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62 Aircraft
The coefficients
follow from the former equations.
ηf =0.85 − 0.43|δf | = η0 − η1|δf |
Lf = 1
sin( π
2 ℓf
2/3 )
ℓf
Xf =(1− ℓf )(1+ℓf − 5ℓ 2 f +3ℓ 3 f )
Mf = −0.85 1
(1 − ℓf )2 (1 − ℓf )ℓf
ℓf
Df =0.9 ℓ 1.38
f
For three-dimensional aerodynamic loads, these two-dimensional coefficients are corrected by
using the three-dimensional lift-curve slope, and multiplying by the ratio of flap span to wing span bf /b.
Then the wing load increments caused by flap deflection, in terms of coefficients based on the wing area,
are:
ΔCL = Sf
S CLαLf ηf δf
ΔCM = Sf
S Mf δf
ΔCD = Sf
S Df sin 2 δf
where Sf /S is the ratio of flap area to wing area.
7–9 Drag
ΔCLmax = Xf ΔCL
Δαmax = −(1 − Xf ) ΔCL
Each component can contribute drag to the aircraft. A fixed drag can be specified, as a drag area
D/q; or the drag can be scaled, specified as a drag coefficient CD based on an appropriate area S. There
may also be other ways to define a scaled drag value. For fixed drag, the coefficient is CD =(D/q)/S
(the aerodynamic model is formulated in terms of drag coefficient). For scaled drag, the drag area
is D/q = SCD. For all components, the drag (D/q)comp or CDcomp is defined for forward flight or
cruise; typically this is the minimum drag value. For some components, the vertical drag ((D/q)V comp or
CDV comp) or sideward drag ((D/q)Scomp or CDScomp)isdefined. For some components, the aerodynamic
model includes drag due to lift, angle-of-attack, or stall.
Table 7-2 summarizes the component contributions to drag, and the corresponding reference areas.
If no reference area is indicated, then the input is only drag area D/q. An appropriate drag reference
area is defined for each component, and either input or calculated. Wetted area is calculated for each
component, even if it is not the reference area. The component wetted areas are summed to obtain the
aircraft wetted area. Some of the weight models also require the wetted area. The component drag
contributions must be consistent. In particular, a rotor with a spinner (such as on a tiltrotor aircraft)
would likely not have hub drag. The pylon is the rotor support and the nacelle is the engine support. The
drag model for a tiltrotor aircraft with tilting engines would use the pylon drag (and no nacelle drag),
since the pylon is connected to the rotor-shaft axes; with non-tilting engines it would use the nacelle
drag as well.
Optionally the aircraft drag can be fixed. The quantity specified is the sum (over all components)
of the drag area D/q (minimum drag, excluding drag due to lift and angle-of-attack), without accounting
for interference effects on dynamic pressure. The input parameter can be D/q; or the drag can be scaled,
specified as a drag coefficient based on the rotor disk area, so D/q = ArefCD (Aref is the reference rotor
CLα