User avatar
hOMSaR
Posts: 1342
Joined: Tue Jan 19, 2010 4:47 am

Which Way The Engines Spin

Mon Feb 27, 2012 3:40 am

Is there any particular advantage to having the engines spin one way or the other, or is it pretty much up to the whim of whoever designs the engine, and everything else just follows from that?
The plural of Airbus is Airbuses. Airbii is not a word, and doesn't even make sense.
 
User avatar
Starlionblue
Posts: 17055
Joined: Fri Feb 27, 2004 9:54 pm

RE: Which Way The Engines Spin

Mon Feb 27, 2012 3:45 am

In jets, there's an advantage where they all spin the same way. That way you only have to make one variant, and you only need parts for one variant.

In props (at least in twins), it helps if they both spin in opposite directions since there is much more gyroscopic effect. The down stroke should be on one side for engine out controllability but I can't remember which side or why.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
futureualpilot
Posts: 2402
Joined: Thu May 25, 2000 10:52 am

RE: Which Way The Engines Spin

Mon Feb 27, 2012 4:15 am

Quoting Starlionblue (Reply 1):
In props (at least in twins), it helps if they both spin in opposite directions since there is much more gyroscopic effect. The down stroke should be on one side for engine out controllability but I can't remember which side or why.

A number of piston twins used primarily as trainers have contra-rotating props, where the descending blade is closest to the fuselage in order to make them easier to control in engine out situations.

Generally, the descending blade generates more thrust than the ascending blade. On many twins the props rotate clockwise (as viewed when you're sitting in the aircraft facing the nose) which positions the descending blade further from the fuselage on the right engine. This makes the failure of the left engine "worse" in that the descending blade on the still operating right engine is further from the fuselage. This thrust is further from the aircraft centerline and contributes to an increase in adverse yaw, making the aircraft more difficult to control. If you have engines that rotate towards the fuselage, the descending blade is always the one closest to the aircraft centerline, making it easier to control the airplane should an engine fail. The downside is you do lose commonality between the two engines.

As far as jets go, my understanding is that because the engine "straightens" the airflow as it passes through the engine, it doesn't matter which way they spin. They go the same direction to keep the parts the same, as mentioned by another poster.

[Edited 2012-02-26 20:19:11]

[Edited 2012-02-26 20:20:33]
Life is better when you surf.
 
User avatar
DocLightning
Posts: 19624
Joined: Wed Nov 16, 2005 8:51 am

RE: Which Way The Engines Spin

Mon Feb 27, 2012 4:39 am

IIRC RR jet engines use a counter-clockwise N1 and GE and PW use clockwise.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Which Way The Engines Spin

Mon Feb 27, 2012 6:12 am

Quoting futureualpilot (Reply 2):
As far as jets go, my understanding is that because the engine "straightens" the airflow as it passes through the engine, it doesn't matter which way they spin.

It still matters for gyroscopic effects but few jets are capable of achieving pitch or yaw rates high enough that it makes much difference to handling (roll is usually the fastest axis and there's no gyro effect from the engines in roll). You do need to take it into account when figuring out the loading the engine mounts must withstand.

There are huge torques involved inside the engine but, as long as the flow going in and the flow going out aren't swirling (which you want to do for efficiency reasons anyway) then all the torques balance out and there's no torque to the airframe. Unless you get a rotor lock...that's bad.

Tom.
 
Fly2HMO
Posts: 7207
Joined: Sat Jan 24, 2004 12:14 pm

RE: Which Way The Engines Spin

Mon Feb 27, 2012 7:02 am

Quoting futureualpilot (Reply 2):

A number of piston twins used primarily as trainers have contra-rotating props, where the descending blade is closest to the fuselage in order to make them easier to control in engine out situations.

To further expand on that, those that are conventional have a "critical engine", so if you loose said engine, you will have your performance and controlability much more compromised than if you lost the other one.

http://en.wikipedia.org/wiki/Critical_engine

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

The P-38 is kinda of the odd freak since it has outward (from the fuselage) counter rotating props.
 
