Prop jet qualified pilots, ...?

Question

Once heard ( credibility of source?) that a turboprop engine always puts out the same RPM's, you increase power or decrease powere by changing the pitch of the prop. Believeable or Bull---. Thanks

Answer 1

Power is not adjusted by the prop levers on a Pratt and Whitney PT6 turboprop engine.

In flight, when the throttle is advanced more fuel is added to the combustion chamber. When this happens the compressor section of the engine turns faster. The speed of the propeller will not change will not change because the governor on the prop will adjust the pitch of the blades to mantain RPM. It is possible for the compressor section to speed up while the prop mantains a constant RPM because the prop and the turbines that turn it aren't mechanically connected to the rest of the engine.

In the aircraft I fly the Prop RPM must be set between 1200 and 1700. Usable power settings in flight result engine RPMs of 77% to 98% of max allowable rpm.

Answer 2

There's too many different rpms to talk about for that question to be answered simply.

In a gas-coupled turboprop engine like the PT6 or CT7, you have basically two entirely separate systems - a gas generator (just a turbine engine) and the propeller. The only thing that connects the two is gas pressure/flow. Think about a pinwheel in the wind. The wind is the accelerated flow out the back of the turbine engine, and the pinwheel is the propeller turbine that would be connected to the propeller. You could start this kind of engine while holding the propeller still, and it would run just fine. (Granted it may take some superhuman strength to hold it, I don't know, I've never tried.)

So, the propeller rpm can be controlled two ways - change the amount of air flowing over the propeller turbine, or change the amount of resistance on the propeller by changing prop pitch.

While all of that is going on, you have the gas generator that is providing the pressure/flow for the propeller. It's spinning at it's own rpm (up to 44,000 rpm in the CT7 on my airplane.)

I could go in to beta/alpha modes now and confuse you more, but I have to go to work.

Answer 3

I think everyone is right here depending on the make and model engine they operate. What’s important to keep in mind is a turboprop engine will keep the prop at a constant RPM at different pitch angles. This is accomplished by inputs to the ECU (on most models) that controls the fuel to the engine. I have flown with GE 700 and 701 engines and developed 110% engine output under heavy load conditions, but the prop (rotor speed) RMP stayed constant.

On some models the fuel can be controlled manually, which makes it hard on the pilots to constantly be monitoring the gauges.

Answer 4

Timeless is right in regard to the PT6 - but lets talk about Allison T56-14(x) - there are verious versions depending on the aircraft they are fitted to.
The power section runs at 100% RPM at all times - from start to shut down.
Horse power or torque is controlled by the propeller.
As the power lever is advanced, pitch is applied to the propeller at he same time fuel is added to the combustion section of the engine.
The amount of fuel added is 120% of engine requirements, but to offset this the fuel control knows the Turbine Inlet Temperature and reduces the fuel flow to control the turbine temp.
I could go on for a week on this - but to answer the question - its right - there is an engine that runs at constant speed.

Nick and timeless are spot on with what they say - but they are not answering the question.
The Allison T56 - 14 is a constant speed engine, in both alpha and beta range. i.e. ground range and flight range.

Answer 5

Believable. The Garret TPE-331 is like that.