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Engine Group 131
The engine group produces a jet thrust FN, acting in the direction of the engine; a momentum drag
Daux, acting in the wind direction; and a nacelle drag Dnac, acting in the wind direction. The engine
group is at location z F . The force and moment acting on the aircraft in body axes are thus:
F F = ef FN + edDaux + edDnac
M F = Δz F F F
where Δz F = z F −z F cg, ef is the engine-thrust direction and ed is the drag direction. The velocity relative
to the air gives ed = −v F /|v F | (no interference). The engine axes are C BF = UiVψ, where U and V
depend on the nominal direction, as described in table 17-1. The engine direction is ef = C FB ef0.
For a tiltrotor aircraft, one of the aircraft controls is the nacelle angle, with the convention αtilt =0
for cruise and αtilt =90degree for helicopter mode. The engine incidence angle is then connected to
αtilt by defining the matrix Ti appropriately. If the engine nominal direction is defined for airplane mode
(+x), then i = αtilt should be used; if the engine nominal direction is defined for helicopter mode (−z),
then i = αtilt − 90 should be used.
Table 17-1. Engine orientation.
nominal (F axes) ef0 incidence, + for force yaw, + for force CBF = UiVψ
x forward i up right YiZψ
−x aft −i up right Y−iZ−ψ
y right j aft up ZiX−ψ
−y left −j aft up Z−iXψ
z down k aft right Y−iX−ψ
−z up −k aft right YiXψ
17–3 Nacelle Drag
The engine group includes a nacelle, which contributes to the aircraft drag. The component drag
contributions must be consistent. 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.
The nacelle drag acts at the engine location z F . The nacelle axes are the engine axes, hence C BF is
calculated as described previously (see table 17-1). For the engine nominal direction forward (+x-axis),
the nacelle z-axis is downward and the x-axis is forward; zero incidence angle corresponds to zero
angle-of-attack; and 90 degree incidence angle corresponds to 90 degree angle-of-attack (vertical drag).
The velocity, angle-of-attack, and dynamic pressure are calculated at the nacelle (without interference).
The reference area for the nacelle drag coefficient is the nacelle wetted area. The wetted area per engine
is input, or calculated either from the engine system (engine, exhaust, and accessories) weight or from
the engine-system plus drive-system weight:
Swet = k 2/3 w/Neng
where w = WES or w = WES + Wgbrs/NEG and the units of k are feet 2 /pound 2/3 or meter 2 /kilogram 2/3 .
The reference area is then Snac = NengSwet. The nacelle area is included in the aircraft wetted area if