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MODELING
Glider with water cannon. Tips for a modeller
Directory / Radio control equipment What kind of engines do not use ship modelers for their models! Among them are the simplest - rubber-engine, and the most complex - internal combustion. They install on models and electric motors, and miniature steam engines, and even pulsating jet engines ... We want to acquaint ship modelers with another simple engine - a hydropneumatic water cannon. In addition to simplicity, its advantages include the fact that it has no moving parts and, therefore, practically does not wear out - a kind of "perpetual" motion machine. Modellers could have received such a water cannon much earlier - say, twenty or thirty years ago, but at that time there were no light and durable cylinders capable of withstanding sufficiently high pressure. Readers probably guessed that we are talking about bottles made of transparent PTF plastic - all kinds of carbonated drinks are now sold in such bottles.
To turn a bottle into an engine, it is enough to equip it with a special fitting, which simultaneously performs the function of a jet nozzle. The bottle, approximately 2/3 filled with water, is installed in a special slot in the hull of the ship model and filled with air compressed to several atmospheres using a car pump. Next, the model is placed on the water, the fitting is released from the tip of the car pump - and the reaction of the water jet from the nozzle accelerates the glider to quite a decent speed. For a small glider of the simplest design, about 600 mm long, a 1-liter bottle can become the basis of a hydropneumatic water cannon - its length is usually about 325 mm and a diameter of about 80 mm. For a fitting, a metal (preferably duralumin) tube about 140 mm long with an outer diameter of 8 mm and a wall thickness of 1 mm is quite suitable. A thread M8x0,5 is cut from one end of the tube; you will also need two nuts with the same thread for attaching the fitting to the bottle cap. The joint is sealed with two washers and an automotive self-hardening sealant. That part of the fitting, which is inside the bottle, has a curved shape - this allows you to almost completely use the so-called "working fluid" - in this case, water poured into a plastic bottle. The ship model itself is a kind of trimaran, assembled using a simplified technology from PS foam and plywood frames.
At the beginning, the frames are cut out - for this, plywood about 3 mm thick is used. It should be noted that from the side of the deck it is necessary to leave a technological allowance with a width of 10 mm. Next, a slipway board with dimensions of 200x700x30 mm is prepared, on which the axis of symmetry of the hull and the axes of the frames are marked. Then, using a circular saw, cuts are made in the board along the axes of the frames to a depth of 10 mm. The width of the cuts should be such that the frame can be easily inserted into them and fixed here with miniature wooden wedges. Thus, all the frames are exposed on the slipway, the space between them is filled with foam blocks, which are fixed in place with epoxy glue. After the polymerization of the resin, the excess foam is cut off with a sharpened knife, and the resulting body is processed with skins of various grain sizes and pasted over with two layers of fiberglass using an epoxy binder. After the resin has cured, the body is removed from the slipway and a recess for the hydropneumatic engine is selected in it, and the technological allowance is cut off from the frames. It remains to paste over the deck with fiberglass, prime the hull with epoxy, sand it and paint it with car enamel of a suitable color. The steering wheel of the model is cut out of a duralumin profile "taurus" or "corner" with a thickness of shelves of 1-2 mm. If desired, a fairing lantern can be installed on the body - the easiest way is to make it from a two-liter plastic bottle by cutting out a suitable section of the shell from it. Having adjusted the fairing to the plane of the deck, you should outline the junction with a pencil, cut the foam plastic with a sharp knife to a depth of 3 - 5 mm and fix the shell in the slot with the same epoxy glue. It is best to paint such a lantern from the inside - then the outside will turn out to be glossy and uniform. It is preferable to launch the model on a closed reservoir. It should be taken into account that its speed, as well as the duration of the operation of the hydropneumatic water jet, will depend on the flow section of the nozzle. To choose the optimal one, it is most convenient to glue a plastic nozzle into the jet and make a series of test runs, gradually expanding the hole with drills of increasing diameter. In the process of experiments, it remains only to record the data of the boat - the speed and duration of movement.
It should be warned that before launching it makes sense to check the plastic cylinder for strength by inflating it with air under pressure 0,5 atmospheres higher than the working one. As mentioned above, the bottle should be filled with water to 2/3 of the volume - in this case, the optimal duration of swimming and the speed of movement of the model are obtained. For pumping air, it is best to use a foot-operated car pump equipped with a pressure gauge and a long hose with a “frog” tip at its end. The latter, in addition, is a kind of starting device - after filling the cylinder with water and pumping air, the model is installed on the water and only after that, by turning the "frog" lever, it is released from the filling hose. Author: I. Terekhov
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