The speed meter is a popular device that is used for various purposes. It measures the speed of movement of objects and substances in kilometers per hour or meters per second.
Types of speed meters
The speed meter is a very accurate equipment that is used almost everywhere in various industries and households. Its design has been repeatedly modernized for specific purposes. There are the following types of speed meters:
- Speedometer.
- Radar.
- Anemometer.
- Chronograph.
- Gas flow meter.
- Speedometer for water.
Speedometer
The speedometer is a device for measuring the speed of wheeled vehicles. It is installed on the instrument panel of cars, agricultural machinery, special equipment and trains. It is mechanical, electronic and electromechanical.
Mechanical the device is equipped with a cable that acts as a drive. The cable is connected to the gearbox or directly to the wheel axle. One of its revolutions corresponds to the revolution of the wheel and, accordingly, the passage of a certain distance. A special mechanism with gears quickly calculates the correspondence between the distance traveled for a certain period of time and the speed in kilometers per hour. Such equipment is equipped with a digital scale and an arrow that indicates the achieved speed. Mechanical speedometers are still in use today. Their main disadvantage is the periodic wear of the cable, which must be changed. In addition to the current speed indication, mechanical models have a dial built into the case, showing the mileage of the vehicle since the start of its operation.
Electronic speedometers are equipped with sensors that transmit information electronically to the dial on the instrument panel. It is displayed as glowing numbers. The absence of arrows allows for a more comfortable visual assessment of the speed indicators.
Electromechanical speedometers are a hybrid of the two types. In them, the removal of indicators is carried out by an electric sensor, but the output of data on the developed rate of movement is carried out using an arrow.
Radar
Radar is a device designed to measure the speed of a moving object without physical contact with it. Typically, such equipment is used by law enforcement agencies, as well as sports referees. The principle of operation of the device is that it creates a radio signal that is directed to a moving object. After reaching the wave to the car or other object, the wave is reflected and returned to the sensitive element of the device. Based on the characteristics of the reflected wave, the device calculates the speed at which the object was moving. There is also a device where a laser beam is sent instead of a radio signal. The speed displayed on the dial is expressed in kilometers per hour.
This equipment is not ideal and gives a small error, which is indicated by the manufacturer. Radars differ among themselves not only in the accuracy class, but also in the measurement distance. It all depends on the power of the emitter and the sensitive element that receives the reflected signals.
Modern radars differ significantly from the first devices of this class. The fact is that due to the presence of fines for speeding, the production of so-called radar detectors began to protect against such troubles. These equipments allow you to jam radio signals and shoot down the indicators that the radar gives out. In this regard, police speed meters began to be equipped with an encryption system with a special technology for sending impulses and their perception. This is not to say that this gives a 100% guarantee against error, but at least it allows you to ignore jamming from most devices that suppress signals.
Anemometer
An anemometer is a measure of the speed of movement of air and gas flows. The principle of its operation is the presence of blades similar to those used in fans or in aviation. When wind passes through the diffuser of the anemometer, the blades begin to rotate. A special mechanism measures the rotational speed and determines the speed of the flow in kilometers per hour or meters per second. Such equipment is commonly used by meteorologists to calculate weather changes. According to the characteristics of the wind movement, it is determined how long the cyclone will reach a certain area.
In everyday life, anemometers have found their application in aviation. They are installed at airfields to determine the parameters of wind force in order to correct pilots by controllers when landing aircraft. Anemometers are used by military snipers to correct the direction of bullet flight. With the help of special tables, the drift angle of a bullet is determined by the wind during flight. The weaker the air flow, the smoother the trajectory you need to fire the bullet. This indicator is important when shooting at long distances.
Anemometers are used in ventilation systems. With their help, fans are adjusted to fine-tune ventilation without creating drafts. The output of speed indicators is carried out with the help of an arrow, as in conventional speedometers for a car, or on a dial if the device is electronic or electromechanical.
