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MODELING
Drop on the cordodrome. Tips for a modeler
Directory / Radio control equipment A lot of things were in the "fate" of a racing car! Even now, when all of them are built almost according to the same scheme, in the conversations of athletes one can hear the terms "boat", "drop", "arrow", meaning certain directions in the layout. Let's remember what a "drop" was. The name itself speaks of the shape of the hull. Aerodynamics has always argued that drop-shaped bodies have the least drag, to overcome which a considerable part of the engine power is spent. The advantage of the "drop" is that the center of gravity of the car is well located on it: it is very close to the driving wheels. And this means that a model of this type will react much less to the irregularities of the cordodrome. But this scheme has not stood the test of time and practice. The fact is that the midsection of such a model turned out to be prohibitively large due to the lack of special engines, and therefore the advantages of better streamlining simply could not appear. Yes, and the "stuffing" was mounted very inconveniently: there was a lot of free space, which should not be on a modern model. Now, when many athletes have learned to make engines that are in no way inferior to the best production models, the “drop” has again attracted attention. The fact is that you can make the motor no longer than the width of a conventional motor with mounting feet. Therefore, it is easy to place it so that the axis of the shaft becomes perpendicular to the longitudinal axis of the model. As a result, again a "drop", but with completely new properties. Carefully comparing the models of the modern school and the proposed option, one cannot but pay attention to the following. The engine can almost be "put", then the area of the midsection is reduced by about half. Aerodynamic calculations show (taking into account the change in the aspect ratio of the hull, nose and total wetted surface) that this reduces the drag by a factor of 1,89. Is it a lot? Yes, if we take into account that, according to experimental data, on a model with an internal combustion engine with a working volume of 10,0 cm3, moving at a speed of 300 km / h, the aerodynamic drag of the body is approximately 0,5 kg. It takes 0,55 liters to overcome it. With. engine! Don't you need "extra" 0,26 liters. With.! With the new layout, spur gears are "asked" for the model. According to manuals on mechanical engineering, the efficiency of such a transmission is 2,5% higher than the conical one (with the same quality of finishing). You can move on to installing a two-stage spur gearbox that is more convenient with the new scheme without power loss. The advantage of the "drop" scheme is retained - a small distance between the center of gravity of the model and the leading axle. And this is a condition for stable passage of the track.
The angle between the cylinder axis and the exhaust pipe is optimal. Studies of large internal combustion engines have shown that when the exhaust ports are opened by the edge of a downward moving piston, the exhaust gases flow out at an average angle of 30 ° relative to the cylinder axis. Exhaust conditions are the same on model engines, so the new position of the nozzle eliminates the kind of exhaust throttling caused by the sharp turn of the exhaust jet in the usual version. At the same time, the operating conditions of the resonant tube in combination with the engine are improved. The pressure wave reflected from the reverse cone of the muffler directs the gas flow not to the booster channel, but almost along the wall of the sleeve. Optimum conditions for blowing the motor are created. The head is cooled efficiently and, most importantly, evenly, which is not the case with traditional blowing. What has not been done before, trying to avoid engine warping caused by uneven cooling of the engine with a rear exhaust! They tried to use both ribs stretched back, and powerful fins on special nozzles on the exhaust pipes, bore the crankcases in a special way, tried to close the front of the cylinder jacket, blowing only the back. But ... uneven heating still led to different thermal expansion of the crankcase sections and caused a change in the geometry of the sleeve and piston, distortion of the parallelism of the axes of the shaft and piston pin. Because of this, mechanical losses increased, and the connecting rod began to "slide" off the crank. The new scheme provides, in addition to uniform cooling of the head, also the fact that the crankcase section near the exhaust windows is not in the "shadow" of the shirt, but in a clean air stream. All this reduces warping. Any knowledgeable modeler will tell you the value of this airflow option. It is especially important that as a result of warping, only an extremely small deaxia (displacement of the cylinder axis relative to the crankshaft axis) can appear, which will not affect the operation of the engine. Let's go further in our reasoning. If it is decided to make the drive axle sprung, then ... along the way, it becomes possible to use energy that was previously extinguished in springs or rubber suspension bushings. And here are the advantages of a cylindrical gearbox. There is no need to install a cardan, which was a mandatory element of the transmission. Now he will no longer "eat" his percentage of engine power. The influence of the gyroscopic moment of the rotating flywheel on the load distribution between the rear and front axles of the wheels of the model disappears. Only heeling moments remain, which are easy to deal with by selecting the attachment point of the cord strap. There are no connections between the axes and the body. Since the moto-installation together with the drive axle is assembled separately, the influence of body deformations during the race is eliminated.
