MD10PLT

This is an excerpt from an article from Boeing.com commercial magazine.

Why in the world would anybody build an airplane which flies with neutral stability. I know, it saves gas, but is the reduction in handling characteristics worth it.

Huck

The writer has it backwards.

The computer (LSAS) is used to compensate for neutral stability.

The CG is indeed moved aft in cruise, but the LSAS makes it stable.

The problem in the early days was, the LSAS kicked off in the flare. It doesn't now.

The FAA would not certify an aircraft that did not have positive static and dynamic stability.

(p.s. Boeing hates the MD-11's. The first thing they did after the buyout of Long Beach was pile up all the tooling and run over it with a trackhoe.....)

Cubdriver

I do not know a lot about MD11's and it would seem obvious no airplane should have any neutral or unstable characteristics in any axis (roll pitch or yaw), but a statically stable airplane can lose a lot in terms of controllability if it is overly stable. Fly-by-wire controls and artificial stability systems permit optimizing stability to better suit a set of requirements. Transport aircraft like the MD-11 tend to be fairly maneuverable and they handle well enough in statically stable configurations, but fighters do not and it is common to make them statically unstable in one or more axes. I wouldn't assume the FAA refuses to certify designs that are statically unstable without any augmentation, only that they are safe with augmented controls.

ptarmigan

The writer of the article is correct, but it does not mean what you think it does. They designed it so the LSAS will hold a pitch attitude, which, of course, is gives the appearance of neutral stability, when, in fact, the aircraft is positively stable.

ptarmigan

Also, the LSAS incorporates low and high speed protection, which will pitch the aircraft as appropriate, so there is no degradation in safety by having the apparent neutral stability.

72Bluestreak31

Someone's been staying at a Holiday Inn Express...

Three Green

We have a winner, winner.

Carlos Abundis

...............

MD10PLT

After reading the responses above, I was kind of wondering if the CG was designed to be aft or are they were just referring to the aft CG as a result of tail fuel management.

Also, another question; did they build it with an aft CG, with the plan of having LSAS compensate for the handling characteristics.

While looking for the answer I came across the following except from airlinesafety.com

So for those that have flown lots of civilian airliners is the above sentence correct. I have no clue since the MD-11 is the only large civilian airplane I've flown.

Just thought I would ask some questions related to the airplane I fly, along with trying to get off the FedEx TA wheel of drama for a while.

jungle

Cubdriver has it right, the MD-11 and a few other designs-like the Airbus 340 use tail fuel management to configure for optimum CG during cruise. This reduces drag by limiting the need for stab trim at cruise. An aft CG tends to limit stabilizer drag. This is true for most fixed wing aircraft.

During descent the fuel is drained from the tail and the aircraft enters a normal mid MAC CG condition. Tail or stabilizer size has nothing to do with it.

The aircraft is never unstable or even neutral in stability.

Handling in pitch and yaw is quite precise in all flight regimes, roll can be somewhat truckish(over stable) at slower speeds.

The smaller stab/ elevator is a result of a longer moment arm when compared to its DC-10 siblings.

The MD-11 does have a fairly high wing loading, and this results in both high approach speeds and higher than normal(for most aircraft) rates of descent on a typical 3 degree glide path. Flare and glidepath control is a little more critical. Nothing unstable, it just takes a bit more attention than other designs.