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Chapter 15
Fuel Tank
15–1 Fuel Capacity
The fuel-tank capacity Wfuel−cap (maximum usable fuel weight) is determined from designated
sizing missions. The maximum mission fuel required, Wfuel−miss (excluding reserves and any fuel in
auxiliary tanks), gives
Wfuel−cap = max(ffuel−capWfuel−miss,Wfuel−miss + Wreserve)
where ffuel−cap ≥ 1 is an input factor. Alternatively, the fuel-tank capacity Wfuel−cap can be input. The
corresponding volumetric fuel-tank capacity is Vfuel−cap = Wfuel−cap/ρfuel (gallons or liters), where ρfuel
is the fuel density (input as weight per volume).
For missions that are not used to size the fuel tank, the fuel weight may be fallout, or the fuel weight
may be specified (with or without auxiliary tanks). The fuel weight for a flight condition or the start of
a mission is specified as an increment d, plus a fraction f of the fuel-tank capacity, plus auxiliary tanks:
Wfuel = min(dfuel + ffuelWfuel−cap,Wfuel−cap)+ NauxtankWaux−cap
where Waux−cap is the capacity of each auxiliary fuel tank. The fuel capacity of the wing can be estimated
from
Wfuel−wing = ρfuel fctbtwbw
where ctb is the torque box chord, tw the wing thickness, and bw the wing span; and f is the input fraction
of the wing torque box that is filled by primary fuel tanks, for each wing. This calculation is performed
in order to judge whether fuel tanks outside the wing are needed.
15–2 Geometry
The fuel tank is at position z F , where the inertial forces act.
15–3 Fuel Reserves
Mission fuel reserves can be specified in several ways for each mission. Fuel reserves can be
defined in terms of specific mission segments, for example 200 miles plus 20 minutes at Vbe. Fuel
reserves can be an input fraction of the fuel burned by all (except reserve) mission segments, so Wfuel =
(1 + fres)Wfuel−miss. Fuel reserves can be an input fraction of the fuel capacity, so Wfuel = Wmiss−seg +
fresWfuel−cap. If more than one criterion for reserve fuel is specified, the maximum reserve is used.