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Topic Author
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DC-10/L-1011 Rudder Design.

Wed May 30, 2001 8:10 pm

Some time ago, someone in a forum talked about some problems with the size of the rudder in the MD-11. They pointed out that it was too small for the size of the plane. I was wondering if the DC-10 suffered from this problem, as the MD-11 was based on the design of the DC-10.
I also wonder if Lockheed postioned the no2 engine on the tail of the L-1011 as opposed to the fin because this allowed for a larger rudder. Does this design increase drag compared to the DC-10?
Are there any pilots who had experience with both types and found any problems with either?

Thank you in advance.
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RE: DC-10/L-1011 Rudder Design.

Wed May 30, 2001 11:24 pm

I remember reading something about this - I think it was actually relating to the horizontal stabiliser, not the rudder. Apparently the MD11 is not very stable on approaches etc because of constant need for trim and changes to the stabiliser settings. I might be wrong, but I must have got this from somewhere!
Topic Author
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RE: DC-10/L-1011 Rudder Design.

Thu May 31, 2001 12:56 am

My apologies, I do believe it was the horizontal stabiliser that was too small, thanks LeeJ. I feel a little bit embarrased! I thonk my memory failed me on this one.
Now that I think of it, the tailplane in the DC-10 is bigger than the MD-11, so why did MD make the tailplane smaller on the MD-11?
I still wonder why Lockheed put the no.2 engine on the L-1011 at the tail as opposed to the fin. This arrangement seems to me to have more drag. Could any one explain this to me?

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RE: DC-10/L-1011 Rudder Design.

Thu May 31, 2001 12:18 pm

I can give you my own thoughts about it. When Douglas put the engine on top of the fuselage instead of in it, like the L-1011, it forced a smaller area for the vertical stabilizer and rudder I believe due to height limits of hangars limiting the height of the tail. Douglas then had to design a small but articulated 4 piece rudder which is less effective than the L-1011 large one piece. You can see this by the location of the engines on the wing. Because the a/c must perform with a wing engine inop, the rudder has to have enough effectiveness to correct for the assymetric thrust. On the DC-10 the wing engines are closer in to the fuselage than the L-1011 so that it requires that the rudder will need less force. This also seemed to force Douglas to go with a cranked wing in order to give proper ground clearance for the closer in wing engines with the gear design. Just look at a DC10 from the rear and you will see what I mean. The DC-10 even has a minimum control ground speed, which also relates to rudder effectiveness, while the L-1011 does not. All in all, although the L-1011 had problems with the reliability of the RB211 engines, the overall design of the a/c seems to be less complicated with fewer compromises. It shows the different ways designers try to achieve similar goals.

Allan Rossmore
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RE: DC-10/L-1011 Rudder Design.

Thu May 31, 2001 12:36 pm

I read somewhere (I think that is in this forum) that the Douglas DC-10 was created at the beginning with only 2 engines but theirs clients (AA I think...) resquested them to made the plane able to take-off from existing runways, so Douglas needed to modify the 10 desing and it was less difficult to plave the third engine in the tail than to redesign the beck of the plane.

This is the evolution of the DC-10 in an A-300 competitor, Douglas planned to release it back in '70. Sorry, forgot the web site where I picked out this picture...

Lockheed builded the TriStar with 3 engine at the startup so they didn't need to modify anything.
It's also the way that 727 and Trident were designed (with an engine in the fin, like the TriStar), so that design was prooved to be efficient.

Hpe that this can help

"For radar identification throw your jumpseat rider out the window."
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RE: DC-10/L-1011 Rudder Design.

Thu May 31, 2001 4:58 pm

The DC-10 and MD-11 have the following "problems".

1.- Due to the tail engine configuration the rudder is smaller so it is less effective to counteract the asymetric thrust in an engine failure. For this reasons, DC-10 engines must be placed closer to the fuselage, resulting in a "curved" wing (to get minimum ground clearance) and more cabin noise.

2.- The distance between the wings and the horizontal stabilizer is very short, because of the center of gravity being so far to the rear. This makes the airplane fairly unstable and difficult to fly. It is VERY difficult to control the DC-10 and MD-11 at cruise speed without the autopilot. Trying to control the airplane manually could easily result in wide up-and-down oscillations. This actually happened few years ago. The autopilot of an MD-11 disengaged at cruise speed and it the pilots quite a while to regain control, with several up-and-down swings of several thousand feet each. A couple of passengers got injured.

3.- Both Lockheed and MD thought about twin engine configuration, but some airlines were skeptical about the ability of a twin engine aircraft to take off with full load from a high airport (for example Denver) and fly to the West Coast over the Rockies, specially in the event of in-flight engine failure. For that reason, both manufacturers preferred to play safe and go with a three-engined airplane.

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RE: DC-10/L-1011 Rudder Design.

Thu May 31, 2001 10:37 pm

The up and down oscillations you are referring to are known as "phugoids" by pilots of the DC-10/MD-11. Captain Al Haynes experienced this phenomenon before the 1989 Sioux City Iowa Crash of his United DC-10, which lost hydraulics. I read somewhere that the plane entered into a phugoid on short finals and crashed because of it.
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RE: DC-10/L-1011 Rudder Design.