Such equipment is not always mechanically driven. There are also anemometers with a heat-sensitive element, which begins to deform when cooled. When the air flow moves, the sensitive element is blown, and its temperature decreases. At the same time, complex calculations are carried out by the equipment, as a result of which accurate wind speed indicators are displayed, adjusted for the temperature of the air itself. One of the latest inventions was ultrasonic anemometers, which analyze the dissolution of sound sent against the movement of air masses.
Chronograph
A chronograph is a versatile piece of equipment that can be used for a variety of purposes. One way to use it is to measure the speed of a bullet fired from an airgun or firearm. The main features of such devices are that they give accurate indicators of the speed of movement of small objects. Such a speed meter will make it possible to take indicators of the characteristics of the movement of an arrow fired from a bow, a bolt from a crossbow or a pebble from a slingshot.
The chronograph records the characteristics of the flight of a bullet or other small object in meters per second. Also, individual models may have the ability to switch indicators to kilometers per hour. Chronographs are complex and very sensitive. Those devices that are used to measure the speed of movement of bullets and other ammunition are made in two versions - muzzle and frame.
The muzzle chronograph is mounted on the muzzle of an airgun or firearm. With it, it will be possible to determine the initial velocity of the bullet. According to this indicator, one can judge the power of the weapon and its penetrating power at a certain distance. To connect the chronograph to the muzzle of a weapon, a special adapter is required. The adapter is different for different types of weapons, but the bullet velocity meter itself can almost always be used. Chronographs that are used for pneumatic weapons have a measurement range of up to 350-400 m/s. Firearms equipment has a much larger range of sensitivity.
Frame chronograph is more versatile. It is made in the form of a frame in which you need to aim so that the bullet flies between the walls. With this chronograph, you can measure the speed of almost any small object. It can be an arrow and even a stone thrown by hand. Such equipment is more dimensional, but due to its versatility it is very popular.
Gas flow meter
There are also speed meters for gas and air flows that move inside pipes. These devices are fixed on pipelines and are equipped with an impeller that rotates when in contact with the medium. Such equipment has much in common with gas meters, but unlike them, it does not show how much volume was passed in total, but allows you to calculate how much gas can be carried out at such a pumping intensity in a certain period of time. Such equipment gives indicators not only in meters per second, but also in volume. It can be liters or cubic meters.
The intensity of pressure on the impeller in different gases is different. In this regard, the equipment is calibrated by the manufacturer for the environment with which it will work. Thus, if the speed meter is designed for natural gas, it will not give accurate readings in the case of carbon dioxide. In addition to equipment for substances in the liquid state, there are meters for gaseous media such as air and even steam.
Speedometer for water
The water velocity meter has a similar design as for the gaseous medium. It is used in exceptional cases when you need to know the speed of the water flow, and not the volume of pumping. This indicator is important when testing equipment for fire extinguishing, water guns and for other purposes. Such a speedometer is an elongated tube that is connected to a flexible hose or pipeline. In addition to devices with a rotating impeller, readings can be carried out with a laser or ultrasonic waves.
Speedometers
The speedometer informs the driver about the speed of the car and the distance traveled, and combines two measuring devices - a speed indicator and a distance meter called an odometer.
The speedometer is an important control and measuring instrument, as it informs the driver about the safe mode of movement, therefore, the operation of a car with a faulty speedometer is prohibited by the rules of the road.
It is believed that the speedometer (from the English "speed" - speed) was invented in 1801 by our compatriot, a self-taught serf mechanic Yegor Kuznetsov. He adapted a counter of his own design to the horse-drawn carriage, which allows not only to count the number of fathoms and miles traveled, but also the speed of movement.
The curiosity, which was called the "verstometer" was shown to Emperor Alexander I and for some time amused the courtiers.
Then, as often happened in Russia, the "verstometer" was forgotten for a long time.
And only two hundred years later, employees of the St. Petersburg Hermitage discovered this unique device in one of the vaults of the famous museum. It was restored and exhibited in the museum exhibition.
The first device for measuring speed was installed on a car in 1901. Until 1910, the speedometer was considered an outlandish thing and was installed as an optional option, only years later, car factories began to include it in the mandatory equipment of cars.
The design of the speedometer, invented in 1916 by Nikola Tesla, has survived to this day, practically unchanged.
