Archived

This topic is now archived and is closed to further replies.

ahinterl

MD-11: CoG

Recommended Posts

The MD-11 has been designed with "relaxed stability", so LSAS was implemented to compensate for this. The reason was to have a CoG moved to the aft limit during cruise to minimize induced drag. Can someone please explain (in an understandable way please, I'm no aerodynamic engineer nor a master of physics :( ) the relationship between CoG and drag and what that means for the MD-11?Andreas

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

Changing the center of gravity does not affect drag directly. What it does is to reduce the stability of the airframe, and so reduce the force required by the control surfaces to change the orientation of the airframe. Since control of the airframe comes from the control surfaces pushing against the airflow (and hence causing drag), reducing the forces required, reduces the drag. For the MD-11, that means the airframe uses less fuel to overcome drag and so is cheaper and more efficient then more stable designs, at the cost of being less stable.

Share this post


Link to post
Share on other sites

Thank you for the informations.According to www.airsafe.com, the MD-11 isn't that dangerous, it has only a handful of fatal accidents in its history. Maybe the pilots pay special attention because of the "known" pitch instability issues...Andreas

Share this post


Link to post
Share on other sites

A very interesting article!I guess ELF has a direct relation to what's described there. It's unclear to me what ELF does in detail. I guess it creates an artificial feeling of elevator force since the Boeing article says the CG is kept at the neutral point (where no force is needed). And maybe ELF compensates as well the effect of column movements generate higher pitch changes at higher altitudes.Andreas

Share this post


Link to post
Share on other sites

The Boeing article explains the issue of improved efficiency of flight with aft CG. What they do not state is the size of the stability issue. A pilot can manually fly an aircraft statically and dynamically unstable in the longitudinal mode. It requires more corrections to maintain pitch attitude. Additiaonlly there is less forward control power, namely reduced ability to pitch down, but for a transport this is of little significance. Last issue is reduced tail clearance during takeoff, flare and landing. The undesrigned is an experimental test pilot with experience in the study of such areas on variable stability research aircraft.

Share this post


Link to post
Share on other sites
The MD-11 has been designed with "relaxed stability", so LSAS was implemented to compensate for this. The reason was to have a CoG moved to the aft limit during cruise to minimize induced drag. Can someone please explain (in an understandable way please, I'm no aerodynamic engineer nor a master of physics :( ) the relationship between CoG and drag and what that means for the MD-11?Andreas
Everything in aerodynamics goes back to Angle of Attack. So to answer ur question the relationship between CoG and Drag is simple. The further aft the CoG the lower the AoA has to be to maintain that current altitude. If the CoG is further aft more of the weight of the airplane is trying to push the nose down already thus requiring less pitch(AoA) to maintain that altitude. If you have a lower AoA you have also lowered the induced drag thus requiring less power to maintain the same speed( Save fuel). the opposite happens when u move the CoG forward, higher AoA=higher induced drag= more power required= more money. This really only matters in cruise for long periods of time.Kevin Henchey

Share this post


Link to post
Share on other sites

Having seen the last post, I find it necessary to comment:aft CG means that the tail surfaces must now provide positive lift, so the total lift of the airplane is now the sum of wing lift and tail lift. Otherwise tail lift works downward which forces the wing to produce more lift, and this increases drag, therefore thrust required, and cost.When one acquires profound knowledge of flight mechanics, not everything is about AOA as someone put it.

Share this post


Link to post
Share on other sites
Oh, hi there Opher.I see you find a new place to troll ever since you got banished from the forum at tapuzhaha you're one pathetic little **
Not sure what that's about, but Opher is right.Leaving handling qualities and stability issues aside, the main purpose of loading an aft CG is to get the stab trim to be as close to "0" during cruise as possible. Flying around with several units/degrees of ANU trim adds unnecessary drag.Regards,Nick

Share this post


Link to post
Share on other sites