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By F. G. Irving

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It is generally more convenient to define an "equivalent airspeed", Ff such that \PoV? 37) where p 0 is the standard sea-level atmospheric density. Alternatively, Vi=V J^^VyJa. 38) V Po Since σ, the local relative density of the atmosphere, is generally less than unity, the equivalent airspeed Vx is generally less than the true airspeed F. The merit of using the equivalent airspeed is that it combines two variables V and p in a single quantity. It follows from eqn. g. 39) ~ip0Vt S' so that for an aeroplane in steady level flight at a given weight, CL is simply a function of V{.

B) The "stick-free" case. Strictly, this should be termed "elevator free", since practical elevator circuits are seldom free from friction. It is assumed that the elevator and tab are adjusted so that the aeroplane is trimmed at a certain lift coefficient and the elevator hinge moment is zero. The static stability is then assessed when the aeroplane is subjected to a small disturbance from the equilibrium state and the elevator is left freely floating. In the disturbed condition, it takes up an angle determined by the condition that the hinge moment is always zero.

The consequences of eqn. 10) are noticed by the pilot as a variation of stick position with speed. This is shown in the following analysis. Write dltrlm dr 1trim dCL -dîï—dëT-dïT (611) For an aeroplane in steady level flight at an equivalent airspeed K . 13) *· = _*» W _/ 4W \Kn K dViί VTa2-p0Vt3S \VTa2p0SjV? 15) xs, the stick displacement, being taken as positive away from the pilot. The slope of the stick position-equivalent airspeed curve is therefore {blb) àVt me\VTa2p0Sj ■ V? Again, this result is generally true, provided that the appropriate values of VT, a2 and Kn are taken.

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