Speedometers are driven by an electric drive or a flexible shaft (a mechanical drive, commonly referred to as a "speedometer cable"). The type of speedometer drive depends on the remoteness of the device and the place of its connection to the vehicle transmission.
Flexible shafts for the drive are recommended to be installed if the length of the route does not exceed 3.55 meters. For longer runs, an electric drive is recommended.
The speedometer drive is carried out from the driven shaft of the gearbox or transfer case. To do this, in the node from which the drive is carried out, a gearbox is installed, the gear ratio of which is selected depending on the gear ratio of the main gear and the rolling radius of the car wheel.
The gearbox is connected to the speedometer either mechanically (flexible shaft) or electrically (using a special sensor). The signal from the gearbox (or the sensor driven by the gearbox) is sent to the speedometer, where it is converted into the corresponding information.
Additional information about automobile speedometers and their drives can be obtained.
Speedometers with a mechanical drive (from a flexible shaft)
All speedometers driven by a flexible shaft have the same principle of operation and differ only in the design features of the high-speed and counting units and in their external design.
On rice. 1 shows a mechanically driven speedometer (from a flexible shaft), which is driven from the input roller 1 with a square section socket into which the square tip of the flexible shaft is inserted. A permanent magnet is fixed at the other end of the input roller. 5 and thermal compensation washer (magnetic circuit) 4 . Magnet 5 magnetized so that its poles are directed towards the edges of the disk.
Rice. 1. Flexible shaft speedometer: 1 - input roller; 2 - felt wick; 3 - stub; 4 - washer; 5 - magnet; 6 - coil; 7 - screen; 8 - axis; 9 - lever; 10 - spiral spring; 11 - arrow; 12, 13 - rollers
on axle 8
, freely rotating in two bearings, an arrow is fixed on one side 11
, and on the other hand, a coil 6
. The coil is most often made in the form of a bowl, which covers the magnet with some clearance. 5
. The coil is made of a non-magnetic material such as aluminium. outside coil 6
covered by a screen 7
made of soft magnetic material that concentrates the magnetic field of the magnet 5
in the coil area.
From arrow side to axis 8
coil spring attached at one end 10
. The other end of the spring is attached to the lever 9
, by turning which you can adjust the tension of the coil spring.
When the car moves from the flexible shaft, the input roller is rotated 1
and with it a magnet 5
. At the same time, its magnetic flux, penetrating the coil 6
, induces eddy currents in it, which cause the formation of a magnetic field of the coil.
Two magnetic fields (magnet and coil) interact with each other in such a way that a torque acts on the coil, the direction of which is opposite to the moment created by the spring. As a result, the coil, together with the axis and the arrow, will turn through an angle at which the increasing moment of the elastic forces of the spring will become equal to the moment of the magnetic forces acting on the coil.
Since the torque of the coil is proportional to the speed of rotation of the magnet, and, consequently, the speed of the car, the angle of rotation of the coil and the arrows increase with increasing speed.
Thermal compensation washer 4 installed with a magnet 5 , neutralizes the effect of changes in ambient temperature on the resistance of the coil. An increase in the resistance of the coil leads to a decrease in the currents induced in it and the magnetic flux caused by them. Washer 4 at the same time, it provides an increase in the magnetic flux penetrating the coil by changing the magnetic permeability.
Roller 1 most speedometers are equipped with an oiler installed in the tail of the speedometer. It consists of a plug 3 with a hole, and a felt wick located under it 2 , which is impregnated with oil and lubricates the roller.
The drive of the counting unit is carried out from the input roller 1 through rollers 12 And 13 by means of three reduction worm gears connected in series. Worm gears provide gear ratio 624 or 1000 .
By design, counting units come with external and internal engagement of counting drums. Typically, the counting unit contains six drums that are loosely mounted on one axis.
With external engagement ( rice. 2) each drum 7
on the one hand has 20
teeth 4
, which are in constant engagement with the teeth of the triboks 8
also freely rotating on their own axis.
On the side opposite to the gear, the drums, except for the extreme left, have two teeth 5
with a gap between them. Each tribka has six teeth. Three prongs on the side of two prongs 5
drums are shortened in width through one.
