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Btriple7
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Which Way The Fan/Propeller Blades Turn

Mon Sep 04, 2006 10:39 pm

Hello,

I have always wondered if there is any particular rhyme or reason as to deciding which way a propeller or jet-fan blade turns on a twin-engined aircraft? I have heard that the blades turn in the same direction in order to reduce vibration, but I have also heard that they turn in the opposite direction for some aerodynamic reason. I'm not sure which is right or for what reason, and I have never actually payed attention whenever I'm at airport.

So it begs the question. Which way do the blades on twin-engined aircraft turn and why? And while I'm at it: What about on single-, tri-, and quad-engined aircraft too.

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Btriple7
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joness0154
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RE: Which Way The Fan/Propeller Blades Turn

Mon Sep 04, 2006 11:02 pm

Well, on some multi engine aircraft the propellors rotate the same way. On others, they rotate opposite ways, known as counter-rotating props.

Planes with propellors rotating the same way have what is known as a critical engine. Normally it is the left. Because of the torque/p-factor/etc of an engine has certain adverse affects on a plane, loss off the critical engine results in the worst config for a multi engine aircraft. Maybe this diagram will help:

http://upload.wikimedia.org/wikipedia/en/7/76/Criticalengine1.jpg

But a plane with counter-rotating props doesn't have a critical engine. They are mostly used on training aircraft, etc. Why not put counter rotating props on all multi engine aircraft? Because the gearbox has to be changed, etc.
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RichardPrice
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RE: Which Way The Fan/Propeller Blades Turn

Mon Sep 04, 2006 11:14 pm

Quoting Btriple7 (Thread starter):
I have always wondered if there is any particular rhyme or reason as to deciding which way a propeller or jet-fan blade turns on a twin-engined aircraft? I have heard that the blades turn in the same direction in order to reduce vibration, but I have also heard that they turn in the opposite direction for some aerodynamic reason. I'm not sure which is right or for what reason, and I have never actually payed attention whenever I'm at airport.

So it begs the question. Which way do the blades on twin-engined aircraft turn and why? And while I'm at it: What about on single-, tri-, and quad-engined aircraft too.

Jet engines turn the same way on both engines, with usually their cores turning the opposite way to counteract the gyroscopic effects.

Why? Because they are all the same engine - it would cost more to produce an engine for each wing.
 
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Starlionblue
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RE: Which Way The Fan/Propeller Blades Turn

Mon Sep 04, 2006 11:42 pm

Quoting Joness0154 (Reply 1):

But a plane with counter-rotating props doesn't have a critical engine.


That depends on the direction of rotation. The P-38 Lightning has 2 critical engines.

[Edited 2006-09-04 16:43:03]
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Tristarsteve
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RE: Which Way The Fan/Propeller Blades Turn

Tue Sep 05, 2006 12:42 am

Quoting RichardPrice (Reply 2):
Jet engines turn the same way on both engines, with usually their cores turning the opposite way

Sorry but what do you mean? All certified jet engines have all spools rotating in the same direction. Bearing technology has just got to the stage where counter rotating spools can be used and RR and GE are both introducing them.
 
RichardPrice
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RE: Which Way The Fan/Propeller Blades Turn

Tue Sep 05, 2006 1:02 am

Quoting Tristarsteve (Reply 4):
Sorry but what do you mean? All certified jet engines have all spools rotating in the same direction. Bearing technology has just got to the stage where counter rotating spools can be used and RR and GE are both introducing them.



Quote:

The Trent 900 offered an interesting challenge from the certification point of view as it introduces a new scale paradigm for commercial jet engines, with civil sector innovations such as a counter-rotating compressor.

http://www.easa.eu.int/home/wsnews_trent900.html

http://www.prnewswire.co.uk/cgi/news/release?id=137955

I was under the impression that it was a current thing employed in most engines, so you are indeed correct to a degree.

The gyroscopic forces on a 777 GE90 must be tremendous then.
 
