Trying to understand Prop Feathering, Constant speed props etc

[ryanbatc 12,774]

Well I disabled the engine damage on the Duke pffff! Maybe it took this long for me to have greater appreciation for TP's...

[N1G 88]

Correct you select prop rpm with the prop lever. If you add power by increasing throttle the blade angle will increase too hold the rpm.

Ok, so let me see if I have this right.The Power/Thrust Lever will change the pitch and the prop lever will change the pitch. Is this correct?Bob

[Tom Wright 438]

The Power/Thrust Lever will change the pitch and the prop lever will change the pitch. Is this correct?

The prop lever sets the required RPM by adjusting blade pitch.However, if you increase power on the throttles there will be more power turning the prop than there was before. So what will happen? The speed of the propeller will increase. So, as an automatic response to an adjustment in engine speed (throttle), the pitch of the blades will adjust again to maintain the required prop RPM.There isn't a direct connection between the prop lever and the blade pitch, or the throttle and the blade pitch. All the prop lever does is allows the pilot to set his desired RPM, and all the throttle does is controls the speed of the engine. The blade pitch itself is controlled by a computer which responds to some input - either movement of the prop lever or a change in engine speed. Edited January 12, 2012 by twright

[JoTayhen 6]

Hey Bob,You're almost there. The prop levers are there to allow you to manually adjust the prop pitch thereby adjusting the RPM up or down. In addition to the prop levers, which of course are under your control, there exists an additional device which, for lack of a better term, "governs" the prop RPM. This device is placed in the linkage between the prop and the turbine. You do not have manual control over this device. It is there to insure the prop RPM does not waiver up or down from the level you have dictated by using your prop levers when you use the throttle. In other words, once you set the prop RPM using your prop pitch levers you then can adjust the throttle and the prop RPM will be maintained by the device mentioned above. This enables you to establish various throttle settings and be comfortable the prop RPM settings previously set by your prop pitch lever will be maintained.Hope this helps.John

[N1G 88]

The prop lever sets the required RPM by adjusting blade pitch.However, if you increase power on the throttles there will be more power turning the prop than there was before. So what will happen? The speed of the propeller will increase. So, as an automatic response to an adjustment in engine speed (throttle), the pitch of the blades will adjust again to maintain the required prop RPM.There isn't a direct connection between the prop lever and the blade pitch, or the throttle and the blade pitch. All the prop lever does is allows the pilot to set his desired RPM, and all the throttle does is controls the speed of the engine. The blade pitch itself is controlled by a computer which responds to some input - either movement of the prop lever or a change in engine speed.

Hey Bud,I watched the video. AA did a very nice job with this, and thank you for taking the time to post, it really did help. This is indeed is a new area for me and a real eye opener. I am beginning to understand the situation of the pilot who failed to understand his plane and it cost him his life and 4 others. If he had been flying a fixed prop pitch aircraft, of course the only thing to do would have been to apply power and clean up the aircraft. With the constant speed prop, he must have come in with low thrust and low rpm I am guessing, which meant he would have had to increase thrust and rpm. Maybe he added the power, but was still at a low rpm, thus having a relatively low airspeed, he could not get any climb. I think that must have been the situation. What do you think? My post on this pilot error is above.RegardsBob

[PIC007 82]

GUMP check is standard for constant speed prop planes and should always be used on approach...Gas...fuel selector on correct tanks, pumps on if requiredUndercarriage... gear down and '3 green'Mixture....full richProps....full forwardKeep the props full for takeoff and landing

[N1G 88]

GUMP check is standard for constant speed prop planes and should always be used on approach...Gas...fuel selector on correct tanks, pumps on if requiredUndercarriage... gear down and '3 green'Mixture....full richProps....full forwardKeep the props full for takeoff and landing

Nice acronym. I can remember that. ThanksBob

[Guest jahman]