Rice. 2. Counting unit with external gearing: 1, 3 - long pinion teeth; 2 - the tooth of the pinion shortened in width; 4 - teeth of the drum; 5 - two drum teeth; 6 - notch, shortening the tooth of the tribka; 7 - drum; 8 - tribka
The rightmost drum is constantly driven by a worm gear. When two teeth 5
approach the shortened prong of the tribe, they grab it and turn it on 1/3
turnover. At the same time, the next drum rotates by 1/10
turnover.
The turned pinion after turning is installed so that during the next pass of the teeth 5
they will again capture the shortened prong.
The tribka cannot stop in another position, as this is prevented by long teeth sliding along the cylindrical part of the drum.
This ensures that each drum is rotated by 1/10 with full rotation of the previous one. With this design, every 100 thousand revolutions the initial (right) drum, the full rotation of which corresponds to 1 km vehicle mileage, all drums return to their original position, and the reading starts from zero.
On rice. 2 the device of the speedometer 16.3802, installed on UAZ vehicles, is shown. Speedometer 16.3802 mechanical, driven by a flexible shaft from the transfer case. Consists of a pointer indicator of the speed of the car and a total counter of the distance traveled. Equipped with high beam indicator.
Rice. 2. UAZ car speedometer: 1 - drive roller; 2 - felt with a supply of lubricant; 3 - hole for lubrication; 4 - permanent magnet; 5 - coil; 6 - return spring arrows; 7 - adjusting spring tension plate; 8 - pointer axis bearing; 9 - bracket drums; 10 - arrow; 11 - arrow axis; 12 - axis of drums; 13 - counting drum gear; 14 - mechanism body; 15 - intermediate worm roller; 16 - horizontal worm roller; 17 - screen; 18 - arrow stand; 19 - tribka bracket; 20 - tribe; 21 - counting drum; 22 - locking plate
The main characteristics of the speedometer 16.3802:
- Speed indication range, km/h: 0-120;
- Value of division, km/h: 5;
- Distance meter capacity, km: 99999.9;
- The number of revolutions of the drive shaft corresponding to 1 km mileage: 624 ;
- Housing diameter ( mm): 100 ;
- Mounting dimensions with flexible shaft, mm: M18×1.5 square 2,67 ;
- Weight, kg: 0.54.
Electric speedometers
Electrically driven speedometers have the same magnetic induction and counting units as mechanically driven speedometers.
The speedometer electric drive consists of a sensor that is installed on the gearbox, an electric motor that rotates the drive roller of the magnetic induction assembly of the pointer, and an electronic motor control device. The electric motor and control device are mounted in one housing with a magnetic induction unit.
The electric drive sensor is a three-phase alternator, the rotor of which is a permanent four-pole magnet. Like the flexible shaft, the encoder rotor is driven by the output shaft of the gearbox.
When the rotor rotates in each phase of the stator connected by a "star" ( rice. 4), a variable sinusoidal EMF is generated, the frequency of which is proportional to the speed of the gearbox shaft, and hence the speed of the vehicle. Each stator phase signal drives the transistors VT1, VT2 And VT3 operating in the electric key mode.
The collector-emitter circuits of the transistors are included in the circuits of the phase windings of a three-phase synchronous motor. The rotor of the electric motor is a four-pole permanent magnet. When a positive EMF half-wave arrives from the phase winding of the sensor to the base of the corresponding transistor, it opens, and current will flow through the corresponding phase winding of the motor.
Since the phase windings of the sensor are shifted by 120
˚, then the opening of the transistors will also be shifted in time. Therefore, the magnetic field of the stator of the electric motor, created by its windings, also shifted by 120
˚, will rotate at the speed of the encoder rotor.
The rotating magnetic field of the stator, acting on the permanent magnet of the rotor, causes it to rotate at the same frequency.
Resistors R1-R6 in the electronic key circuit, the conditions for switching transistors are improved.
