For governor installation and operation please refer “Installation and Operation Manual”.
On top of the information contained in applicable manuals in the following text is described additional operational experience.
Lost motion or too much friction in governor push-pull control or quadrant friction lock makes precise speed setting difficult. To reduce the effect of lost motion and friction the following points should receive careful consideration:
- The engine end of the push-pull control must be secured to the governor or engine so that movement of the engine in its mounts will not change the governor speed setting.
- Avoid sharp bends and poor angularity at the attachments points of the push-pull control.
- The quadrant friction lock should prevent propeller control lever slip due to vibration. The change in RPM is very gradual and may require observation of the speed setting knob for several minutes to detect lever movement.
- The governor control lever spring is designed to hold the existing speed setting or move the control lever in the maximum RPM direction in the event of failure of the governor push-pull control or cable.
JIHOSTROJ GOVERNOR IS THE ONLY GOVERNOR THAT FITS SMOOTHLY TO NEW ENGINE ROTAX 912 iS
Transfer Ring Leakage.
Propeller control oil passes from the governor through a groove or „transfer ring“ in the engine’s front main bearing, through a passage in the crankshaft, to the propeller piston. Oil leakage between the governor and propeller cause a shift in the engine rpm from a given governor rpm setting. The required propeller control oil pressure is determined by the propeller characteristics.
Transfer ring leakage in an uncounterweighted propeller results in an increase in rpm greater than the governor is set for, and is most noticeable at high airspeeds and increased oil temperatures. This is due to the increase in propeller control pressure required at high blade angles by the uncounterweighted propeller as airspeed increases. If transfer ring leakage equals governor pump output, loss of propeller control will result. The propeller will go into flat pitch and the throttle must be closed to regain propeller control.
Maximum control oil pressure in a counterweighted propeller is required at maximum rpm, full throttle, and zero airspeed. As airspeed increases and throttle or rpm is decreased, control pressure decreases. The required control pressure may vary from relief valve pressure at take-off to 70 PSI during cruise (depends on propeller design). The pressure variation or „scale“ has only a slight effect on the governor control rpm, except when oil leakage from the propeller control line lowers the pressure available at the propeller control piston. Change in oil viscosity with temperature adds to this indesirable shift in control rpm. A loss of 25 to 50 rpm at take-off may result from an increase in oil temperature, but may not be noticed because of the short duration of maximum power application. Loss of control oil pressure will allow the propeller counterweights and feathering spring to rotate the propeller blades to their high pitch position. If leakage between the governor and the propeller is high because of excessive bearing clearance, or a transfer tube seal leak develops, governing will be affected. In extreme cases, the propeller may feather when the engine is idled with red line oil temperature.
Excessive propeller friction results in poor rpm control and in extreme cases may cause surging. For precise rpm control – especially in cruise – the propeller must respond to small control oil pressure changes from the governor. If a large pressure change is necessary to increase or decrease pitch, a change of 10 to 20 rpm may result in no apparent corrective action by the governor. A „dead band“ exists in which the propeller acts like a fixed pitch propeller. The visual evidence of this is the large speed change required before the propeller control pressure builds up enough to move the propeller blades.
There are several causes of high propeller friction:
- Lack of lubrication that leads to tight blade pilot bearings on the propeller. Blade thrust bearings are heavily loaded in flight and may have damaged bearing balls or races that can be detected only by visual inspection.
- Propeller piston seal friction is a common cause of trouble after a long flight in smooth air at fixed power and airspeed. Under these conditions the propeller piston seal tends to squeeze the oil from under it and bind it on the cylinder. A gradual increase in airspeed will result in an increase in rpm or vice versa until the pressure change in the propeller overcomes the seal friction. Propeller piston seal friction is usually not serious and can be corrected by simply exercising the propeller with a change of throttle large enough to cause a blade angle change. One of the most serious causes of propeller friction is sludge in the propeller piston assembly. Sludge, carbon or engine chips entering the propeller, collect as in a centrifuge on the inside surface of the propeller cylinder and may slow or prevent full piston movement. The best way to prevent this is to change rpm setting in flight at periodic intervals. This should discharge any accumulation of sludge trough the governor to the engine sump.
