To get the correct governor type for a fixed-shaft turboprop, you need to set both the engine type and the failure mode of the prop governor correctly. Then, the prop must be configured for sensible blade angles to achieve correct alpha and beta. Correct behavior of the governor requires correct set up of the prop first. The below guide assumes that:
- you have a “fixed turboprop” type on the engines 2 page.
- you have selected the loss of oil pressure position of the governor to “startlock” on the prop engines spec page.
- you have “constant speed” as the type of the propeller on the props 1 page.
- you have ticked the boxes to allow the prop to go into beta as well as reverse on the engine 1 page.
- you have set up the minimum alpha pitch as fine pitch on the prop 1 page. With the prop at this pitch, it should generate a substantial forward thrust at 0 airspeed.
- you have set up the beta pitch on the engine 1 page. At this pitch, the prop should be generating close to 0 thrust at 0 airspeed.
- you have set up the reverse pitch aft of the beta pitch on the engine 1 page so that the prop can generate negative thrust.
Setting up governed speeds
There’s two governing functions that control the speed of the shaft: The prop governor controls it in alpha mode, and the fuel delivery control controls it in beta mode. The prop governor usually only controls the speed within a range of 96% to 100% of nominal RPM, while the fuel delivery control varies the engine speed between around 60% to 96% of nominal RPM. To set up the governing speeds you need to:
- Provide the nominal (100%) speed in RPM as the “redline engine RPM” speed
- Provide the same 100% speed as the “top of the green arc engine RPM” speed (for correct indication, does not effect governor simulation)
- Provide the minimum alpha speed for the prop governor as the “bottom of green arc engine RPM” – this is usually 96% of the nominal RPM. This value will be used as the lowest selectable for the prop governor in alpha mode and also for the underspeed governor that will increase fuel flow to keep the RPM up in alpha mode.
- Provide the lower limit for beta operation (65-73% of nominal RPM on most installations) as “minimum governor speed engine RPM”. This is the minimum RPM achievable in beta with the prop speed lever as far back as to not cross over into feather. This is the lowest RPM that can be set with fuel delivery control.
Setting up the engine idle fuel adjustments
Fuel delivery control works by manipulating the “idle_speed_ratio” actuator, so it changes the idle speed between “low_idle_ratio” and “high_idle_ratio” to govern the engine speed in beta mode. In order to do so effectively, you must tweak the low and high idle to be suitable for the governor. To do so, start up X-Plane and run these two tests:
- Place the power lever just below flight idle, in the tiniest bit of beta you can get. Basically, toggle into beta mode and then advance to beta_ref=0.01 or just the tiniest bit of beta. This way, the prop will be at or very close to the minimum alpha angle, thus grabbing the highest load it can get without crossing over into alpha fully. With the engine speed lever fully forward, the fuel delivery control must now achieve bottom of green arc RPM – usually around 96%. It will do so by running up the “idle speed ratio” towards high idle. Now tweak the “high idle ratio” parameter such that the governor sets an idle speed ratio close to 1.0. In other words, we want the fuel delivery control to be using close to all of its idle fuel here. The engine should be operating very close to the maximum idle ratio. Note the “high_idle_ratio” dataref now.
- Place the power lever as far back into beta without getting reverse thrust. With a properly set up prop, that means beta_ref=1.0. The prop is now at full beta pitch and should be producing very little thrust now and provide the lowest load on the engine. Observe that fuel delivery control will have lowered the idle speed ratio somewhat because it doesn’t need as much fuel flow now to keep the engine turning at min green RPM. Now pull the speed lever back as far as it goes without going into feather. You should be at about 2% travel on the prop axis to achieve minimum RPM short of feather. This will cause the fuel delivery control to call for a very low idle speed ratio to get the RPM down to the minimum governed RPM now, which will typically be between 65-73%. Now, tweak the “low idle ratio” so that the governor will use about 0.3 of “idle speed ratio” to achieve this lowest governed RPM (we need about 30% room below for the engine to spool down). Write down that “low idle ratio” dataref now.
Now back in Plane Maker, on the engines 1 page, set the parameters “hi idle fuel adjustment” and “low idle fuel adjustment” to the two values obtained.
Only with the idle parameters set sensibly, you will get proper response going in and out of beta mode, i.e. when going from flight to ground idle.
Finally, you probably will find that the “throttle available at max reverse lever position” should be low. During reverse, as the prop moves from beta pitch towards reverse pitch, it picks up more load, and this is counteracted by fuel delivery control, keeping engine speed up at the bottom of the green arc. Only if you find that at your reverse pitch you need additional throttle (more than fuel delivery control will provide) to keep the RPM up, should you allow for more throttle here.