Old 12th Feb 2012, 23:11
  #1319 (permalink)  
Join Date: Feb 2008
Location: In the Old Folks' Home
Posts: 402
A Bit of History

What does the equivalent diagram look like for other aircraft? Do you have to hold the stick back to maintain a nose high attitude or will neutral stick do it?
Positive aircraft stability means that when you let go of a control, the nose of the aircraft returns to the speed that you had when you first moved the control. This speed is called the trim speed. Neutral pitch stability means that when the nose is moved, the pitch angle remains where you put it.

When the Wright brothers started developing aircraft a century ago, they initially worked with the premise that aircraft should exhibit positive stability in yaw but be neutrally stable in pitch and roll. As the first test pilots, they quickly discovered that it is easier to fly if you have positive stability in pitch. If you pull or push on the stick in pitch and let go, the aircraft returns to the trim speed and settles down. This is particularly helpful in turbulence. If a gust displaces the nose in pitch (or yaw) the plane simply returns to the trim speed without help from the pilot. Accidentally bumping the stick or yoke is no problem.

Some PPRuNers have asked why the AF447 PF didn't simply let go of the side stick and let the plane resume the trimmed speed. Why indeed? In every plane those PPRuNers had flown, letting go would have allowed the plane to return to trim speed. In the AB system, letting go, which neutralizes the stick, does nothing but stop the movement in pitch. It does not cause the aircraft to resume the previously trimmed speed because the autotrim has set a new trim speed. The trim speed set by the autotrim in AF447 (in response to the PF's side stick input) was below stall speed.

With the AB flight control system, because it incorporates autotrim in pitch, the aircraft essentially becomes neutrally stable in pitch because the autotrim follows the sidestick command. The airplane does still retain positive stability in pitch when the nose is moved by external aerodynamic forces, but if you move the stick in pitch, the nose stays at the pitch angle commanded.

With a century of flight experience under our belts, why did AB decide to alter the flight control philosophy? Good question.
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