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MODELING
Biplane training aircraft Po-2. Tips for a modeller
Directory / Radio control equipment Electric motors in conjunction with batteries are increasingly gaining ground in aircraft modeling. Today, the growing power of motors and improved battery parameters make it possible to create full-fledged, well-flying aircraft models, including radio-controlled ones. Not too big a problem is the acquisition of remote control equipment. Beginners may well use even inexpensive "toy" equipment, which is equipped with radio-controlled cars. As a rule, such equipment is two-three-channel, which is quite enough to control a simple flying radio model. When disassembling the car, you should carefully isolate those conductors that go to the engine and to the steering mechanism - on the model aircraft, they will be used to drive the electric motor and ailerons. Po-2 (until 1944 - U-2) designed by N.N. Polikarpov is a single-column braced biplane of a wooden structure, made mainly of pine and plywood with linen sheathing. Biplane "box" is assembled from two pairs of detachable consoles, almost identical in design. The frame of each of the wings consists of two spars and sixteen ribs. The wing profile is of the TsAGI-541 type, flat-convex, with a relative thickness of 8,12%. Ailerons are provided on both the upper and lower wings. The wing struts were first made of steel pipes with wooden fairings, and later on from drop-shaped duralumin pipes.
The fuselage is of rectangular section, rounded at the top. It is based on pine spars connected to the front part by a frame frame and a system of racks. In front of the fuselage, a sub-engine frame welded from pipes is fixed, and the tail section is a truss of pine spars and struts, reinforced with wire braces. The tail unit is of a normal layout, with rudders of direction and height of an increased area. Chassis - with rubber cord depreciation. Spoked wheels, covered on both sides with canvas. At the rear of the fuselage, a controlled tail spike made of ash with steel fittings is installed, which is deflected simultaneously with the rudder. The power plant of the biplane consists of a five-cylinder star-shaped air-cooled engine M-11 designed by A.D. Shvetsov, equipped with a wooden propeller with a diameter of 2,4 meters. Po-2 aircraft were painted, as a rule, in dark green, the lower surfaces of the wings, plumage and fuselage - in blue. Currently, the most popular material for the manufacture of electric aircraft is foam, more precisely - foam ceiling panels with a thickness of 3 to 6 mm. When choosing panels, preference should be given to plates with a smooth front surface, without a convex ornament. Almost all airframe parts can be made from them - from wing spars and ribs to skin. I note that 6 mm thick ceiling tiles can be cut with an incandescent nichrome wire into two 3 mm plates. In addition to ceiling panels, Styrofoam food trays are excellent blanks with a smooth surface - such are used in grocery stores for packaging goods; their flat part has a thickness of 3 - 4 mm, which is quite suitable for the manufacture of frames and fuselage skin.
The most suitable for joining foam parts is Titan glue - the adhesive seam turns out to be almost equal in strength with the foam and quite elastic, the same as the foam itself. The designer of the model should keep in mind that a number of parts of the model will have to be strengthened by pasting them with whatman paper. A few words about this technology. Facing is best done on a slab of laminated chipboard, the surface of which is covered with plastic wrap. Another of the same plate will be needed to evenly press the cladding to the foam. Please note that the glue must be applied in a thin layer to both the foam and paper. After 5 minutes of drying in air, the Whatman paper is evenly pressed against the foam blank. Next, a clamp is placed on the "sandwich", on top of which a not too heavy load is placed. The adhesive gains full strength in a day, but it will be possible to continue working with the workpiece in half an hour. In the manufacture of the force-bearing elements of the model from foam plastic, in particular the wing spars, it should be taken into account that their rigidity and bending strength are low. Strengthening these elements is not too difficult - just use low-stretch fiberglass threads or, better, synthetic threads that can be "extracted" from modern clothesline sold in hardware stores (it consists of a bundle of heavy-duty threads enclosed in a vinyl sheath) . These threads should be molded with Titan glue to the upper and lower surfaces of the spars, as well as to the wrong sides of the foam facing panels, placing them diagonally - this measure will make the parts more torsionally rigid with almost the same mass. So, we proceed to the manufacture of a model-half-copy of the Po-2 aircraft. The semi-copy class gives the model constructor some leeway when creating the model. The focus here is on the recognizability of the model, rather than the careful reproduction of its individual elements. In particular, the engine, landing gear, wing struts, tail skid and cockpits are rather conventionally copied on the Po-2 model. Instead of a flat-convex wing profile, an asymmetric doubly-convex profile of the R-II type with good load-bearing properties is used. Only the lower wing is equipped with ailerons - this is quite enough for a light model of an electric aircraft. In addition, the vertical tail does not have a rudder, since this control is practically not used when piloting the model - to turn the model, they create a roll with the ailerons and at the same time use the elevators to introduce a half-copy into a turn.
The fuselage of the model is assembled from two sidewalls, top and bottom panels, as well as three frames. All of the above parts are cut out of 3 mm sheet foam, and the frames are also pasted over with whatman paper. Each of the sidewalls is edged at the top and bottom with lime slats with a section of 4x4 mm. Styrofoam and wood are glued together with "Titan" glue, the edgings are fixed on the rails with tailor's pins during the gluing process. The top panel is bent on a heated steel pipe of suitable diameter. Three half-frames (foam plastic panels lined with whatman paper) and edgings made of 4x4 mm linden rails are glued to it with Titan glue. The bottom panel consists of two foam parts - front and back. When assembling the fuselage, it should be taken into account that the top panel is removable, so a simple fixing lock must be provided for its fastening. It is also necessary to provide for the installation of a landing gear spring support made of a lime block, a lime support for the front landing gear struts, a tail spike support, as well as panels for mounting steering machines on them, a remote control equipment receiver and a power supply unit consisting of 12 batteries. In front of the fuselage, on the first frame, a clip is fixed for the installation of an electric motor, glued from whatman paper. When making it, it makes sense to use the motor itself as a mandrel (by the way, electric motors like SPEED-500 or SPEED-400 are quite suitable for this model) - you just need to protect it from glue with a couple of layers of plastic film. From the outside of the clip, a foam plastic fairing and dummies of the M-11 engine cylinders are glued. The latter are easiest to turn on a lathe from linden, however, good imitations of these motor elements can be glued together from washers cut with a sharpened steel pipe of a suitable diameter from linden veneer 1 mm thick.
