Page 25 - Volume 15 Number 4

WHY BETA?
Did you ever wonder why the word “Beta” is used when discussing reversing propellers? It is common that engineers use Greek letters when labeling angles. When variable-pitch propellers were invented, the first letter of the Greek alphabet – Alpha – was chosen to represent these variable angles. An engineer might say that the lowest Alpha value is 15 degrees and the highest Alpha value is 28 degrees in a typical constant-speed propeller found on, say, a Bonanza. When the range of blade angles was widened as reversing props made their appearance, the second Greek letter – Beta – was used to describe this new range of blade angles that had not previously existed.
When I first started instructing at the Beech factory in the early 1970s, Beta generally referred to all of the blade angle positions that could be selected when the power levers were behind Idle. We referred to the first “half” of this range as “Beta for Taxi” and the latter half as “Beta Plus Power.” Residual thrust is reduced when we allow the propeller to go to flat pitch. Of course, propeller speed increases as the blade angle flattens since there is less resistance to rotation. If, however, the blade angle continues past flat pitch and starts pushing air forward, then the extra resistance to rotation will slow the propeller, leading to a loss of reverse effectiveness. To remedy this undesirable situation the “Beta Cam Box” – to which the power lever’s cable connects – is engineered such that the fuel control unit will increase N1, increase power, as the LPS starts going into negative angles. The outcome of this design is that the reduction in forward thrust and the increase in negative thrust should change in a rather linear fashion as the power levers move aft through the Beta and Reverse ranges.
For the last many years “Beta for Taxi” and “Beta Plus Power” have generally been replaced by the terms “Beta” and “Reverse.”
When a propeller is exhibiting actual constant speed – the RPM is remaining constant even while airspeed and power changes are being made – it implies that the propeller blade angle is variable – able to be changed. Increased power and/or airspeed are balanced by more resistance to rotation as the governor makes blade angle increase. Likewise, reductions in power and/or airspeed are balanced by the governor decreasing blade angle to achieve less resistance to rotation.
When the propeller slows down below the selected governor speed it is because the LPS has been reached. The governor is incapable of reducing rotational resistance because the blade is as flat as it can now go so the constant speed propeller has – at least for now – reverted to a simple, fixed-pitch prop with the blade angle being at the LPS setting.
In the simplest of terms, reversing is nothing more nor less than moving the LPS. I’ve said it before and I’ll say it again: A pilot cannot force the propeller to reverse; he can only allow it to do so.
He allows it to do so by moving the LPS. Thus, when the propeller is underspeeding due to a low power setting combined with a low airspeed, the governor flattens the blade angle until it finds the LPS ... which may now be in the negative range since the pilot has moved the power lever aft behind the Idle stop. It makes sense that the Pilot’s Operating Handbook (POH) warns the pilot that moving the power levers aft of Idle is only permitting when the airplane is on the ground.
As all of you know, Beta and Reverse are selected by lifting up on the power levers when they hit the Idle stop in the power quadrant and then pulling them further back. That is how we reposition the LPS. Since the power levers must be at Idle before they can be lifted, it means that engine power will be low at this time ... High Idle (70% Ng) at most. Thus, one of the two factors
APRIL 2021
KING AIR MAGAZINE • 23