The CH-47C Chinook model has a maximum cargo hook capacity of 20,000 pounds. The CH-47C has only a single cargo hook below the center of the aircraft. When hooking a single load, soldiers use the main hook. They must coordinate closely with the aircrew as to which hooks to use when carrying multiple loads. The planning figure for the fore and aft hooks is 10,000 pounds each. The Army's continued need for further performance improvements lead to the development of the CH-47C. Designed to meet an Army requirement to transport a 15,000 pound sling load over a 30 mile radius, the C model boasted an increased gross weight to 46,000 pounds, increased fuel capacity, the Lycoming T55-L11 engine developing 3750 shp, and addition structural improvements. The first C model flew in late 1967 and became the mainstay of the Chinook fleet until the advent of the CH-47D. Production of the C model continued until 1980 with improvements such as the crash worthy fuel system and fiberglass rotor blades being incorporated into the fleet.
Starting in 1969, when CH-47 C-model helicopters (with T55-L-7C engines) were converted to the full "C" model by incorporating T55-L-II engines, variations in engine and rotor shaft torque were observed. These torque oscillations, at a frequency of 4.1 cycles per second (dual engine) and 4.7 cps (single engine), were audible and disconcerting to the pilot and crew. The significant differences between the -7C and -11 engines were a 50% increase in inertia and a new fuel control configuration. Figure 29 shows a schematic of the drive/fuel control/engine arrangement.
This problem was first discovered due to the magnitude of the noise associated with the oscillations. The forward transmission sounded like it was loading and unloading. Oscillations were evidept on the ground and in hover, and rotor speed fluctuations -.f + 2 rpm were noted during the oscillations. Recorded data indicated large rotor shaft torque oscillations (forward and aft hubs in phase) at the same frequency as engine fuel flow fluctuations. The only difference between the "C"-model prototypes experiencing oscillation and the "C" model which exhibited no perceptible oscillation was the incorporation of the -11 engines, with significantly higher inertia and a more responsive fuel control than the -7C engine.
A parametric study was made during which various system parameters, such as rotor shaft spring rate, turbine inertia, hub inertia, centrifugal spring, and engine gain, were varied in an attempt to reproduce thce oscillation. It was found that the only parameter change that could induce a significant oscillation was an increase in the preload slope of the lag damper force-velocity curve.
A method of improving attenuation is by lowering the fuel control response break frequency, or, equivalently, by increasing the control time constant. With this method of attenuation, steady-state gain is relatively unaffected, but the frequency at which the control may follow has been reduced significantly, hence reducing the effective gain at 4.1 cps. The time constant is defined as the inverse of two times pi times the response break frequency. A set of fuel controls with 70% gain and .10 sec time constant were flown with lag dampers which incorporated low temperature modifications.
The reduced gain/increased time constant fuel control fix provided satisfactory torsional stability for the CH-47C production fleet. However, several early production aircraft reported instances of a "pseudo-torque oscillation". This phenomenon is a torque split, followed by a low amplitude torque oscillation of the high torque engine. The problem was traced to high levels of vibration affecting the internal workings of the fuel control. Vibration at cross shaft frequency caused an instantaneous increase in the effective gain of the control, increasing its torque output with respect to the other engine and making it susceptible to torsional instability. The problem was resolved by closely monitoring cross shaft vibration, and with minor fuel control component modifications.
Field service reports had indicated increased engine failures since the introduction of the T55-L-II and -IIA engines in the CH-47C helicopter. Engine inlet housing fatigue cracks had occurred and engine mounting system components wore and failed. The engine mount consists of two forward engine mounts, both of which react vertical and longitudinal loads (one of which also reacts lateral loads), and a vertical link at the aft end which reacts vertical loads only. To brace the forward outboard engine mount support, a link connects the forward outboard engine mount to the fuselage structure aft of the forward mounts.
Testing led to the conclusion that the approach which would yield the most favorable results was to "soften" the-drag linik. The drag strut was needed to assure cross shaft alignment in severe maneuvers; therefore, it could not be eliminated.
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