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Beta defined.

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paul(at)eucleides.com
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Posted: Fri Nov 28, 2008 10:28 am Post subject: Beta defined.

On Fri, November 28, 2008 7:06 am, Noel Loveys wrote:

 	  Quote: 	
		   Lynn:



 Beta, â, is the second letter of the Greek alphabet.  Alpha, á, 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’t 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’s an autocoarsen (AC) system, which will detect this. It then

proceeds to feather the dead engine automatically. There’s 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.



 	  Quote: 	
		   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.



 	  Quote: 	
		   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.



 	  Quote: 	
		   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

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Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Fri Nov 28, 2008 11:38 am Post subject: Beta defined.

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 “Beta” 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  

   

   

--

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_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

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