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How Does The Torque indication system measure Torque in the helicopters and turbo prop aircrafts?

Asked by: 17587 views Aircraft Systems, Helicopter

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3 Answers



  1. Kris Kortokrax on Apr 29, 2014

    Don’t know how complex an answer you are looking for.

    For the Rolls Royce 250 engine used in helicopters, there is a torquemeter gear. On the output of the power turbine there is a helical gear which engages the torquemeter gear. On the output side of the torquemeter gear there is a helical gear which engages the Power Output gear. An increase in torque causes the torquemeter gear to move longitudinally along the shaft. That increases the opening size in the piston inlet port. This then increases the pressure in the oil line to the torquemeter which registers as a higher torque reading.

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  2. Andrew D. Gee on May 02, 2014

    I agree with Kris above, don’t know how complex an answer you are looking for but I’ll expand a little more on the explanation Kris gave above. I am currently a Helicopter Maintenance Examiner on Bell 206 and Military OH-58 helicopters which use the Rolls Royce 250-C20 turbine engine…

    The torquemeter is incorporated in the accessory gearbox to provide a hydraulic pressure signal which is directly proportional to output torque. The power turbine gear train has two stages of helical gearing. The 1st stage reduction is accomplished by the helical power train drive gear driving the larger diameter gear on the helical torquemeter gearshaft. The 2nd stage reduction is accomplished by the smaller diameter gear on the helical torquemeter driving the helical power take-off gearshaft. Helix angles are such that both stages of reduction produce a forward axial thrust on the helical torquemeter gearshaft. If friction is neglected, this axial thrust is directly proportional to the torque transmitted through the gears.

    The torquemeter assembly incorporates two roller bearings which provide radial support for the helical torquemeter gearshaft. One bearing fits around the support shaft, and the other around the shaft support. Each bearing has a two-piece inner race. The accessory gearbox housing has a cast passage which delivers pressure oil to a cavity formed by the support shaft and shaft support. Pressure oil is ported from this cavity to the two roller bearings and to the torquemeter piston. Thus, the roller bearings are lubricated, and the torquemeter is provided with regulated engine pressure oil. The torquemeter piston has a jet which delivers oil from the torquemeter oil chamber onto the ball bearing.

    A ball bearing transmits helical gear thrust from the rotating helical torquemeter gearshaft to the non-rotating torquemeter piston. The piston has an anti-rotation pin which contacts an anti-rotation pin on the flange of the support shaft. The piston is free to slide axially on the non-rotating and axially retained support shaft. The sliding piston serves as a variable position valve which admits regulated engine pressure oil from the support shaft to the oil chamber formed by the piston and the flange on the support shaft
    The support shaft and piston are arranged such that an increase in gear thrust increases the piston inlet port opening. Since the outlet openings of the piston remain constant, the pressure inside the oil chamber is directly proportional to torque. Pressure in the oil chamber is directed to the torquemeter pressure sensing port on the front side of the accessory gearbox. The helicopter torque indicating system then converts the pressure sensed at the port into an indication of torque output.

    During a stabilized power condition, the axial thrust force, acting on the piston, is counterbalanced by the torquemeter pressure in the oil chamber. Let us assume that the torque output of the engine is increased. This results in an increased axial thrust acting on the piston and an unbalanced condition on the piston. The piston moves forward slightly to increase the piston inlet port opening. With a larger opening, there is less restriction to the flow of oil into the oil chamber thus the pressure in the oil chamber increases. The pressure in the oil chamber continues to increase until its force, acting on the piston, counterbalance the axial thrust acting on the piston. When the forces are equal, the piston stops moving, and the pressure in the oil chamber will be higher than it was prior to the torque increase. The torquemeter indicating system, sensing a higher pressure, registers the increased torque. The indication from the torquemeter system to the Indicator in the cockpit is a “wet-line meaning that if you disconnect the indicator in the cockpit you will have oil coming from the line, this results in a real-time indication of the torque indication coming from the torquemeter.

    Hope I didn’t confuse you…

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  3. David Weston on May 09, 2014

    Andrew,

    On my almost new Allison 250 B17/2 I see some torque indicator needle vibration at torque settings around 80-90%. Should that worry me? It is smooth at higher cruise altitudes where temperature limits me to 70 percent or so.

    D

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