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Propellor Planes

Tue Feb 19, 2002 3:08 pm

i was wondering, does propellor plane have the thing called reverse thrust?
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RE: Propellor Planes

Tue Feb 19, 2002 3:17 pm

Somewhat, yes. Although it doesn't work exactly the same as on jet engines, they do perform the same purpose. In simple terms, many aircraft with variable pitch props can change the pitch (angle of the blades) so the thrust is forward instead of backwards.
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RE: Propellor Planes

Tue Feb 19, 2002 3:18 pm

Well, kind of, in the right plane. In a constant velocity prop (adjustable pitch) the prop can be 'feathered' to not provide much thrust. In short no.

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RE: Propellor Planes

Tue Feb 19, 2002 4:40 pm

Careful peter.. you missed that one by a mile. Most modern turbo props have reverse thrust. It is provided by moving the blades of the propellor into a range below 0 degrees...somewhere from 0 to -4 degrees (some can be up to -7 degrees). Blade pitch is maintained by oil pressure acting on a piston inside the hub of the propellor acting against spring pressure from springs on the pitch rods. (Not always true... the Aeroproducts propellor used on Alison 501's on airplanes like the CV-580 and the C-130 centrifugal force overcome by oil pressure generated by electric pumps inside the prop hub.. but they are weird and don't count.) So, turbo props have basically four thrust ranges: reverse, which I explained. Beta, which is flat pitch or a zero thrust position used mostly while taxiing the aircraft, fwd which is any blade angle above beta which provides fwd thrust and feather, which is not intended to provide thrust like peter said, but to reduce drag for one of two purposes. In the event that the engine is shut down or fails in flight, the amount of drag produced from an unpowered propellor in fwd blade angles would be tremendous. So, propellors are designed to feather when oil pressure is lost. They move to an angle that is streamed with the airflow to minimize drag. Feather is also used on free turbine engines to minimize the load on the engine during starts, thereby reducing start temps and starter wear. This is the reason Pratt and Whitney engines are superior, but that is fodder for another thread.... Smile/happy/getting dizzy
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Reverse Pitch Picture

Tue Feb 19, 2002 4:45 pm

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RE: Propellor Planes

Tue Feb 19, 2002 4:46 pm

Big recips also had reversable props as well.

The reason light aircraft don't have reverse is largely because they don't need it at their low approach speeds.

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RE: Propellor Planes

Tue Feb 19, 2002 11:29 pm

a few points.

First off, on many larger aircraft the propeller is driven towards coarse as well as fine pitch by hydraulic oil, while counterweights will push the propellet towards typically 50 degrees of pitch if it is left without oil pressure. A reservoir of oil that can only be used by the feathering pump (which is generally electrically driven) is kept to make sure that you'll always be able to feather the propeller even if the oil system develops a leak somewhere.

Turboprops can go far below -7 degrees in reverse mode. The aircraft I work with go to -16 (typically only using -15.5 since it is in constants speed mode in the reverse region) and -16.5 degrees.

In beta, you most certainly don't have flat pitch. The beta range typically goes from the zero-thrust pitch setting up to 14 degrees or so. In this region, the control lever setting controls the propeller pitch directly rather than controlling engine power and leaving pitch to the pitch control unit. This is described in more detail below along with the reason to have a beta range in the first place.

Free turbine engines are just that, free turbine engines. They don't care much about the pitch setting during startup. With turboprops which aren't free turbine and recips it is a good idea to have the propeller in flat pitch for the reasons you mentioned. Perhaps you got them mixed up?

As for how anything of what you'd wrote would make a P&W engine superior in any way... well, clearly you know something the rest of the world doesn't and I'll leave it at that.

Below is an excerpt from a supplement to my thesis, explaining the very basics of engine management in the SAAB 340.