Tachometers
Instruments that measure the speed of the crankshaft are divided into tachometers, fixing the number of revolutions per minute at the moment, and tachoscopes - counters that show the number of revolutions of the shaft at a certain point in time. Tachoscopes are used when testing engines after overhaul, and are not installed on cars.
Tachometers are used on cars if there is a need to control the engine speed. According to the principle of operation, pressure gauges are centrifugal, electric, electronic (pulse), magnetic (induction), stoboscopic, etc. On cars, electric tachometers are most widely used, providing remote measurement of the crankshaft speed.
On diesel engines, the tachometer is driven from the engine camshaft using a flexible shaft or an electric drive. Tachometers of magnetic induction type, installed to control the frequency of rotation of the crankshaft of a diesel engine, have an electric drive. Their design is similar to that of an electrically driven speedometer. They differ in the absence of a counting node.
On carburetor engines, electronic tachometers are usually installed to control the speed of the crankshaft, the principle of which is based on measuring the frequency of pulses that occur in the primary circuit of the ignition system when the primary circuit is opened.
Electronic tachometer circuit ( rice. 5) provides measurements of the frequency of current interruption in the primary circuit of the ignition system.
Rice. 5. Electronic tachometer circuit
The circuit consists of three nodes: a node for generating triggering pulses, a node for generating measuring pulses and a pointer magnetoelectric device.
The tachometer receives an input signal I from the primary circuit of the ignition system. Trigger pulse forming unit, consisting of resistors R1, R2, capacitors C1, C2, C3, C4 and zener diode VD1, extracts from a damped sinusoidal signal I signal II, having the form of a single pulse, which enters the base of the transistor VT1 unit for the formation of measuring pulses.
In the initial state, the transistor VT2 open, because through resistors R11, R10 And R5 base current flows through it, and the capacitor C5 charged.
Transistor VT1 at this time is closed, since the potential of its emitter, caused by a significant voltage drop across the resistor R5, more base capacity.
When a positive momentum II goes to the base of the transistor VT1, it opens. Capacitor C5 discharged through an open transistor VT1, creating on the basis of a transistor VT2 a negative offset that locks it.
Transistor VT1 maintained by open base current flowing through resistors R11, R9, R8 And R5. open transistor VT1 allows current to flow through the meter through the resistors R11, R7, R3 And R5.
Pulse duration III current flowing through the measuring device is determined by the discharge time of the capacitor C5.
After the discharge of the capacitor C5, the transistor VT2 opens, as the negative bias at its base disappears, and the transistor VT1 closes.
Pulse frequency III current is equal to the frequency of openings of the primary circuit of the ignition system. Effective value of current pulses I ef, proportional to their frequency, shows the device.
variable resistor R7 when setting, adjust the amplitude of the pulsed current.
Thermistor R3 compensates for the temperature error of the device.
Diode VD2 serves to protect the transistor VT1.
zener diode VD3 provides stabilization of the supply voltage of the device.
Instruments for measuring speed and flow 10- 8
Instruments for measuring speed
Hydrodynamic tubes, hot-wire anemometers and hydrometric turntables are used to measure local velocities.
The determination of velocities with the help of hydrodynamic tubes is based on the measurement of velocity pressure equal to the difference between the total 
and static pressure in the flow. The total head is measured by a full head tube, which is a tube bent at a right angle with its open end facing upstream (Figure 4).
AND
,
where 
Picture 4 - Tubes of full and static pressure
The tube of full head and static head are structurally combined in one device and represent a hydrodynamic tube. Pitot-Prandtl (Figure 5). The full pressure receiver is the hole 1 of the axial channel of the cylinder, which communicates through the full pressure tube 6, placed in the holder, with the fitting 9. To receive static pressure 
grooves 7 are made on the side surface of the cylinder, covered by a casing 4 with slots 3.
Figure 5 - Pitot-Prandtl hydrodynamic tube with a spherical toe
Hydrodynamic tubes of a different design are also used. Local speed (speed at a point) is determined by the formula
,
Where 
- correction factor determined by calibrating the tube.
Hydrodynamic tubes are applicable for measuring velocities over 1 m/s.