Friction in the governor may result from normal wear, sludge and varnish deposited from oil. Rust or premature wear from high torsional vibration of the drive may also result. Dirty oil, or chips in the oil from a bearing failure in the engine or some accessory add to fiction problem.
Torsional vibration in the governor drive may become excessive because of a weak cylinder or bad accessory drive gears. This is one of the most difficult governing problems to recognize and remedy. Not only will a rough engine be hard to govern, but in an extreme case may cause premature wear and even complete failure of either the engine or governor drive.
The proper governor mounting gasket with screen is to be used at all times. The standard governor mounting gasket screen (figure 1) will not prevent fine metal particles from entering the governor. However, it will keep out large particles which can lodge in the pilot valve port.
In case of a bearing failure in the engine, the governor should be disassembled and cleaned before it is returned to service. Failure to do this will cause rapid wear, and a runaway propeller may result from a chip holding the governor pilot valve open to pressure.
Under normal operating conditions a piston engine type governor requires no service between major engine overhaul periods. Tightness and security of all external screws, nuts and levers must be checked as a part of each routine engine inspection. The exception occurs in cases of poor engine maintenance.
Dirty oil will cause more serious wear in a governor than any other single thing. Governor pump pressures range from 250 to 450 psi and running clearances range from .001 to .003 of an inch. Fine abrasive particles in the oil will be forced through the pump journals and cause rapid wear, because of the close tolerances in the governor and the high oil pressure.
ENGINE EFFECTS ON OPERATION
The propeller governor senses only engine rpm, but its ability to maintain a constant rpm under varying flight conditions is influenced by several things. If the engine becomes rough due to poor ignition, carburetion or burned valves, this may result in erratic governing. Intermittent ignition or carburetor trouble will be more noticeable with a constant speed propeller than with a fixed pitch propeller because the governor will try to correct for rapid torque changes and the result will be „over control“ rather than smoothing out rpm variations.
When cruise conditions are favorable to the formation of carburetor ice, poor rpm control may result. Ice formation may not cause serious loss of power, but only variations in power due to build up and dissipation of the ice on the carburetor throttle plate. This is a problem only with float type carburetors because of the vaporization temperature drop in the carburetor Venturi. Since this temperature drop may be as much as 60 degrees F (15.5 Celsius), icing may occur even on a hot day if high relative humidity is present. With the pressure type carburetor, the fuel discharge nozzle is located downstream from the throttle valve and vaporization ice formation on the throttle valve is eliminated.
PROPELER BLADE ANGLE SETTING
On a single engine aircraft, several considerations determine not only the low propeller blade angle limit. Current FAA regulations permit the low propeller blade 105% maximum rated rpm static when used with a propeller governor that is adjusted to limit maximum rpm to 100% of rated take-off value. Some manufacturers set the low blade angle to a higher value than the minimum angle established by FAA regulations so that take-off rpm is not reached until the aircraft has some forward speed. There are two advantages to this practice. First, the maximum static rpm can be used as a check on developed horsepower as with a fixed pitch propeller. Any loss of maximum power is readily apparent during a preflight check. Second, if the propeller remains in flat pitch after take-off due to some malfunction, the higher value low pitch setting will permit enough power to be developed to fly the aircraft without overspeeding the engine.
On the other side the high pitch blade angle must be elevated enough so that at high airspeeds and high increased horsepower, as during a let down, engine speed can still be controlled by the governor. It is desirable that the high blade angle not be more than required by the particular aircraft. If due to some malfunction during flight the propeller should go to full high pitch, it will still be possible for the engine to develop horsepower enough to allow continued flight. In a typical single engine aircraft with a take-off rpm of 2625, the required high blade angle (as previously discussed) will permit approximately 1750 rpm at full high pitch with full throttle and the air speed stabilized at 150 mph. As airspeed is decreased, rpm will also decrease to about 1250 rpm at zero airspeed. For this reason, if a failure of this type should be experienced, the airspeed should be kept as high as possible by keeping the aircraft clean until a safe landing is assured.
When good operating practices are observed the governor assembly will require complete overhaul only at regular engine overhaul time. Engine oil changes at frequent periods will greatly extend the useful life of the governor and engine. The propeller and engine manufacturers lubrication and service instructions are important in maintaining satisfactory propeller, engine, and governor operation.