Turboprops, can anyone help me?

[DJXL]

Hiya,

I was watching a program on the tele the other day about the Do328 and how it all works. As a frequent visitor to airports where turboprops roam free I was wondering if there is a website that anyone knows of which explains the operation of all the different systems etc. Just want to learn a bit more see. Now, ask me about PA34s and A320s and I could probably teach you how to fly them, but turboprops have so far eluded me...

Cheers

[ft]

What differences did you see on the telly that puzzled you? As the systems are varying quite a bit I think finding a website that explains everything about all systems on all planes is a bit optimistic. If you can be a bit more specific I'm sure we can help you out here though.

Basics: Prop attaches to turboshaft engine. Spins around. Pulls plane forward. ;)

Cheers,
/ft

[DJXL]

Well, my confusion is about the props really. OK, basic principles are fairly easy but it's all the other bits and bobs that I can't get to grips with, probably because I only understand props in the PA34 sense.

Just things like thrust reversers and how the props can still carry 20 tonnes of airliner through the air at 250IAS at FL250 where there is bugger all air. See what I mean?

I dunno why I have suddenly thought of all this stuff but it would be nice to know about it all.

[Mr moto]

Well, you've got a jet engine(efficient at high altitudes) and a propeller(most efficient at low levels).
The propeller, which is just a constant speed unit which can go negative, is usually driven by a free turbine; its not actually connected to the engine. The turbine is driven by the exhaust gases and then through an axle to the front of the engine where there is a reduction gearbox and other bits and bobs.

The trick is that when you pull the power levers back further than ground idle the pitch of the propeller gets finer and finer and then negative. Then you get an increase in power from the engine blowing more air forward. It must also be possible to do this with a reciprocating engine with some jammy rotary bell cranky thing but you wouldn't get a smooth power application. The turboprop's is via a fuel control unit.

FL250 is about the economic limit due to the requirement of 8.33Mhz radio spacing and the carriage of oxygen and drop-down masks above this level.

Hope we're getting somewhere!

[ft]

I thought I'd give a "brief" rundown of the various modes of operation of the plane I know the best, the SAAB 340.

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 (duh!)
- 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.

I think that’ll do for now. Of course there are lots of other systems, features and details but these are the most notable ones in the cockpit (unless I’m forgetting something, it has been known to happen :)).

Cheers,
/ft

[ 22 August 2001: Message edited by: ft ]

[ 22 August 2001: Message edited by: ft ]

[DJXL]

Thanks for the response guys! It's certainly cleared a lot of things up for me. It's a whole new can of worms this turboprop lark isn't it?

Cheers

[mstram]

FT,

Great info !

My flight time on the Saab 340 is so far only in MS FS200 ;)

Ater takeoff, what kind of power settings do you use in the climb ?
.. for that matter ,what is the climb rate ?

What are your approach and landing speeds, flap settings etc ?

What do you love / hate about the plane ? ;)

Mike

[ft]

mstram,
I'll find the numbers for you next week when I'm back at work. I'm in engineering (didn't break things often enough to be a pilot! :D)so I don't know them off the top of my head and right now I'm moving so it's all in boxes. :)

I know of one gripe pilots have with it. It's slick and hard to slow down at times. When ATC request high speed on the approach, it can apparently be a bit of a problem getting the speed off in time. When I put food on the table working as a handler I wished they could have put the GPU socket somewhere more convenient too... and a cargo bay door that doesn't hit you in the head when you're in a hurry. ;) OTOH, most cargo doors like to do that...

Of course I'm in love with the machine. Even when I'm objective about it. Great little plane!

Cheers,
/ft