Propeller Control for Turbo-Prop Engines

Propeller Control for Turbo-Prop Engines By Dan Ankarlo April 1999 Controlling propeller RPM is only one of the functions of a governor installed on the turboprop engine. Indeed, other more complicated functions are performed by the governor as...

Putting the propeller into reverse mode (beta mode) and maintaining RPM control in reverse is accomplished through the use of the beta valve located at the front of the governor. The beta valve is directly connected to the propeller through the beta linkage and propeller beta ring. The primary functions of the beta valve are (1) to prevent the propeller from entering reverse uncommanded and (2) to control blade angle (and RPM) while in reverse command. As propeller blades approach reverse blade angles, the beta valve is pulled open through direct linkage to the prop. This action diverts high pressure oil from the propeller in much the same fashion as a feather lift rod. Oil is then diverted to the sump and the propeller blade angle is not allowed to travel further into reverse. Commanded reversing is accomplished by "overriding" the beta valve function. When the operator moves the power lever over the idle stop and into beta range; he resets the beta linkage and allows oil to continue to be ported to the propeller. This allows the propeller blades to travel into reverse angles.

The inherent problem with a reversing propeller system is that at some point between positive and negative blade angles, the propeller will be generating zero thrust. This is the point where the engine will be "off loaded" and attempt to overspeed. To prevent an overspeed of the engine at this point, the PT-6 governor makes use of an internal pneumatic air section. The pneumatic air section within the governor can allow PY air to bleed and thus signal the fuel control unit to decrease fuel and slow turbine speed. This ability of the governor is used in two conditions. During normal operation if a system failure allows the propeller to overspeed; the airbleed yoke inside the governor is raised and PY air is bled. This commands the fuel control unit to decrease fuel to the engine and thus reduce power. Also, during reversing operations the power turbine speed must be held to approximately 95% in order to avoid overspeed. This is accomplished by resetting the airbleed yoke manually inside the governor. When the operator commands the power lever into beta range the airbleed yoke is automatically reset through the beta linkage and engine power is thus limited to 95%.

The feather plunger operates independent of the pilot valve.

An additional function of some models of turboprop governors is to synchronize engine RPMs on multi-engine applications. Commonly known as Type II synchronizing, this system eliminates the need for rod end trimming devices. This is accomplished by use of a speed bias coil within the governor. The coil, when energized, will exert a pull on the metering valve within the governor. Changes in metering valve position are relatively minute allowing for accurate RPM control between the propellers. Of course, the operator can manually override the synchronizing system at any time. The synchronizing computer, more commonly known as the "box", will compare RPMs of the engines and increase or decrease voltage to the coils within the governors to obtain accurate speed control. Speed sensing inputs to the box can be obtained from electrical pickups on the overspeed governors or from the propellers themselves.

Although not directly controlling engine power or propeller RPM; a mention should be made about the overspeed governor within the governing system. The overspeed governor (OSG) is a separate control component located on the front of the engine. In the event of primary governor failure, the overspeed governor will limit propeller speed to somewhere between 104% and 106% of rated propeller RPM. The overspeed governor is directly in line between the primary prop governor and the propeller servo. All oil traveling from the primary prop governor to the propeller servo must travel through the overspeed governor. Using the same mechanism as the primary prop governor (i.e. flyweights and metering valve) the overspeed governor is preset to divert high pressure oil in the event of a propeller overspeed. The problem in checking overspeed operation is that since the OSG is not activated until 104%, its operation cannot be checked because the primary prop governor will hold prop RPM to 100%. Therefore, a reset condition is incorporated which will change the speed setting of the OSG to a point below 100% prop rpm. This is accomplished through the use of a solenoid valve and reset piston within the governor. During reset mode, the overspeed governor is reset to 92% of rated prop speed on most applications. This allows the governor to be checked for proper operation.

Propeller Control for Turbo-Prop Engines

By Dan Ankarlo

April 1999  The beta valve controls propeller rpm in reverse, or "beta" mode. Synchronizing is performed by an electric speed bias coil in the cover.

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