February 22, 2017

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

By F. G. Irving

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7), whence CMG = εΜο-£ι,Κη-ντ(αίητ + α2η + α3β). 1) Lines representing the variation of CMG with CL could therefore be plotted for various values of η (see Fig. 2), and the aeroplane would be in trim at the lift coefficients corresponding to points A, B and C for elevator angles ηΐ9 η2 and η3 respectively. From eqn. 1) and Fig. e. deflections increasing in the downward sense. FIG. 2. Pitching moment coefficient curves for three values of the elevator angle. When the aeroplane is in trim, CMc = 0, and the elevator angle to trim at a certain lift coefficient, Y\trim, can be found from eqn.

In the process GENERAL STABILITY CONSIDERATIONS 37 of obtaining this dimensionless equation, lTST, which has the dimensions of a volume, is divided by another "volume" based on the wing dimensions Sc. The quotient is called the "tail volume" coefficient, symbol V. Also, since CMovvb is the only moment coefficient of significance, the suffix wb will now be omitted. The final form of eqn. 2) is therefore CMG = CMo + (fe-/z 0 )C Lwb - VCLT. 4) As explained in the previous chapter, it will become necessary to differentiate this expression with respect to CL, where CL relates to the whole aircraft.

The components of aerodynamic force acting perpendicular to the mean aerodynamic chord are the same as the lift forces. Since lift is defined as acting perpendicular to the free-stream direction, this is equivalent to assuming small angles of incidence. 4. The effects of slipstream or jet efflux are neglected. 5. The air in which the aeroplane isflyingmay be regarded as an incompressible fluid, so that aerodynamic force coefficients for an aeroplane of given geometry depend only on the incidence of the wing and the angular settings of the tail and control surfaces.

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