Engine controls on turboprops

[Luke Mc]

Hello
Are the engine controls on a turboprop similar to those for piston aircraft, but minus the mixture and carb heat controls, i.e. there is a set of throttle and prop levers?
Luke

[Capt Claret]

Luke, it depends on the turbo prop. Kingair & Twin Otter are fitted with PWC PT6 and have separate Power (throtle), Pitch and Fuel levers. The fuel levers only turn fuel on or off, no leaning as you would have with piston engines.

The Dash 8 (PWC PW100 series) and Nomad (Alison 250) have Power levers (throttles) and Condition Levers. The first part of forward movement of the condition lever from the aft position, turns the fuel on, then the condition lever acts like a conventional pitch control.

I believe F50's have Fuel levers and Power Levers only, the prop RPM being controlled electronically by some other method.

PFM :D

[Amateur Turbines]

Luke, I think the most important thing for you to learn here is engine indicators. That will really introduce turbine engines. Ex. N1, Torque Gauges, Fuel Flow... Understanding these will give you the basics of turbine flying.

[Keith.Williams.]

TURBOPROP ENGINES AND TURBOPROP FUEL CONTROL SYSTEMS

TYPES OF TURBOPROP ENGINES
Turboprop engines can be categorised according to the method employed to provided propeller drive. These are as follows:

COUPLED POWER TURBINES
This is the most simple adaptation of the turbojet engine in that the propeller drive is provided by means of an extension of the drive shaft of a single spool engine. Operating the propeller at constant speed means operating the engine at constant speed.

COMPOUNDED ENGINES
The compounded engine is similar to the coupled turbine but employs two independent gas generator spools. It therefore exhibits the benefits of flexibility and efficiency provided by multi-spool turbojet engines. Propeller drive is provided by an extension of the low pressure spool shaft, through a reduction gearbox. Operating the propeller at constant speed means operating the low pressure compressor at constant speed. The high pressure compressor RPM varies to match the power selection.

FREE POWER TURBINES
In this form of engine the gas turbine engine serves the function of a gas generator providing a high energy gas stream to a separate free power turbine. The free power turbine is not connected to the compressor in any way, but drives the propeller through a reduction gearbox. Power to drive propeller is extracted from the hot gasses produced by the gas generator. Because the compressor is not connected to the free turbine each can assume its optimum speed independently of the other. This ensures maximum operating flexibility and efficiency. Furthermore because of the absence of propeller drag on the compressor, the free turbine engine is easier to start than a coupled or compounded engine.


TURBOPROP ENGINE FUEL CONTROL SYSTEMS
Three forms of turboprop engine fuel control systems are currently in use. These are:

INTEGRATED RPM AND FUEL CONTROL
This system is suitable for coupled turbines and compounded engines. This system typically employs an engine condition lever to switch fuel on and off, and a power control lever. Propeller RPM and power output are selected by a power control lever which simultaneously adjusts the fuel flow to match the selected RPM. Up to maximum RPM increases in power demand increase both RPM and fuel flow. Power demands above maximum RPM are achieved through increases in fuel flow, with the constant speed unit increasing blade angle to maintain constant RPM

DIRECT FUEL CONTROL
This method is suitable for use in free power turbine engines. This system typically has one lever to control propeller RPM and another to control fuel flow and hence power output. The RPM lever controls the propeller constant speed unit and is typically set in the flight condition and left there throughout flight. The power output is controlled by a power lever which varies fuel flow to match power demand. The fuel flow and gas generator RPM vary in response to changes in power lever setting. As power selections change, the speed of the propeller / free power turbine is maintained by the constant speed unit, which alters blade angle to match increases or decreases in power output.

DIRECT CONTROL OF BLADE ANGLE(BETA CONTROL)
This method can be employed for the control of any type of turboprop. An engine condition lever or HP cock is used to switch fuel on and off. This is typically set in the flight condition and left there throughout flight. The power control lever selects blade angle directly, while the fuel system automatically adjusts fuel flow to maintain propeller RPM.

CONTROL OUTSIDE FLIGHT RANGE
When outside the normal flight range and particularly in the reverse thrust range, the engine/propeller combination is usually controlled by the beta system. The transition between this and the normal control system is usually indicated by a stop or detent in the throttle lever quadrant.

The majority of gas turbine engines do not employ manual mixture control. There are however exceptions to every rule. A few use it to aid starting.

[ft]

Here's a hands-on example of how to operate turboprop controls that I posted a while ago in reply to a similar question. Can't find the original post since the search functions are out, out and out.

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 safetguard making it impossible to feather both engines in flight should this system fail. In short, the feathering signal to the right prop control unit pulls a relay cutting off the feathering signal to the left. 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 (although in reality, the conditions are such that the system will probably latch until action is taken).

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.

Cheers,
/ft

[hopharrigan]

Not relevant here, but it reminds me of an incident where I was starting the Nomad and was interrupted during the preflight check, leaving the power levers in reverse. Start up was normal, until the transfer to the power levers from the start control, and the engines both ran up at a quite fantastic rate to full reverse! Made the old ticker work at a fantastic rate too! I don't know who was shocked more, me or the ground engineer out front holding the fire bottle.

[crackerjack]

Sorry, could you remind me of the question again?

[moggie]

I trained on Astazou powered jetstreams in the RAF ad they had stubby little throttle lever that allowed you to control between 100% (for flight) and about 88% - if I remember correctly- for ground use. They also had power levers which controlled fuel supply which in turn (via the constant speed prop) controlled thrust more or less directly.

You started up and taxied at ground rpm, moved the throttles to 100% just before take-off (and then balanced them using little thumbwheels) and then advanced the power levers to leap skyward in a haze of vibration.

the other main control was a ground/flight interlock that allowed you to pull the power levers far enough back to go into reverse pitch.

all very fiddly but designed by the French and Handley-Page - so are you surprised?

As for indications (16 years ago so expect errors) RPM, TGT, fuel flow and blade angle (no torque if I recall). In prcatice, you used power levers to set a blade angle - small blade angle = little power, big blade angle = (yep, you guessed it).

Probably no help but I like to feel as if I've contributed.

PS- engines ran at 43,089 RPM (yes forty three thousand and eighty nine) for 100%, prop ran at 1,800 RPM and engine produced 1,089 SHP. Not bad for something about the size of 2 rugby balls (3 if you count the gearbox).

PPS- No electronic control at all - the engine lubricating-oil pressure was used to control pistons which adjusted prop blade angle. If the engine tried to accelerate (with fuel increase) the increase oil pressure increased blade angle until the prop dragged the engine back to normal RPM. Converse for fuel flow reduction. However, total oil pressure loss (engine failure) caused the blades to go full coarse into the feather. All very cunning. Small, lightweight and powerful - not at all like me.

[ 24 October 2001: Message edited by: moggie ]

[ 24 October 2001: Message edited by: moggie ]