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Jetlagged
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RE: Which Way The Fan/Propeller Blades Turn

Tue Sep 05, 2006 1:19 am

Quoting Tristarsteve (Reply 4):
All certified jet engines have all spools rotating in the same direction.

The RR Pegasus (on the P-1127 and Harrier) has had counter rotating spools for a long time. Other than that you are right, most gas turbine spools turn in the same direction.

Quoting Starlionblue (Reply 3):
That depends on the direction of rotation. The P-38 Lightning has 2 critical engines.

You can't have two critical engines. If the props counter rotate, than asymmetry is the same regardless of which engine fails. It may be that the direction of rotation on the P-38 made the asymmetry worse, but it would do so in both directions equally.

I did hear a story from a Fokker 50 pilot (not sure whether it's true or not) regarding the Fokker 50 and F.27. The fin of the F.27 was set at a slight angle to allow for the rotating slipstream of the two Dart engines. The PW125 engines on the Fokker 50 rotate in the opposite direction to the Darts, but the fin angle was not reversed, leading to worse asymmetric handling on the F50 than the F.27, the difference between critical engine and non-critical engine becoming much greater.
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Starlionblue
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RE: Which Way The Fan/Propeller Blades Turn

Tue Sep 05, 2006 3:20 am

Quoting Jetlagged (Reply 6):

Quoting Starlionblue (Reply 3):
That depends on the direction of rotation. The P-38 Lightning has 2 critical engines.

You can't have two critical engines. If the props counter rotate, than asymmetry is the same regardless of which engine fails. It may be that the direction of rotation on the P-38 made the asymmetry worse, but it would do so in both directions equally.

The fact that the P-38 had two critical engines is widely documented. If you lost an engine during take-off and instinctively increased power, the resulting roll would easily flip the airplane over. Procedures were implemented which had the pilot initially reducing power on the remaining engine in engine out situations on take-off.
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FredT
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RE: Which Way The Fan/Propeller Blades Turn

Tue Sep 05, 2006 6:29 am

Quoting Joness0154 (Reply 1):
But a plane with counter-rotating props doesn't have a critical engine.



Quoting Starlionblue (Reply 7):
The fact that the P-38 had two critical engines is widely documented. If you lost an engine during take-off and instinctively increased power, the resulting roll would easily flip the airplane over. Procedures were implemented which had the pilot initially reducing power on the remaining engine in engine out situations on take-off.

It all depends on the definition. If you define critical engine as "the engine which rotates in such a direction that the P factor asymmetrical thrust will be at a maximum", both are critical. If you define it as "the engine which will cause the most asymetrical thrust if it quits", it doesn't have one.

As for reducing power, I'd say that's another issue alltogether, with the aircraft taking off below Vmc for the power setting used... which is probably above Vmc, period, but I'm too lazy to check the regs tonight.  Smile

Cheers,
/Fred
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Starlionblue
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RE: Which Way The Fan/Propeller Blades Turn

Tue Sep 05, 2006 6:39 am

Quoting FredT (Reply 8):
It all depends on the definition. If you define critical engine as "the engine which rotates in such a direction that the P factor asymmetrical thrust will be at a maximum", both are critical. If you define it as "the engine which will cause the most asymetrical thrust if it quits", it doesn't have one.

Hehe. Well done.
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leebird
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RE: Which Way The Fan/Propeller Blades Turn

Tue Sep 05, 2006 9:30 am

According to the US FAR's:

"CRITICAL ENGINE means the engine whose failure would most adversely affect the performance or handling qualities of an aircraft."
 
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Starlionblue
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RE: Which Way The Fan/Propeller Blades Turn

Tue Sep 05, 2006 9:39 am

Quoting Leebird (Reply 10):
According to the US FAR's:

"CRITICAL ENGINE means the engine whose failure would most adversely affect the performance or handling qualities of an aircraft."

