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Mass balance

 Mass balance

  • Control surfaces are often balanced in quite a different sense. A mass is fitted in front of the hinge.
  • This is partly to provide a mechanical balancing of the mass of the control surface behind the hinge but may also be partly to help prevent an effect known as ‘flutter’ which is liable to occur at high speeds.
  • This flutter is a vibration which is caused by the combined effects of the changes in pressure distribution over the surface as the angle of attack is altered, and the elastic forces set up by the distortion of the structure itself.
  • All structures are distorted when loads are applied.  If the structure is elastic, as all good structures must be, it will tend to spring back as soon as the load is removed, or changes its point of application.
  •  In short, a distorted structure is like a spring that has been wound up and is ready to spring back. An aeroplane wing or fuselage can be distorted in two ways, by bending and by twisting, and each distortion can result in an independent vibration.
  • Like all vibrations, this flutter is liable to become dangerous if the two effects add up. The flutter may affect the control surfaces such as an aileron, or the main planes, or both.
  • The whole problem is very complicated, but we do know of two features which help to prevent it – a stiff structure and mass balance of the control surfaces.


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