The upper and lower wings of the model are almost the same, the difference is only in the span and the absence of ailerons on the upper. Like the prototype, the wing is two-spar, the spars themselves are made of 4-mm foam sheet, pasted over on both sides with whatman paper. Top and bottom of each spar, as mentioned above, low-stretch threads should be glued. The ribs of the wing are cut out of 3 mm thick foam plastic, pasted over on both sides with whatman paper. Each of the ribs consists of three parts, which, when assembling the wing, are respectively glued between the leading edge of the wing and the front spar, between the front and rear spar and the rear spar and the trailing edge. The leading and trailing edges of the wing are lime, and the leading edge consists of two rails glued together, forming the letter "T" in cross section. The leading edge is steamed in boiling water and bent in accordance with the shape of the wing in plan, thus forming an elliptical tip. The wing skin is foam plastic, made of 3 mm thick sheet. It consists of five elements - central, two end and two main. It is advisable to specify the shape of the foam panels using a pre-made pattern from drawing paper. The lower wing is equipped with ailerons suspended on the rear spar with loops, which are pieces of nylon fishing line with a diameter of 1,2 mm. Fixing the fishing line in the spar and aileron with "Titan" glue - you only need to first make transverse notches on the fishing line with a soldering iron. Both the upper and the lower wing are equipped with brackets for fixing the racks, with the help of which a biplane box is formed. Each of them is made from a piece of a knitting needle, one end of which is flattened when heated, and a hole with a diameter of 1.2 mm is drilled in it, and an M3 thread is cut on the other, designed to ensure that the bracket is glued into the linden boss. The drive of the ailerons is carried out using duralumin torsion bars - they will require knitting needles with a thickness of 2,5 - 3 mm. In the place where the torsion bar passes through the ribs, plastic bushings are glued into the latter - segments of the used core of a gel ballpoint pen. The lower wing is fixed on the fuselage with a central beech rod 5 mm in diameter (glued into the bridge between the wing consoles) and is attracted by rubber rings to a pair of beech rods 5 mm in diameter, fixed in the lower part of the fuselage The upper wing is attached to the lower wing by means of and with the help of a set of racks and brackets made of duralumin knitting needles with a diameter of 2,5 mm. The brackets are segments of knitting needles with flattened (after heating in a candle flame) ends, in which holes with a diameter of 1,2 mm are drilled. Each of the brackets is glued into a linden boss fixed during assembly of the wing on the front and rear spars. The length of each of the mini-biplane racks is determined "in place", for which the upper and lower wing are fixed with wooden bars so that the planes of the chords of the consoles are parallel to each other, and the forward extension of the upper wing relative to the lower one is 44 mm. The ends of each of the racks are flattened on both sides (also after heating) and holes with a diameter of 1,2 mm are drilled into them; the connection of the racks with the brackets on the wing is carried out using pieces of nylon fishing line of the same diameter - they are passed through the holes in the brackets and racks, after which the ends of the fishing line are given the appearance of balls using an electric soldering iron.
Racks and brackets in the central part of the mini-biplane are mounted in the same way. The finished model is painted - it is best to do this with alkyd enamel, which does not dissolve the foam. The centering of the biplane within certain limits can be changed by moving the battery pack or, in extreme cases, by shifting the upper wing forward or backward. True, the last measure will require the alteration of all billan racks. Before the first flight, it makes sense to practice by "flying" on the spot, for which you need to wait for a good wind, hang the model on a strong thread from a tree branch or, for example, to a rope stretched between a pair of poles. The suspension point should be located near the center of gravity of the model. By controlling the joysticks of the remote control transmitter, you will learn how to control the model in both pitch and roll as a first approximation. It is far from always possible to find a suitable airfield for a radio-controlled model, so you will have to take an assistant with you on flights - at your command, he will launch the biplane into the air with a throw - strictly horizontally and against the wind. Some radio modellers (those who did not have an assistant) use a catapult consisting of a flat board and a rubber shock absorber - a tourniquet made of aircraft model rubber. The model is fixed on the catapult with the help of a simple stopper (for example, it is tied to a nail hammered into the board with twine tied "in a bow"), and a stretched rubber band is attached to it. After checking the equipment and turning on the electric motor, the pilot pulls the string, the knot is untied - and the electric plane soars into the air. In principle, a model with a well-chosen centering and not having wing and tail warps will be able to carry out a straight flight without your help - you just have to carefully adjust its position in space. Landing at first is best done in parachuting mode, for which the model is brought to the ground at low gas (also against the wind!), After which the engine is turned off, and at a height of half a meter you begin to slowly pick up the elevator joystick towards you, until the model finally will lose speed and will not go into parachuting mode. Well, if this happens over a lawn overgrown with grass - in this case, the model will have a better chance of surviving, and you will have another flight. Author: I. Khoroshevsky
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