Argh. My lovely theory beaten down by a simple fact. Big grin

Anyway lots of sources call them both critical. Go yell at the sources  Wink
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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RE: Which Way The Fan/Propeller Blades Turn

Tue Sep 05, 2006 5:03 pm

Quoting RichardPrice (Reply 2):
Jet engines turn the same way on both engines, with usually their cores turning the opposite way to counteract the gyroscopic effects

On Twin Engined Aircraft both Engines rotate in the same direction.

However Engine manufacturers can use either Clockwise or Counterclockwise as the direction depending on the model.
eg JT8Ds rotate clockwise & RB211-535s rotate counterclockwise.Remember as viewed from Aft.
regds
MEL
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Btriple7
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RE: Which Way The Fan/Propeller Blades Turn

Tue Sep 05, 2006 11:43 pm

Okay. Thank you everyone for your responses, but I am still a bit confused about this critical engine business. How could one engine be more critical for configuring an aircraft during an engine-out situation. Shouldn't the manufacturers build the engines so that neither one of the engines is more important than the other? After all, one doesn't get to choose which of his engines is going to fail, right?

Regards,
Btriple7
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Starlionblue
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RE: Which Way The Fan/Propeller Blades Turn

Wed Sep 06, 2006 3:21 am

Quoting Btriple7 (Reply 13):
Okay. Thank you everyone for your responses, but I am still a bit confused about this critical engine business. How could one engine be more critical for configuring an aircraft during an engine-out situation. Shouldn't the manufacturers build the engines so that neither one of the engines is more important than the other? After all, one doesn't get to choose which of his engines is going to fail, right?

Just because you are left with the critical engine doesn't mean you are going to die. It means the adverse torque effects of the engine out are more severe, but still manageable.
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RE: Which Way The Fan/Propeller Blades Turn

Wed Sep 06, 2006 4:49 am

Quoting Starlionblue (Reply 14):
Just because you are left with the critical engine doesn't mean you are going to die. It means the adverse torque effects of the engine out are more severe, but still manageable.

I understand that it is not disastrous if your critical engine goes out, but still why do they make one engine more...critical, for lack of better words? Is there any particular reason?

Regards,
Btriple7
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FredT
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RE: Which Way The Fan/Propeller Blades Turn

Wed Sep 06, 2006 6:06 am

Quoting Btriple7 (Reply 15):
I understand that it is not disastrous if your critical engine goes out, but still why do they make one engine more...critical, for lack of better words? Is there any particular reason?

To avoid it, you have to have counterrotating propellers. This means having to have handed engines, which is a logistics nightmare. You have to stock twice as many of most engine parts, as you effectively have two types of engine per aircraft. This increases the cost of maintaining the fleet and keeping the servicability up tremendously. It also opens up a whole new spectrum of MX mishaps and mistakes.

"Why not just have a gearbox to reverse the direction of rotation", many people ask. A gearbox always comes with a power loss, which is unacceptable, along with space and weight penalties, maintenance etc.

Rgds,
/Fred
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Starlionblue
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RE: Which Way The Fan/Propeller Blades Turn

Wed Sep 06, 2006 6:26 am

As FredT says, handed engines are a logistical nightmare. The critical engine problem is not so great as to make engine outs unmanageable, so it's not a good enough reason to make handed engines.
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SlamClick
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RE: Which Way The Fan/Propeller Blades Turn

Wed Sep 06, 2006 7:57 am

Quoting Btriple7 (Reply 13):
How could one engine be more critical for configuring an aircraft during an engine-out situation.

The illustration in reply #1 is a very good depiction of this. In this case the LEFT engine is 'critical' (which is really typical for US light twins) because if you lose it, you still have the RIGHT engine running.

The problem is mostly this: The propeller is spinning, it is therefore descending on one side and ascending on the other side. The descending blade will always have slightly greater effective pitch when the plane is descending, or flying somewhat nose-up, and therefore it will have more thrust.

In the illustration above the descending blade is to the RIGHT side of the centerline of EACH engine. The right side of the left engine is closer to the centerline of the airplane than the right side of the right engine is. So if you have only the right engine operating the thrust is taking effect at a greater distance or 'longer arm' from both the center of gravity and the rudder. This means that it requires greater force from the rudder to counteract this asymmetry of thrust than it would require if the LEFT engine was the one operating.