The crankcase is made of heat-treated steel 30KhGSA. It has a fairly simple shape, but the small thickness of the walls somewhat changes the technology of its processing. Bypass channels are milled in the rough-bored workpiece, and the outer shape is finished on the same machine. Then follows the thermal process of normalization, and only after it - the boring of cylindrical inner surfaces and landing ends. Bearing wall made of the same steel. It has sockets for bearings No. 1000900 (10x22) of the crankshaft. The fastening tabs are processed on a milling machine. The wall is attached to the crankcase with M4 screws. The distribution wall is also made of steel 30KhGSA. The inlet channel is drilled, with the help of cutters it is given the required shape. The part is fastened in the same way as the bearing part: with four M4 screws. It is possible to underestimate the inner end of the wall by 0,5 mm, leaving intact only a two-millimeter-wide belt around the inlet and along the edge of this end. The crankshaft is made of steel 38HMYuA, nitrided, heat treated and ground in a mandrel. Its features are a toothed belt, threaded sockets in the shaft and crank pin, a conical entrance to the socket in the pin is subjected to preliminary knurling. The preparation of the crankshaft ends with pressing on the sealing belt, which simultaneously plays the role of an additional flywheel. Gear: Z = 30, module - 1.0. The connecting rod in both heads has pressed bronze bushings, made on one side (outer when the model moves along the ring track of the cordodrome) with a shoulder. The spool is a figured polished plate made of alloy steel with a thickness of 0,4-0,5 mm. A flange is welded to it, the cone of which is pressed with an M3,5 screw to the knurled surface of the socket in the crank pin. When assembling the engine, make sure that there is a gap of 0,08-0,1 mm between the spool and the end of the distribution wall. Piston pin lightened by drilling, heat treated and ground. Material - ШХ15. The piston is extremely light. Large recesses in the skirt are designed not only to reduce weight, but also to effectively cool the flow of fresh mixture entering the crankcase. The threaded ring fixing the insert with the pin and the connecting rod simultaneously protects the pin from axial movement. The maximum relief of the piston group makes it possible to make centrifugal loads longitudinally small, increase the operating speed of the motor and limit vibrations. Balancing is carried out by pressing plugs from VNM type alloy into the cheek of the crank. Piston material - Al-26. Sleeve made of brass LS-62. The working surface (mirror) is chrome-plated and lapped. It is inserted freely into a steel shirt. The head to reduce stress in the sleeve is made to be screwed onto the shirt. It differs from conventional designs in relatively powerful radial cooling fins and vortex-forming grooves on the inner surface. Made on the material AK4-1T.
