Propeller Beta & Reverse Models

Everybody always asks, and few ever understand, how the propeller beta and reverse models work in X-Plane.

Everything here applies to single and multi-engine propeller planes in X-Plane, and also applies to REAL KingAirs and Lancair Evolutions and the like (planes with PT-6 engines) because they work mostly the same way. To simulate non-PT-6 airplanes, use the info in this document to see how X-Plane works, and then tweak the inputs into Plane-Maker as needed to suit your propeller-driving engine.

So let’s start with the REAL airplane description, and then move on to entering data into Plane-Maker after that.

Real Airplanes

The Red Knob: the Condition Lever

If this is all the way forward, then you are at FLIGHT idle. This dumps enough fuel into the engine, even at IDLE throttle, to keep the flame from going out.

If this knob is about halfway back, then you are at GROUND idle. This is a lower idle speed, designed for the taxi-only operations, where a flame-out would not be catastrophic.

If this knob is all the way back, then you are at CUT-OFF: All fuel to the engine is cut, with predictable results.

The Blue Knob: Prop RPM

This knob does not control fuel to the engine, but instead commands the RPM that the prop WANTS to turn.

If the prop is turning SLOWER than the RPM commanded by the blue knob, then the prop will FLATTEN.

If the prop is turning FASTER than the RPM commanded by the blue knob, then the prop will INCREASE its pitch to slow down.

As long as there is enough power and/or airspeed through the prop disc, then the RPM that you have commanded with the blue knob will be followed by the prop.

Now, if the blue knob is all the way FORWARD, then the prop will adjust pitch to try to hold REDLINE RPM (maximum rpm for take-off or fast cruise). As you drag the blue knob AFT, you get progressively less and less commanded RPM, until you get the blue knob all the way aft, at which point the prop will FEATHER (if this is a feather-able prop–some are, some aren’t), where it goes to the feathered pitch of the prop, which will result in basically zero rpm.

The Black Knob: Throttle

And now, finally, the BLACK KNOB, which we commonly refer to as the ‘throttle’, but will also control the prop in certain cases, as we will show below.

When you are doing regular flying, you have the throttle at or forward of the idle stop. As long as the throttle is at or forwards of the idle stop, the propeller is in what is called ALPHA mode.

In ALPHA MODE, the propeller acts as described above, adjusting pitch to try to hold the RPM commanded by the blue knob (redline when all the way forward, minimum or feathered when all the way back). This means that it tries to hold the RPM commanded by the BLUE KNOB, and it does this by adjusting prop pitch as needed to hold that commanded RPM. This is how you taxi and fly.

Now, sometimes, as you taxi in a PT-6 turboprop, your taxi speed starts to build up too high for comfort even with the engine at IDLE!

This, perhaps, is not surprising when you have an engine of about 1,000 horsepower on a light airplane, and being a turbine, it even idles fast to keep the desired airflow running through the engine. (This is needed, interestingly, to keep the engine from overheating! If a PT-6 were to idle too slowly, then the blanket of air that isolates the flame of combustion from the engine wall would deteriorate, allowing the flame to touch the engine wall and melt the engine! So we actually have to idle fast in a PT-6 to keep the combustion flame from reaching the engine wall and destroying it!)

When taxiing, we have a hugely powerful engine running at a high idle, so even at ground idle, the taxi speed might be too fast for comfort.

In that case, you lift the black knob aft of the idle stop and drag it back.

As you do so, the engine stays at idle, and the farther you drag the handle aft, the more the prop pitch moves from flat pitch to beta pitch. This is beta.

If you keep moving the throttle back even FARTHER, then the prop pitch moves from BETA pitch to REVERSE pitch, and the throttle starts coming up as well! This is reverse.

NOTE: Beta and reverse only work if the blue knob is all the way forwards. For whatever reason, Pratt and Whitney just designed the system this way.

Here is a table diagram:
(NOTE: all transitions between the states should be smooth and continuous. There is no sudden jump anywhere.)

