Leading Edge Slats - Operation

Operation

The chord of the slat is typically only a few percent of the wing chord. The slats may extend over the outer third of the wing, or they may cover the entire leading edge. Many early aerodynamicists, including Ludwig Prandtl believed that slats work by inducing a high energy stream to the flow of the main airfoil thus re-energizing its boundary layer and delaying stall. In reality, the slat does not give the air in the slot high velocity (it actually reduces its velocity) and also it cannot be called high-energy air since all the air outside the actual boundary layers has the same total heat. The actual effects of the slat are:

  • The slat effect: The velocities at the leading edge of the downstream element (main airfoil) are reduced due to the circulation of the upstream element (slat) thus reducing the pressure peaks of the downstream element.
  • The circulation effect: The circulation of the downstream element increases the circulation of the upstream element thus improving its aerodynamic performance.
  • The dumping effect: The discharge velocity at the trailing edge of the slat is increased due to the circulation of the main airfoil thus alleviating separation problems or increasing lift.
  • Off the surface pressure recovery: The deceleration of the slat wake occurs in an efficient manner, out of contact with a wall.
  • Fresh boundary layer effect: Each new element starts out with a fresh boundary layer at its leading edge. Thin boundary layers can withstand stronger adverse gradients than thick ones.

The slat has a counterpart found in the wings of some birds, the alula, a feather or group of feathers which the bird can extend under control of its "thumb".

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