The driving axle does not differ from the usual ones. The fork (30HGSA) is as light as possible and has bronze bushings. Drive axle bearings - No. 1000098 (8x19). The same is true for the intermediate gear. Its axle, which simultaneously serves as a fork hinge pin, is made of steel and is pressed into the crankcase eye during assembly. Has sockets for fixing bolts on both sides. Gears - from steel 40X. Hardness after carburizing and quenching 45HRC. Intermediate Z \u40d 45, and driven Z \u4,3d XNUMX. The width of the crown in all cases is XNUMX mm. Driving wheels of lightweight design, duralumin, with vulcanized rubber. Attached to the hubs with screws. The lightening windows are sawn and, after balancing the wheels, they are sealed with balsa plugs, which significantly reduces the disturbances introduced into the flow around. Description of the model The frame (pallet) is milled from a bar of duralumin (D16T). It differs relatively large in comparison with the dimensions of the entire body height. This allows you to make it more rigid and well-designed engine mount. When processing the frame, pay special attention to the seats of the power plant: a recess for the bearing wall and a hole for the distribution wall pin. An air intake for the carburetor is also machined from the pallet material. The gap between them is well sealed. This will ensure full use of the energy of the oncoming flow for boosting the engine. it is not so small - the velocity head when the model reaches 300 km / h is 0,04 atm. Do you want to imagine what kind of pressure it is? Then try to clamp the outlet of the home vacuum cleaner with your hand - the pressure is the same there. The upper fairing (body) is hollowed out of linden and glued on all sides with 0,2 mm thick fiberglass fabric on epoxy resin. The channel for supplying cooling air to the motor head is made of three layers of the same fiberglass on a foam mandrel and glued into the body. Knife-shaped front wheels are made using conventional technology. Their distinctive feature is their small diameter and rather large distance between them. The first allows you to aerodynamically correctly solve the nose of the model (the most important in terms of flow) and reduce the weight of unsprung parts, although it imposes increased requirements on rubber. The second makes it possible, oddly enough, to get rid of excess air resistance. The fact is that closely spaced wheels rotate the entire annular layer of the flow located between the disks. The result is the same as if we put one nose wheel, but with a treadmill width equal to the distance between the discs. The front axle is a conventional pendulum fork with bored bearing seats No. 1000095 (5X13). Carefully consider the selection of the material of the shock absorber. The cross-section and elasticity of the rubber from which it is made determine the operation of the entire bridge on cordodroms of varying quality. The tank with a volume of 80 cm3 is soldered from 0,4 mm thick sheet metal. It is mounted on rubber gaskets that dramatically reduce fuel foaming. The stop device works on the principle of a latch, pinching the rubber supply tube. The shock absorber of the drive axle is a conventional spring type. It is necessary to provide for the possibility of adjusting the tightening of the spring itself - this will come in handy when debugging the "projectile". Pay attention to two more characteristic details. The first - the air intake is located on the inside of the model, and the exhaust channel of the resonant pipe is bent into the outside. This is done in order to try to avoid passing the distance in the exhaust stream. After all, our "shell" is in the same place in a matter of fractions of a second! And the ingress of exhaust gases into the air intake can not be considered useful. Second, there is no flywheel on the model. Calculations have shown that the existing rotating parts are quite sufficient for normal operation. So, if you're concerned about the light weight of this microcar, use an extra load to increase the traction of the wheels on the track. In conclusion, I would like to turn to those who are not afraid to experiment. We want to propose a scheme with front drive wheels. Using a horizontally located engine with a resonant exhaust pipe bifurcated in the area of these wheels (or smoothly enveloping them) can dramatically improve the aerodynamics of the model. The compressed front drive wheels are closed in front from below with a wedge-shaped air divider, most of them are inside the body, and the protruding top is hooded with a well-streamlined aircraft-type "lantern". This allows you to make the midship area even smaller with a successful elliptical cross-sectional shape. Fully closed wheels will not mix the incoming air even with their sides and create a vortex wake in which the entire tail section of the model is hidden, nullifying the influence of its shape on the value of the total aerodynamic drag. But the creation of such a vortex "tail" takes a lot of energy! If the wheels are closed and only a small part of them meets the oncoming stream with their rectangular "foreheads", it makes sense to make the rear part of the body slick as well. By the way, this scheme of the model has another advantage: it does not have a tendency to lift its nose, tearing off the wheels from the track of the cordodrome. An unusual motor mount using the most promising crankshaft design, a two-bearing one, will also be of some interest. The advantage of such a motor is that the volume of the crankcase can be reduced to a minimum, which will provide the most rational shape of the bypass channels. If you think that this is not necessary, then make the connecting rod longer: in this way, the side loads on the piston will decrease compared to conventional motors. Suction will go through the reed valve ending in the cavity of the booster channel. In addition, with a slight increase in the track, you can go to a single-stage spur gearbox, fully using the advantage of such gears. Gain with a motor power of 3 liters. With. compared to bevel gears will be almost 0,1 l. With.! True, one cannot do without a flywheel, since the mass and number of rotating elements are small. However, it is simple to perform it - it is necessary to press a ring machined from an alloy of the VNM type, which has a large specific gravity, onto the cheek of the crank. Author: V.Tikhomirov
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