Thermoelectric anemometers
The action of hot-wire anemometers is based on the use of the relationship between the electrical resistance of conductors and their temperature. A hot-wire anemometer is a wire made of an inert metal (platinum, tungsten, nickel) soldered to two electrodes fixed in a holder (Figure 6). Wire thickness 0.005-0.01 mm, length 1-3 mm. The wire is placed in the stream and heated by an electric current. The flow around the wire cools it, while the electrical resistance of the wire changes by a certain amount depending on the flow rate, fixing this change with the help of appropriate electrical circuits, it is possible to determine the value of the local flow rate normal to the wire.
Figure 6 - Electrical circuit diagram and calibration curve
hot-wire anemometer operating according to the constant current method:
- flow rate; 
- voltage
Hydrodynamic pinwheel
It is a paddle wheel placed in the stream and driven by it into rotation (Figure 7). During the measurement, the velocity of the oncoming flow is fixed. The turntable is pre-calibrated and supplied with a calibration schedule 
Figure 7 - Hydrometric spinner
Instruments for measuring flow and quantity of liquid
A means of measuring the flow or amount of liquid is called flow transducer.
By type of measured medium Distinguish between liquid, gas and steam flow meters. The same flowmeter model cannot be used to measure different media - the physical parameters are too different.
Liquid means any type of dripping liquids (water, fuel oil, oil, and other technical liquids)
Gas refers to natural (methane) or technical (oxygen, hydrogen, etc.) gas, as well as compressed air.
Steam can be used dry saturated or superheated. For wet steam correct flow measurement impossible. The maximum steam pressure and temperature are specified.
By output signal– with analog, pulse or digital output.
|
According to the principle of action |
|
|
measured containers (calibrated tank, tank) | |
|
measured weirs (float flow meters) | |
|
with variable cross-sectional area - rotameters | |
|
variable differential pressure - diaphragms, nozzles and Venturi tubes | |
|
tachometric | |
|
electromagnetic (induction) | |
|
ultrasonic * 1 | |
|
vortex | |
|
coriolis | |
Measuring containers
With the volumetric method of measuring the flow of liquid, the liquid enters a carefully calibrated reservoir (mernik), while filling time is fixed 
a certain amount 
. The volume flow is
.
The method of measuring flow using a measuring tank is the most accurate. It is widely used in laboratory practice for experimental studies and verification of flow meters.
Weirs
Serve to measure water flow in laboratories and irrigation systems. An example is a triangular weir with a thin wall in a lab.
Variable pressure flowmeters
Variable differential pressure flowmeters are measuring complexes based on the dependence of the pressure drop created by a device installed in the pipeline on the flow rate of a liquid or gas.
The composition of the complex:
Primary flow converter (hydraulic resistance, Pitot tube);
primary communication lines - connecting tubes and auxiliary devices on them (settling vessels, air collectors);
primary measuring device - differential pressure gauge;
secondary communication lines (electrical wires)
electronic converter (recording, indicating)
|
Variable pressure flowmeters |
|
|
with narrowing device |
Standard - orifice, nozzle, venturi tube - do not require individual calibration. |
|
with hydraulic resistance |
for example - ball packing |
|
with pressure device |
The principle of operation is based on measuring the pressure drop that occurs when kinetic energy is converted into potential energy. Example - Pitot-Prandtl tube or averaging pressure tubes installed across a pipeline |
|
centrifugal flow meters |
based on the dependence of the flow rate on the pressure drop formed on the rounded element of the pipeline (elbow) under the action of centrifugal forces |
Figure 8 – Variable differential pressure flowmeters:
a - diaphragm; b - nozzle; c - Venturi pipe
The fluid flow rate is determined by the formula
or 
Where 
- flow rate,
- area of the passage section of the narrowing device;
- difference of static pressures,
.
- pressure difference before and after the narrowing device
- density of the measured medium (depends on temperature and pressure)
High-speed meters are most often used to control the amount of water consumed in water supply systems. There are high-speed counters with a vertical impeller (vane) and with screw turntables (turbine).
The vane counter consists (Figure 9) of the impeller 1 and the transmission mechanism 8 associated with the counting mechanism 9. The transmission and counting mechanism is a series of gears in series.