You have two levers for each engine, Power Lever (PL) and Condition Lever (CL). The range of the CL is divided into

- Fuel off where the engine goes to feather (83.5 degrees pitch) and the fuel is cut off
- Start, where you are supplying fuel to the engine but the prop is still feathered
- UNF, UNFeathered, where the prop is out of feathered and basically in constant speed mode trying to maintain 1180 RPM but without the bottoming governor (more on that later)
- Min to max constant speed (CS) range where the prop RPM is controlled to be within 1180 RPM (min) and 1384 RPM (max).
- T/M (torque motor) lockout, which will lockout, the engine control unit (ECU, or digital ECU, DECU, in B model a/c) if it malfunctions. Once T/M lockout is activated, you have to shut down the engine (put the CL in fuel off) to reactivate it.

The power lever range goes from full reverse through ground idle (GI) to flight idle (FI) and then on up to full power. Below FI you are operating in the beta range where the PL position (unless the CL is in feather or you feather manually) directly controls the prop pitch from -16.5 to +10 degrees. Above FI there is a minimum pitch stop ranging from +10 (FI) to +25 (full power) degrees pitch. As you go from PL full aft to PL full forward, more and more fuel is added to the engine (naturally) through signals to the Hydro-Mechanical Unit (HMU). At low power settings (below approx 30%), this amount of fuel is not enough to spin the propeller up to the commanded 1180 RPM at the pitch setting commanded by PL in beta range or at the minimum pitch stop.
Why do we have a beta range? Due to the slow response to throttle setting changes in turbo engines it is very impractical to use the throttle to control movement on the ground. You would have to wait for the gas generator to spin up (Ng increase), providing more torque through the power turbine (PT) increasing the prop RPM (Np). The prop CS governor would then tell the pitch control unit (PCU) to increase the prop pitch and then you would get additional power. In beta mode, you change the pitch first instead using the inertia in the propeller system to provide thrust, letting the Ng accelerate or decelerate in response to Np to keep Np constant.
If the amount of fuel burned below 30% won’t keep the prop spinning at 1180 RPM, what keeps it at constant speed in the beta range? This is where the previously mentioned bottoming governor (BG) comes into play. The BG is active when the CL is above UNF and will send a signal to the HMU to add fuel above what the PL setting is dictating to keep the Ng up. The normal reference Np for the BG is 1040 RPM but to give more power in full reverse the BG reference will change to 1200 RPM Np when the pitch goes below –10 degrees (<-10 on both engines on older versions).

Early on it was discovered that the torque set in the beginning of the take-off roll would increase as the ram air effect increased with airspeed. To avoid having to stare at the torque (Nq) reading during the entire takeoff roll, decreasing the PL setting to keep it at 100% and not above a CTOT (Constant Torque on Take-OFF) system was added. When active, this system will signal to the HMU through the ECU to add fuel until the preset Nq is reached as soon as you set the PL above a certain position.

4 AC
If an engine dies there’s an autocoarsen (AC) system, which will detect this. It then proceeds to feather the dead engine automatically. There’s an inbuilt safety making it impossible to feather both engines in flight should this system fail. The AC system continues to monitor a failed engine and will bring it out of AC mode should the engine parameters used to detect a flameout increase above the threshold values again.

340B a/c has something called automatic power reserve (APR) which when one engine goes into AC during CTOT operation automatically adds 7 percent units of torque to the other engine to compensate for the loss of thrust.

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RE: Propellor Planes

Wed Feb 20, 2002 1:06 am

Thanks for the post Fred, it was very informative. I also wanted to ask a question. When you have the PL above the flight idle position, does it control both engine power and propeller pitch? Or is the pitch set by the CL at this point?

Thanks again for your input.
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RE: Propellor Planes

Wed Feb 20, 2002 5:11 am

Thank you, good to hear that I was at least somewhat understandable.  Big grin

Above flight idle, the PL controls the fuel flow through the HMU. Np (prop RPM) is then governed through the pitch setting to the RPM set by the CL - if that can be done without going below the min pitch stop. The engine will be able to provide Np 1180 at around 30% of the PL's travel range above FI.

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