The minimum speed (VMC) at which the plane can be controlled with takeoff power on the operative engine is higher (worse) if the LEFT engine is inoperative.

If all engine power is lost (both engines) you are going to land somewhere just ahead of where you are right now. If one engine is lost, then control is lost due to VMC you are going to CRASH. Huge difference there! This is what makes light twins so potentially hazardous to less experienced or less proficient pilots.

A better outcome may be to pull some power OFF the remaining engine. The inability to climb is much better than the inability to control the plane. I've lost friends to this phenomenon - rolled in and impacted on the windshield. It is ugly! On the P-38 discussed above there was a surplus of power and factory pilot Tony LeVier went around to airbases demonstrating engine-out procedures including pulling power off as above.
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SlamClick
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RE: Which Way The Fan/Propeller Blades Turn

Wed Sep 06, 2006 8:08 am

Another factor worthy of discussion is changing direction of rotation.

For the next couple of weeks I'm going to be watching a lot of Hawker Sea Fury race planes.

View Large View Medium
Click here for bigger photo!

Photo © Brian Spurr


Note that the original Dowty-Rotol five-bladed prop turned anti-clockwise as viewed by the pilot. Some of the planes that will race here at Reno have been re-engined with American powerplants including the R-4360 on "Furias" seen below; all of which turn the other way.


I'm sure this presents rigging problems. I talked to other air tanker pilots after the 1970 crash of a B-17 equipped with Rolls-Royce Dart engines. Some of them were convinced that the opposite torque was a factor.
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Starlionblue
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RE: Which Way The Fan/Propeller Blades Turn

Wed Sep 06, 2006 8:43 am

Quoting SlamClick (Reply 18):
On the P-38 discussed above there was a surplus of power and factory pilot Tony LeVier went around to airbases demonstrating engine-out procedures including pulling power off as above.

Including rolls close to the ground with one engine feathered. That's an EXCESS of power. But I can understand that the intuitive reaction is to ADD power to the remaining engine.
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SlamClick
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RE: Which Way The Fan/Propeller Blades Turn

Wed Sep 06, 2006 9:07 am

Quoting Starlionblue (Reply 20):
I can understand that the intuitive reaction is to ADD power to the remaining engine.

Right, without proper examination of the idea it is easy to conclude that power is your friend. I think I was lucky to have an IP in the T-42 who did a terrific VMC demonstration.

He'd have us shut down the critical engine, then put some significant amount of power on the good engine and slow down. He would explain as we got into a nose-high mushing with gobs of rudder in, that one might instinctively think "STALL" when directional control was lost, because the plane is going to roll and go nose-down. So when the break came (well above stall speed) he'd have us go full power on the good engine. So I can claim to have done snap rolls in a Baron.

As it all happens with fairly low dynamic airloads I don't think it stressed the airframe too badly. One thing is certain though - it didn't just straighten up and fly away! We had to get the power off both engines, keep the nose down to get some directional control, then start feeding power back in to recover to level flight. Very dramatic demo.

All that and I've never lost an engine in a light twin to make it worthwhile.

edit: It would be a very difficult thing, and a bitter pill for the owner of a light twin to have to decide to crash his airplane in order to save his passengers. There are times when the remaining engine will not get you to an airport. Arguably you are in violation of FAR 91.9 at that moment but I have to believe that a violation is better than a funeral. I've seen it happen within five miles of my house three times that I can think of, in a Baron, a C-401 and a C-310. The better choice in all thee cases would have been to pull off the other engine and land in a street or a farmer's field.

[Edited 2006-09-06 02:10:40]

[Edited 2006-09-06 02:11:42]
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Btriple7
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RE: Which Way The Fan/Propeller Blades Turn

Wed Sep 06, 2006 11:24 pm

Quoting SlamClick (Reply 18):

Thank you! This clears a lot of stuff up.

