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N464AT 09-13-2002 10:02 PM

MD-11 Stabilator Question
 
I just dusted off my GJ Delta MD-11 and set beside my GJ National DC-10-30 and I noticed a drastic difference between the size of the stabilators, The MD-11 is almost half the size. Does this carry over to the full size versions or is it a mistake in scaleing?

Sentinel Chicken 09-13-2002 10:27 PM

There is a 30% reduction in the size of the MD-11 horizontal tailplane compared the tailplane of the DC-10-30. I could be wrong on the reasoning for the size difference, but I seem to recall it has something to do with the longer moment arm from the center of gravity (or is it center of lift?) to the tailplane thanks to the longer fuselage of the MD-11.

AlphaSigOU 09-13-2002 10:29 PM

Re: MD-11 Stabilator Question
 
Quote:

Originally posted by N464AT
I just dusted off my GJ Delta MD-11 and set beside my GJ National DC-10-30 and I noticed a drastic difference between the size of the stabilators, The MD-11 is almost half the size. Does this carry over to the full size versions or is it a mistake in scaleing?
No mistake -- it does carry over into the full-scale aircraft. The MD-11's fuselage is longer, and does not require such large horizontal stabilizers as its older brother, the DC-10.

David Hingtgen 09-13-2002 10:36 PM

Being longer does reduce the need for big stabs, as does putting a bunch of heavy fuel inside--BUT: the stab was reduced TOO much. A BIG reason for making it so small is to reduce drag. It's too small--MD-11's are quite unstable, pitch-wise. ('Tis why every MD-11 crash is on approach) Uses a special computer-controlled longitudinal stability program. And it's ALWAYS on, like a yaw damper. If it fails, you're not totally screwed, but you will have a hell of a time keeping the right attitude. (An F-16 would instantly fall out of the sky if it lost it's FBW controls, the MD-11 will simply become very hard to handle). If it had DC-10 stabs, it'd fly better, but use more gas.

FLYFRONTIER 09-13-2002 10:36 PM

Quote:

Originally posted by Sentinel Chicken
There is a 30% reduction in the size of the MD-11 horizontal tailplane compared the tailplane of the DC-10-30. I could be wrong on the reasoning for the size difference, but I seem to recall it has something to do with the longer moment arm from the center of gravity (or is it center of lift?) to the tailplane thanks to the longer fuselage of the MD-11.
The relation of the horizontal stabilizer to the CG is one factor in the reason that the stabilizer of the MD-11 is smaller than that of the DC-10. Basically, the further the two are from each other, the more authority the horizontal stabilizer will have. You can see how the opposite is true on the 747SP which has a noticeable larger horizontal and vertical stabilizer. But that is only half the story. As I understand it, the MD-11 stores fuel in it's horizontal stabilizer and is able to transfer it forward and aft in order trim the aircraft in flight and obtain the optimal CG location for flight. Because the MD-11 operates in a much tighter CG range, it does not need as large of a horizontal stabilizer to counteract large nose down pitching moments. As the others have mentioned, by reducing the size of the horizontal stabilizer you reduce drag making the aircraft more efficient. Also by moving the CG aft by pumping fuel into the tail of the aircraft, you will also make the aircraft more fuel efficient (but at the price of stability).

Sentinel Chicken 09-13-2002 11:39 PM

I don't know if there are any MD-11 pilots on this forum, but from what I have heard from some airline pilots is that the MD-11 is a real b*tch of an aircraft to fly with overly complex systems. Most of the instability stems from what David pointed out, the horizontal tailplane being too small and the pitch instability most apparent at low speeds in approach. Supposedly (and again, I'm open to any differing opinions on this) it's also a real dog when manually flown at cruise as well.

There are some who are of the opinion that this is a critical design flaw in the MD-11 that's likely had a role in several accidents already in which pilots overcontrolled the aircraft thanks to its aerodynamics which shift the CoG aft to reduce drag and fuel burn and exacerbate the problems caused by the smaller-than-ideal horizontal tailplane.

AirDar 09-14-2002 01:24 AM

Saftey first!
 
Gee... in light of this information relating inherant instability in the MD-11 series, I've decided to remove the horiz stabs from all of my MD-11 models and begin scavenging DC-10 models for their tailplanes... I have enough accidents in my house without MD's doing groundloops while on approach to the coffee table!

Sgt Caribou 09-14-2002 02:02 AM

Copied Info
 
This copied from a website, http://www.airlinesafety.com/faq/faq9.htm, and basically reiterates what was said above. This site does make for some interesting reading.

[The MD-11 was designed with a smaller horizontal stabilizer than other airliners. That, plus the shifting of its center of gravity further aft, all to reduce drag and thus fuel burn, causes it to be unusually light on the controls. That design, known as “relaxed stability,” is common to fighter planes but is not normally found in the pitch axis of a civilian airliner. It makes it more likely that the pilot will overcontrol and exacerbate the situation, during a recovery attempt after a high altitude upset or during a bounced/hard landing.

I have never flown an MD-11. However, I do have a description of its handling characteristics from a pilot that has had considerable experience in that cockpit:

“The MD-11 is not fly-by-wire. It is, however, fly by CONSTANT pilot input. The geniuses at MD decided to make the empennage 40% smaller than the DC-10 to save on both parasitic drag and induced drag by keeping the c.g.[center of gravity] near the aft limit during high-speed cruise.

This airplane doesn't really have a "slot" when you are on final; it doesn't seem to really stay at a trimmed AOA [angle of attack] /deck angle at a specific power setting/airspeed. As such, the pilot is constantly making little corrections, like flying a dynamically unstable fly-by-wire fighter with the computer out. This is unlike any transport aircraft I've flown. Part of the problem is a system called the Longitudinal Stability Augmentation System (LSAS) which is a computer that constantly trims the stab to make up for the shortcomings of the tail size. The landing is also unique. As soon as the plane touches down I have to push on the yoke to counteract a severe pitchup from the spoilers coming to 2/3 extension. Less than a second later, the autobrakes kick in, so you have to pull back on the yoke to gently lower the nose to the runway.

Somebody once said they should let Lockheed design all the airplanes, Boeing build them...and McDonnell-Douglas market them! And let the French guys stick to making Citroens and Peugeots...”

In my view, that unstable pitch mode constitutes defective design, which is directly responsible for all the deaths and injuries that have occurred during high altitude upsets and the resulting violent pitch oscillations.

I also think it reasonable conjecture that the Anchorage, Newark and Hong Kong landing accidents might not have happened at all if the MD-11 was designed with a stable pitch mode as are Boeing airliners. The kinds of conditions encountered in those accidents (heavy weights, short runway, wake turbulence, gusting crosswinds), are personally known to airline pilots of high experience. That is when the skills of the pilot are put to the ultimate test, and the design quality of the plane is revealed. It is precisely the time when the pilot, and all on board, need everything going for them.]

The pilot must be skilled enough to instantly recognize what control inputs are needed, and the plane must be designed so as to respond instantly to those commands without excessive oscillations. The brain of the pilot and the brain of the design engineer must be simpatico. If they are not, then in my opinion, you get what happened in some of the accidents listed above.

N464AT 09-14-2002 07:47 PM

Thanks guys! I love it you can ask a simple question and get a wealth of knowldege. It's what makes this site so cool. I appreciate everyones imput


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