The primary purpose of the main rotor transmission is to reduce engine output rpm to optimum rotor rpm. This reduction is different for the various helicopters. As an example, suppose the engine rpm of a specific helicopter is 2,700. A rotor speed of 450 rpm would require a 6:1 reduction. A 9:1 reduction would mean the rotor would turn at 300 rpm.

Dual Tachometers

Most helicopters use a dual-needle tachometer or a vertical scale instrument to show both engine and rotor rpm or a percentage of engine and rotor rpm. The rotor rpm indicator is used during clutch engagement to monitor rotor acceleration, and in autorotation to maintain rpm within prescribed limits. It is vital to understand that rotor rpm is paramount and that engine rpm is secondary. If the rotor tachometer fails, rotor rpm can still be determined indirectly by the engine rpm since the engine supplies power to the rotor during powered flight. There have been many accidents where the pilot responded to the rotor rpm tachometer failure and entered into autorotation
while the engine was still operating.

Look closer at the markings on the gauges in Figure 4-17. All gauges shown are dual tachometer gauges. The two on the left have two needles each, one marked with the letter ‘T’ (turbine) the other marked with the letter ‘R’ (rotor). The lower left gauge shows two arced areas within the same needle location. In this case, both needles should be nearly together or superimposed during normal operation. Note the top left gauge shows two numerical arcs. The outer arc, with larger numbers, applies one set of values to engine rpm. The inner arc, or smaller numbers, represents a separate set of values for rotor rpm. Normal operating limits are shown when the needles are married or appear superimposed. The top right gauge shows independent needles, focused toward the middle of the gauge, with colored limitation areas respective to the needle head. The left side represents engine operational parameters; the right, rotor operational parameters.

Figure 4-17. Various types of dual-needle tachometers.

Figure 4-17. Various types of dual-needle tachometers.

Many newer aircraft have what is referred to as a glass cockpit, meaning the instrumentation is digital and displayed to the pilot on digital screens and vertical scale instruments. The bottom right gauge in Figure 4-17 replicates a vertical scale instrument. The dual tachometer shown displays rotor rpm (NR) on the left and engine rpm (NP) on the right side of the vertical scale. Corresponding color limits are present for each component parameter.

Structural Design

In helicopters with horizontally mounted engines, another purpose of the main rotor transmission is to change the axis of rotation from the horizontal axis of the engine to the vertical axis of the rotor shaft. [Figure 4-18] This is a major difference in the design of the airplane power plant and power train whereas the airplane propeller is mounted directly to the crankshaft or to shaft that is geared to the crankshaft.

Figure 4-18. The main rotor transmission reduces engine output rpm to optimum rotor rpm.

Figure 4-18. The main rotor transmission reduces engine output rpm to optimum rotor rpm.

The importance of main rotor rpm translates directly to lift. RPM within normal limits produces adequate lift for normal maneuvering. Therefore, it is imperative not only to know the location of the tachometers, but also to understand the information they provide. If rotor rpm is allowed to go below normal limits, the outcome could be catastrophic.

51l0aN891BL._SX396_BO1,204,203,200_Are you ready to start your journey learning to fly helicopters? Learning to Fly Helicopters, Second Edition, provides details on the technical and practical aspects of rotarywing flight. Written in a conversational style, the book demystifies the art and science of helicopter flying.

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