Regards,
Btriple7  wave 
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Jetlagged
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RE: Which Way The Fan/Propeller Blades Turn

Wed Sep 06, 2006 11:39 pm

Quoting SlamClick (Reply 19):
I'm sure this presents rigging problems.

Absolutely, but anyone crazy enough to fly one of those monsters at Reno would not be bothered by a nicety like that, just as long as there is enough rudder control to actually get airborne  Smile

Must be quite a sight though, compared to the Red Bull style of air racing.
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Fly2HMO
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RE: Which Way The Fan/Propeller Blades Turn

Mon Sep 11, 2006 3:22 am

Quoting Joness0154 (Reply 1):
Because the gearbox has to be changed, etc

The PA-44 doesn't have a gearbox. AFAIK, the one engine is mounted "backwards" in relation to the other one.

I'm not 100% sure though, been a while since I've read the POH. Big grin
 
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RE: Which Way The Fan/Propeller Blades Turn

Mon Sep 11, 2006 4:02 am

Quoting FLY2HMO (Reply 24):
The PA-44 doesn't have a gearbox. AFAIK, the one engine is mounted "backwards" in relation to the other one.

I'm not 100% sure though, been a while since I've read the POH.

All the opposed-style airplane engines I've ever seen have a propeller shaft at one end and an accessory gearbox at the other. I don't see how one could be mounted backwards. I'd assume that with valve and ignition timing changes, an engine might be made to run in opposite rotation though.

Anyone shed any more light on that?
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Fly2HMO
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RE: Which Way The Fan/Propeller Blades Turn

Mon Sep 11, 2006 7:58 am

Quoting SlamClick (Reply 25):
I'd assume that with valve and ignition timing changes, an engine might be made to run in opposite rotation though.

Thats how I think they did it. Also, I've seen both engines myself, and the thing I'm 100% sure of is that neither one has a gearbox.
 
bohica
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RE: Which Way The Fan/Propeller Blades Turn

Mon Sep 11, 2006 10:45 am

Quoting FredT (Reply 16):
Quoting Btriple7 (Reply 15):
I understand that it is not disastrous if your critical engine goes out, but still why do they make one engine more...critical, for lack of better words? Is there any particular reason?

To avoid it, you have to have counterrotating propellers. This means having to have handed engines, which is a logistics nightmare. You have to stock twice as many of most engine parts, as you effectively have two types of engine per aircraft. This increases the cost of maintaining the fleet and keeping the servicability up tremendously. It also opens up a whole new spectrum of MX mishaps and mistakes.

"Why not just have a gearbox to reverse the direction of rotation", many people ask. A gearbox always comes with a power loss, which is unacceptable, along with space and weight penalties, maintenance etc.

The BAE J41 has counter rotating propellers. The number one propeller spins clockwise and the number two counter-clockwise (as viewed from the aft). Both turbine sections of the Garrett TPE331-14 engines spin the same direction. The gearbox in the #2 engine is geared so the propeller can spin the opposite direction.

The only extra parts needed were propellers and gearbox components. At my fromer airline, the marginal increase in costs of having a few extra parts were made up by having less operating costs as compared to a EMB-120 or a Saab 340.
 
timz
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RE: Which Way The Fan/Propeller Blades Turn

Tue Sep 12, 2006 1:47 am

Quoting Bohica (Reply 27):
the marginal increase in costs of having a few extra parts were made up by having less operating costs as compared to a EMB-120 or a Saab 340.

Why do c/r props cost less to operate?
 
vinwow
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RE: Which Way The Fan/Propeller Blades Turn

Thu Apr 01, 2010 6:27 am

What will happen to the 'torque' effect if the propeller RPM is reduced while engine power is kept same?
Will the 'torque' effect increase?

regards
vin
 
DH106
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RE: Which Way The Fan/Propeller Blades Turn

Thu Apr 01, 2010 9:44 am

Quoting SlamClick (Reply 25):
All the opposed-style airplane engines I've ever seen have a propeller shaft at one end and an accessory gearbox at the other. I don't see how one could be mounted backwards. I'd assume that with valve and ignition timing changes, an engine might be made to run in opposite rotation though.

