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Rear Engine-out Handling Characteristics  
User currently offlineSpeedracer1407 From United States of America, joined Dec 2004, 333 posts, RR: 0
Posted (7 years 8 months 2 weeks 2 days 9 hours ago) and read 2885 times:

A common sense layman understanding (like mine) of engine-out handling charactersitics suggests that aircraft with wing mounted engines require higher Vmc and/or restricted power settings on the good engine than aircraft with rear mounted engines.

However, I seem to recall reading that the longitudinal moment arm between rear mounted engines and the rudder is so small that it's yaw authority is limited in engine-out situtations; thus, planes with rear mounted engines are not necessarily more controlable (require less rudder trim compensation) than those with wing mounted engines.

Am I making stuff up, or is there something to this?

thanks for any replies,

O


Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
20 replies: All unread, jump to last
 
User currently offlineFlametech21 From United States of America, joined Sep 2006, 47 posts, RR: 0
Reply 1, posted (7 years 8 months 2 weeks 2 days 3 hours ago) and read 2816 times:

Quoting Speedracer1407 (Thread starter):
However, I seem to recall reading that the longitudinal moment arm between rear mounted engines and the rudder is so small that it's yaw authority is limited in engine-out situtations; thus, planes with rear mounted engines are not necessarily more controlable (require less rudder trim compensation) than those with wing mounted engines.

You are somewhat correct. The moment arm is extremley small making the rudder not as effective at controllong it as on a wing engine failure.

However, the engines are so close to each other that it only creates a very small change in lateral moment arm. I have flown an md80 with engine 2 out, and I only needed minimal rudder trim to correct it. Theoretically it is within limits to fly it without any change in rudder trim at all, although I would not reccomend it.

In my experience, it is very easy to control a rear engine out. If you weren't looking at the EICAS or the airspeed indicator you would hardly know that you had lost an engine!  bigthumbsup 



They build them to a higher standard at Long Beach!
User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 2, posted (7 years 8 months 2 weeks 2 days 3 hours ago) and read 2810 times:

Note that the it's not the moment arm between rudder and engines that's important but the rudder's moment arm from the aircraft's CG. However, most rear engined planes do have a somewhat shorter moment arm with the CG (and hence relative wing position) being more rearward set than an aircraft with wing mounted engines.


...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 3, posted (7 years 8 months 2 weeks 2 days 3 hours ago) and read 2804 times:

Quoting Speedracer1407 (Thread starter):
However, I seem to recall reading that the longitudinal moment arm between rear mounted engines and the rudder is so small that it's yaw authority is limited in engine-out situtations; thus, planes with rear mounted engines are not necessarily more controlable (require less rudder trim compensation) than those with wing mounted engines.

The distance between the engine and the rudder has nothing to do with it.

What matters (when looking at yaw) is the orthogonal (minimum) distance in the (world) xy plane between the engine thrust line and the center of drag of the aircraft (i e for yaw and with thrust lines roughly parallell to the fuselage the distance from aircraft centerline to the engine). With an engine out, this distance sets up a torque couple acting around the yaw axis. This torque couple is the distance center of drag to thrust line times the thrust of the engine. For wing mounted engines, the distance will be quite a bit larger and the torque couple will increase proportionally.

This is counteracted by another torque couple set up by the lateral force created by yaw and the lateral force on the rudder. The placement of the engines will not have any effect on this torque couple.

No, the center of gravity does not matter as long as you are wings level and flying straight ahead, as gravity will not act in the same plane.

Rgds,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 4, posted (7 years 8 months 2 weeks 2 days 3 hours ago) and read 2796 times:

I seem to recall that this effect of an engine-out effected the design of the L1011 / DC-10 due to their #2 engine placement. In the L1011, the placement of the #2 engine allows for engines 1 & 3 to be spaced further out on the wings, thereby reducing noise. On the DC-10 however, the placement of the #2 engine means that 1 & 3 have to be much closer to the fuselage.

User currently offlineN243NW From United States of America, joined Jul 2003, 1630 posts, RR: 20
Reply 5, posted (7 years 8 months 2 weeks 1 day 19 hours ago) and read 2699 times:

Quoting N231YE (Reply 4):
In the L1011, the placement of the #2 engine allows for engines 1 & 3 to be spaced further out on the wings, thereby reducing noise.

Although I don't doubt the idea that having the engines further outboard makes for a slightly quieter cabin, the main reason for this placement was to improve wing loading and make the aircraft more fuel efficient.