The flow rate of the liquid is determined by the ratio of the volume of liquid passed through the meter 
for a certain time to time 
.
The rotameter (Figure 10) is a conical transparent glass tube 1 (taper angle from 35 to 5 o 35 //) with a float 2 placed inside it.
Figure 9 - Vertical impeller meter Figure 10 - Rotameter
The rotameter is installed on a vertical section of the pipeline. If the force acting on the float exceeds the weight of the float, then the float floats, increasing the area of the slot for the liquid to flow, while the force acting on the float from the liquid decreases. When the hydrodynamic force becomes equal to the weight of the float, its ascent stops.
Flow measurement with a rotameter is based on the relationship between flow and float position. The nature of this relationship depends on the taper angle of the tube, the shape and weight of the float, the viscosity of the liquid and is usually established by individual calibration of rotameters.
Rotameters are used to measure liquid and gas flow rates in a wide range, starting from small ones, on the order of 0.1 cm 3 /s. The measurement error does not exceed 6%. Their disadvantage is the dependence of readings on the physical properties of the liquid and the inability to measure time-varying costs.
1Note: Not "ultra" but "ultra"!
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For aircraft, a distinction is made between true, airborne, indicated airspeed and ground speed.
True airspeed is the speed of the aircraft relative to the air.
The indicated (or indicator) airspeed is the true airspeed reduced to the normal (mass) air density. This speed characterizes the magnitude of the aerodynamic forces acting on the aircraft.
Ground speed is the speed of the aircraft relative to the ground. It is equal to the geometric sum of true airspeed and wind speed.
In addition to speeds, a pilot in flight also needs information about the relative speed of flight, that is, about the number M.
On airplanes and helicopters there are corresponding sensors and indicators of the speeds mentioned above.
To measure air speeds, the most widely used aerodynamic method is based on measuring the total and static pressure of the oncoming air flow.
Ground speed is measured by radio, inertial and other systems.
Air pressure receivers (APS) fig. 167. It has a full pressure tube 1 and a static pressure chamber 2. The full pressure tube is open at the front and is installed in the direction of flight.
The static pressure cavity has side openings connecting it to the atmosphere. These holes should be located
where a is the speed of sound. 6*
The scale graduation of the true airspeed meter is determined by the following expression:
V = "I / , (2.23)
where y l is the air density at flight altitude H.
Or when dividing formula (2.23) by (2.21) we get
V = Vnp V~Tn (2'24)
Because the? = , then instead of formula (2.24) we can write
Consequently, the true speed is obtained from the airspeed after correcting it for the static pressure p and temperature T at a given flight altitude H, i.e., corrections for the change in air density with a change in flight altitude.
All of the above expressions are taken into account when creating a device design. On fig. 168 shows a schematic diagram of the instrument and airspeed meter. With an increase in flight speed under the action of a pressure difference ptot - Pst, the membrane box 1 turns the arrow 2 of the indicated airspeed indicator through the rod. At the same time, the center of the box 1 moves the rod 3 and, consequently, the arrow 5 of the true speed indicator.
If the flight altitude increases, then the aneroid box 4 expands and also turns the rod 3, overcoming the force of the spring I. In this case, the length of the arm I of the arrow 5 decreases, and it turns to an additional angle, taking into account the change in air density.
On fig. 169 shows a structural diagram of a combined speed meter with a measurement range of up to 2,000 km / h (KUS-2,000). The movement of the center of the gauge box 6 through the axles, leashes 7 and 8, sector 3 and tube 9 is transmitted to the wide arrow 2 of the indicated speed and simultaneously through a number of leashes, axles and sector 10 is transmitted to the narrow arrow 1 of the true speed. With a change in flight altitude, the position of the center of the aneroid box 5 changes, which causes a displacement of the leash 4 and a change in the gear ratio between the M and A axes. The M axis is connected to the gauge box, and the A axis is connected to the true airspeed arrow.
To take into account the change in air temperature with flight altitude (it is assumed that the temperature changes in accordance with the standard atmosphere), the characteristic of the aneroid box 5 is selected accordingly.