Piston Engines - Propellers — Page 186, Lesson 237

I want to walk you through the Beta range operation of a propeller system, because this is where the propeller stops being just a thrust producer and starts acting as an aerodynamic brake. This is a ground-only mode, and it's critical you understand exactly when and how it's used. First, let's define the two ranges of propeller operation you'll encounter on turboprop engines. The system is controlled by what we call "Alpha" and "Beta" ranges. The Alpha range is your high-speed range. It's used during the take-off run at high speed, in flight, and during the initial high-speed part of the landing roll-out. So think of Alpha as your normal, in-flight, forward-thrust operating range. The Beta range, however, is used only on the ground. It's selected during the landing roll-out, and the key mechanical event is the removal of the flight fine pitch stop, which sits inside the propeller's pitch change cylinder. That stop is what normally prevents the blades from going too fine in flight. When you withdraw it, you allow the blades to move to a much finer pitch setting, and that setting is called "Ground Fine Pitch." Now, how does the pilot make this selection? Most propellers do it via a lever on the central control console, and sometimes that lever is the throttle lever itself. When you make that selection, warning lights illuminate to indicate that all propellers have carried out the selection. That's your confirmation that every engine has actually moved to ground fine pitch. Here's the important effect: as the propeller goes into ground fine pitch, there will be a significant aerodynamic braking effect. The blades are now presenting a much flatter angle to the airflow, so they're catching the air and slowing you down rather than pulling you forward. Now, power control remains normal while you're taxiing and during the initial part of the take-off run. But here's the clever part — later in the take-off run, as your true airspeed increases, the normal process of pitch coarsening kicks in. As TAS rises, the propeller naturally wants to coarsen its pitch, and this causes the flight fine pitch stop, which is inside the pitch control unit, to re-engage automatically. So the system resets itself for flight without you having to do anything. Now, later propellers have an even greater range of blade movement in the Beta range. This extended range goes from around +8° to -3° of pitch, and -3° is full reverse. It's selected at the same time as the older system — during the high-speed, initial part of the landing roll-out. But in this case, the braking effect from reverse pitch is much better than what you'd get from merely ground fine. So you're actually driving the blades into negative pitch, pushing air forward and creating a powerful reverse thrust to stop the aircraft. Let me show you the typical feathering installation so you can see how the pitch change cylinder and the stops fit together in the propeller hub. So to summarise the sequence: Alpha range for flight and high-speed ground operation, Beta range only on the ground, selected by withdrawing the flight fine pitch stop, moving to ground fine pitch for braking, and then automatically re-engaging that stop as speed builds on the take-off run. And on later propellers, you get the extended Beta range down to full reverse at -3° for much stronger braking.