Of the myriad of dials that clutter the typical military airplane's instrument panel, only six are sufficiently vital to merit the classification of primary flight group. These furnish sufficient basic information to the pilot for blind or instrument flying. Except for the operational instruments that report aircraft functions, all other equipment merely supplements these six. They are the turn and bank indicator, the rate-of-climb, sensitive altimeter, compass, directional gyro and air speed indicator.
Besides the obvious function of measuring the airplane's speed through the air for navigational purposes, it acts as an index of the ship's fore and aft angle by indicating increases and decreases in forward speed at a constant power output.
In principle, the air speed indicator is a differential pressure gage, set to measure forces generated in a pitot-static tube which is mounted on the wing or some other place where the air flow will not be disturbed by the propeller wash (turbulent air coming off the propeller).
The pitot-static tube consists of two separate tubes, one of which is open to receive the full impact of the air as the ship moves forward through it. The other tube is closed at the forward end, but has a series of small holes some distance back from this closure. These serve to transmit to the inside of the tube the static pressure of the air at that point which may be more or less than the static pressure inside the fuselage where the instrument mechanism is located.
Two lines of tubing connect the pitot-static tube with the instrument. The chief motivating element inside the instrument case is a hollow two-surface diaphragm, while the static line connects with the inside of the pressure-tight case. The pressure of the inrushing air is exerted on the inside of the diaphragm while the air in the instrument itself is determined by the static-line's stabilizing influence. The amount which the diaphragm expands is proportional to the difference between these two pressures. This movement is transmitted through gears and levers to a pointer which indicates on a circular face on which its movement is calibrated in miles per hour.
Since the air speed indicator is a pressure actuated unit, it has to be calibrated under "standard atmosphere" conditions to indicate the speed of the airplane relative to the surrounding air at sea level. As the airplane rises, a variation will be experienced, between the readings of the air speed indicator and the true speed, which is directly proportional to the increase in altitude. Thus, as the ship climbs higher, the differential pressure will decrease, and the indicated speed will be slower than the true air speed. This correction is made according to the formula: in which Vt is the true air speed, Vi the indicated air speed, Ts is the normal or standard temperature, 59° F or 15° C, Th the temperature at flight altitude, Ph the air pressure at flight altitude and K, a fixed quantity whose value is 459.4 for temperature in Fahrenheit and 273 in Centigrade.
A graphical solution for this formula was evolved by the National Advisory Committee for Aeronautics. It is reproduced here and is workable to altitudes of 30,000 feet. With indicated airspeed as ordinates and altitude, corrected to standards as abscissae, the true air speed can be read directly at the point where the two intersect, by interpolating at a glance with respect to the nearest line marked "true air speed" running across the chart.
The same solution can also be arrived at by the use of a special circular slide rule. Most good navigational computers have this built in as one of the features.
Like anything mechanical, the airspeed indicator develops certain operational bugs. While major ailments should be corrected by instrument technicians in regular factory or depot shops, minor malfunctions can be corrected in the field. For instance, if the hand is off zero when the plane is at rest, several things may be wrong. There may be water in the connecting tube. In this case, disconnect both lines from the instrument, open drain plugs and blow the lines out with a moderate pressure air hose, a pump or even lung pressure. Check to see that no other instrument, altimeter or rate-of-climb indicator is connected to the same static line. The hand may also be on the staff wrong, or there may be other mechanical maladjustment in assembly. This, however is a shop function and should be attempted only by an expert. Before condemning the instrument as inaccurate, check to see that it is not headed into a strong wind and that nature's air speed isn't being registered on the dial.
If the pilot complains that the airspeed is reading lower than normal, check first to make sure that he has made the correct allowances for changes in altitude. Then check to see if the pitot-static tube is bent or damaged, or if the tubing connections are airtight. If all of these are found to be satisfactory, the trouble is most likely to be found in the instrument whose case is not airtight, or is in need of mechanical adjustment. Both of these are base shop functions. Loose tube connections, leaky cases or mechanical maladjustment are also likely to cause too high a reading.
Slip a combination rubber tube and brass collar with a nipple over the pitot tube so that the collar is tight over the static holes. The bore of the collar should be slightly larger than the diameter of the tube. Apply suction to the static holes to give an indication of one third the arc and pinch off the tube. If the pointer does not hold, examine all connections for leaks. If after all connections are made tight, the indicator still drops faster than 1 mph. per minute, the case is probably leaky and should be sent back to the shop for repairs.
This article was originally published in the May, 1943, issue ofAir Tech magazine, vol 2, no 5, pp 16-18, 70.
Photos credited to Arnold, Kollsman, Boeing, Pioneer, Groenhoff.
The PDF of this article includes photos showing pitot static-tube locations on a P-39, AT-15 and AT-7, indicator dials of Pioneer and Kollsman instruments, and diagrams of the Pioneer and Kollsman instrument, the graph above, the structure of a Kollsman pitot static-tube assembly and procedure for draining a system.