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elevator mass moment
  • what is ment by the term "elevator mass moment"  and what happens when it is not at the hinge point?
  • That's a great question. The elevator mass is just how much it weighs. I believe the elevator mass moment is referring to the center of gravity (CG) of the elevator.

    If the center of gravity or mass of the elevator is not at the hinge line, then the servo may have to work harder to move it. It can also lead to flutter.

    Another important point is the aerodynamic center (AC) of the elevator. Think of the elevator as a small airfoil. Like a wing, the AC will be close to the 25% chord point on the elevator, too. Having both the CG and the AC on the hinge line is ideal.

  • After more investigation I now have the answer.  Elevator mass moment is the vertical line of force exerted to the aircraft (up or down) when the control surface is moved off of horizental plane.   Vintage aircraft(before hydraulics) to ease the inputed control force needed to move the stablizer, there is a portion of the moving airfoil the is in front of the hinge.  This forward surface catches the air flow and negates the force needed to displace the airstream by the control surface behind the hinge,  The problem arises when the forward part of the stablizer is larger than needed and moves the vertice line of force in front of the hinge.  What happens then  is that the forward area catches the air stream and overpowers the control input.  The pilot or servo than drives the surface in the other direction and again the forward surface catches the airflow and drives the elevator in the other direction.  This condition is known as galloping.  The solution is to reduce the forward hinged surface area, or increase the rear surface area, moving the moment to, or behind the hinge - behind is stable condition - ahead is unstable. 

  • Galloping? Never heard of it. It must have taken quite a bit of digging to find it!

    A moment is just a twisting or rotating force. Think of CG balancing as static balancing and AC balancing as dynamic balancing.


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