YQBexYHZBGM
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Small Rudder Size On DC10 / MD11

Mon Sep 16, 2013 10:32 pm

This is a question I've been wondering about for some time:

Obviously, the tail-mounted engine limits the size of the rudder on the DC10 / MD11, but how did they get away with using such a small rudder on such a large aircraft? I'm surprised they didn't require a much larger vertical stabilizer to accommodate both the engine and a larger rudder. The small size of the rudder and the vertical stabilizer as a whole is very evident as compared with the L1011.

Also, is there a special technical name for the rectangular piece that extends rearward from the tail above the engine but below the rudder? What is its aerodynamic function?

Al
YQBexYHZBGM
 
YQBexYHZBGM
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RE: Small Rudder Size On DC10 / MD11

Mon Sep 16, 2013 11:15 pm

My apologies, I just discovered there was a similar thread in 2002. But, if anyone wishes to continue the conversation, please go ahead.

Al
 
PITrules
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RE: Small Rudder Size On DC10 / MD11

Mon Sep 16, 2013 11:23 pm

Quoting YQBexYHZBGM (Thread starter):
The small size of the rudder and the vertical stabilizer as a whole is very evident as compared with the L1011.

The engines on the L-1011 are further out on the wing than that of the DC-10. This would create more yaw during an engine failure on the L-1011 necessitating a larger rudder.
FLYi
 
zanl188
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RE: Small Rudder Size On DC10 / MD11

Mon Sep 16, 2013 11:34 pm

DC-10 & MD-11 rudders are double hinged. Makes them more effective than a conventional rudder of the same size.


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md11sdf
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RE: Small Rudder Size On DC10 / MD11

Tue Sep 17, 2013 1:17 am

The structure that extends aft, below the rudder contains the aft mount/support for the number two engine.
It serves zero "aerodynamic-function"

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YQBexYHZBGM
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RE: Small Rudder Size On DC10 / MD11

Tue Sep 17, 2013 2:22 am

Thanks all! That's about the fastest any of my questions have ever gotten answered -- 3 posts, done!  

-Al
 
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jetmech
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RE: Small Rudder Size On DC10 / MD11

Tue Sep 17, 2013 4:06 am

Quoting PITrules (Reply 2):
The engines on the L-1011 are further out on the wing than that of the DC-10. This would create more yaw during an engine failure on the L-1011 necessitating a larger rudder.

I was of the impression that it was the larger rudder itself that permitted the L1011's engines to be further out on the wing?

IIRC, the lower mounting of the L1011 centreline engine allowed for a rudder of much greater height. This in turn allowed the engines to be further out on the wing, the benefit of this being extra bending moment relief that ultimately provided the opportunity for a lighter wing structure.

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Starlionblue
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RE: Small Rudder Size On DC10 / MD11

Tue Sep 17, 2013 4:17 am

Quoting jetmech (Reply 6):
IIRC, the lower mounting of the L1011 centreline engine allowed for a rudder of much greater height. This in turn allowed the engines to be further out on the wing, the benefit of this being extra bending moment relief that ultimately provided the opportunity for a lighter wing structure.

Indeed. The lower rudder mounting also meant less rudder-induced roll moment.
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Max Q
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RE: Small Rudder Size On DC10 / MD11

Tue Sep 17, 2013 5:24 am

Quoting YQBexYHZBGM (Thread starter):
This is a question I've been wondering about for some time:

Obviously, the tail-mounted engine limits the size of the rudder on the DC10 / MD11, but how did they get away with using such a small rudder on such a large aircraft? I'm surprised they didn't require a much larger vertical stabilizer to accommodate both the engine and a larger rudder. The small size of the rudder and the vertical stabilizer as a whole is very evident as compared with the L1011.

As Zan said, MD had to use a double hinged rudder to compensate for it's smaller size.