User avatar
HAWK21M
Posts: 29867
Joined: Fri Jan 05, 2001 10:05 pm

RE: Which Way The Engines Spin

Mon Feb 27, 2012 9:10 am

Quoting DocLightning (Reply 3):
IIRC RR jet engines use a counter-clockwise N1 and GE and PW use clockwise.

To add when looking at the Inlet.....as In Aviation terminology it would be opposite as looking from aft.
I may not win often, but I damn well never lose!!! ;)
 
DH106
Posts: 592
Joined: Wed Jun 15, 2005 5:32 pm

RE: Which Way The Engines Spin

Mon Feb 27, 2012 9:59 am

Quoting tdscanuck (Reply 4):
There are huge torques involved inside the engine but, as long as the flow going in and the flow going out aren't swirling (which you want to do for efficiency reasons anyway) then all the torques balance out and there's no torque to the airframe. Unless you get a rotor lock...that's bad.

Tom.

I've often wondered about torque (if any) produced by a jet.
If you discount bearing friction as trivial and effectively imparts no no torque to the housing, surely you then have fixed stators straightening out the flow that would generate some significant torque?
...I watched c-beams glitter in the dark near the Tanhauser Gate....
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Which Way The Engines Spin

Mon Feb 27, 2012 2:57 pm

Quoting dh106 (Reply 7):
I've often wondered about torque (if any) produced by a jet.
If you discount bearing friction as trivial and effectively imparts no no torque to the housing, surely you then have fixed stators straightening out the flow that would generate some significant torque?

The fixed stators in the compressor generate a huge torque...which is opposite the torque that the turbine stators experience. The fan also imparts huge torque to the air, which is balanced out by the fan outlet guide vanes. It all nets out to zero (assuming rotation free jet coming out the back).

It's really easy to get confused around all the individual forces inside the engine but it's much easier to just step back and look at it from afar...the air isn't rotating going into the engine and it's not rotating coming out the back. As a result, the net torque on the air going through the engine must have been zero. Due to equal and opposite reactions, the torque on the engine from the air going through it must also be zero.

Tom.
 
User avatar
DocLightning
Posts: 19624
Joined: Wed Nov 16, 2005 8:51 am

RE: Which Way The Engines Spin

Mon Feb 27, 2012 6:57 pm

Quoting futureualpilot (Reply 2):
Generally, the descending blade generates more thrust than the ascending blade.

I didn't know this. Why?
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
User avatar
akiss20
Posts: 763
Joined: Tue Sep 04, 2007 9:50 am

RE: Which Way The Engines Spin

Mon Feb 27, 2012 7:27 pm

Quoting DocLightning (Reply 9):
Quoting futureualpilot (Reply 2):
Generally, the descending blade generates more thrust than the ascending blade.

I didn't know this. Why?

I don't know for sure but my guess is that the ascending blade experiences the tip vortex of the previous blade. The previous blade sheds a vortex and gravity pulls it down into the next blade. On the descending side, the vortex is pulled clear of the next blade by gravity.

Again, I'm not sure, but that makes sense to me at a first glance.
Change will not come if we wait for some other person or some other time. We are the ones we've been waiting for. We are
 
User avatar
glen
Posts: 259
Joined: Thu Jun 30, 2005 4:43 pm

RE: Which Way The Engines Spin

Mon Feb 27, 2012 7:51 pm

Quoting DocLightning (Reply 9):
Quoting futureualpilot (Reply 2):
Generally, the descending blade generates more thrust than the ascending blade.

I didn't know this. Why?

We can not say generally, as in normal cruise the propeller axis should be aligned with the axis of the airflow.
With lower speed however (climb) the statement is true - it is called the P-factor.
At lower speed with the aircraft angle of attack increasing, the propller axis has also an increasing angle towards the axis of airflow. Therefore the descending blade has an increasing AOA towards the airflow around the propeller and the ascending blade a decreasing AOA.
Thus the descending blade generates more thrust.
"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
 
KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

RE: Which Way The Engines Spin

Mon Feb 27, 2012 8:49 pm

Quoting DocLightning (Reply 9):
I didn't know this. Why?