Turbo-props are slow to respond at low power settings because you have to wait for the "gas generator" (compressor and turbine, a.k.a. N1) to spool-up. Therefore you fly the last part of the approach in high drag configuration (full flaps) to avoid approaching with engines at idle. Then if you go-around your engines will respond a lot quicker when you add throttle.Constant speed props all have a prop rpm governor. This means the prop lever is not setting pitch, rather, um, rpm! Prop lever forward: More rpm. Prop lever back: less rpm. That's it.What the prop governor does is change pitch as you change throttle so the rpm remain constant. Thus the name constant-speed prop. When you add throttle, the prop will seek to increase rpm, thus the governor will increase blade pitch to increase drag, so prop rpm remain again constant. Of course as the governor increases prop pitch, thrust also increase. Thus an increase in throttle leads to an increase in thrust while prop rpm remains constant.Now as you increase throttle, while the prop rpm remain constant, what you are increasing is torque. So you can increase torque by increasing throttle or you can increase torque by lowering prop rpm (remember, at constant throttle, for a lower rpm setting the prop gorvernor will increase blade pitch to slow the prop, and increased pitch will increase torque).Now power (hp) is torque times rpm, so you see you can have the same power at high torque and low rpm or at low torque and high rpm. Of course you will have max power at both high torque and high rpm.With a piston constant speed prop, the idea is to lower rpm to reduce engine wear and fuel consumption, The limit is the maximum torque for a given rpm, as piston engines do not take to high torque and low rpm kindly.Conceptually the prop lever acts as a gear shift on your car or bicycle: High rpm is low gear (you pedal fast on your bike!) then as you increase airspeed low rpm (you pedal slower.)Finally, when approaching in a turboporp, learn to listen to the gas generator high-pitched whine frequency to develop a feel for torque, as torque is a proxy for throttle setting (the prop rpm remain fixed at the high setting during approach).Cheers,- jahman.

[N1G 88]

Turbo-props are slow to respond at low power settings because you have to wait for the "gas generator" (compressor and turbine, a.k.a. N1) to spool-up. Therefore you fly the last part of the approach in high drag configuration (full flaps) to avoid approaching with engines at idle. Then if you go-around your engines will respond a lot quicker when you add throttle.Constant speed props all have a prop rpm governor. This means the prop lever is not setting pitch, rather, um, rpm! Prop lever forward: More rpm. Prop lever back: less rpm. That's it.What the prop governor does is change pitch as you change throttle so the rpm remain constant. Thus the name constant-speed prop. When you add throttle, the prop will seek to increase rpm, thus the governor will increase blade pitch to increase drag, so prop rpm remain again constant. Of course as the governor increases prop pitch, thrust also increase. Thus an increase in throttle leads to an increase in thrust while prop rpm remains constant.Now as you increase throttle, while the prop rpm remain constant, what you are increasing is torque. So you can increase torque by increasing throttle or you can increase torque by lowering prop rpm (remember, at constant throttle, for a lower rpm setting the prop gorvernor will increase blade pitch to slow the prop, and increased pitch will increase torque).Now power (hp) is torque times rpm, so you see you can have the same power at high torque and low rpm or at low torque and high rpm. Of course you will have max power at both high torque and high rpm.With a piston constant speed prop, the idea is to lower rpm to reduce engine wear and fuel consumption, The limit is the maximum torque for a given rpm, as piston engines do not take to high torque and low rpm kindly.Conceptually the prop lever acts as a gear shift on your car or bicycle: High rpm is low gear (you pedal fast on your bike!) then as you increase airspeed low rpm (you pedal slower.)Finally, when approaching in a turboporp, learn to listen to the gas generator high-pitched whine frequency to develop a feel for torque, as torque is a proxy for throttle setting (the prop rpm remain fixed at the high setting during approach).Cheers,- jahman.

Ah..... very nice indeed jahman!!! Thanks for the contribution.Geez, so much to learn. It never ceases to amaze me that by asking a question, so many are ready to jump in.Bob

[Tom Wright 438]

Maybe he added the power, but was still at a low rpm, thus having a relatively low airspeed, he could not get any climb. I think that must have been the situation. What do you think? My post on this pilot error is above.

