I would appreciate some basics on Turboprops like the one in the Kodiak. I gather that the engine is spinning at a constant RPM and the thrust is determined by the blade pitch or bite into the air. Correct? Or does the RPM vary?
The Kodiak is the first Turboprop that I’ve flown with a functioning prop (blue) lever I think. But I’m not sure what the proper use of it is and the relationship with the throttle (black lever)
What is each lever doing exactly?
What is Beta? Is that just reverse pitch/thrust? Why not just call it reverse? Should that be used on all landings or only STOL situations? Can it be used to back up on the ground?
Feathering is where the blades are facing into the airstream to reduce windmilling and drag in the event of an engine failure… right? I’ve read you should also feather the prop on shutdown to stop it spinning sooner. Is there any other use for feathering?
What do the engine metrics like NG, ITT, etc refer to?
A special thanks to anyone who takes time to reply
All good questions and I will let those more familiar attempt to answer them. Here is a short piece with some answers.
There is also a pretty good explanation of turboprop behaviour in the Twin Otter manual which comes with the Aerosoft Twin Otter for msfs.
You should also be aware that the current implementation of msfs does not really simulate turbo props very well. It is apparently on Asobo’s list of things to improve. In the meantime, third party devs need to employ a number of workarounds to try to simulate actual turboprop behaviour.
Thanks… that helped. I think I’ve figured out part of it… at least for the Kodiak…
Power Lever in Alpha controls fuel which increases power output of the engine which will tend to increase RPM of the prop, but since the prop has a governor, it will maintain the optimal RPM by adjusting the angle-of-attack of the blades which will increase thrust. So in short: power lever adjusts fuel, which increases power, which increases RPM, which is controlled by adjusting blade pitch which increases thrust.
Power Lever in Beta directly controls prop pitch from idle thrust to zero thrust, to reverse thrust.
The Prop Lever has two settings: Max (for flight) and Feather (for engine failures and shutdown).
Alpha range is the propeller pitch range from max angle to the flight idle pitch stop. In the Kodiak, that stop is at 11.4 degrees. Below 11.4 you are in beta.
The power lever in alpha varies in different planes, but overall it controls fuel flow->shaft hp->torque. In the Kodi the prop will begin at 11.4 degrees of pitch and rpm will increase along with the fuel flow. Once the rpm reaches 1900, the prop governor starts working and it will adjust pitch to keep rpm steady at the selected speed.
That speed is 2200rpm when the prop lever is fully forward, to 1900rpm fully pulled.
Feathering is another function of the prop lever, which will set the prop for minimum drag.
Beta: the power lever controls pitch & fuel flow directly. As you pull back, pitch reduces. RPM should stay idle until you reach the discing/zero thrust point. As the prop will want to increase rpm, the power lever reduces fuel flow to keep it at idle rpm.
Past the discing point, the prop will go into reverse pitch and push you back. The fuel governor will limit fuel flow to make sure the prop doesn’t exceed 95% rpm (2090 for the Kodiak).
All the tables in the Kodiak PoH seem to reference 2200 RPM and all instructions I could find on the use of the Propeller Lever refer to MAX or feather. In fact, this is a bit of text from the POH…
“The lever has two main positions: MAX and FEATHER. The MAX position is used when a high RPM is desired and governs the propeller speed at 2200 RPM. The FEATHER position is used during normal shutdown of the engine to assist in stopping the rotation of the power turbine and front section of the engine.”
Under what conditions would you use less than max RPM… like 1900?
In the Aerosoft twin otter (manual for which is mentioned in another post above), lower RPMs are used in cruise for increased fuel efficiency. High RPM is only really used on takeoff and below 500 feet AGL on landing (in preparation for go-around, if needed).
The Aerosoft twin otter manual is an English translation of a German language book that explains in layman’s terms how to fly the twin otter, as well as some of the history of the type, so it’s a pretty good read.
Edit: I should add that I wouldn’t recommend the Aerosoft Twin Otter unless you’re willing to put up with some foibles and weirdness, but some have suggested that the manual makes the price of the mod worth it.
Lower RPM will allow you to achieve more fuel economy, lower noise and once you reach your max speed, lowering the RPM will allow you to go even faster.
The reason for the speed increase is propeller tip mach. As the propeller rotates around and moves forward at the same time, the total speed of its tip approaches 1 Mach which will reduce its efficiency. Around 0.8-0.9M of tip speeds most propellers reach their maximum thrust and going further only produces drag.
Lowering RPM will mean that the propeller spins slower. The total speed (forward+rotation) will then lower, allowing you to get a little more thrust out of it and travel faster. That difference can be 5-15kts, depending on the propeller, engine and aircraft.
Mind you that lower RPM doesn’t always mean higher speed, as that will depend on the propeller design itself. In the Kodiak, for example, the propeller does achieve higher speeds at 2000rpm vs 2200rpm, but I don’t remember seeing any data in the manual about 1900rpm. What will happen at that setting is less noise and probably less fuel burn, but the speed will have to be determined experimentally.
Ryan in his videos does mention his organisation flies at 2000 RPM not 2200 . Presumably for lower noise and fuel consumption, at the expense of cruise speed and climb power.
Thanks for the question and all the great answers! I am completely new to turboprops and just got the Kodiak.
What does the condition lever do? It seems to have only 3 positions. I read somewhere that you use low idle for taxi, but if someone could explain a bit more, I would be very grateful!
The fuel condition lever in the cockpit is connected through airframe linkage to a combined lever and stop mechanism at the top of the fuel control unit(FCU); this is connected by the FCU linkage to the cut-off lever on the side ofthe unit. The lever and stop also function as a hi-idle stop. The fuel conditionlever performs the function of CUTOFF, LOW-IDLE and FLIGHT-IDLE. TheCUTOFF position shuts off all fuel to the engine fuel nozzles. LOW-IDLEpositions the control rod stop to provide a gas generator RPM of 52% nFLIGHT IDLE positions the control rod stop to provide a gas generator RPMof approximately 68% n