It's also why the wing engines are as close as possible to the fuselage (unlike the L1011) so the engine failure induced yaw case would be the lowest possible that this relatively 'weak' rudder would have to cope with.
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PGNCS
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RE: Small Rudder Size On DC10 / MD11

Tue Sep 17, 2013 2:47 pm

Quoting jetmech (Reply 6):
Quoting PITrules (Reply 2):The engines on the L-1011 are further out on the wing than that of the DC-10. This would create more yaw during an engine failure on the L-1011 necessitating a larger rudder.
I was of the impression that it was the larger rudder itself that permitted the L1011's engines to be further out on the wing?

IIRC, the lower mounting of the L1011 centreline engine allowed for a rudder of much greater height. This in turn allowed the engines to be further out on the wing, the benefit of this being extra bending moment relief that ultimately provided the opportunity for a lighter wing structure.
Quoting Starlionblue (Reply 7):
Quoting jetmech (Reply 6):IIRC, the lower mounting of the L1011 centreline engine allowed for a rudder of much greater height. This in turn allowed the engines to be further out on the wing, the benefit of this being extra bending moment relief that ultimately provided the opportunity for a lighter wing structure.

Indeed. The lower rudder mounting also meant less rudder-induced roll moment.

Indeed, gentlemen. All of which had the nice side effect of less FOD potential for the L-1011 (as they are higher off the ground as they moved out and up the wing) and a quieter cabin as they are farther from the fuselage. I can personally testify that the rudder on the L-1011 is EXTREMELY effective.
 
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RE: Small Rudder Size On DC10 / MD11

Tue Sep 17, 2013 4:00 pm

Another factor is that rudders are sized for an engine out condition on takeoff. With the trijet configuration, the effect of an engine out is less dramatic than on a twin, or a quad with the outboard engine inoperative. The smallest rudders are always on airplanes with tail mounted engines. The 777 with its gigantic engines has to have a very large rudder because so much yaw is induced at takeoff thrust with an engine out. Similarly, despite being less powerful, the moment generated from losing an outboard engine on a 747 is quite large, and requires quite a bit of rudder.
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PGNCS
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RE: Small Rudder Size On DC10 / MD11

Tue Sep 17, 2013 6:40 pm

Quoting roseflyer (Reply 10):
Another factor is that rudders are sized for an engine out condition on takeoff.
Quoting roseflyer (Reply 10):
The smallest rudders are always on airplanes with tail mounted engines.

Very concisely stated. The fact that the L-1011 was able to easily have such a giant rudder due to the lower placement of engine 2 allows far more asymmetric thrust moment than could be tolerated by a DC-10 even with it's articulated rudder, hence the relative engine positions on engines 1 and 3 on the two types.
 
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RE: Small Rudder Size On DC10 / MD11

Tue Sep 17, 2013 7:48 pm

Quoting roseflyer (Reply 10):
Similarly, despite being less powerful, the moment generated from losing an outboard engine on a 747 is quite large, and requires quite a bit of rudder.

...and this is why the 747SP's vert stab is even larger than the non-SP - since the SP fuselage is shorter, the "tail" has to be bigger (in this case they just made it a few feet taller) to compensate. Further, I suspect this is why the SP's rudder is double-hinged since it appears NOT to be taller than that of the non-SP 747 - only the vert stab itself is taller.
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RE: Small Rudder Size On DC10 / MD11

Tue Sep 17, 2013 8:01 pm

Quoting Western727 (Reply 12):
...and this is why the 747SP's vert stab is even larger than the non-SP - since the SP fuselage is shorter, the "tail" has to be bigger (in this case they just made it a few feet taller) to compensate. Further, I suspect this is why the SP's rudder is double-hinged since it appears NOT to be taller than that of the non-SP 747 - only the vert stab itself is taller.

Same story with the A318, the taller fin provides lift, and thus directional stability, when there is an off-center angle of attack over the fin, e.g. engine out. The lower rudder on the 748i is likewise double-hinged, so they could retain most of the structure from the 744.
 