-N243NW Big grin



B-52s don't take off. They scare the ground away.
User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 6, posted (7 years 8 months 2 weeks 1 day 4 hours ago) and read 2644 times:

Quoting N243NW (Reply 5):
Although I don't doubt the idea that having the engines further outboard makes for a slightly quieter cabin, the main reason for this placement was to improve wing loading and make the aircraft more fuel efficient.

I never knew that. Looks like I learned something today  biggrin 


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17027 posts, RR: 67
Reply 7, posted (7 years 8 months 2 weeks 1 day 1 hour ago) and read 2598 times:

Quoting N231YE (Reply 6):
Quoting N243NW (Reply 5):
Although I don't doubt the idea that having the engines further outboard makes for a slightly quieter cabin, the main reason for this placement was to improve wing loading and make the aircraft more fuel efficient.

I never knew that. Looks like I learned something today

Shorter gear too, but that's probably a secondary reason.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 8, posted (7 years 8 months 2 weeks 23 hours ago) and read 2570 times:

Quoting N243NW (Reply 5):
the main reason for this placement was to improve wing loading

Outboard placement would improve wing bending stress relief, but would have little effect on 'wing loading' per se, except in that a fractionally lighter wing structure might yield a slightly lighter overall weight.



...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineMoose135 From United States of America, joined Oct 2004, 2314 posts, RR: 10
Reply 9, posted (7 years 8 months 2 weeks 18 hours ago) and read 2531 times:

Quoting N243NW (Reply 5):
Although I don't doubt the idea that having the engines further outboard makes for a slightly quieter cabin, the main reason for this placement was to improve wing loading and make the aircraft more fuel efficient.

Yes, the L-1011 has its engines further out on the wing, which makes it more efficient structurally, and they were able to achieve that because the #2 engine was located down low. This allowed a larger rudder, resulting in additional rudder force available to counteract an inop wing engine.



KC-135 - Passing gas and taking names!
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17027 posts, RR: 67
Reply 10, posted (7 years 8 months 2 weeks 18 hours ago) and read 2529 times:

Quoting Moose135 (Reply 9):
Quoting N243NW (Reply 5):
Although I don't doubt the idea that having the engines further outboard makes for a slightly quieter cabin, the main reason for this placement was to improve wing loading and make the aircraft more fuel efficient.

Yes, the L-1011 has its engines further out on the wing, which makes it more efficient structurally, and they were able to achieve that because the #2 engine was located down low. This allowed a larger rudder, resulting in additional rudder force available to counteract an inop wing engine.

Indeed. But it's all about compromises. The DC-10 straight duct is more efficient than the L-1011 S-duct. On the gripping hand though, the DC-10 duct requires a beefier tail structure.

As I said, all about compromises. And center tail engines were just such a complex exercise in compromises that they eventually dissapeared.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineThePinnacleKid From United States of America, joined Feb 2005, 725 posts, RR: 8
Reply 11, posted (7 years 8 months 4 days 19 hours ago) and read 2376 times:

I find the ERJ to be slightly touchy with V1 cuts, etc.. it tends (in the sims at least) to want to get into little dutch roll tendencies easy if the incorrect amount of rudder input is used... that being said.. once it is undercontrol and stable.. trim the bird up.. she's awesome and really becomes a non-event other than going through LONG QRH procedures and the resulting landing with flaps 22 instead of 45 and thus a higher landing speed... but thats life.. all in a day's work.

Chris



"Sonny, did we land? or were we shot down?"
User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 12, posted (7 years 8 months 4 days 16 hours ago) and read 2351 times:
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DATABASE EDITOR




Quoting FredT (Reply 3):
The distance between the engine and the rudder has nothing to do with it.

Well, but there are more forces at play than just torque. How about P-factor? If the descending prop blade experiences a greater angle of attack during climb, and thus, produces more thrust than the ascending blade, adverse yaw will occur.

With respect to this particular source of yaw, a greater distance/moment arm between the engine and the rudder(s) would reduce the amount of force required to counteract it....would it not?


2H4





Intentionally Left Blank
User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 13, posted (7 years 8 months 4 days 9 hours ago) and read 2319 times:

Quoting 2H4 (Reply 12):
Well, but there are more forces at play than just torque. How about P-factor? If the descending prop blade experiences a greater angle of attack during climb, and thus, produces more thrust than the ascending blade, adverse yaw will occur.

With respect to this particular source of yaw, a greater distance/moment arm between the engine and the rudder(s) would reduce the amount of force required to counteract it....would it not?