Most flight instructors explain it to their students in simple terms: relative to the oncoming airflow, the higher you lift the nose, the flatter the ascending prop blade becomes to the relative airflow, and thus creates more drag   Right or wrong, this is how it is usually explained to private pilot students... (and this being Tech/Ops, I'm sure someone will shoot down this explanation   ). So it's not actually about thrust production, but rather drag production...
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
CaptainKramer
Posts: 256
Joined: Thu Feb 09, 2012 8:12 pm

RE: Which Way The Engines Spin

Mon Feb 27, 2012 9:10 pm

While at the Farnborough Airshow I asked a Rolls Royce rep on the stand to settle an argument with a collegue who said that the port engine span opposite to the starboard engine so that the yaw would be neutralised, which I wasn't so sure about. The Rolls Royce rep replied that both port and starboard spun in the same direction, but interestingly added that the main electronics and plumbing were routed, where possible, away from the engine opposite to reduce the chance of damage from a blade out event.

According to the Rolls rep the wiring and plumbing are routed differantly outside the core on the port and starboard engines, is the case with the other manufacturers?

[Edited 2012-02-27 13:22:04]
 
jetlife2
Posts: 179
Joined: Sun Jul 09, 2006 11:32 am

RE: Which Way The Engines Spin

Tue Feb 28, 2012 12:18 am

Quoting CaptainKramer (Reply 13):
According to the Rolls rep the wiring and plumbing are routed differantly outside the core on the port and starboard engines, is the case with the other manufacturers?

For GE under-wing mounted engines the answer is no, the port and starboard are the same. This gives the customers complete flexibility.

For aft-fuselage side-mounted engines there are differences due to the engine mounts.
My views are not necessarily the views of the GE Company
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Which Way The Engines Spin

Tue Feb 28, 2012 4:24 am

Quoting AKiss20 (Reply 10):
I don't know for sure but my guess is that the ascending blade experiences the tip vortex of the previous blade. The previous blade sheds a vortex and gravity pulls it down into the next blade. On the descending side, the vortex is pulled clear of the next blade by gravity.

Vortices have no meaningful density (and hence buoyancy) difference so they're essentially unaffected by gravity. They move due to induced flow from other bits of the airplane (including other vortices), not due to gravity.

Quoting CaptainKramer (Reply 13):
According to the Rolls rep the wiring and plumbing are routed differantly outside the core on the port and starboard engines, is the case with the other manufacturers?

That would only be true of the engine-build-up (the airframer stuff that's on the engine), not the engine by itself. Otherwise you'd end up with right-hand and left-hand jet engines and you basically never see that for under-wing mounted engines.

Tom.
 
redflyer
Posts: 3881
Joined: Thu Feb 24, 2005 3:30 am

RE: Which Way The Engines Spin

Tue Feb 28, 2012 4:23 pm

Quoting tdscanuck (Reply 4):
There are huge torques involved inside the engine but, as long as the flow going in and the flow going out aren't swirling (which you want to do for efficiency reasons anyway) then all the torques balance out and there's no torque to the airframe. Unless you get a rotor lock...that's bad.
Quoting tdscanuck (Reply 8):
the air isn't rotating going into the engine and it's not rotating coming out the back.

Is this paradigm different with a radial flow turbine? I know some of the early jets were radial flow and I believe there are some radial flow turbines still in use, albeit none are wing-mounted. Would be curious to know if the torque would be considerably more.
My other home is a Piper Cherokee 180C
 
rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

RE: Which Way The Engines Spin

Tue Feb 28, 2012 10:57 pm

Quoting redflyer (Reply 16):
Is this paradigm different with a radial flow turbine? I know some of the early jets were radial flow and I believe there are some radial flow turbines still in use, albeit none are wing-mounted. Would be curious to know if the torque would be considerably more.

Radial flow? Do you mean turbines with centrifugal compressors? No, those straighten out the airflow as well.