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RE: Small Rudder Size On DC10 / MD11

Wed Sep 18, 2013 2:23 am

Quoting Western727 (Reply 12):
Quoting roseflyer (Reply 10):
Similarly, despite being less powerful, the moment generated from losing an outboard engine on a 747 is quite large, and requires quite a bit of rudder.

...and this is why the 747SP's vert stab is even larger than the non-SP - since the SP fuselage is shorter, the "tail" has to be bigger (in this case they just made it a few feet taller) to compensate. Further, I suspect this is why the SP's rudder is double-hinged since it appears NOT to be taller than that of the non-SP 747 - only the vert stab itself is taller.

In addition to the 5 ft. taller vertical stabilizer, the SP's horizontal stabilizer is also much larger than on other 747 models. Span is 10 ft. greater (82.9 ft. instead of 72.9 ft.) I think the elevators are the same size but the stabilizer structure extends another 5 ft. on each side beyond the end of the elevator. Illustrated on the photo below. Can also see the SP's much simpler single-slotted flaps which lack the big canoe fairings for the mechanism that other 747 models have.



[Edited 2013-09-17 19:25:18]

[Edited 2013-09-17 19:27:36]
 
bohica
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RE: Small Rudder Size On DC10 / MD11

Wed Sep 18, 2013 2:50 am

I have noticed a "dip" on the -SP fuselage at the tail which is not on the other 747 variants. You can see it in this picture:


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Is that dip also to give the -SP extra vertical stabilizer area without adding too much height?
 
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Starlionblue
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RE: Small Rudder Size On DC10 / MD11

Wed Sep 18, 2013 3:07 am

Quoting bohica (Reply 15):
Is that dip also to give the -SP extra vertical stabilizer area without adding too much height?

Yepp.
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RE: Small Rudder Size On DC10 / MD11

Wed Sep 18, 2013 3:14 am

Quoting Starlionblue (Reply 16):
Yepp.

Thanks.  
 
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RE: Small Rudder Size On DC10 / MD11

Wed Sep 18, 2013 4:26 pm

Quoting Starlionblue (Reply 16):
Quoting bohica (Reply 15):
Is that dip also to give the -SP extra vertical stabilizer area without adding too much height?

Yepp.

Does it also help with area ruling?
 
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RE: Small Rudder Size On DC10 / MD11

Wed Sep 18, 2013 11:23 pm

Quoting Starlionblue (Reply 16):
Yepp.

What? Are you sure?

My understanding of the "dip" was that it was a design compromise to use existing structure. Since the rear fuselage of the SP was so much shorter there wasn't enough length to use the existing keel "slope" AND have the crown of the fuselage line up. So the entire rear fuselage sits lower compared to the rest of the fuselage. Note how much lower the H-stab sits on the SP as compared to the full length 747 (line it up with door and window belt).
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RE: Small Rudder Size On DC10 / MD11

Wed Sep 18, 2013 11:49 pm

Quoting zanl188 (Reply 19):
Quoting Starlionblue (Reply 16):
Yepp.

What? Are you sure?

My understanding of the "dip" was that it was a design compromise to use existing structure. Since the rear fuselage of the SP was so much shorter there wasn't enough length to use the existing keel "slope" AND have the crown of the fuselage line up. So the entire rear fuselage sits lower compared to the rest of the fuselage. Note how much lower the H-stab sits on the SP as compared to the full length 747 (line it up with door and window belt).

You may be right, but I have never heard this.

Quoting LH707330 (Reply 18):
Quoting Starlionblue (Reply 16):

Quoting bohica (Reply 15):
Is that dip also to give the -SP extra vertical stabilizer area without adding too much height?

Yepp.

Does it also help with area ruling?

No idea. Frankly at the tail area ruling goes out of the window a bit given the empennage.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
zanl188
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RE: Small Rudder Size On DC10 / MD11

Thu Sep 19, 2013 12:35 am

Quoting Starlionblue (Reply 20):
You may be right, but I have never heard this.

