Cutaways applied to aircraft instruments

It has been a common practice in the past for manufacturers to point out the constructional and operational features of various mechanical products through the medium of special models sectionalized or cutaway to expose the hidden parts and materials used. These cutaway models usually create considerable interest and prove educational to the potential customer. Although their use in teaching is usually restricted to individual students or small groups because of size, their desirable features nevertheless give them broad popularity as instructional aids.

In a course of aircraft instrument servicing where structural features and principles of operation are explained, the problems concerned with devising suitable cutaway models are few but important. First, the model selected should be sectioned partially or totally (if necessary) to expose the hidden essential parts to view. Second, the relationship of the moving parts and their actuation should be shown wherever possible; this feature creates interest and makes the operation of the instrument more self-explanatory. Third, the exposed parts should be protected against dirt, dust, and improper handling; this is particularly true of aircraft instruments where parts are usually small and delicate.

If the first two features are to be incorporated in the cutaway model, a compromise is sometimes made, as it is often impossible to completely sectionalize a unit and still be able to actuate it. Where no motion is desired and the parts remain static, it may be completely sectionalized or cut away to expose every part to view. In the case of the examples illustrated here, it was desired to retain the operating qualities for demonstration use wherever possible and to substitute charts where detailed explanations were required.

Steps taken in the construction of models described herein are as follows: After an instrument has been selected for sectionalizing, it is dismantled. The case is cut away in such a manner that it exposes the mechanism to view. When finished, the edges, where cutting was performed, are polished, if metal, and painted red if of plastic.

The general methods of sectionalizing and housing are somewhat standard but the methods of simulating the actuating force for the different representative mechanisms presents various problems. Pressure used for simulating the operation of the airspeed indicator and rate of climb indicator cutaways is developed by a short length of soft rubber tubing, one end sealed and the opposite end connected to the instrument diaphragm. The tube is squeezed between two flat strips of wood. This produces the slight pressure necessary for actuating the diaphragm mechanism. This rubber tube assembly is concealed at the rear of the display case and actuated by means of a small push rod extending to the outside. Over-expansion is eliminated by this method, as only a limited amount of pressure is allowed to act on the diaphragm.

The moving action of the parts and the resultant action of the pointer are thus shown each time the push rod is depressed. No attempt has been made to operate the altimeter by pressure. However, the setting knob shaft has been extended to the outside of the case where it may be rotated to show the action taking place during adjustments for changes in barometric pressure.

An actuating means was devised for the tachometers simply by extending the driving shaft through a bushing in the end of the case and providing a small crank for manual rotation. The tachometer generator is completely exposed, having been cut almost in half lengthwise. It is connected to a sectionalized indicator and the set is demonstrated as a whole.

Because of the apparent complexity and compactness of the chronometric tachometer, simplification of the cutaway model is desired. To accomplish this, the mechanism as a whole is extended by the use of additional plates and posts and extending several staffs. The three basic parts, namely, the driving, watch, and counting mechanisms, may thus be viewed separately. This modification can be performed with- out sacrificing the operating characteristics of the instrument. Clearness is also enhanced by the use of transparent mechanism plates.

Autosyn instruments, which essentially consist of two parts, the transmitter and indicator, are placed in two separate display cases. The units are sufficiently sectionalized to demonstrate the moving parts and are wired to external plug connections. The two cases are connected together, supplied with the proper amount of RC power, and demonstrated. A flap-position transmitter is used, and the operation of the unit is controlled manually by means of an external handle. An additional inoperative unit of a different type is placed in each display case. This equipment has proven very effective in demonstrating the various results of improper wiring connections.

Working cutaways of gyroscopic flight instruments are accomplished by removing enough of the original instrument case to expose the gyro rotor and its linkage mechanism. The display case for each cutaway gyro instrument is provided with a vacuum connection and relief valve. An external air filter is provided at the air inlet. Essentially the display case is nothing more in principle than an enlarged instrument case. In order to draw the air to the rotor, the air inlet of the display case is extended to the original instrument case connection which leads to the jets. Air flows directly to the instrument mechanism. causing the gyro to spin, and hence passes out through the vacuum connection. Heavy plexiglass is used on the windows of the cases to withstand the differential pressure involved during the operation of these instruments. Considerable care and ingenuity was displayed in the process of sectionalizing the flight indicator. Small windows are inserted in the rotor case to view the action of the rotor which is spotted with paint of a contrasting color. Additional windows enable the various air passages to be viewed and traced. Manipulation of the caging knob is made possible by means of a knob on the end of a shaft which extends through a packing gland on the plastic window of the case.

This article was originally published in the December, 1943, issue of Air Tech magazine, vol 3, no 6, pp 50, 52.
The original article includes 2 uncaptioned photos of examples of the models described.
Photos credited to US Army Air Forces Training Command.