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Old 10th Apr 2019, 01:34
  #3789 (permalink)  
fdr
 
Join Date: Jun 2001
Location: 3rd Rock, #29B
Posts: 681
Originally Posted by yanrair View Post

As you accelerate the plane pitches up due to lift being in front of cg and you need to trim nose down to fly level. And vice Versa. Lesson 1 of type rating course on sim covers this.
back on planet earth, where Newtonian physics still reigns, you may find that for an aircraft to be statically stable, mass is always ahead of the center of pressure. Now your B2 may be unstable but pretty much no airliner is unstable, including the Max8. It has a reduced stick force gradient in part of the envelope, which MCAS addresses, just as STS deals with another gradient issue in a different part of the envelope.

Nect time you you are in a pub with a dartboard, throw a dart feathers first and observe status stability play out.

The acceleration issue as quoted before this comment, where in the excessive out of trim AND case is so that the aircraft approaches the speed that it has been miss-trimmed to, and near that point, the stab forces between section Cm and the elevator countering load will approach a minimum, allowing for movement of the stab with less force.

Now the the non linear longitudinal stability arises from the component of lift arising from the engine nacelle, which happens to be forward of the center of mass, so as additional lift is generated, that results in a pitching moment being added to the total body pitching moment. The fuselage is a lifting component, as are the wings. The stabiliser normally generates a downforce to counter the pitching moment of the total aircraft.

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