Please explain. To do as you suggest would have required a redesign of the ENTIRE rear fuselage when a simple plug at the base of the v-stab would have provided any additional area required.
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RE: Small Rudder Size On DC10 / MD11

Thu Sep 19, 2013 1:16 am

Quoting zanl188 (Reply 21):
Quoting Starlionblue (Reply 20):
You may be right, but I have never heard this.

Please explain. To do as you suggest would have required a redesign of the ENTIRE rear fuselage when a simple plug at the base of the v-stab would have provided any additional area required.

Shifting the burden of proof?  

As I said, you may be right. I may be wrong. I'm not sure of anything anymore! 
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zanl188
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RE: Small Rudder Size On DC10 / MD11

Thu Sep 19, 2013 1:51 am

Quoting Starlionblue (Reply 22):

This is Tech Ops, please explain why the dip to enhance v-stab area.

Section 48 of the SP is structurally unchanged from a standard 747. Because of this the proceeding barrel section had to be recontoured, thus the dip. Please check the usual 747 books and sources for confirmation.
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RE: Small Rudder Size On DC10 / MD11

Thu Sep 19, 2013 2:29 am

Quoting zanl188 (Reply 23):
Quoting Starlionblue (Reply 22):

This is Tech Ops, please explain why the dip to enhance v-stab area.

Section 48 of the SP is structurally unchanged from a standard 747. Because of this the proceeding barrel section had to be recontoured, thus the dip. Please check the usual 747 books and sources for confirmation.

And I'm saying I was sure it was to increase area without adding too much height, but given the information you presented it turns out I was wrong.

I checked my book "Boeing 747 - Design and Development since 1969" by Guy Norris and Mark Wagner and you are indeed correct. The seemingly lower tail attachment is due to revised fuselage contouring. Boeing removed part of the tapered section so the tail sat lower.
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JAAlbert
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RE: Small Rudder Size On DC10 / MD11

Tue Oct 22, 2013 2:44 am

I recall reading somewhere that the MD-11's rudder was too small as initially designed and had to be enlarged. Is this correct? I also read that even as re-designed, some felt the rudder still wasn't sufficient. Anyone know more about this?
 
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RE: Small Rudder Size On DC10 / MD11

Tue Oct 22, 2013 7:18 pm

Quoting JAAlbert (Reply 25):
I recall reading somewhere that the MD-11's rudder was too small as initially designed and had to be enlarged. Is this correct? I also read that even as re-designed, some felt the rudder still wasn't sufficient. Anyone know more about this?

I don't know about that, what I do know is that answer 4, that the structure below the tail engine serves no aerodynamical purpose is wrong. ALL vertical surfaces forward and aft of the CG serves to stabilize (aft) or un-stabilize (forward) the airframe in yaw so this part of the fin and even the fin engine nacelle are important parts of the directional stability of a MD11. The rudder is needed to increase the yawing moment when one engine goes inop. The aft stabilizing area can be reduced if one employes a yaw damper but the airplane must also be able to fly with an inop yaw damper and still be directionally stable (not dutch rolling to much).
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Pihero
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RE: Small Rudder Size On DC10 / MD11

Tue Oct 22, 2013 10:20 pm

Quoting zanl188 (Reply 3):
DC-10 & MD-11 rudders are double hinged. Makes them more effective than a conventional rudder of the same size.

In fact the main reason is to reduce at high speeds the rudder-induced rolling moment and the stress on a short-chord surface.

Quoting PITrules (Reply 2):
The engines on the L-1011 are further out on the wing than that of the DC-10. This would create more yaw during an engine failure on the L-1011 necessitating a larger rudder.

With respect, you have it backward : It's because of the greater yaw authority of the larger verttcal tail ande rudder that the engines could be placed farther away from the fuselage ( and the cabin )

Quoting md11sdf (Reply 4):
The structure that extends aft, below the rudder contains the aft mount/support for the number two engine.

... and its reverser.

Quoting PGNCS (Reply 9):
I can personally testify that the rudder on the L-1011 is EXTREMELY effective.

