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72 Fuselage
9–3 Aerodynamics
The aerodynamic velocity of the fuselage relative to the air, including interference, is calculated in
component axes, v B . The angle-of-attack αfus, sideslip angle βfus (hence C BA ), and dynamic pressure
q are calculated from v B . The reference area for the fuselage forward-flight drag is the fuselage wetted
area Swet, which is input or calculated as described previously. The reference area for the fuselage
vertical drag is the fuselage projected area Sproj, which is input or calculated as described previously.
9-3.1 Drag
The drag area or drag coefficient is defined for forward flight, vertical flight, and sideward flight.
In addition, the forward-flight drag area or drag coefficient is defined for fixtures and fittings, and
for rotor-body interference. The effective angle-of-attack is αe = αfus − αDmin, where αDmin is the
angle of minimum drag; in reverse flow (|αe| > 90), αe ← αe − 180 signαe. For angles of attack less
than a transition angle αt, the drag coefficient equals the forward-flight (minimum) drag CD0, plus an
angle-of-attack term. Thus if |αe| ≤αt
and otherwise
CDt = CD0 (1 + Kd|αt| Xd )
CD = CDt +
CD = CD0 (1 + Kd|αe| Xd )
Sproj
CDV − CDt
Swet
π
sin
2
|αe|−αt
π/2 − αt
and similarly for the transition of payload drag (D/q)pay and contingency drag (D/q)cont. Optionally
there might be no angle-of-attack variation at low angles (Kd =0), or quadratic variation (Xd =2). With
an input transition angle, there will be a jump in the slope of the drag coefficient at αt. For a smooth
transition, the transition angle that matches slopes as well as coefficients is found by solving
2Xd
− 1 α
π Xd
t − Xdα Xd−1
t + (Sproj/Swet)CDV − CD0
=0
KdCD0
This calculation of the transition angle is only implemented with quadratic variation, for which
αt = 1
⎛
⎝1+
a
1 − a (Sproj/Swet)CDV
⎞
− CD0 ⎠
KdCD0
with a =(4/π) − 1; αt is, however, required to be between 15 and 45 degree. For sideward flight
(v B x =0) the drag is obtained using φv = tan −1 (−v B z /v B y ) to interpolate between sideward and vertical
coefficients:
CD = CDS cos 2 φv + Sproj
CDV sin
Swet
2 φv
Then the drag force is
D = qSwet CD + CDfit +
CDrb + q (D/q)pay +(D/q)cont
including drag coefficient for fixtures and fittings CDfit and rotor-body interference CDrb (summed over
all rotors); drag area of the payload (specified for flight state); and contingency drag area.
9-3.2 Lift and Pitch Moment
The fuselage lift and pitch moment are defined in fixed form (L/q and M/q), or scaled form
(CL and CM, based on the fuselage wetted area and fuselage length). The effective angle-of-attack is