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  • Please see the most recent updates in the "Where did the .com name go?" thread. Posts number 16 and 17.

    Post 16 Update

    Post 17 Warning

A question about turbo props.....

Navy Chief

Navy Chief

Senior Member
Seems like I recall that the prop blades on a P-3C Orion would adjust/turn, with relation to engine RPM? (constant speed, variable pitch)

Do all turbo props do that?

And when (in the case of the P-3C for example) reverse thrust is applied, is it simply the prop blades turning around to cause the direction of airflow to reverse?

P-3s have such an awesome sound when they land.

NC
 
Think of a tubroprop like a high-bypass ratio turbofan engine (which is very similar to a ducted fan version of a turboprop). The gas turbine engine is serving as a generator, so the core speed is directly related to the power produced. The prop speed is typically held constant while the pitch is adjusted with engine speed. The greater the pitch angle, the greater the drag produced and associated torque required to turn the prop. So, the faster the core speed of a turbofan, the more energy can be drawn for propulsion and this leads to a greater sustainable pitch angle. Hope that was hopeful, I'm happy to extrapolate if anything was unclear.
 
Turboprops are, from what I have studied, very unique powerplants. They are powerful (very powerful) and require management most when it comes to max throttle (temps and torques) and high RPM setting (red condition lever, high idle setting, max), and proper management of the prop.

When you are about to take-off and land, you MUST go to full max red condition lever.

One of the beauties of a Turboprop with an adjustable prop is that you can use it to slow down, but ONLY when you have landed. Never use that in flight as that might be your last time. For instance, if you suddenly had a plane move out into the runway as you are approaching and you must go to full throttle and you are giving the bird a bit of reverse thrust to maximise your decelleration to land, then you have that much less time to set up a go-around.

The reason you use full condition/max high idle, is that if you need to do a go-around, you have the RPM that much higher then usual which might mean saving your life if you go to full throttle and need that thrust 'now'. Turboprops have a slightly slow 'spool-up' and that moment of spool up can cost a person or crew.

Usually on the ground, you would select like 40% condition lever(s) to manage taxiing around. Full condition (like on the Epic LT) will make the plane a bit squirrelly and tricky to manage as the spool up with high idle will find you sitting there one moment, then the craft wanting to lunge forward the next as it transitions into a higher RPM.

I 'have' heard of DH pilots adding a bit of reverse prop on landings (before touch down) but... I dont know how much of that is true. Also, some have the ability to backup with reverse thrust (like the Quest Kodiak) but it can kick up debris into the front of the engines which will damage the turbine or at least wear it out. You can do this on alot of airliners as well, but the main reason you shouldnt is the debris factor. Its fine shooting back, but going in front of the engine, small stones and twigs and things, is very bad...

Also, you can cook Turboprops. You have to manage the turbine's temp very closely. Take-offs are usually never full throttle in TP's. Also, lastly, torque. On some single engines with the PW-67, the turbines would rip loose from frequent full throttle take-offs due to extreme torque events on the engine mounts. TB lost one turbine as it ripped loose from the firewall during testing with a PW 67A. They then installed a 50% lockout throttle over-ride which turns off after a certain altitude. This keeps the 'warp core' from doing an auto-jettison. The Epic LT 'does not' have this feature, and so the guys at Epic would have a new Epic pilot just drive around a full day in an Epic to teach him how to handle the things power and spool ups, which can at first be very nerve racking. (Not all TP planes do that).


Thats the two cents I have learned on them.

Reverse thrust does sound cool... Nothing like seeing a plane backup into its parking spot though. :d
 
Also found these diagrams sitting on my computer that chronicle the transition from Turbojet to Turboprop. Shown are:

1: Single spool Turbojet (non-AB)

2: Dual spool Turbojet (non-AB)

3: Low Bypass Turbofan with mixed exhaust

4: High Bypass Turbofan with separate exhaust

5: Turboprop
 
On most turboprops, the propellers work in essentially the same way that a constant speed prop on a piston aircraft would, in that the propeller governor constantly tries to adjust the blade angle to maintain the desired RPM.

Exactly what tells the propeller's what RPM to aim for varies depending on the aircraft. On aircraft like the King Air or Twin Otter, the pilot has a set of propeller controls that directly tell the governors what RPM setting they're supposed to be at.

On other aircraft (like the Jetstream 4100 series), the propeller pitch cannot be controlled by the pilot (aside from feathering the blades), and is adjusted automatically when the power levers are moved.

As for reversing the props, that's done by simply rotating the blades slightly beyond the feathered position, so they direct thrust forwards. When the power levers are moved into the reverse range, the engine core accelerates slightly past idle to produce a little more reverse thrust, but it's nowhere near the full power output of the engine.

Many aircraft have a switch that senses whether there is weight on the landing gear before the propellers can be commanded into the reverse range, but it is possible to get some aircraft into reverse while airborne, doing so is extremely dangerous and is almost always prohibited by the manufacturer.
 
azflyboy said:
As for reversing the props, that's done by simply rotating the blades slightly beyond the feathered position, so they direct thrust forwards. When the power levers are moved into the reverse range, the engine core accelerates slightly past idle to produce a little more reverse thrust, but it's nowhere near the full power output of the engine. Many aircraft have a switch that senses whether there is weight on the landing gear before the propellers can be commanded into the reverse range, but it is possible to get some aircraft into reverse while airborne, doing so is extremely dangerous and is almost always prohibited by the manufacturer.
Click to expand...

Perfect; thanks all!

NC
 
azflyboy said:
Many aircraft have a switch that senses whether there is weight on the landing gear before the propellers can be commanded into the reverse range, but it is possible to get some aircraft into reverse while airborne, doing so is extremely dangerous and is almost always prohibited by the manufacturer.
Click to expand...

One aircraft type that is cleared for selecting reverse in flight is the Pilatus PC-6. :)
 
I Like to Keep Things Simple

I picture the reversing prop simply by comparing it to a ceiling fan. If the fan is turning one way it pushes air down, if you flip the switch the fan blades turn the other direction and the air is pushed in the other direction. A similar thing happens when you push / pull the prop lever in a constant speed propeller equipped airplane...the air is pushed one way or the other....push it toward the rear for thrust or push it the other way for braking. (All propellers are not 'constant speed' (or variable pitch), many (on less complex aircraft) are fixed pitch....easier to maintain with fewer parts to break!) I'm just a simpleton.
 
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