Thinking on the fly here (aerodynamists feel free to jump in...), but all forces/torques effectively resolve themselves around the aircraft's CG in various planes (yaw/pitch/roll). So the key factor in rudder authority is the rudder moment arm from the CG.
Now, the engine's placement with respect to the CG (which presumably runs through or very near the aircraft's centreline) will clearly affect the magnitude of 'engine out' yaw that needs to be corrected by the rudder. So both the engine and rudder's position is important with respect to the CG, but not de-facto to each other.

Interestingly, the yaw effect that 2H4 describes in which a propellor at an angle of attack produces a yaw - the yaw effect actually decrases with distance from the centreline. The direction of the yaw depends on the direction of prop rotation - the downward moving blade is at a higher angle of attack and thus produces more lift (thrust) than the upward blade. I would imagine though that this effect would probably be swamped by the obvious asymmetry in an engine out situation?



...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 14, posted (7 years 8 months 4 days 4 hours ago) and read 2276 times:

Quoting 2H4 (Reply 12):
With respect to this particular source of yaw, a greater distance/moment arm between the engine and the rudder(s) would reduce the amount of force required to counteract it....would it not?

Afraid not. If you apply a torque to a rigid body, the effect is the same regardless of where you apply the torque. Consider the case of drilling in a board. If the board comes loose, it will start to spin about the center of gravity if you are drilling in the center of mass (center of gravity). If you are drilling in the end, it will still try to rotate about the center of mass, kicking
your drill to the side.

Apply the P-factor torque in the nose, at the wing tip, at the top of the stabilizer or at the tail and the effect will be the same.

If the net torque on a rigid body about an axis is nil, as it will be in straight flight, the effect is nil. There will be no angular acceleration.

Only when you have angular acceleration does the center of gravity come into play as the axis of that acceleration will pass through it. When there is no angular acceleration around the vertical axis and no acceleration in the horizontal plane, the CoG can be anywhere relative to rudders and engines alike and it will not make a difference when considering only the torque around the vertical axis.

The length of the empennage will, as I described above, factor in as it will move the tail surface away from the lateral aerodynamic center of the rest of the aircraft, thus increasing the torque generated by a given amount of lateral force at the rudder, thus decreasing the amount of side slip required to counter a given amount of thrust assymetry. The distance between the engines and the rudder will not factor in.

Quoting DH106 (Reply 13):
Interestingly, the yaw effect that 2H4 describes in which a propellor at an angle of attack produces a yaw - the yaw effect actually decrases with distance from the centreline

Are you really, really sure about that statement?  Smile



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offline113312 From United States of America, joined Apr 2005, 571 posts, RR: 1
Reply 15, posted (7 years 8 months 4 days 4 hours ago) and read 2266 times:

Sorry Fred T, you do not understand propeller aerodynamics. It's not the same as inserting a drill bit into a board. I suggest that you read up on propeller aerodynamics and then review aerodynamics related to multi-engine aircraft. You will find that the thrust produced by a propeller is not symmetrical when the propeller disk has an angle of incidence other than zero.

User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 16, posted (7 years 8 months 4 days 4 hours ago) and read 2263 times:

Quoting FredT (Reply 14):
Quoting DH106 (Reply 13):
Interestingly, the yaw effect that 2H4 describes in which a propellor at an angle of attack produces a yaw - the yaw effect actually decrases with distance from the centreline

Are you really, really sure about that statement?

Yes, you've got me thinking now.....
By your arguments above you should be able to move the engine anywhere along a wing and the torque will be reacted in the same way. I agree.


There's also a secondary effect - which I think is the effect I was addressing - that stems from the fact that the inner blade is closer to the CG than the outer (whether they be rising or falling). In twins/quads with engines that all turn in the same direction (most, but not all aircraft) there'll be a moment imbalance because on one wing the descending blades will be outer blades and create more moment than the descending blades on the other wing which will be inners. This yaw moment would decrease as the ratio of engine distance from CG to distance between rising and falling blade increases.



...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 17, posted (7 years 8 months 4 days 2 hours ago) and read 2229 times:

Quoting 113312 (Reply 15):
Sorry Fred T, you do not understand propeller aerodynamics. It's not the same as inserting a drill bit into a board. I suggest that you read up on propeller aerodynamics and then review aerodynamics related to multi-engine aircraft. You will find that the thrust produced by a propeller is not symmetrical when the propeller disk has an angle of incidence other than zero.

If you are of the opinion that I have claimed that there is no thrust assymetry inherent in a propeller under the influence of P-factor, I suggest that you reread my posts in this thread. The entire argument revolves around (yaw) torque (pun unintentional), which is exactly what you get when P-factor comes into play.

