One of the standard instruments which appears almost universally on aircraft equipped for instrument operation is the vertical speed or rate-of-climb indicator. As its name implies, it is used to indicate the speed in feet per minute at which the airplane is gaining or losing altitude. While it does not indicate the attitude of the plane, its primary function in flying at night or under conditions of poor visibility is to show whether ship is in level flight or not.
The instrument, basically, is a sensitive differential pressure gage consisting of a metal diaphragm the outside of which is exposed directly to the atmosphere. Its inside is connected to a capillary leak tube. Barometric pressure alterations that occur as the ship gains or loses altitude expand or contract the diaphragm. This movement is multiplied through a suitable mechanism and transferred to a pointer on the face of the instrument.
There are two general types of vertical-speed indicators. One is a self-contained unit, built by the Kollsman division of the Square D Corp. The other, an exterior chamber type, is built by the Pioneer instrument division of Bendix.
The Bendix instrument (see fig 1) consists of a metal cell or diaphragm (D), whose outside surface is subjected to barometric pressure changes through the large vent hole (V). The inside of the diaphragm is connected by a capillary tube (C) to a thermally insulated tank (T), usually a variety of ordinary thermos bottle. The capillary tube in practice, consists of a glass cylinder with a very small hole through the center.
When the pressure on the diaphragm remains static, as in level fight, the pressures inside and outside the diaphragm remain the same, since the inside of the diaphragm is connected to the outside air by capillary (C). When the airplane climbs, it immediately gets into a zone of lower pressure. Because the outside atmospheric weight is reduced, part of the air inside the instrument case leaves through the vent. The air inside the diaphragm, however, is practically at the pressure corresponding to the previous elevation, since the pressure of the volume of air inside the diaphragm and the tank cannot equalize rapidly, due to the small size of the capillary connecting it to the outside air.
As long as the ship keeps climbing, pressure inside the diaphragm remains higher than that on the outside, because it cannot "catch up" with the drop in pressure. The pressure differential which is proportional to the rate of climb, causes the diaphragm to expand. This moves the mechanism as shown by the arrows and the hand or arrow shows climb. When the ship levels out, the pressure equalizes and the pointer returns to zero. When the ship loses altitude, the reverse process and indication occur.
The Kollsman unit operates on the same principle as the Pioneer unit, differing only in structure and installation. It is a single, self-contained unit, with no outside insulated cell. Note in the cut-away drawing (fig. 2) the atmospheric pressure enters the opening in the rear of the case (B), circulates around the thermos bottle (A) inside which the mechanism is contained so as to overcome the effect of extreme temperature changes on the mechanism.
Circulating around to the front plate the pressure enters the hole D and a corresponding hole in other parts of the front plate and enters tube (K) to the indicating diaphragm (M); tube (L) to the over-pressure diaphragm (E) and the capillary restriction metering unit (J). This metering unit automatically makes any corrections for variations in the viscosity of the air and allows it to escape into the space surrounding the mechanism.
Changes in altitude with the corresponding difference in pressure cause movement of diaphragm (M) which moves the balance lever arm (H) and transmits the movement through the gears (C) to the pointer. This movement is restricted, however, by an arm connected to the diaphragm which presses against the restraining springs at point (F). Along these springs are calibrating screws which are adjusted so as to restrict the movement of the springs and the diaphragm to coincide with the markings on the dial. In this way correct calibration and the logarithmic scale (wide graduations at zero, closer at the extreme range) also is obtained.
The overpressure diaphragm (E) has a release mechanism so the pressure extremes outside the limits of the instrument's indicating range are immediately offset without harm to the instrument.
As the instrument remains in service for some time, shifts occasionally occur in the mechanism which change the zero point. An adjustment (I) is therefore provided so that this can be corrected.
While the general rule for field maintenance of instruments ship it back for expert repair still holds for the rate-of-climb, the following are typical ailments that occur and their probable causes, which may act as a guide if field repairs are necessary. If the pointer is off zero, the mechanism has probably shifted, and requires simple readjustment. If it is off zero and cannot be brought back by the zero adjusting screw, the pivot is probably broken and requires expert replacement. If the instrument indicates less than actual climb, its case is probably leaking. Obvious friction may be caused by dirty or broken pivots or jewels or by improper clearances in assembly. If the pointer sticks, it is probably rubbing up against the glass, the dial or dial screws. Dirt in the sector or pinion teeth may also cause this sticking.
This article was originally published in the March, 1943, issue of Air Tech magazine, vol 2, no 3, pp 44-45.
Photos credited to Kollsman, Bendix, Army.
The PDF of this article includes photos showing the faces of the two types of instruments and of typical installations in the instrument panels of a B-18 and a P-36.