>
> Well, that is not true. Beta is the angle of the propeller at 3/4 radius. A negative
Beta would result in reverse thrust.
>
> Propeller pitch can be specified in two different, but equivalent, ways. The
first is the blade angle, and the second is the pitch.
Not quite right, Alpha is the blade Angle of Attack
The term Beta in this usage means the following
On Turboprop aircraft with a constant speed prop, power lever settings between
flight idle and maximum power allow the prop governor to select the appropriate
blade pitch to maintain a set RPM and this is called the Alpha mode of operation.
When the power lever is moved below flight idle the lever directly controls
the pitch angle of the blades and this is referred to as Beta mode where the
pitch can be changed from the flight idle position to full reverse, also known
as Full Beta.
For those interested in propeller aerodynamics
In aerodynamics the Greek letter alpha is commonly used to denote the Angle of
Attack of an aerofoil (wing shape), that is the angle between the chord line and
the relative airflow. This is also true for propellers as they are really just
aerofoils in rotation, in this case the reference Alpha is taken at 75 percent
span from the centre of the hub.
Note that this is a completely separate measure to the blade pitch angle which
is a geometric angle of the blade relative to the plane of rotation of the prop.
Where Angle of Attack (or Alpha) will change with both the airspeed and RPM
the pitch angle remains constant,hence the term "fixed pitch". Constant speed
props operate differently by continually changing the blade pitch angle to achieve
a constant RPM.
As Alpha changes so too do the lift and drag properties, the most efficient use
of power is when the blade achieves its best lift to drag ratio. Every blade
design will have its own optimum Alpha for best lift to drag which, at a given
pitch angle, is dependent on relative airspeed and RPM. Thus a fixed pitch prop
is only operating at its most efficient at a given RPM and airspeed. One can
choose the best operating regime by setting the blade pitch angle to give max
efficiency at that speed and power setting. Ground adjustable props make this
a once per flight deal but In-flight adjustable (or variable pitch) props allow
the pilot to select the best pitch angle, and in doing so optimize Alpha for
the conditions.
What may surprise people is that a negative blade AoA can occur at positive blade
pitch angles because it is relative to the airflow and not the aircraft. At
zero airspeed a nearly flat pitch angle would be required to get negative Alpha
but as airspeed increases it can be achieved at coarser angles because as forward
airspeed increases at a given RPM the AoA of the blade decreases. Hence
at higher airspeeds the pilot will require a coarser blade pitch to keep the
prop working at its most efficient Alpha. Also because higher forward airspeeds
decrease AoA, if you take your in-flight adjustable prop in cruise flight and
send it to fully fine too quickly there is a good chance that it will achieve
negative thrust and provide a rather effective airbrake until the airspeed reduces
sufficiently and equilibrium is once again reached. This might not be too
good for your engine though.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=216855#216855
