THE USE OF TABS

TRIM TABS

In order to fly at a given speed, or CL, it has been shown in Sec. 6.4 that a certain elevator angle detrlm is required. When this differs from the free- floating angle <5efree, a force is required to hold the elevator. When flying for long periods at a constant speed, it is very fatiguing for the pilot to maintain such a force. The trim tabs are used to relieve the pilot of this load by causing d and <5efree to coincide. The trim-tab angle required is calculated below.

When Che and Cm are both zero, the tab angle is obtained from (6.5,2) as ^trim = — T trim + Metrim) (6-7,1)

°3

On substituting from (6.4,13), (which implies neglecting dCmlddt) we get = – £ [e*.. + °-f – b2CLJ + ^ (GheCms – 62Cma)j

which is linear in Cxtrim for constant h, as shown in Fig. 6.25. The dependence on h is simple, since from (6.6,11) we find that

– hCmJ = -a(h – K)

and hence

= “ [<k. + ~ ~ b*GiJ ~ – K)cLtrim

This result apphes to both tailed and tailless aircraft, provided only that the appropriate values of the coefficients are used. It should be realized, of course, in reference to Fig. 6.25, that each different C£trlm, n a real flight situation corresponds to a different set of values of M, JpF2, and CT, so that in general the coefficients of (6.7,2) vary with CL, and the graphs will depart from straight lines.

Equation (6.7,2) shows that the slope of the <5( vs GL curve is pro­portional to the control-free static margin. When the coefficients are constants, we have

b, Д

The similarity between (6.7,3) and (6.4,13c) is noteworthy, i. e. the trim-tab slope bears the same relation to the control-free N. P. as the elevator angle slope does to the control-fixed N. P. It follows that flight determination of h’n from measurements of ddt^^JdCj is possible subject to the same

restrictions as discussed in relation to the measurement of hn on p. 221.