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MODELING
Hang glider threshold speed alarm. Tips for a modeler
Directory / Radio control equipment Hang gliders do not need to be told how important the speed indicator is to flight safety. Such a device is especially valuable for beginners: its hint will protect the novice pilot in a timely manner from an irreparable mistake in controlling the aircraft. In hang gliding, sound speed indicators are used. But due to the complexity of assembly and the difficulty of acquiring the US-250 device, it has not yet received mass use. We bring to the attention of readers a description of the hang glider threshold speed signaling device, which is distinguished by its simplicity and reliability. It is quite possible to make it from improvised materials at home. In the pilot's headphones, the device receives sound signals of two tones - high and low, reporting the excess of the maximum allowable speed or its decrease below the minimum allowable value. The device consists of a pneumometric type sensor with a contact group (Fig. 1) and a sound generator with headphones (Fig. 3). The device is powered by a Krona VTS battery. Velocity pressure, perceived by the sensitive element of the sensor, is a parameter on which the aerodynamic forces and moments acting on the wing of a hang glider directly depend. This parameter, noted by the device, uniquely determines the planning angle and the position of the control knob, regardless of the air density (its temperature and pressure). The values of the limiting speeds depend on the correspondence between the rigidity of the working springs of the rod (contact-opening group) and the area of the diaphragm of the sensitive element of the device. These values, equal to 28±2,5 and 70±2,5 km/h, are selected by adjusting the degree of compression of the springs by changing the position of the sensor contacts.
The sensor housing (Fig. 1) consists of flat covers - blind and drained - and a cylindrical side wall, cut in height into two parts. An air pressure receiver tube is attached to the first cover with rivets, and a contact group with lead wires for connecting to a sound generator is attached to the second cover. A rubber diaphragm with a thin duralumin disk glued is sandwiched between the parts of the side wall. The structure is fastened with screws, nuts and washers. In the center of the disk, a rod with an internal spring is fixed with M4 nuts. The smooth shank of the rod moves freely in the hole of the guide bracket riveted to the bottom cover. Its upper part with a thread and a locking contact washer exits the sensor housing through the central hole of the drained cover. Together with the disc mounting nut, it fixes the internal spring, the ends of which rest against the disc and the centering plate. To reduce friction, the rod section moving in the central hole of the cover should have a smooth surface. The stud intended for the rod is turned and ground by holding an electric drill in the chuck. The resulting backlash of the rod in the hole of the centering plate, as practice has shown, does not impair the performance of the device. The electrical circuits of the signals corresponding to the minimum and maximum speeds commute the contact washer and the upper end of the rod. The first is an ordinary brass washer soldered to the gank. The fixed contact system consists of a metal bracket with an adjustable screw and a getinax sleeve. This assembly is attached to the top cover with screws and ganks using electrically insulating washers and bushings. The foil surface of the getinax plate comes into contact with the contact washer, to which the minimum speed signal wire is soldered. The maximum speed signal wire terminal is secured with the adjusting screw nut. Between it and the upper end of the rod there is an external spring centered by a contact washer and an electrically insulating sleeve of the screw. When installing the sensor on the side tube of the hang glider trapezoid, it is necessary that the axis of the rod be parallel to the ground plane - then the influence of the weight of the moving parts of the rod on the accuracy of the sensor will be minimal. The discrepancy between the sensor signal and the given values of the minimum and maximum hang glider speeds resulting from this does not exceed ±2,5 km/h. When the contact washer touches the foil surface of the plate, the outer spring is fully unclenched, and the inner spring is so compressed that, in the absence of an excess pressure drop across the diaphragm (zero speed), a force equal to the product of the effective area of the diaphragm and the minimum velocity head corresponding to the admissibly small hang glider speed value. As the speed increases, the force of the disc from the excess pressure drop exceeds the force of the compressed internal spring and the rod, moving, opens the minimum speed signal circuit. A further increase in this parameter and the movement of the rod causes compression of the outer spring. When the hang glider is flying at its maximum allowable speed, the force on the disk from the excess pressure difference is balanced by the forces of both compressed springs until the stem touches the end of the adjusting screw. The full stroke of the stem is 6,5 mm. It does not make sense to set the exact dimensions of all parts of the sensor, since most of them can be done arbitrarily, taking into account the available materials at hand. We give only the data of those parts on which the performance of the device depends. The disc is made of D16T sheet material 0,5 mm thick. The diaphragm is made of sheet rubber 0,5 mm thick, for example from the cuff of a surgical glove. The presence of a corrugation on the diaphragm (Fig. 2), which does not prevent the movement of the disk in the housing, serves as a guarantee of the sensor's operability. Such a corrugation can be molded using both parts of the side cylindrical wall as follows. The workpiece, cut along the outer diameter of the side wall (Ø 136 mm), is firmly attached with 88N glue to the end face of one of the halves of the cylindrical wall. A central round hole Ø 40 mm is cut out in the diaphragm. Then, a layer of 88H glue is applied to the surfaces to be glued between the diaphragm and the disk and dried slightly (until it sticks to the fingers). Further, after the glue has completely hardened, a 2 kg weight is placed on the disk to stretch the diaphragm. In this case, the edge of the central hole is shifted to the periphery of the disk. The corrugation obtained in this way is quite suitable for the operation of the diaphragm in the sensor.
Parts of the cylindrical side wall can be cut out with a jigsaw from plywood, and both body covers can be cut out of D16T sheet 2 mm thick. For the sensor, springs from the brushes of electric motors of vacuum cleaners are suitable; they can be made independently from steel wire Ø 4 mm. The diameter of the coil of the inner spring is 8 mm, the pitch of the coil is 2 mm, the length in the expanded state is 27 mm, the compression force is 110 g to a size of 16,5 mm. The outer spring has the same diameter and coil pitch as the inner one. But the length of ce in the expanded state is 22,5 mm, and the compression force to a size of 16 mm is 70 g. The air pressure receiver tube is made of a pipe (D16T) 12x1 mm in size. The stem and adjusting screw are made of brass or steel studs Ø 4 and Ø 6 mm respectively. To protect the contact group from contamination and mechanical damage, it is covered with a protective cover, for example, a plastic cap from an aerosol can (shown in Figure 1 by a dotted line). In the device, you can use an audio signal generator, made according to any of the two circuit diagrams (Fig. 3). The tone of the audio signal is selected using variable resistors R2, R3 (option A) and R1, R2 (option B).
The sound generator, together with the power source - the Krona battery (option A), fits in a case 30x60x80 mm in size and has a mass of 100 g. The moments of operation of the contacts, depending on the specified value of the air speed, are selected by the adjusting screw and by changing the positions of the contact washer and disk on the rod. Before installation on a hang glider, the sensor is blown in the oncoming air flow on a moving car (motorcycle), controlling its operation by signals in the head phones in accordance with the speedometer readings. Author: V.Morzobaev
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