Beta range props common on turboprops?

Snow

Snow

'Not a new member'
I've been doing a little systems study and on the Saab 2000, there's a thing called Beta range, which is between the ground and flight idle positions where moving the power levers changes the prop pitch while maintaining a constant rpm for taxing purposes. First time I've ever heard of such a thing and was wondering if this a common thing on turboprops in general?

Also, why would you start the engines with the props feathered if there's no disconnect for the props? It says for less noise, but i would think a feathered prop would be rather unaerodynamic while its turning and make even more noise, a 0 deg blade pitch would make more since to me, anyone know the reasoning behind this?
 
Yeah, most turboprop engines use Beta. I fly a King Air and when we're taxiing with the condition levers in high idle, we taxi with the power levers in Beta.

I don't know what kind of engine is on the Saab 2000, but on the King Air 90 (PT6 engines), you can start the engines with the props in feather because the PT6 is a free turbine engine. When you engage the starter, you're not turning the power turbine, only the gas turbine.

Chris


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I've been doing a little systems study and on the Saab 2000, there's a thing called Beta range, which is between the ground and flight idle positions where moving the power levers changes the prop pitch while maintaining a constant rpm for taxing purposes. First time I've ever heard of such a thing and was wondering if this a common thing on turboprops in general?

Also, why would you start the engines with the props feathered if there's no disconnect for the props? It says for less noise, but i would think a feathered prop would be rather unaerodynamic while its turning and make even more noise, a 0 deg blade pitch would make more since to me, anyone know the reasoning behind this?

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I've been doing a little systems study and on the Saab 2000, there's a thing called Beta range, which is between the ground and flight idle positions where moving the power levers changes the prop pitch while maintaining a constant rpm for taxing purposes. First time I've ever heard of such a thing and was wondering if this a common thing on turboprops in general?

Also, why would you start the engines with the props feathered if there's no disconnect for the props? It says for less noise, but i would think a feathered prop would be rather unaerodynamic while its turning and make even more noise, a 0 deg blade pitch would make more since to me, anyone know the reasoning behind this?

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Part 1, yes, most if not all turboprops have beta range.

Part 2, we start our props in feather. It is MUCH more quiet than in min (unfeather). Just because of the RPMs the props are moving at in feather as compared to min. It's like 800 RPM difference. Also, when in feather, the props aren't producing any forward thrust, so it's better when on slick ramps and what not. Plus, you can move the power levers up to get a high Ng (gas generator) setting to run the High Pressure bleeds for heat or AC during taxi. If you were in Min/Max, you'd be producing a lot of thrust at that point.
 
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I've been doing a little systems study and on the Saab 2000, there's a thing called Beta range, which is between the ground and flight idle positions where moving the power levers changes the prop pitch while maintaining a constant rpm for taxing purposes. First time I've ever heard of such a thing and was wondering if this a common thing on turboprops in general?

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Yup.

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Also, why would you start the engines with the props feathered if there's no disconnect for the props? It says for less noise, but i would think a feathered prop would be rather unaerodynamic while its turning and make even more noise, a 0 deg blade pitch would make more since to me, anyone know the reasoning behind this?

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Depends on the type of engine. Direct drive engines are different, since the prop, gearbox, and drive shaft are all directly connected. On the oddball King Air (B100) that I fly, we don't (and can't) start the engines with the props feathered because it has Garrett engines, which are direct drive. We can't start with the props feathered, because it would create too much load on the starter, and it wouldn't be possible to attain a high enough RPM for lightoff.

To keep the props from feathering after shutdown, we have what are called "start locks". On shutdown, before the engine spools down to less than 30% RPM, we put the power levers in reverse and leave them there. As the RPM winds down, the lack of oil pressure causes the blades to go from reverse to normal pitch, however at the same time the start locks (pins) are no longer held in their housing by centrifugal force and so they pop out and stop the blades from going into feather (which they would if you did not reverse on shutdown). Once the blades are on the locks and the engine has slowed below about 15%, the power levers are moved from reverse to start (or should be...because starting it with them in reverse will get your attention). At the next start, the blades will be on the locks until you unlock them after start by bringing each one from start, back to reverse, and then to ground or flight idle. This relieves the tension on the pins, and allows them to be "sucked" back into their housing by centrifugal force.
 
Yeah, it tends to spool up quicker, but not significantly. Only on startup really, I don't notice a difference in flight but most of my turbine time is with Garretts.
 
Ok thanks for the replys, so I guess with an indirect drive turboprop the props don't turn until combustion occurs and the engines are running?

Here's what it has

SAAB 2000 ENGINES
The aircraft is powered by two Rolls-Royce Allison AE 2100A turboprop engines, each rated at 3,096kW and fitted with Lucas Aerospace full authority digital engine control (FADEC). The engine cowlings are manufactured by Westland and Hispano-Suiza.

The six-bladed Dowty propellers, type R381, are constant speed propellers with auto-feathering and reverse pitch. The blades are fitted with electrical de-icing.
 
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