the upper rail, the nose auxiliary strip 52 and the end strip 53. After drying, the framework is removed from the building board and the nose and end strips are sanded to shape. Before the planking, the plywood parts 66, 67, 79 for surface fastening, the two motor supports 72 and the ground triangular strips 60 must be glued in with epoxy glue. Fit and glue part 78, glue partial ribs 68 and web 69. The motor cables also need to be laid now. Make sure the cables have a sufficient cross-section and are of sufficient length for the motor connection. Now the paneling parts 53, 56, 70 can be attached to the top and, after drying, to the bottom. The surface must be fixed on the building board.
After the planking has dried out, the protruding parts are sanded off and then the nose strip 51 and the edge arches, which were previously roughly sanded to shape, are glued on. The raw parts of the edge arches can be made from three 10 mm balsa boards that are glued together with a hard adhesive. Commercially available or self-made parts can be used to drive the ailerons. To install the drive, appropriate recesses and slots must be made in the end strip and in part 64 and then glued together with two-component adhesive. The two ailerons are ground into shape and drilled for the drive. The cutout for the servo is made in the middle part of the surface. The cutouts for the engine mounts also need to be made in the paneling above the engine mount. First there is a small one..."
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User commentsHello there, Way back when I build this plane. Two Speed 600 engines, micro switch for on/off and 20 steaming NC cells, and that for only a few minutes flying time. It was stowed away until a few years ago when I installed a proper ESC and a 2S Lipo battery. Unfortunately, this didn’t work. The used cables were too thin and got very hot. So, flying with this configuration was not an option. Changing into thicker leads was not possible without opening the wing. Then I decided to pull of the two brushed engines and replace it for a 3536 / 1100 Kv brushless type. A 4400 MaH 4S Lipo completed the setup.
Hendrik - 12/02/2024
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