  
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PITrules
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RE: Small Rudder Size On DC10 / MD11

Wed Oct 23, 2013 7:25 am

Quoting jetmech (Reply 6):
I was of the impression that it was the larger rudder itself that permitted the L1011's engines to be further out on the wing?

IIRC, the lower mounting of the L1011 centreline engine allowed for a rudder of much greater height. This in turn allowed the engines to be further out on the wing, the benefit of this being extra bending moment relief that ultimately provided the opportunity for a lighter wing structure.
Quoting Pihero (Reply 27):
With respect, you have it backward : It's because of the greater yaw authority of the larger verttcal tail ande rudder that the engines could be placed farther away from the fuselage ( and the cabin )

Seems like a bit of a Chicken/Egg argument. I can see advantages to having the engines further out (cabin noise, less wing bending, etc), and I see some advantages of having them closer in, such easier access for maintenance (closer to the ground), and less engine-out yaw requiring less rudder input equating to less drag.

Lockheed obviously thought one configuration was optimum, while McDD and Boeing (i.e. 737) thought having engines closer in was the way to go.
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Starlionblue
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RE: Small Rudder Size On DC10 / MD11

Wed Oct 23, 2013 8:46 am

All things considered, I think that having the engines further out will win since it enables a lighter wing. The question is just how big you can make the rudder. 
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jetsetter1969
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RE: Small Rudder Size On DC10 / MD11

Wed Oct 23, 2013 9:47 am

what about the depth of the tail fin? when comparing say the 744 and the 777 the 777 tail fin seens anorexic compared to the 747.
 
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RE: Small Rudder Size On DC10 / MD11

Wed Oct 23, 2013 9:53 am

Quoting jetsetter1969 (Reply 30):

what about the depth of the tail fin? when comparing say the 744 and the 777 the 777 tail fin seens anorexic compared to the 747.

It went on a diet. 

I think that's just a question of 25 years evolution in aerodynamics. More efficient with the same area. How they did it I have no idea.
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sturmovik
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RE: Small Rudder Size On DC10 / MD11

Wed Oct 23, 2013 11:53 am

Quoting PITrules (Reply 28):
and Boeing (i.e. 737) thought having engines closer in was the way to go.

Having the engines further out on the wing also allows for greater fan diameter, provided the wing is dihedral. Of course, the designers of the 737 could not have foreseen that back then.
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Pihero
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RE: Small Rudder Size On DC10 / MD11

Wed Oct 23, 2013 12:32 pm

Quoting PITrules (Reply 28):
I see some advantages of having them closer in, such easier access for maintenance (closer to the ground), and less engine-out yaw requiring less rudder input equating to less drag.

1/- These pics reveal no significant difference between engine height, do they ?

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2/- The drag due to rudder is quite negligeable in the sum of all aerodynamic and thrust forces involved : around 1/15 th of the rudder -cum- vertical tailplane side "lift".

No, the #2 engine location decision caused the tailplane surface to be dimensioned so, hence the ideal engine location in that decision. Certainly not a chicken-and-egg argument.
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LH707330
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RE: Small Rudder Size On DC10 / MD11

Thu Oct 24, 2013 3:30 am

Quoting Starlionblue (Reply 31):
More efficient with the same area. How they did it I have no idea.

The higher aspect fin and rudder certainly help, I suspect part of the consideration with the 747 was limiting total height. I can't think of a civil airliner with a lower aspect fin off the top of my head, even the DC-8 and 707 have fins shaped closer to the newer ones than the 747 does....
 
Max Q
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RE: Small Rudder Size On DC10 / MD11

Thu Oct 24, 2013 6:27 am

Quoting jetsetter1969 (Reply 30):
what about the depth of the tail fin? when comparing say the 744 and the 777 the 777 tail fin seens anorexic compared to the 747.

It looks way too small and out of proportion to me, maybe something to do with the constant 'tail wag' of the -200 series ?
Don't know about the -300, never been on one.