Fri Jun 01, 2001 2:02 am


Not exactly the same thing. The "phugoids" will happen on most airplanes with its horizontal stabilizer improperly trimmed. This is about the fine balance between power, speed, weight and stabilizer trim. The mechanism as follows:
1.- Let's say that the airplane is flying level. You move the stabilizer trim a little bit.
2.- The airplane starts to climb, and looses some speed because of that.
3.- Because of the lower airspeed, the airplane begins to descend.
4.- Because it descends, the speed goes up, and were back to point number 2.

In order to avoid this, the airplane must be kept level by correcting the up or down pitch with the elevators. Some airplanes are more stable than other, but this can happen on any aircraft, starting with the humble Cessna 172, all the way up to the 747.

The instability of the DC-10/MD-11 is a little different problem. Because the center of gravity so far to the rear and the stabilizer so close to it, at cruise speed even a small movement of the control surface will produce a very pronounced change in pitch, sending the aircraft up or down. The fine adjustments of the horizontal stabilizer required to keep the aircraft level are easily done by the autopilot, but it is difficult to control by hand. It is TOO easy to over-control and send the aircraft repetitively up and down while trying to control the pitch manually. This also contributes to the fact that it is necessary to adjust the trim frequently during descend accordingly to the speed and flaps configuration. Other airplanes with a longer "tail" are simply less sensitive and easier to handle. But any of them could go into "phugoids" with inoperative horizontal stabilizer.
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RE: DC-10/L-1011 Rudder Design.

Fri Jun 01, 2001 5:04 am

Thanks for clearing that up, KateKebo!  Smile

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RE: DC-10/L-1011 Rudder Design.

Fri Jun 15, 2001 8:05 pm

If I may interject, having flown hundreds of hours in the flight deck of the DC-10, I do not find many of your statements to be true. Any aircaft, regardless of design, is inherently unstable at high speed/high altitude in the hands of a less than perfect pilot, but you are forgetting that Douglas addressed these issues with the -10/11. First, you have the Elevator load feel system. As airspeed increases, so does the force required to move the elevators up and down. This is a huge difference from take off speed, where you can move the elevators rather easily, to cruise speed of 300 knots, where some muscle is required to pull on the column. This prevents pilots from overcorrecting and stalling and/or oscillating. The second safety messure is the variable rate horizontal stabilizer control. The stab control speed is based on altitude and airspeed. Below 250kts, the stab can be moved continuoulsy by holding the trim switches, but above 34000ft and /or 250kts, holding the stab trim switch will result in a one second on - two seconds off stab motion, which effectively nudges the stab slightly instead of swinging it greatly. I have also flown many test flights in the A300-600/A310-200, and on approach, those aircraft required constant retrimming down the glidepath. I have not seen this to be the case in every DC-10/KC-10 I've flown in.
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RE: DC-10/L-1011 Rudder Design.

Fri Jun 15, 2001 10:11 pm

To clear the topic on the 1011 stabilizers - they were designed in a peculiar way. Actually, the 1011 has a variable-incidence tailplane, BUT:

- responses to pushing/pulling the control column resulted in moving the WHOLE tailplane and not just the "elevators"

- the "elevators" were controlled by the trim wheel

The whole system is kinda a "reverse" implementation of the (now common) tailplane construction where the elevators are controlled by the stick's movements and the whole tailplane swung up or down for trim. Lockheed opted the construction to prevent runway overshoots on takeoff due to improper trim and because the 1011 was developed mainly to match AA's requirements(and the runway in La Guardia was pretty short). Later, they demonstrated the abilities of this construction by letting a LOADED 1011 takeoff with "full down"-elevator trim, and surely, the takeoff went fine and without a hitch.

As for the DC-10 and the 1011 being born as twins - YES, THIS IS TRUE. The DC-10 was developed with 2 engines and became known as the "jumbo twin". Unfortunately, there was NO ETOPS regulation so far and the FAA required a third engine for extended range operations over land and long overwater flights as well. So, both DC-10 and 1011 saw the world as TriJets, because the Airlines and Manufacturer's lobby weren't able to establish what later became known as ETOPS regulation that quick.

Sadly, in the dawn of the DC-10 and 1011, both Lockheed and MD together with Airlines were lobbying for giving twins the clearance for long range (overwater) operations - the thing which was later established as ETOPS. Now, over 20 years later, both DC-10(and the MD-11) and 1011 are replaced by the new widebody twins. Getting a bit centimental, but it sounds like being beaten by your children.
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RE: DC-10/L-1011 Rudder Design.

Fri Jun 15, 2001 10:32 pm

I must take issue with a past post which stated that the DC10 has a minimum control ground speed and the L1011 doesn't due to the DC10's less effective rudder.
All aircraft (even the L1011) have a Vmcg as no airliner that I know has an effective rudder without sufficient airspeed.
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