Sponsor Message:
Civil Aviation Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
Propellor Planes  
User currently offlineWadha From United Arab Emirates, joined Mar 2000, 185 posts, RR: 0
Posted (12 years 5 months 2 weeks 2 days 14 hours ago) and read 1307 times:

i was wondering, does propellor plane have the thing called reverse thrust?

8 replies: All unread, jump to last
 
User currently offlineFlyf15 From , joined Dec 1969, posts, RR:
Reply 1, posted (12 years 5 months 2 weeks 2 days 14 hours ago) and read 1278 times:

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.

User currently offlinePmk From United States of America, joined May 1999, 664 posts, RR: 2
Reply 2, posted (12 years 5 months 2 weeks 2 days 14 hours ago) and read 1268 times:

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.

Peter


User currently offlineApathoid From , joined Dec 1969, posts, RR:
Reply 3, posted (12 years 5 months 2 weeks 2 days 13 hours ago) and read 1254 times:

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

User currently offlineBlatantEcho From United States of America, joined Sep 2000, 1903 posts, RR: 1
Reply 4, posted (12 years 5 months 2 weeks 2 days 13 hours ago) and read 1251 times:


Click for large version
Click here for full size photo!

Photo © Glenn Alderton




They're not handing trophies out today
User currently offlineSllevin From United States of America, joined Jan 2002, 3376 posts, RR: 6
Reply 5, posted (12 years 5 months 2 weeks 2 days 13 hours ago) and read 1246 times:

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.

Steve


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 6, posted (12 years 5 months 2 weeks 2 days 6 hours ago) and read 1221 times:

Apathoid,
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).

3 CTOT
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.

5 APR
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.

Cheers
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineLapa_saab340 From Spain, joined Aug 2001, 390 posts, RR: 4
Reply 7, posted (12 years 5 months 2 weeks 2 days 4 hours ago) and read 1202 times:

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.


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 8, posted (12 years 5 months 2 weeks 2 days ago) and read 1161 times:

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.

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
Top Of Page
Forum Index

This topic is archived and can not be replied to any more.

Printer friendly format

Similar topics:More similar topics...
FAA: Planes Must Be Built To Take An Attack posted Tue Jan 9 2007 20:00:10 by KarlB737
Why Are People Allowed Knitting Needles On Planes? posted Fri Jan 5 2007 15:06:10 by Gh123
Best Place To Watch Planes At O'Hare posted Wed Jan 3 2007 03:19:26 by TWA1985
2 WN Planes Bump Tails In San Diego posted Wed Dec 27 2006 08:20:13 by Lt-AWACS
What Happens To Sims Of Outdated Planes? posted Tue Dec 26 2006 18:59:31 by Airfoilsguy
What Happens To Planes When Flights Are Canceled? posted Fri Dec 22 2006 07:55:32 by Fbm3rd
RJ, New Planes And Routes posted Mon Dec 18 2006 11:43:59 by Manni
TUI Orders 65 Boeing Planes posted Fri Dec 15 2006 11:17:50 by N1786b
9-11 Planes Deregistration posted Fri Dec 15 2006 04:58:15 by Encore
Ipod On Planes posted Wed Dec 13 2006 17:27:37 by Qwame