I stand to be corrected but I believe the vertical stabilizer on the 747 is so large and effective that it can be dispatched with no yaw damper ?



Certainly couldn't do that on the B707 / 727 !
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747classic
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RE: Small Rudder Size On DC10 / MD11

Thu Oct 24, 2013 3:10 pm

Quoting Max Q (Reply 35):
I stand to be corrected but I believe the vertical stabilizer on the 747 is so large and effective that it can be dispatched with no yaw damper ?

On the 747 two yaw damper systems are installed (one for the upper rudder and one for the lower), one may be unservicable for dispatch.
Actually I have flown the 742 and 743 with both yaw dampers off during acceptance flights, performed after a D-check :
No dutch roll was noticed.

The max rudder deflection of the 747 classic series is 25 , at the 744 serie increased to 30, finally at the 748 series the lower rudder is made double hinged to cater for the further increase of installed engine thrust levels.(despite the increase in length of the 748)

Regarding the MD11 rudders the following : I remember that our company (KL) had troubles in the past with the initial operation of the MD11 out of SXM, with a wet runway.(and it rains a lot over there) and the availability of only full thrust TL tables for SXM.
Operating the 747 we always used full thrust T/O rolls (with packs off) towards the fast sloping terrain (hill) at the end of the runway. In case of an engine failure we could make it just right of the top of the hill. No problems with a wet runway.

The MD11 was not able to handle the combination of a relative low TOW, full thrust and a wet runway in an engine out situation in the low speed regime during the initial T/O roll ). Later special (reduced thrust) TL tables were used.

On the DC10 we didn't have the same problem (less installed thrust).

[Edited 2013-10-24 08:36:39]
Operating a twin over the ocean, you're always one engine failure from a total emergency.
 
KC135Hydraulics
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RE: Small Rudder Size On DC10 / MD11

Fri Oct 25, 2013 5:25 am

Quoting 747classic (Reply 36):
The MD11 was not able to handle the combination of a relative low TOW, full thrust and a wet runway in an engine out situation in the low speed regime during the initial T/O roll ). Later special (reduced thrust) TL tables were used

Was it due VMCG at these configurations being just too high to be safe?
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Max Q
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RE: Small Rudder Size On DC10 / MD11

Fri Oct 25, 2013 6:49 am

Quoting 747classic (Reply 36):


Quoting Max Q (Reply 35):
I stand to be corrected but I believe the vertical stabilizer on the 747 is so large and effective that it can be dispatched with no yaw damper ?

On the 747 two yaw damper systems are installed (one for the upper rudder and one for the lower), one may be unservicable for dispatch.
Actually I have flown the 742 and 743 with both yaw dampers off during acceptance flights, performed after a D-check :
No dutch roll was noticed.

Thanks 747Classic, always good to get the straight story from you.


Best wishes.
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747classic
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RE: Small Rudder Size On DC10 / MD11

Fri Oct 25, 2013 11:39 am

Quoting KC135Hydraulics (Reply 37):
Was it due VMCG at these configurations being just too high to be safe?

Yes.

Remark : I never operated the MD11, but followed this embarrassing story after several intermediate stops of the then just introduced MD11 at SXM with many hours delay (until the runway was dry again) or the stop was sometimes even ommited at the AMS-SXM-CUR-AMS schedule.
Several SXM flight were actually flown with the 747 as a substitute, before the adapted MD11 TL tables (with reduced thrust) were made available for SXM.
At that time it was a nice subject to talk about with the MD11 crews at our carribean en route stations (underneath the palm trees with a nice cool beer.........that were the days), especially because we were the lucky ones not to operate " that aircraft with the mini DC10 tail ".

Nice climb-out profile of KL MD11 from SXM (low T/O weight , full thrust, high terrain in front of aircraft)

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Photo © Teemu Heikkonen



[Edited 2013-10-25 05:28:16]

[Edited 2013-10-25 05:29:25]
Operating a twin over the ocean, you're always one engine failure from a total emergency.

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