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Which Way The Fan/Propeller Blades Turn  
User currently offlineBtriple7 From United States of America, joined Sep 2005, 1160 posts, RR: 9
Posted (7 years 10 months 3 weeks 2 days 13 hours ago) and read 11909 times:

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.

Regards,
Btriple7


Just...fly.
35 replies: All unread, showing first 25:
 
User currently offlineJoness0154 From United States of America, joined Nov 2005, 667 posts, RR: 0
Reply 1, posted (7 years 10 months 3 weeks 2 days 13 hours ago) and read 11925 times:

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.



I don't have an attitude problem. You have a perception problem
User currently offlineRichardPrice From , joined Dec 1969, posts, RR:
Reply 2, posted (7 years 10 months 3 weeks 2 days 13 hours ago) and read 11919 times:

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.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 3, posted (7 years 10 months 3 weeks 2 days 12 hours ago) and read 11887 times:

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]


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineTristarsteve From Sweden, joined Nov 2005, 3976 posts, RR: 34
Reply 4, posted (7 years 10 months 3 weeks 2 days 11 hours ago) and read 11857 times:

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.


User currently offlineRichardPrice From , joined Dec 1969, posts, RR:
Reply 5, posted (7 years 10 months 3 weeks 2 days 11 hours ago) and read 11846 times:

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.


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2543 posts, RR: 24
Reply 6, posted (7 years 10 months 3 weeks 2 days 11 hours ago) and read 11840 times:

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.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 7, posted (7 years 10 months 3 weeks 2 days 9 hours ago) and read 11808 times:

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.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 8, posted (7 years 10 months 3 weeks 2 days 5 hours ago) and read 11742 times:

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



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 9, posted (7 years 10 months 3 weeks 2 days 5 hours ago) and read 11734 times:

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.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineLeebird From United States of America, joined Sep 2006, 17 posts, RR: 0
Reply 10, posted (7 years 10 months 3 weeks 2 days 2 hours ago) and read 11687 times:

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."


User currently offlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 11, posted (7 years 10 months 3 weeks 2 days 2 hours ago) and read 11681 times:

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."
User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 12, posted (7 years 10 months 3 weeks 1 day 19 hours ago) and read 11621 times:

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



Think of the brighter side!
User currently offlineBtriple7 From United States of America, joined Sep 2005, 1160 posts, RR: 9
Reply 13, posted (7 years 10 months 3 weeks 1 day 12 hours ago) and read 11567 times:

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



Just...fly.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 14, posted (7 years 10 months 3 weeks 1 day 9 hours ago) and read 11528 times:

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.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineBtriple7 From United States of America, joined Sep 2005, 1160 posts, RR: 9
Reply 15, posted (7 years 10 months 3 weeks 1 day 7 hours ago) and read 11508 times:

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



Just...fly.
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 16, posted (7 years 10 months 3 weeks 1 day 6 hours ago) and read 11487 times:

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



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 17, posted (7 years 10 months 3 weeks 1 day 5 hours ago) and read 11482 times:

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.


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 18, posted (7 years 10 months 3 weeks 1 day 4 hours ago) and read 11451 times:

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.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 19, posted (7 years 10 months 3 weeks 1 day 4 hours ago) and read 11446 times:

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.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 20, posted (7 years 10 months 3 weeks 1 day 3 hours ago) and read 11433 times:

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.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 21, posted (7 years 10 months 3 weeks 1 day 3 hours ago) and read 11420 times:

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]


Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineBtriple7 From United States of America, joined Sep 2005, 1160 posts, RR: 9
Reply 22, posted (7 years 10 months 3 weeks 12 hours ago) and read 11342 times:

Quoting SlamClick (Reply 18):

Thank you! This clears a lot of stuff up.

Regards,
Btriple7  wave 



Just...fly.
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2543 posts, RR: 24
Reply 23, posted (7 years 10 months 3 weeks 12 hours ago) and read 11335 times:

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.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 24, posted (7 years 10 months 2 weeks 3 days 8 hours ago) and read 11135 times:

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


25 SlamClick : 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 co
26 FLY2HMO : 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.
27 Bohica : The BAE J41 has counter rotating propellers. The number one propeller spins clockwise and the number two counter-clockwise (as viewed from the aft).
28 Timz : Why do c/r props cost less to operate?
29 vinwow : 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 vi
30 DH106 : 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
31 Arrow : 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 thoug
32 DC8FriendShip : What will happen to the 'torque' effect if the propeller RPM is reduced while engine power is kept same? W But that would add cost to the engine becau
33 timz : Unless you didn't. On all? production P-38s, the prop blades were moving outward (toward the wingtips) at the top of their arcs.
34 rwessel : 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
35 Northwest727 : 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 o
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