Throttle Position Prop Mode Fuel Flow
1 forward alpha mode max!
2 idle stop alpha mode ground or flight idle
3 beta beta pitch ground or flight idle
4 reverse reverse pitch full reverse fuel flow

Setting up Plane Maker

Now, on to Plane-Maker, which you can set up to perfectly match the PT-6 described above.

Go to the ENGINE tab in Plane-Maker.

For the critical altitude upper-left, enter the maximum altitude at which that maximum power is actually available.

PT-6’s are typically flat-rated, so you can enter some altitude above 0 here… most PT-6’s continue to give full power as they climb until they hit a temp-limit and then they lose power as they climb above that point. Whatever point you first have to drop below max torque at max RPM due to hitting a temp-limit–that is the critical altitude: The highest you can fly and still get full power.

Below that you see the box for the FADEC. Check it if you have it–some PT-6’s have that, some don’t.

Throttle available at max lever is the throttle available when the throttle is all the way forwards. This is usually 1.00, but if your Ng (called N1 in X-Plane) goes over 100% in the real plane, then enter a slightly higher number here.

Below that, you see your low and high idle adjustments: Simply adjust these ‘idle screws’ up or down a bit as needed to idle at just the right Ng, or torque, when you are at low or high idle.

Now, below that, you see “Go to BETA PITCH below this throttle lever position”. This is the position shown in line 2 in the throttle position map up above.
If you are dragging the throttle with the mouse or have a hardware throttle, then below this value for that device, we start going into BETA.

Now, below that, you see “Go to REVERSE below this throttle lever position”. This is the position shown in line 3 in the throttle position list up above.

If you are dragging the throttle with the mouse or have a hardware throttle, then below this value for that device, we start going into REVERSE, and the throttle starts coming up as well!

Now, below that, you see “throttle available at max reverse lever position”. This is simply the percentage of maximum throttle that is available when you pull the lever all the way back to the aft stop. It is the throttle available from the engine in line 4 above.

Now, here is something interesting: If you want your physical throttle handle to be set up like the throttle in a real KingAir or Evolution or the like, then you will use the values above to scale the throttle accordingly.

But here is where it gets bogus: In the REAL airplane, there is a GATE at the idle stop of the throttle that keeps you from going into beta by mistake: You have to lift a trigger to get over the gate and into beta to keep you from going into beta by mistake in flight. But you probably don’t have that gate on your physical throttle quadrant, or want to mess with trying to find that exact point when moving the throttle, or even operating the throttle with the mouse on the monitor. You probably want to just drag your throttle all the way back to go to idle, since that is the way any normal person would want to descend! So, in Plane-Maker, you could easily enter ZERO for the “Go to BETA PITCH below this throttle lever position” and “Go to REVERSE below this throttle lever position,” so X-Plane NEVER goes into beta or reverse when you drag the throttle all the way aft! This way, you can just slap the throttle back to the aft stop to go to idle. This is the way anyone would want to fly the airplane.

Then, to go into beta, hit the / key. To go into reverse, hit the ? key. (And hit again to toggle out). And, while in reverse, you can run the throttle right back up again to go farther into reverse! This way the ergonomics are not like the real airplane, but you can really control what is going on. If you want the ergonomics to be the same as the real plane, then enter the throttle positions as mentioned above… but you better not drag the throttle all the way back in flight, or you will go into beta and then reverse in flight! The REAL plane has a gate at the idle stop to stop this from happening!

In the main screen at right, for the maximum power box, enter the maximum continuous power allowed by the engine. This is the maximum power allowable at sea level (not maximum thermodynamic, which you would never use or you would wreck your gear-box!).

For the prop pitch numbers, the top is the feathered pitch of the blade, as you get with the blue knob all the way back… maybe 60 degrees.

Then the coarse pitch is the maximum the prop can open up in alpha mode… maybe 45 degrees.

The fine pitch is the flattest the prop can go in alpha mode… maybe 10 degrees.

The beta pitch is the flattest the prop can go in beta mode… maybe 0 degrees.

The reverse pitch is the flattest the prop can go in reverse mode… maybe -45 degrees.

So, that is how a real PT-6 works, and how to set it up in X-Plane.

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