Engineering Transportation

An Introduction to the Longitudinal Static Stability of by F. G. Irving

By F. G. Irving

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If the aeroplane is completely conventional and has afixedtailplane and an elevator and trim tab separately controlled by the pilot, it is convenient to consider two limiting conditions. (a) The "stick-fixed" case. , which connect the control column or "stick" to the elevator is rarely very rigid. It is assumed that the elevator and tab are adjusted so that the aeroplane is trimmed at a certain lift coefficient; the static stability is then assessed when the aeroplane is subjected to a small disturbance from the equilibrium state, the settings of the control surfaces being left unchanged.

And by the shift of datum. , the resultant moment MG is given by : MG = M0wb + (h-h0)cLwb + M0T-lTLT. 1) 35 36 LONGITUDINAL STATIC STABILITY FIG. 1. Forces and moments acting on a conventional aeroplane in steady level flight. Now the moment Μ0τ is usually very small. It arises as a consequence of "camber" of the tail and since the basic tail section is usually symmetrical, any effective camber is due to elevator deflections. Except near the stall, these are usually small; moreover, the tail area is not very large compared with the wing area.

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. It is also assumed that the 45 46 LONGITUDINAL STATIC STABILITY tab is always set at the angle appropriate to the equilibrium condition.

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