Paul:
All though your post is factual and well backed up, I think you missed
the
boat on this one.
I think what Lynn was looking for was how the term Beta came around.
Something that would make =93Beta=94 easier to remember
Still, I liked your only slightly plagiarized description of the
operation
of the 340 engine. Most do not realize the difference in % rpm between
ground idle and flight idle is only a few %. The ground idle has to be
so
high to keep the engine running efficiently.
Noel
-----Original Message-----
From: owner-kitfox-list-server@matronics.com
[mailto:owner-kitfox-list-server@matronics.com] On Behalf Of Paul Franz
-
Merlin GT
Sent: Friday, November 28, 2008 2:57 PM
To: kitfox-list@matronics.com
Subject: RE: Beta defined.
<paul@eucleides.com>
On Fri, November 28, 2008 7:06 am, Noel Loveys wrote:
> Lynn:
>
>
>
> Beta, , is the second letter of the Greek alphabet. Alpha, =E1, 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% won=92t 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 there=92s an autocoarsen (AC) system, which will
detect
this. It then
proceeds to feather the dead engine automatically. There=92s 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
