A question about turbo-props
 

Hi

I have a couple of questions about the workings of a turbo-prop that I would appreciate somebody answering, or can someone direct me to a link?

In a jet cockpit there is just one set of thrust levers, but on a turbo-prop there are two sets of levers-I think one is the throttle and the other is the prop "condition"?! What does each lever do and what settings would be used at each stage of flight?

Thanks in advance!:ok:

Turbines vary in how their controls are setup, the following is for the PW123 fitted to Dash 8s.

The power lever controls fuel flow to the engine and therefore engine power/torque. The condition levers control the propellor RPM and feathering, and also acts as the fuel shutoff to shut the engine down.

On the ground the condition levers are at "max" rpm (1200) and the power lever is used to control taxy speed (on the ground it's actually controlling the propeller blade angle rather than engine power directly.)

Shortly after take-off the condition levers are reduced to "min" which gives 900 rpm for the props, they remain at this setting for the remainder of the flight and the power levers are used to increase or decrease engine power for climb, cruise and descent. The condition levers are increased to "max" just prior to landing so that maximum power is available in the event of a go-around.

Saab 340 (CT7) is very similar. It like the PW120 series is a free turbine (dual shaft) engine.

Some older turboshafts (T64) have the prop rpm on a separate lever from the condition lever, which controls only flight/ground idle and stopcock.

On the Be-1900 (PT6-A), we have six levers.

First are the power levers which control fuel to the engines through the FCU, thus controlling engine torque (power). The power levers also control the beta valve which changes the low pitch stop for the propellers. Basically, beginning with ground idle and moving the levers aft toward reverse, the power levers control both prop pitch and fuel to the engines. The power levers are used to control the engines from full reverse, through ground fine, ground idle, and all the way to max power.

Next to the throttles are the propeller levers which control prop RPM basically through a govenor setup like you see in reciprocating engines. The prop levers are set to taxi for ground ops, are full forward (1700 RPM) for T/O and landing, and generally we pull the props back to around 1500-1550 for cruise.

Lastly we have the condition levers which basically adjusts the idle speed of the engine from 65% N1 to 72% N1. Normally, the condition levers are set to the low idle setting and are not touched for the duration of the flight. They are basically used for starting and shutting down the engines.

...and as a result the prop is feathered on every shutdown. This is typical of most free-turbine engines.

Single-shaft engines (T56, TPE331 etc.) must go to full low pitch (fine pitch if you prefer) on a ground shutdown, to permit the engine to be restarted without overtemp. :8

Now that's interesting. I did MCC in Cheyenne III simulator and I was told that putting condition levers to low idle inflight is asking for trouble as at lo idle there's nothing to stop paddles from going into beta range. How is Kingair protected from low pitch in flight?

Strange I must say...
I guess it's really type specific what exactly the condition levers/speeds/prop levers do. On the 340 going into beta range is protected by the flight idle stop below which you must lift guards on each power lever. When below flight idle on the PL, prop RPM is governed by the bottoming governor that maintains a constant RPM of 1040 to reduce noise levels. When going into reverse, prop RPM is increased to 1200 automatically by the bottoming governor. In beta range (guarded), the PL controlls blade angle directly I believe and the DECU forks in as much fuel as is required to maintain the prop RPM at 1040 or 1200 depending on PL position.
Do you control reverse power with the power levers or with the condition levers on the B1900 respectively Chyenne III? This might be a clue as to why this procedure is used on the Beech and not on the Chyenne.

/LnS

Ok this is my understanding of it. (very simplistic)

Ok lets say that the power levers are set to ground idle, or ground fine, whatever you want to call it. And the condition levers are fully forward, which for arguments sake gives you 100% RPM. (but no forward thrust)

As you advance the power levers (increase fuel flow) the natural tendancy is for the prop RPM to increase. Now here's the clever bit. As the engine torque (twisting force produced by the gas turbine) increases, the propeller govenor adjusts the blade angle so that they are biting the air and are generating a forward thrust. This "bite" induces drag which is balanced by the increase in torque. Or to put it another way, the increased torque is transferred into thrust, rather than used to increase the prop RPM.

Pulling the power levers into reverse activates beta mode. The power levers are now controlling blade angle (reverse), while the underspeed govenor regulates fuel flow to maintain RPM.

