The airspeed indicator shows how fast an airplane is traveling relative to the air through which it is flying. The dial reading tells the pilot the indicated airspeed of his plane. He uses this figure in controlling the performance of the aircraft.
Every airplane has a definite range of rated airspeed values which are related to safety and efficiency of operation. For example, there is a rated minimum airspeed and any effort to operate an airplane below its stalling speed., as it is called, may result in the airplane becoming unmanageable. Also, there is a rated maximum airspeed for each airplane beyond which it may not safely go: the pilot uses this figure to keep his aircraft within design limitations as to speed in dives. In addition, there is a rated cruising airspeed for each individual aircraft; and, to assure efficient engine performance, this figure should not be exceeded in level flight.
For navigational purposes, indicated airspeed must be converted into true airspeed. These two are identical under normal sea-level pressure and temperature 29.92 inches of mercury at 59° . At higher altitudes, true airspeed is derived from indicated airspeed by correcting the latter to account for lowered atmospheric pressure and for temperature changes.
The required correction may be rather closely approximated by adding two per cent to the indicated airspeed for each thousand feet above sea-level. For example, if an airplane is flying at an altitude of 30,000 feet and at an indicated airspeed of 200 mph, its true airspeed is approximately 320 mph. However, for precise navigational usage, a more accurate true airspeed figure is required; hence tables, calculators or charts are used in making the conversion
In addition to its many other uses, the airspeed indicator is helpful in determining under conditions of blind flight whether an airplane is pointed up or down, since a nose-up position causes a decreased airspeed while a nose-down position will produce increased airspeed.
In construction (see cutaway), the indicator consists primarily of a diaphragm assembly (1) and a linkage system; the latter converts linear diaphragm deflection into angular motion of a pointer. The linkage system is, in turn, composed basically of a rocking shaft (2), a sector (3), and a pinion (4). The tapered staff (5) carries the pointer (6) which moves around the dial. All units are contained in a case.
The airspeed indicator is essentially a sensitive differential pressure gauge which measures the difference between impact air pressure and static air pressure. Impact pressure is supplied by the pitot side of the pitot-static tube. Static pressure or pressure of the surrounding, undisturbed air is led to the inside of the instrument case by the static side of the pitot-static tube.
As the speed of the airplane increases, impact pressure becomes greater and greater, and this results in an increasingly wider differential between impact and static pressures. The diaphragm expands more and more as the pressure inside it is built up; measurement of the amount of expansion gives the indicated airspeed.
The linear motion of the diaphragm is not very great and, therefore, the linkage system must amplify as well as transmit this minute motion.
The linkage system operates very simply. When the diaphragm (1) expands, the rocking shaft (2) picks up the motion, transmits it to the sector (3); the latter turns the pinion (4) and the tapered staff (5). The pointer (6), which is attached to the tapered staff, moves around the calibrated dial, thus giving the indication of airspeed. The hairspring (7) keeps the linkage taut, and causes it to follow a contracting diaphragm movement as the airspeed decreases.
This article was originally published in the January, 1945, issue of Flying magazine, vol 36, no 1. p 72.
The PDF of this article includes the cutaway above and a photo of the indicator in the instrument panel of an AT-6 Texan, with inset of an expanded image of the dial face.
Photo credited to Bendix.