If there's a gearbox on the rear of the engine then the crankshaft comes out of both ends of the engine. If you were to make the fitting at each end identical, then in theory you could mount the engine 'backwards' and drive an opposite rotation prop from what was effectively the rear of the engine. The accessory gearbox would then be driven from the 'front' of the engine (now at the rear) but this would also be rotating in reverse fashion and would have to be designed to work like this.
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Arrow
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RE: Which Way The Fan/Propeller Blades Turn

Thu Apr 01, 2010 4:26 pm

Quoting Starlionblue (Reply 3):
The P-38 Lightning has 2 critical engines.
Quoting Jetlagged (Reply 6):
You can't have two critical engines. If the props counter rotate, than asymmetry is the same regardless of which engine fails. It may be that the direction of rotation on the P-38 made the asymmetry worse, but it would do so in both directions equally.

Meaning each engine would tend to torque the aircraft into a spin, rather than counteract it? Why would they do something that stupid? I always thought "critical" engine on a twin -- assuming they both turn in the same direction -- referred to the torque effect of the running engine. If you are going to install counter rotating props, surely you would do it in such a manner that the torque pulled outboard, not inboard.

Quoting SlamClick (Reply 19):
I talked to other air tanker pilots after the 1970 crash of a B-17 equipped with Rolls-Royce Dart engines.

Holy cow! They put Darts on a B-17?
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DC8FriendShip
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RE: Which Way The Fan/Propeller Blades Turn

Thu Apr 01, 2010 4:58 pm

What will happen to the 'torque' effect if the propeller RPM is reduced while engine power is kept same?
W

Quoting DH106 (Reply 30):
If there's a gearbox on the rear of the engine then the crankshaft comes out of both ends of the engine. If you were to make the fitting at each end identical, then in theory you could mount the engine 'backwards' and drive an opposite rotation prop from what was effectively the rear of the engine. The accessory gearbox would then be driven from the 'front' of the engine (now at the rear) but this would also be rotating in reverse fashion and would have to be designed to work like this.

But that would add cost to the engine because it woud have to be built to withstand thrust from both ends. a simpler way is to have a left and right crankshaft and cam. That is how it is currently done( in piston engines.)
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timz
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RE: Which Way The Fan/Propeller Blades Turn

Thu Apr 01, 2010 5:32 pm

Quoting Arrow (Reply 31):
If you are going to install counter rotating props, surely you would do it in such a manner that the torque pulled outboard, not inboard.

Unless you didn't. On all? production P-38s, the prop blades were moving outward (toward the wingtips) at the top of their arcs.
 
rwessel
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RE: Which Way The Fan/Propeller Blades Turn

Fri Apr 02, 2010 11:30 pm

Quoting Arrow (Reply 31):
Meaning each engine would tend to torque the aircraft into a spin, rather than counteract it? Why would they do something that stupid? I always thought "critical" engine on a twin -- assuming they both turn in the same direction -- referred to the torque effect of the running engine. If you are going to install counter rotating props, surely you would do it in such a manner that the torque pulled outboard, not inboard.

Because engine-out performance is not always the deciding factor. In the P-38's case, have both engines rotate top-blade-out improved the aircraft's performance as a gun platform, by, as I understand it, reducing the amount of turbulent propwash acting on the horizontal stabilizer.
 
Northwest727
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RE: Which Way The Fan/Propeller Blades Turn

Mon Apr 05, 2010 1:58 pm

Quoting Btriple7 (Thread starter):
I have heard that the blades turn in the same direction in order to reduce vibration

Not sure about fan blades, but with propellers blades, I doubt this to be the case. Often, its more about costs. Its cheaper to fit a twin with two of the same engines/gearboxes/propellers than to fit a twin with a one engine/gearbox/propeller turning one way, and another turning another way.

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