In any event, Tom's point remains - "(if) the air isn't rotating going into the engine and it's not rotating coming out the back" there's no net torque.
 
redflyer
Posts: 3881
Joined: Thu Feb 24, 2005 3:30 am

RE: Which Way The Engines Spin

Tue Feb 28, 2012 11:10 pm

Quoting rwessel (Reply 17):
Radial flow? Do you mean turbines with centrifugal compressors? No, those straighten out the airflow as well.

Yes, that's what I meant. (I thought they are referred to as radial flow as opposed to axial flow?) Perhaps there isn't any effect from those either, but I would imagine there has to be more torque than on an axial flow turbine since the airflow is being "re-routed" from a straight path into a completely different path within the engine.

Anyway, this is getting off topic a bit as I think the OP's original question was answered. I'm just fascinated with engineering and how forces act upon materials and surfaces in ways that most laymen, such as myself, are not aware.

Regards
My other home is a Piper Cherokee 180C
 
User avatar
akiss20
Posts: 763
Joined: Tue Sep 04, 2007 9:50 am

RE: Which Way The Engines Spin

Wed Feb 29, 2012 3:39 pm

Quoting redflyer (Reply 18):

Yes, that's what I meant. (I thought they are referred to as radial flow as opposed to axial flow?) Perhaps there isn't any effect from those either, but I would imagine there has to be more torque than on an axial flow turbine since the airflow is being "re-routed" from a straight path into a completely different path within the engine.

They are sometimes called "Radial flow compressors" and "axial flow compressors" but as a jet engine has a combination of compressors and turbines (and rarely are they ever all centrifugal, I can only think of the original whittle engine that had both centrifugal compressors and turbine) they are not lumped together that way.

What Tom is essentially making is a control volume analysis (see http://en.wikipedia.org/wiki/Control_volume for a brief overview on this concept). What a control volume essentially does is relate the fluxes of a control volume to forces acting on the control volume. One could draw a control volume for the entire gas path of the engine and note that the vorticity (essentially swirl) entering and exiting the flow is both, assuming a perfectly deswirled exit flow which is very close to reality, is zero. There is no velocity along the walls of the entire gas path, thus the vorticity flux there is zero. Thus there is no vorticity flux, thus there cannot be a net torque acting on the gas. If there is no net torque acting on the gas, by newton's 3rd law, there is no net torque on the walls/engine.

My first pass at trying explain this at 10:30AM (for a college student, that's early!   ) My argument might need some refinement, but the gist is there.

Hope that helped!
Change will not come if we wait for some other person or some other time. We are the ones we've been waiting for. We are
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Which Way The Engines Spin

Thu Mar 01, 2012 12:24 am

Quoting redflyer (Reply 18):
Yes, that's what I meant. (I thought they are referred to as radial flow as opposed to axial flow?)

That's correct. There are both radial and axial flow compressors and turbines. Radial flow turbines are pretty rare in jet engines these days but radial compressors are still common in the smaller engines.

Quoting redflyer (Reply 18):
Perhaps there isn't any effect from those either, but I would imagine there has to be more torque than on an axial flow turbine since the airflow is being "re-routed" from a straight path into a completely different path within the engine.

Radials tend to have a larger pressure ratio, per stage, than an equivalently sized axial so the torque on one radial impeller would typically be larger than an equivalent axial stage. However, you'd have less stages and the efficiency of both types of devices is comparable so the total power input should be about the same. The control volume arguments all still apply...if it's not swirling going in or out, there's no net torque on the engine. There will be *huge* torques inside the engine (e.g. the shaft between turbine and compressor) but the shaft torques can't get to the engine (assuming perfect bearings) and the aerodynamics torques will all balance out.

Tom.
 
User avatar
bikerthai
Posts: 2153
Joined: Wed Apr 28, 2010 1:45 pm

RE: Which Way The Engines Spin

Thu Mar 01, 2012 4:10 pm

Quoting CaptainKramer (Reply 13):
is the case with the other manufacturers?