A torque applied to a rigid body is a torque applied to a rigid body, regardless of the means by which that same torque is applied. You can apply it by thrust assymetry between two engines, or by thrust assymmetry between the upgoing and downgoing blades on a propeller disc. The latter is P-factor. You can also, even if this is rarely done on aircraft in flight, apply it via a drill bit. The net result on the body upon which the torque is acting will be exactly the same if the magnitude and axis direction is the same.

For the purposes of this discussion an aircraft can safely be viewed as a rigid body.

Asymmetrical thrust is thrust with a torque moment added on top of it, as the thrust is not inline with the drag on the airframe. The torque moment is what we need to counteract by having a fin and rudder on the aircraft.

If you do not see this, perhaps you are the one lacking full understanding?



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 18, posted (7 years 8 months 4 days 1 hour ago) and read 2216 times:

Quoting DH106 (Reply 16):
There's also a secondary effect - which I think is the effect I was addressing - that stems from the fact that the inner blade is closer to the CG than the outer (whether they be rising or falling).

Yes, P-factor. I'm sure 113312 is willing to spell out all the gory details.  Wink

The effect depends not on the distance from the CoG but on the downgoing blade getting a higher angle of attack than the upgoing blade if the angle of attack of the aircraft increases. Thus the downgoing blade generates more thrust than the upgoing blade, setting up a thrust assymetry over the propeller disc which generates a yawing force couple (torque).

Quoting DH106 (Reply 16):
This yaw moment would decrease as the ratio of engine distance from CG to distance between rising and falling blade increases.

Ah, this is where you go wrong. The P-factor yaw moment will not be changed by where you put the engine. Let's do the math.

Assume that you have an engine generating thrust T at a distance y from the center of drag.

Assume that the propeller of this engine can be represented by a model having two thrust forces, T_d and T_u 1 m apart, thus at the distances of (y+.5) (T_d) and (y-.5) (T_u) from the center of drag. T_d is the thrust force from the downgoing half of the propeller disc and T_u is the thrust force from the upgoing half of the propeller disc.

At an angle of attack where the propeller will be at a positive angle of incidence, this will make it the critical engine, with more thrust being generated further out along the wing.

Assume that T_d is 0.6*T and that T_u is 0.4*T, for a total thrust of T.

Any moment on a rigid body around any axis can be calculated about any point, for the same result. Let us calculate the moment about the center of drag.

n = T_u * moment arm + T_d * moment arm =
= 0.4*T*(y-.5) + 0.6*T*(y+.5) =
= 0.4*T*y - 0.4*0.5*T + 0.6*T*y + 0.6*0.5*T =
= (0.4+0.6)*T*y + (0.6-0.4)*0.5*T + 0.6*T*y =
= T*y + .2*.5*T

T*y is the torque generated by the thrust, as you can see independent of the thrust assymetry over the disc and thus of the P-factor**.

This leaves the torque generated by the P-factor as .2*.5*T, or the distance between the thrust lines of the propeller halves times the thrust difference between them. The position of the engine is not a factor in this term.

To make it even clearer, replace the arbitrary figures for propeller half thrust line distance and thrust difference with constants. This is left as an excercise for the reader*.

Cheers,
/Fred

**) If you chose to look at the yaw torque about the engine instead, you would not get this term but, if the condition is steady flight and that is the only engine (T=D), an equal term D*y for the torque generated by the drag. Recall that the net torque on a rigid body is constant regardless of what point you calculate it about.

*) As always, this means "I cannot be arsed"...  Wink



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 19, posted (7 years 8 months 3 days 22 hours ago) and read 2189 times:

I think your maths is good Fred, but I'm not convinced we're talking about the same effect. You're defenitely talking about P-Factor - a turning prop creating a yaw moment because it's at an angle of attack.

I'm in two minds as to whether the effect I'm trying to address is 'real', or merely a different interpretation of what you've described above.
The effect I was trying to address was that if you put two/four or more props on wings, all turning the same way (the usual scenario) then P-Factor applies as above, but is there an additional yaw moment created because forces aren't symmetrical about the aircraft's axis. One wing will have the 'increased force' blades INBOARD, the other wing will have them OUTBOARD, leading to slightly different moment arm lengths and hence a small additional yaw moment ?



...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 20, posted (7 years 8 months 3 days 12 hours ago) and read 2155 times:

That's still P-factor, only times four.  Smile

Yes, the net effect will be similar to having the thrust of the critical engine(s) moved somewhat outboard.

Rgds,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
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