I'd say pilot error too. If the prop levers were still set to a low RPM when he maxed the throttles, the engines could likely have failed. Asking the props to maintain a low RPM whilst throttling up the engines can completely destroy the engine internally - you're asking it to do something it doesn't want to do (naturally at high power the propeller wants to turn faster). Edited January 12, 2012 by twright

[N1G 88]

I'd say pilot error too. If the prop levers were still set to a low RPM when he maxed the throttles, the engines could likely have failed. Asking the props to maintain a low RPM whilst throttling up the engines can completely destroy the engine internally - you're asking it to do something it doesn't want to do (naturally at high power the propeller wants to turn faster).

Good point. I never thought of engine failure.Bob

[Wendall 50]

Sorry to hijack this,I have NO experience in realistic turbo-props, and I'm looking for one. I been told the only real options are the J41, Turb. Duke, and (more recently) the Bronco X....anymore contenders?Back on Topic!

[Paul J 349]

Therefore you fly the last part of the approach in high drag configuration (full flaps) to avoid approaching with engines at idle. Then if you go-around your engines will respond a lot quicker when you add throttle.

Just to expand a little on that - to ensure adequate go-round power - on approach the turbine is run at "Flight idle" - which is a turbine and propellor rpm somewhat higher than "ground idle" - and also (courtesy of Airliners.net) bear in mind - turboprop engines come in two basic varieties: "Fixed" Shaft and Free Turbine. On the latter, the turbine stage that is connected to the propeller and it's reduction gear box is not directly connected to the shaft of the gas generator turbine. On those engines, the speed of the gas generator determines the thrust available to drive the turbine connected to the prop. The blade angle and air load determine the speed or revolutions of the propeller. Engines of this type include most made by Pratt & Whitney, the GE CT7, and the Rolls Royce Tyne.The fixed shaft types of engines have a reduction gear off the same shaft that runs the compressor and turbine stages. Of course, the propeller rpm must be reduced to a much lower value than is efficient for the gas turbine section. These engines run in a very limited speed range. Increased fuel flow is translated into a blade angle change rather than a rpm response. Direct control of the propeller, for Beta or Reverse, allows for low or reverse thrust operation on the ground. Engines of this type include the Allison 501, Garrett TPE 331, and (with a nod to Aerosoft's Vickers Viscount) the Rolls Royce Dart. Edited January 12, 2012 by Paul J

[Guest jahman]

Ah..... very nice indeed jahman!!! Thanks for the contribution.Geez, so much to learn. It never ceases to amaze me that by asking a question, so many are ready to jump in.Bob

You're welcome!

Just to expand a little on that - to ensure adequate go-round power - on approach the turbine is run at "Flight idle" - which is a turbine and propellor rpm somewhat higher than "ground idle" - and also (courtesy of Airliners.net) bear in mind - turboprop engines come in two basic varieties: "Fixed" Shaft and Free Turbine. On the latter, the turbine stage that is connected to the propeller and it's reduction gear box is not directly connected to the shaft of the gas generator turbine. On those engines, the speed of the gas generator determines the thrust available to drive the turbine connected to the prop. The blade angle and air load determine the speed or revolutions of the propeller. Engines of this type include most made by Pratt & Whitney, the GE CT7, and the Rolls Royce Tyne.The fixed shaft types of engines have a reduction gear off the same shaft that runs the compressor and turbine stages. Of course, the propeller rpm must be reduced to a much lower value than is efficient for the gas turbine section. These engines run in a very limited speed range. Increased fuel flow is translated into a blade angle change rather than a rpm response. Direct control of the propeller, for Beta or Reverse, allows for low or reverse thrust operation on the ground. Engines of this type include the Allison 501, Garrett TPE 331, and (with a nod to Aerosoft's Vickers Viscount) the Rolls Royce Dart.

Yes, indeed, missed out on the fixed-floating prop shaft bit. Thanks for filling in!Cheers,- jahman.

[N1G 88]

You're welcome!Yes, indeed, missed out on the fixed-floating prop shaft bit. Thanks for filling in!Cheers,- jahman.

Well, that was a little over my head as I must now get a little experience in the JetProp and see if I can have an "I get it" experience.Thanks AllBob