The CFM engines on 737 can be mounted on either wing. You can see this from the bump in the cowling for the generator.

Quoting dh106 (Reply 7):

I've often wondered about torque (if any) produced by a jet.

It been so long since I took Jet Propulsion Class but . . .

Does the main fan shaft and the compressor shaft spin in different directions?
There is certain aerodynamic benefit of doing this for the flow between the fan and the first compressor stage.

For high by pass, the mass moment of the fan may out weigh the mass moment of the compressor stages, but for fighters, the moment difference may be less, thus helping out with the maneuvering forces discussed.

bt
Intelligent seeks knowledge. Enlightened seeks wisdom.
 
dandaire
Posts: 62
Joined: Fri Jul 11, 2008 9:16 am

RE: Which Way The Engines Spin

Thu Mar 01, 2012 5:23 pm

In the Rolls Royce Pegasus engine fitted to the Harriers the HP and LP spools turn in different directions to prevent any gyroscopic problems of the engine affecting the maneuvering of the aircraft at low speed.

In the ALF 502/507 series of hair dryers/engines    the LP turbine turns in the opposite direction to the fan it drives because it goes through a planetary gearbox first, which is novel when you first see it.   
Old age and treachery will triumph over youth and skill.
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Which Way The Engines Spin

Thu Mar 01, 2012 5:26 pm

Quoting bikerthai (Reply 21):

Does the main fan shaft and the compressor shaft spin in different directions?

It depends on the engine. For GE and PW engines the fan and LP compressor go the same direction because they're on a common shaft, for a RR they're on different shafts so they have the choice. Most modern large turbofans have at least two compressor spools so the HP may spin the same or opposite direction as the LP (IP on RR).

Tom.
 
redflyer
Posts: 3881
Joined: Thu Feb 24, 2005 3:30 am

RE: Which Way The Engines Spin

Thu Mar 01, 2012 11:17 pm

Quoting tdscanuck (Reply 23):
Most modern large turbofans have at least two compressor spools so the HP may spin the same or opposite direction as the LP (IP on RR).

Is there an advantage to an opposite spin or is it done only to address some other issue that may be present with certain designs? (Not implying an inherent issue, just wondering if in certain circumstances an opposite turn is better.)
My other home is a Piper Cherokee 180C
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Which Way The Engines Spin

Thu Mar 01, 2012 11:47 pm

Quoting redflyer (Reply 24):
Is there an advantage to an opposite spin or is it done only to address some other issue that may be present with certain designs?

Done properly, it lets you shrink or eliminate a stator stage between the spools.

Tom.
 
MountainFlyer
Posts: 486
Joined: Sat Jul 09, 2005 10:19 am

RE: Which Way The Engines Spin

Fri Mar 02, 2012 7:38 pm

Quoting Fly2HMO (Reply 5):
The P-38 is kinda of the odd freak since it has outward (from the fuselage) counter rotating props.

The P-38 was made that way due to aerodynamic problems caused by the spiraling prop-wash with the dual fuselage/tail design when the engines spun inward.
SA-227; B1900; Q200; Q400; CRJ-2,7,9; 717; 727-2; 737-3,4,5,7,8,9; 747-2; 757-2,3; 767-3,4; MD-90; A319, 320; DC-9; DC-1
 
User avatar
DocLightning
Posts: 19624
Joined: Wed Nov 16, 2005 8:51 am

RE: Which Way The Engines Spin

Mon Mar 05, 2012 6:06 am

Quoting redflyer (Reply 24):

Is there an advantage to an opposite spin or is it done only to address some other issue that may be present with certain designs? (Not implying an inherent issue, just wondering if in certain circumstances an opposite turn is better.)

Part of the way an axial compressor pressurizes is by "throwing" the swirling airflow against the stators, which compresses it. A counter-rotating N2 accomplishes the same and allows you to eliminate a stator stage without increasing overall engine complexity (it's no harder to make it counter-rotate than rotate).
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan

Who is online

Users browsing this forum: No registered users and 12 guests