Effect of Fuselage and Nacelles
A body, such as a fuselage or nacelle, is not a very efficient producer of lift by comparison to a wing or tail surface. Thus, except for missilelike configurations, where the body is relatively large, the contribution of the fuselage or nacelles to the total airplane lift can be neglected. However, in the case of the moment, fuselage and nacelle contributions can be fairly significant and result in a measurable shift of the neutral point. Generally, the
increments to CMa are positive, resulting in a decrease in h„. Depending on the incidence of the wing relative to the fuselage and nacelles, these components can also contribute to Сщ. Thus, including the fuselage and/or nacelles, Equation 8.7 becomes
См — Сщ + Д Сщ + (Сщ + ДСМ>
The increments Д Сщ and ДСщ result from the addition of the fuselage and/or nacelles to the basic wing-tail combination. Thus, the relationships derived thus far are unchanged if the total values for Сщ and CMa are used. An increment to CLa can also be included.
It is somewhat futile to present either data or theoretical results for predicting Д Сщ or ДСщ in view of the many possible fuselage-nacelle-wing – tail combinations. Real fluid effects and lifting surface-body interference effects severely limit any application of theoretical solutions of the body alone.
For approximate purposes, expressions for CL„ and Сщ for bodies alone are presented that are based on material from DATCOM (Ref. 5.5). The lift curve slope of an axisymmetric body, having a length of / and a diameter d, can be expressed in the form
Here, Cl is based on a reference area, S0, which is the body’s cross-sectional area at a station X0 back from the nose. X0 is a function of a station Xt; X, is the station along the body where ds/dx has its minimum value. Usually this means the point of maximum cross-sectional area. In terms of the body length, X0 and X, are related by
Х0 = 0.367 + 0.533Х,
The slope of the body moment curve can be estimated from
Сщв = CL. JXM — XQ+ To)
Cmg is based on a reference area of S0 and the body length, /. Xm is the station for the moment center expressed as a fraction of /. /0 is an average length that, multiplied by S0, gives the body volume ahead of the station X0. Again, T0 is relative to the total body length.
Since Cl„b and Сщв are based on the body geometry, their values must be corrected when they are applied to the total airplane. More specifically, cLaB must be multiplied by the ratio S0IS before being added to Equation 8.11. Similarly, Сщв must be multiplied by (S0l)l(Sc) before being added to equations previously derived from Сщ – It should be emphasized that the expression for the lift curve and moment curve slopes hold only for small angles of attack.