There we go, clear as MUD :ugh: :ugh: :ugh:

With regard to the levers,

The thrust levers control the power, or torque of the gas turbine. And the condition levers control the propeller RPM. A high RPM is normally used during takeoff, landing and reverse. A reduced RPM is used in the cruise, which also reduces noise. And for aircraft that do not have seperate propeller and condition levers, a low RPM is used during taxi.

Pretty darn close from my recollection - I haven't been close to a T-prop for about 20 years. :oh:

On the Metro 23 we have 4 levers. 2 Power levers and 2 Speed levers. However we have Garrett engines which have a fixed gearbox which makes life different. In reality all the Speed lever does is govourne the RPM whilst the Power levers are just fuel taps.

Is it true that the garrett engine is used as an APU in some aircraft?

VNAVSPD, After having read the AOM for the 340B I just get more confused.
This is my understanding after having read the part of the BG operation :8
Between the GND IDLE STOP and the FLT IDLE STOP, advancing the power levers will coarsen the blade angles. When doing this the forward Np sensor will detect a reduction in PRPM and send a signal to the bottoming governor that send a signal to the torquemotor to increase fuel flow and thus increase Ng and in turn maintain PRPM at a reference speed.
Perhaps I shouldn't get so excited, this is really type specific stuff I guess :}
/LnS

Interesting!

The only knowledge I have of turboprops comes from my school boy days. I used to have a Saturday job cleaning a Commander 690C. Little did they know, that I actually used to do very little cleaning. Instead I used to sit in the back reading the POH. he he :}

low n' slow

What you've just said makes sense to me. My example was poor. I should have started with the power levers ahead of FLT IDLE. Would be interesting to see what it says about advancing the power levers past the FLT IDLE STOP. From what I remember about the commander, you had to lift two detent latches to bring the power levers back past the FLT IDLE STOP. And (may have this wrong) but this is when beta mode takes over?? (ie Power levers control pitch, and overspeed/underspeed govenor controls RPM through fuel metering)

It's to keep the AC generator online for anti-icing, IIRC.

I think one advantage of beta mode on a free turbine engine is that on a throttle slam, beta starts the blades moving toward high (coarse) pitch right away; rather than pouring in fuel, driving the prop overspeed, and then having the prop governor catch up to bring prop speed back down. In normal flight, Beta acts as a variable low-pitch stop in case of a governor failure.

But some turboprops fly with just a "Power Lever", the RPM being controlled by a PEC (Propellor Electronic Control) and the "Thrust Rating" being controlled by the EEC (Electronic Engine Control). Flying this sort of aircraft is simplicity itself. Put the power lever in the detent and let the electronics do the work. You only move the levers out of the detent when the "selected" power would be in excess of your requirements, like a low altitude cruise or a descent.

There is a squat switch that contols the beta valve positioning between flight idle and ground idle. Now, you can override that in flight by lifting the power levers into the beta range which, as you can imagine, would be a bad idea and probably ruin your day.

Once on the ground, the props automatically go to the finer pitch setting of ground idle. I guess you could say it works like a poor man's spoilers in a way because of the sudden increase in aerodynamic drag caused by the flatter pitch.

The RR Dart was the best design...one lever and an electric switch to control everything, power and RPM.

Very reliable, too.

An oldie, but a goodie.:)

Reverse is controlled with the power levers on the 1900. With the power levers at idle on the ground, this is the ground idle position. Lifting the power levers into the first detent is called ground fine, which is essentially a zero pitch blade angle. In this position, the power levers are controlling blade angle only. If you lift the power levers over the second detent, you are into the reverse range. The further aft you move the power levers, the more power is added to the engines and the more the blade angle moves toward full reverse. So in this range, the power levers are controlling blade angle and fuel to the engine.

Moving the PL's above the FI STOP makes the PL's govern fuel flow and thus Ng and thereby torque. The propellor pitch will then be controlled by the condition levers and everything works as you would've thought...

Thanks Barit1, I didn't know. Do you mean the AC wild gen's for the inlet anti icing? Makes sense to me as a noise reduction based RPM should've been a lot lower in my opinion :hmm:

/LnS