Their Internal Mechanism Described
THERE was a time when any form of indicator on an aeroplane was regarded as a novelty ; that was when air pilots relied solely on their judgment, air sense, hearing and ” feel ” in flying a plane. Present day conditions and requirements have altered this so that every aeroplane which takes the air is now equipped with at least three different instruments ; in fact no machine is eligible for an Airworthiness Certificate unless it k arries a specired complement of indicators.
The three principal instruments, as everyone who sits in the cockpit knows, are the revolution indicator, the airspeed indicator and the altimeter, and while most people are familiar with the appearance of these, few know of the intricate details of the design and mechanism which lie behind the dials which help to record the behaviour of an aircraft engine, speed, through the air, and the height at which a machine travels.
The Revolution Indicator.
In the evolution of the R.P.M. indicator, various principles have been tried in the system of -working— notably magnetic, hydraulic and centrifugal, but the test of time has resulted in the centrifugal type being the most favoured.
In the head of this instrument a rotating spindle is mounted, which is driven by a flexible shaft coupled to the engine. The spindle carries a form of sloping flywheel which, owing to centrifugal force, tends to run true as the speed of rotation increases. This action is transmitted to a spring-loaded sliding collar (also mounted on the spindle), which, in turn, transfers the movement through a rack and pinion to the pointer on the familiar calibrated dial, and thus the varying speed of the engine is indicated. It goes without saying that the essential quality of an indicator is accuracy, and in consequence, manufacturers work to very fine limits in calibrating their instruments. The Accurate Recording Instrument Co., for example, test and calibrate their R.P.M. indicators with a highly-scientific apparatus known as a Stroboscope. The standard adopted in this machine is a tuning fork of a known vibratory pitch ; this-fork is set humming, and the vibrations are used to make and break the contact of an electric circuit, consisting of a neon tube and a secondary cell. The neon tube is
The internal mechanism of an airspeed indicator. Movement
of the diaphragm is transmitted to the pointer through the link gear and pinions.
mounted in a dark cabinet, at one end of which is a spindle connected to a small variable-speed electric motor ; on the end of this spindle is a disc which is. covered with various geometric designs, and the reverse end of the spindle is coupled through a flexible drive to the instrument under test.
When the motor is revolved, the speed of which is controlled by a rheostat, the disc also revolves at the same speed and being illuminated by the vibrating light from the neon tube, gives the operator a series of designs caused by an optical illusion. The operator knows by the shape and contour of the design the exact speed of the motor, and he can thus accurately calibrate and check the instrument undergoing test.
Airspeed. Modern airspeed indicators may be divided into two classes—those which embody the pitot static head and those of the Venturi type. While the latter is in general use in America and on the Continent, the pitot head instrument is the type commonly installed in machines in England. In reality, the British type of airspeed indicator is a very sensitive pressure gauge, working on the diaphragm principle. An oiled silk or metallic capsule with corrugated walls is mounted in an airtight and
the movement of this capsule, or diaphragm against a spring is transmitted by a highly sensitive rack and pinion device to the pointer on the dial. An alumin; ium tube is taken from the pressure side of the diaphragm mounted on and another phragm to combined tive sides of connected to the pressure head which is leading edge or wing strut of the machine, tube connects the static side of the diastatic side of the pitot tube. Thus a and suction is created on the respecthe diaphragm, which varies according to
the speed of the machine when in the air. Airspeed indicators are usually calibrated and tested by means of a water column pressure gauge, but, of course, they may be, and very often are, checked for accuracy by being placed in a wind tunnel. Considerable ingenuity has been employed during more recent years in the construction of air speed indicators so that they now give dead true readings over a
wide range under the-most-varied conditions of temperature and different altitudes—factors which can easily upset the accurate registering of the instrument. As an instance of the high standard which has now been reached, a test, carried out some time ago, of an indicator made by S. Smith & Sons, showed that it had a accuracy of from one to one and a half miles per hour over a speed range of 40 to 170 m.p.h., and under a range of temperature of 40°C. to —30°C., the error was within one mile per hour. machines operating in all quarters of the world— in intense heat and cold —fabric diaphragms and rubber connections fo the pitot tubes have proved to be a source of trouble in the past, so that allmetal instru
ments are now being used in increasing numbers.
The altimeter, like the air speed indicator, is now regarded as a highly essential instrument for modern flying, and similarly, has now reached a stage of development when its action is extraordinarily accurate and reliable. Its internal mechanism comprises a form of aneroid barometer, with a corrugated metal capsule. There is a partial vacuum inside the capsule and a steel spring is attached to the exterior in such a way that it pulls against the atmospheric pressure acting on the capsule.. and balances the two forces at ” sea-level ” (14t lbs. per square inch). Thus the capsule is sensitive to barometric pressure and tends to expand (aided by the spring) as the altitude increases, the external pressure being
lessened by Vb. for every 1,000 feet. This movement is conveyed to a pointer by magnifying mechanism comprising: a compensating arm, a fulcrum, a fine silver chain, a ” puller ” and a hair spring. The instrument is compensated for variation of tem perature by the expansion of two metals—brass and iron—which are to be found in the compensating lever. Atmospheric pressure varies, of course, at ground level, hence the revolving dial arrangement operated by the
knurled head, which permits the setting of the instrument at zero before the machine to which it is fitted is taken off.
Altimeters are tested by makers by being placed in a chamber which is connected to a standard mercury column gauge. Vacuum or pressure can be introduced to the chamber from secondary tanks, the control being by means of highly sensitive valves and the working of instruments can be checked to an extremely fine degree.