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MODELING
Impeller. Tips for a modeler
Directory / Radio control equipment Every year there are fewer aircraft with piston engines. Lightweight, economical, running on fuel cheaper than gasoline, jet engines are replacing piston engines even from agricultural aviation. But the "piston era" still reigns in any aircraft modeling competition. On the cordodromes you will see masterfully made copies of Ilya Muromets, Nieuport, I-15, Yak-3... Don't just try to look for models of jet planes - the era of fast modern machines in modeling has not yet arrived. The reason for this is the lack of jet microengines, or at least their imitators: piston engines with impellers. Unfortunately, numerous attempts by modellers to create a quasi-jet engine were not successful - as a rule, these devices had an extremely low efficiency and did not develop the thrust necessary for a stable flight of the model. A more successful version of the impeller was built by a veteran of Soviet aircraft modeling, Gorky resident P.P. Smirnov. With a "ten-cc" engine, its two-stage fan with a dead weight of 830 g develops a thrust of 2,25 kg / s. And this is quite enough for the flight of not only a cord model, but also a radio-controlled copy model. We invite the readers of our magazine to join the work on creating a quasi-jet engine of an optimal design, to talk about independently developed designs, to share ideas and ideas. The best technical solutions will be published. When designing the impelper, I was faced with the task of obtaining maximum thrust (and, consequently, the maximum efficiency of the propeller) with a minimum fan diameter. This turned out to be possible only when using a two-stage impeller. The blades of the first and second stages are located in the annular channels between the blades of the straighteners. First, the air flow enters the inlet guide vane - an annular channel, in which 11 radial profiled blades are located. Each of them is set at an angle of 15° to the direction of the air flow, which makes it possible to twist it in the direction of rotation of the rotor. This reduces the flow velocity relative to the rotor blades of the first stage impeller. After the first fan, the air enters the intermediate straightener. Unlike the inlet, it has a larger number of blades (22), the chords of which are parallel to the channel axis. Then the air flow enters the second stage, accelerates and passes through the outlet straightener. The latter is designed similarly to the inlet and has the same number of blades, but the angle of their installation is opposite - this is necessary to equalize the flow. Then the air rushes into the receiver - most of it passes through the nozzle at high speed, and part of it flows around the engine cylinder head, cooling it. The impeller parts are made of magnesium alloys with a density of 1,78 g / cm56 - this made it possible to create a design of relatively low weight. Those who intend to repeat my quasi-jet engine should take into account that magnesium oxidizes very easily, so all parts from it must be oxidized, followed by painting and polishing. I recommend using polyurethane and pentaphthalic enamels with a preliminary primer with EP-XNUMX for this, since other paints dissolve with methanol. Very briefly about the design features. The "ten-cube" glow engine of the impeller is mounted on a frame, which is part of the outlet directing vane. The hubs of both rotors and the nose spinner are pulled together into a single block with a threaded stud screwed into the motor shaft. The front support of the block is a rolling bearing, planted in the housing of the inlet guide vane. The hubs of the rotors are machined in such a way that a gap of 0,5 mm remains between them and the housing of the intermediate straightener, which allows the rotors to rotate freely and at the same time does not create excessive resistance to the air flow. The rotors are composite, each of them is assembled from a hub and twelve blades. In the hubs for fixing the latter, 12 equally spaced radial holes were drilled. The blades have a rather complex profile - the section of each has a variable chord, thickness, and, moreover, the end section of the blade is twisted relative to the section with a chord equal to 21 mm. To more accurately align the blades with respect to the rotor hub, I used a simple template. On both fans, the blades are installed at the same angle - 41°9' to the plane of their rotation relative to the chord.
After the final assembly, the rotors are machined on a lathe so that the outer diameter of each is 98 mm - this provides an annular gap of 0,25 mm between the shells and the fan blades. And finally, the balancing of the rotors, which must be done very carefully, given the high inertial loads. When developing your design for the engine you have, please note that the indicated angle of installation of the fan blades was chosen in accordance with the frequency of its rotation (my engine, in particular, develops 14 thousand rpm), while the thrust was maximum. If your engine has a lower (or higher) speed, then the angle of installation of the blades must be increased (or decreased). The inner and outer shells of the receiver are glued from fiberglass and epoxy resin. It is best to mold them on mandrels carved from wood, pre-lubricated with parquet mastic. When drawing the contours of the shells, please note that the area of the nozzle at the outlet of the receiver should be 75-100% of the area of the annular channel of the impeller. Fastening of the receiver elements to the shells - "electron" magnesium alloy clamps. An 8 mm wide hole is cut in the outer shell, through which air enters to cool the engine. All internal surfaces of the channels must be polished, this significantly increases the efficiency of the impeller and, accordingly, the thrust. To access the spark plug electrode, a hole Ø 10 mm is cut in the fairing shell. The carburetor needle and the engine speed control lever are out of the impeller contours. The rotation of the rotor is counterclockwise when viewed from the side of the bow spinner. It is necessary to start the engine with a starter whose shaft has a trick with a rubber nozzle with an internal cone. After starting, be careful - the impeller sucks in air very intensively through the inlet guide vane. In conclusion, a few recommendations for those who want to make a similar device. You should not be confused by some "overcomplexity" of a quasi-jet engine. This is explained by the fact that my design was supposed to provide the possibility of customization - varying the angles of installation of the fan blades, selection of profiles and installation angles of the straightening vanes. Once matched to engine RPM, many design elements can be greatly simplified using advanced manufacturing techniques such as synthetic resin molding and glass and carbon fiber molding. So, in particular, you can make fan blades or even the entire fan. In general, there is something to break the head of experienced modellers.
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