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Statement I: A dorsal fin increases the fin's contribution to directional stability at small sideslip angles.

Statement II: A dorsal fin delays fin stall at large sideslip angles.

In the light of the above statements, choose the correct answer.

  • A.Statement I is incorrect but Statement II is correct
  • B.Both Statement I and Statement II are correct
  • C.Both Statement I and Statement II are incorrect
  • D.Statement I is correct but Statement II is incorrect

Answer: A

Correct answer: Statement I is incorrect but Statement II is correct

The second statement is right: a dorsal fin's swept, low aspect ratio leading edge sheds a vortex that keeps the flow attached well beyond the fin's natural stall angle, preserving stability in a large sideslip and preventing rudder lock.

The first is wrong. At small sideslip the fin is unstalled and working normally, and the dorsal fillet adds very little side force there - its contribution is negligible until the angles become large. It is a fix for the extreme case, not a way of adding cruise directional stability.

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