On Fri, November 28, 2008 7:06 am, Noel Loveys wrote:
> Lynn:
>
>
> Beta, , is the second letter of the Greek alphabet. Alpha, , is the first
> letter. As props are primarily designed to pull or push a plane through the
> air I'd assume that that would be the primary, #1 or Alpha job. Anything
> else is a second job or Beta.
As I said before, Beta is the prop angle. If negative, that gives reverse thrust,
positive gives forward thrust.
But as I also said there is a colloquial use of the term as it applies only to
a
turboprop setup.
When a turbo prop is started it is at low idle power setting with the pitch flat.
These engines are very slow to respond to throttle settings (power) and the engine
is
advanced to run in the fast idle range. In that range, if you want to get the plane
rolling you do it not by advancing the throttle but by changing the prop pitch
and the
enormous inertia of the spinning engine and propeller allow for a quicker response
and
the plane begins to move. This technique can only be used in a narrow power band
around fast idle. This power range where you can just change the pitch to get the
plane to move has been called Beta, meaning the power range where you can just
change
beta to generate movement.
Here's an excerpt from how it works in a SAAB 340 with PL + CL:
( Happily plagiarized from
<http://www.airliners.net/aviation-forums/tech_ops/read.main/73083/> )
Basic operation
You have two levers for each engine, Power Lever (PL) and Condition Lever (CL).
The range of the CL is divided into
* Fuel off where the engine goes to feather (83.5 degrees pitch) and the fuel
is
cut off
* Start, where you are supplying fuel to the engine but the prop is still feathered
* UNF, UNFeathered, where the prop is out of feathered and basically in constant
speed mode trying to maintain 1180 RPM but without the bottoming governor (more
on
that later)
* Min to max constant speed (CS) range where the prop RPM is controlled to
be
within 1180 RPM (min) and 1384 RPM (max).
* T/M (torque motor) lockout, which will lockout, the engine control unit (ECU,
or
digital ECU, DECU, in B model a/c) if it malfunctions. Once T/M lockout is
activated, you have to shut down the engine (put the CL in fuel off) to reactivate
it.
The power lever range goes from full reverse through ground idle (GI) to flight
idle
(FI) and then on up to full power. Below FI you are operating in the beta range
where
the PL position (unless the CL is in feather or you feather manually) directly
controls the prop pitch from -16.5 to +10 degrees. Above FI there is a minimum
pitch
stop ranging from +10 (FI) to +25 (full power) degrees pitch. As you go from PL
full
aft to PL full forward, more and more fuel is added to the engine (naturally) through
signals to the Hydro-Mechanical Unit (HMU). At low power settings (below approx
30%),
this amount of fuel is not enough to spin the propeller up to the commanded 1180
RPM
at the pitch setting commanded by PL in beta range or at the minimum pitch stop.
Why do we have a beta range? Due to the slow response to throttle setting changes
in
turbo engines it is very impractical to use the throttle to control movement on
the
ground. You would have to wait for the gas generator to spin up (Ng increase),
providing more torque through the power turbine (PT) increasing the prop RPM (Np).
The
prop CS governor would then tell the pitch control unit (PCU) to increase the prop
pitch and then you would get additional power. In beta mode, you change the pitch
first instead using the inertia in the propeller system to provide thrust, letting
the
Ng accelerate or decelerate in response to Np to keep Np constant.
If the amount of fuel burned below 30% wont keep the prop spinning at 1180 RPM,
what
keeps it at constant speed in the beta range? This is where the previously mentioned
bottoming governor (BG) comes into play. The BG is active when the CL is above
UNF and
will send a signal to the HMU to add fuel above what the PL setting is dictating
to
keep the Ng up. The normal reference Np for the BG is 1040 RPM but to give more
power
in full reverse the BG reference will change to 1200 RPM Np when the pitch goes
below
10 degrees (<-10 on both engines on older versions).
CTOT
Early on it was discovered that the torque set in the beginning of the take-off
roll
would increase as the ram air effect increased with airspeed. To avoid having to
stare
at the torque (Nq) reading during the entire takeoff roll, decreasing the PL setting
to keep it at 100% and not above a CTOT (Constant Torque on Take-OFF) system was
added. When active, this system will signal to the HMU through the ECU to add fuel
until the preset Nq is reached as soon as you set the PL above a certain position.
AC
If an engine dies theres an autocoarsen (AC) system, which will detect this. It
then
proceeds to feather the dead engine automatically. Theres an inbuilt safety making
it
impossible to feather both engines in flight should this system fail. The AC system
continues to monitor a failed engine and will bring it out of AC mode should the
engine parameters used to detect a flameout increase above the threshold values
again.
APR
340B a/c has something called automatic power reserve (APR) which when one engine
goes
into AC during CTOT operation automatically adds 7 percent units of torque to the
other engine to compensate for the loss of thrust.
> As far as I know when the prop ceases to deliver thrust it can be considered
> to be in beta operation...
No, beta is the prop angle. In "beta operations", as applied to turboprop engines
the
pitch angle is changed to provide either forward or reverse thrust briefly.
> reverse thrust is usually known as "Full Beta"
Well, maybe, but that would be negative beta or reverse beta. The reverse beta
angle
is usually much higher than the maximum forward or positive beta operation. That
does
not apply to higher power settings where reverse beta is blocked from occurring
and
much higher positive prop angles are allowed.
> all turbo prop engines require full beta operation. Even on floats a Twin
> Otter will use full beta on landing to shorten the landing run out. On
> takeoff beta allows the turbine to spool up without load to deliver the
> torque required to spin the prop.
Some of this is colloquial semantics and not entirely correct, at least from an
engineer's point of view.
--
Paul A. Franz
Registration/Aircraft - N14UW/Merlin GT
Engine/Prop - Rotax 914/NSI CAP
Bellevue WA
425.241.1618 Cell
