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I was having a discussion at work today about how one controls the engines and props on a turboprop aircraft, and pretty much figured out I had no idea how it works. I know that PT-6's and Garretts are controlled differently, and it also depends on the aircraft. So, those of you with some turbine time, how does it work? Do the prop controls go full forward on takeoff, and short final like with a piston? How do you set takeoff power, is it limited by torque?
I'll let the more experienced drivers elaborate, but I know that ITT was the limiting factor for takeoff thrust (at least for the PT-6).
I'm reaching waaaay back into my fuzzy memory for this. I've got a few thousand hours of PT-6 time under my belt, but none of it recent. Here's what I recall:
The prop controls did go full forward for takeoff, back some for climb, and back a little more for cruise. When on the approach, we pushed them back up to the climb setting.
We used torque for power settings. We did have to keep an eye on the temps, but usually we reached the max torque setting prior to hitting the temp limit.
I've never flown Rolls-Royce, Allison, or Garrett turboprops - their systems are different and I don't know how they are operated.
Hope this helps.
I am a current Caravan driver and here's how it works in the C-208B. I am not familiar with a Garrett, but the P&W PT6-114A is not a direct drive engine, it is a free turbine. In simplified terms, the pressure from the compressor section drives the fans in the power section which in turn, drives the prop governor. That is why the prop on a PT6 spins freely when the engine is not running.
There are three power controls in the Caravan similar to most pistons. The throttle, fuel condition, and prop control levers. In the Caravan you leave the prop lever forward all the time to get 1900 RPM. You may back it off to 1600RPM if you want to lower the noise level but a good ANR headset works just as well and you keep the extra 2 to 4 knots airspeed.
You may notice that many turboprops are feathered when shut down. That is the distinctive charactistic of the PT-6. You can tell Garretts apart because they are not feathered when stopped. The reason Pratts are feathered is because if not, the prop will windmill in the breeze, which is bad because there is no oil to lubricate the governor when the engine is not running, causing unwanted wear. PT-6's are considered almost indestructible, the governor is the weak link in the system.
The fuel condition lever serves to introduce fuel during start and set the minimum idle level through a stop on the fuel control unit. On the Caravan there are three settings; cutoff, high idle(65% Ng) and low idle(52% Ng). For reference, 100% Ng is 37,500 gas generator RPMs, red line is 101.6% Ng. High idle is required during flight to maintain a higher idle speed so it will not take so long for the engine to spool back up during a rejected landing (it was a certification requirement). The condition lever is also used to shut down the engine.
The throttle or "power lever" is used to set the desired power level, then the "fuel control unit" provides the appropriate "fuel schedule" for the power required. In the event of a fuel control unit failure, there is an emergency power lever that gives the pilot a direct linkage to fuel flow. However be careful because power response is immediate and you risk over temping the engine.
My company sets lower ITT and Torque limits than those approved by Cessna/Pratt & Whitney. The book numbers are 740 degrees ITT and 1865 Lbs torque. We run at 700 ITT and 1800 torque and after thousands of hours and two overhauls, our engines are like new. We have been approved for extended TBO's.
Wether you torque or temp out first depends on atmospheric conditions and altitude. In the winter, you will hit the 1800 lbs torque with only 640 to 680 ITT at takeoff, while during hot summer days with a low barometer you will hit 700 ITT with torque well below 1800. As you climb, torque drops off and you advance the throttle back to your limit, eventually you will hit the temp limit and that is where the throttle stays.
Hope that answers you question.
I am a Metro 3 pilot ,using the Garrett TPE 311.Two main engine controls ,power levers and speed levers.On the M3 there is a basic computer which controls starting ,ie push a button and fuel and ignition are controlled automatically.Main problem would be low batteries unable to carry the engine through the start, engine "hangs" at a low rpm and eventually over temps and melts down if allowed to continue.
Different engine temp limits apply depending on altitude and outside air temperature ,however to keep things simple the temp gauge always shows redline as 650C ,adusting the indications accordingly.
If the engine is overtemping a valve opens to bypass fuel and cool things down.
Propellors have to be held by locks to prevent them feathering on shutdown as the starter motor would be overloaded trying to start a feathered propellor.These locks must be released after start to allow the propellor normal operation.
In short different to a piston engine ,simpler in some ways but must be thoroughly understood as mistakes are extreamly expensive.
Originally posted by Superfueler
...pretty much figured out I had no idea how it works. ...
Welcome to the club.;)
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