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Small Rudder Size On DC10 / MD11  
User currently offlineYQBexYHZBGM From Canada, joined May 2009, 204 posts, RR: 1
Posted (1 year 2 months 5 days 12 hours ago) and read 6758 times:

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

39 replies: All unread, showing first 25:
 
User currently offlineYQBexYHZBGM From Canada, joined May 2009, 204 posts, RR: 1
Reply 1, posted (1 year 2 months 5 days 11 hours ago) and read 6680 times:

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

Al


User currently offlinePITrules From United States of America, joined Jun 2000, 3235 posts, RR: 6
Reply 2, posted (1 year 2 months 5 days 11 hours ago) and read 6672 times:

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
User currently offlinezanl188 From United States of America, joined Oct 2006, 3562 posts, RR: 0
Reply 3, posted (1 year 2 months 5 days 11 hours ago) and read 6663 times:

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


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User currently offlinemd11sdf From United States of America, joined Dec 2006, 80 posts, RR: 0
Reply 4, posted (1 year 2 months 5 days 9 hours ago) and read 6580 times:

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

Cheers from Louisville (KSDF) home to the UPS Worldport AND the MD-11 fleet.



LOUISVILLE, where your camera may as well be a stinger misslie to the Airport Police.
User currently offlineYQBexYHZBGM From Canada, joined May 2009, 204 posts, RR: 1
Reply 5, posted (1 year 2 months 5 days 8 hours ago) and read 6543 times:

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

-Al


User currently offlinejetmech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 6, posted (1 year 2 months 5 days 6 hours ago) and read 6494 times:

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.

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17108 posts, RR: 66
Reply 7, posted (1 year 2 months 5 days 6 hours ago) and read 6487 times:

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.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently onlineMax Q From United States of America, joined May 2001, 4734 posts, RR: 18
Reply 8, posted (1 year 2 months 5 days 5 hours ago) and read 6458 times:

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.



The best contribution to safety is a competent Pilot.
User currently offlinePGNCS From United States of America, joined Apr 2007, 2834 posts, RR: 45
Reply 9, posted (1 year 2 months 4 days 20 hours ago) and read 6336 times:

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.


User currently offlineroseflyer From United States of America, joined Feb 2004, 9681 posts, RR: 52
Reply 10, posted (1 year 2 months 4 days 19 hours ago) and read 6310 times:

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.


If you have never designed an airplane part before, let the real designers do the work!
User currently offlinePGNCS From United States of America, joined Apr 2007, 2834 posts, RR: 45
Reply 11, posted (1 year 2 months 4 days 16 hours ago) and read 6230 times:

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.


User currently offlineWestern727 From United States of America, joined Jan 2007, 751 posts, RR: 4
Reply 12, posted (1 year 2 months 4 days 15 hours ago) and read 6196 times:

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.



Jack @ AUS
User currently offlineLH707330 From United States of America, joined Jun 2012, 826 posts, RR: 0
Reply 13, posted (1 year 2 months 4 days 15 hours ago) and read 6185 times:

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.


User currently offlineViscount724 From Switzerland, joined Oct 2006, 25838 posts, RR: 22
Reply 14, posted (1 year 2 months 4 days 8 hours ago) and read 6056 times:

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]

User currently offlinebohica From United States of America, joined Feb 2004, 2736 posts, RR: 0
Reply 15, posted (1 year 2 months 4 days 8 hours ago) and read 6044 times:

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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Photo © Raymond Ngu



Is that dip also to give the -SP extra vertical stabilizer area without adding too much height?


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17108 posts, RR: 66
Reply 16, posted (1 year 2 months 4 days 7 hours ago) and read 6034 times:

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

Yepp.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinebohica From United States of America, joined Feb 2004, 2736 posts, RR: 0
Reply 17, posted (1 year 2 months 4 days 7 hours ago) and read 6029 times:

Quoting Starlionblue (Reply 16):
Yepp.

Thanks.  


User currently offlineLH707330 From United States of America, joined Jun 2012, 826 posts, RR: 0
Reply 18, posted (1 year 2 months 3 days 18 hours ago) and read 5883 times:

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?


User currently offlinezanl188 From United States of America, joined Oct 2006, 3562 posts, RR: 0
Reply 19, posted (1 year 2 months 3 days 11 hours ago) and read 5753 times:

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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User currently offlineStarlionblue From Greenland, joined Feb 2004, 17108 posts, RR: 66
Reply 20, posted (1 year 2 months 3 days 11 hours ago) and read 5733 times:

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."
User currently offlinezanl188 From United States of America, joined Oct 2006, 3562 posts, RR: 0
Reply 21, posted (1 year 2 months 3 days 10 hours ago) and read 5714 times:

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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User currently offlineStarlionblue From Greenland, joined Feb 2004, 17108 posts, RR: 66
Reply 22, posted (1 year 2 months 3 days 9 hours ago) and read 5694 times:

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! 



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinezanl188 From United States of America, joined Oct 2006, 3562 posts, RR: 0
Reply 23, posted (1 year 2 months 3 days 9 hours ago) and read 5684 times:

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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User currently offlineStarlionblue From Greenland, joined Feb 2004, 17108 posts, RR: 66
Reply 24, posted (1 year 2 months 3 days 8 hours ago) and read 5668 times:

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.



"There are no stupid questions, but there are a lot of inquisitive idiots."
25 JAAlbert : 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
26 ferpe : 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 vert
27 Post contains images Pihero : In fact the main reason is to reduce at high speeds the rudder-induced rolling moment and the stress on a short-chord surface. With respect, you have
28 PITrules : 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 s
29 Post contains images Starlionblue : 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
30 jetsetter1969 : 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.
31 Post contains images Starlionblue : 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
32 sturmovik : 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 c
33 Post contains links and images Pihero : 1/- These pics reveal no significant difference between engine height, do they ? View Large View MediumPhoto © Justin CederholmView Large View Mediu
34 LH707330 : 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
35 Max Q : 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
36 747classic : 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 ha
37 KC135Hydraulics : Was it due VMCG at these configurations being just too high to be safe?
38 Max Q : Thanks 747Classic, always good to get the straight story from you. Best wishes.
39 Post contains links and images 747classic : Yes. Remark : I never operated the MD11, but followed this embarrassing story after several intermediate stops of the then just introduced MD11 at SX
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