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gravity location has some effect on the static directional stability but very little or
no infiuence on the static lateral stability.
The methods of analyses of static longitudinal stability were extended to simple
maneuvers like pull-up in a vertical plane and steady turns in a horizontal plane.
For an airplane to be safely flyable,it must be capable oftrim and have adequate
levels of longitudinal, lateral, and directional stabilities. In this chapter, we have
studied how these stabilit)r levels are related to various geometric, aerodynamic,
and mass properties (center of gravity location). We have also presented meth-
ods suitable for prelinunary estimation of these characteristics at subsonic and
supersonic speeds for typical air:plane configurations.
Quite often, to improve the performance, the inherent stability of the airplane is
compromised. For safe flyability, such airplanes have to be provided with artificial
(closed-loop) stability. This forms the subject matter of automatic flight control
systems, which we will study in Chapter 6.
The controllability of the airplane is inversely proportional to the level of stabi-
lity. The elevator is the primary longitudinal control, and the ailerons and rudder
are the primary roll and yaw control devices. We found that the stick or pedal
forces associated with the defiection of these control surfaces are direcdy related
to their hinge-moment characteristics. For adequate feel and ease of flying, the
stick or pedal forces must lie within normally acceptable limits.
In Lhe next chapters, we will study the dynamic motions of the airplane. The
basic concepts we have studied here will be very usefulin understanding the more
complex subject of airplane dynamics and control.
References
rHoak, D. E., et al., *^The USAF Stability and Control Datcom:' Air Force Wright
Aeronautical Laboratories, TR-83-3048, OcL 1960 (Revised 1978).
2Munk, M. M., "Aerodynamic Forces of Airship Hulls:' NASA TR 184, 1924.
3Multhopp, H., isAerodynamics of Fuselage:' NASA TM-1036, 1942.
4Abbott, I. H., and Von Doenhoff, A. E., 7heory of dWng Sections, Dover, New York,
1958.
s Civil Airworthiness Spec~cations, Parts 23 and 25, Federal Aviarron ReguLations, U.S.
Govemment Printing Office, Washington, DC, 1991.
6British C/wL Airworthiness Requirements, Section D, Air Registration Board, England.
7Roskam, J.,Airplane Flight Dynamics and Automatic Flight Control, Part I, Roskam
Aviation and Engineering, Lawrence, KS, 1979.
gSova, G., and Divan, P., "Aerodynamic Preliminary Analysis System II:' Part 2, User's
Manual, NASA CR-182077, 1990.
9Seckel, E., Stability and Control of Airplanes and Helicopters, Academic, New York,
1964.
loPamadi, B. N., and Pai, T. G., "A Note on the Yawing Moment Due to Sideslip for
Swept-Back Wings:' Journal of Aircra[r, Vol. 17, No. 5, 1980, pp. 378-380.
Problems
3.1 For an airplane configuration of the type shown in Fig. 3.57, deternune the
low-speed slope.of pitching-moment-coefficient curve using Multhopp's method
STATIC STABILITY AND CONTROL
313
and the following data: Cre - 3.6 m, cr - 2.0 m, c - 3.0 m, S - 43.5 m2,b = 15 m,
A = 5.17, Ac/4 = 3.5 deg,lh = 5.0 m, hH = 0.05bh, and CLcr.WB - 0.06ldeg. The
geometrical data are given in Table P3.1.
TableP3.1 Geometricalparameters
of the :urplane in Exercise 3.1
_- _~--_---
Secoon Ax,m bj,m xi,m
1 0.70 0.30 4.45
2 0.60 0.50 3.90
3 0.60 0.65 3.30
4 0.60 0.70 2.70
5 1.20 0.80 1.80
6 1.20 0.85 1.20
7 1.20 0.85 0,60
8 1.20 0.80 1.80
9 0.60 0.72 2.70
10 0.60 0.62 3.30
11 0.60 0.50 3.90
12 0.60 0.37 4.50
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