wing root to close to the wing tip (a tapered wing tip has a lower volumetric capacity). Fuel storage in the wing is advantageous because it is close to the aircraft’s CG, which lowers the range of the CG shift. If the volume available in the wing is not adequate, especially for thin-winged combat aircraft, then the fuselage space can be used for storage. Typically, fuel storage in the fuselage can be above or below the floorboards and forward, rearward, and/or at the center of the wing. When there are several tanks, it is convenient to collect fuel at a central location before delivering it to the engines. Fuel from tanks at various locations is pumped into a centrally located collector tank following a transfer schedule that minimizes the CG shift. A symmetrical fuel level in the wings also is important. Note the compartmentalization of the wing tank; surge tanks are provided at the wing tips and internal baffles restrict fuel-sloshing. Some long-range aircraft have volume available in the stabilizer to balance the CG shift through an in-flight fuel transfer.

The fuel-tank arrangement for a thin-winged aircraft (i.e, the supersonic type) is complex because there is insufficient volume available for the mission. Therefore, fuel is provisioned in the fuselage wherever space is available. The Concorde example, shown in Figure 15.23, carries a substantial amount of fuel and the CG shift is minimized by in-flight balancing through a fuel transfer from the forward and aft trim tanks. The military aircraft fuel-tank arrangement is similar to the Concorde. There can be as many as sixteen tanks, all interconnected to meet the fuel requirements of the mission.

Fuel tanks can be either rigid, made of metal or composite material, or flexible, made of a rubber-neoprene–like material. Tanks are installed during component assembly. Flexible-tank maintenance requires a change of tanks, which can be a laborious task. Most modern aircraft have wet tanks, in which the skin at the joints is treated with a sealant. A wet-tank system is lighter and more volume-efficient; however, leakage is problematic and these aircraft require strict inspection, especially older aircraft. Sealant technology has improved and wet tanks are favored.

Heat generated in stagnant regions of an aircraft flying faster than Mach 2.4 can be cooled by recirculating cold fuel around the hot zones before being fed to the engine. The preheating of fuel also helps in the combustion process.

15.9.3 Emergency Power Supply

Most midsize and larger aircraft install an APU, which performs many functions. An APU is a small power plant, invariably a turboshaft engine that uses the same fuel (i.e., AVTUR). When ground facilities are not available, the APU can provide an emergency electrical supply and air-conditioning, and it can start the main aircraft gas turbines. It is interesting that an APU exhaust can reduce aircraft drag, regardless of how small. A typical example of an aft-mounted APU is shown in Figure 15.24 (i.e., a schematic layout). The APU and its installation weight range from 100 to 300 kg depending on the size. The size of an APU in a military aircraft depends on user requirements. An APU can be started using onboard batteries.

A ram air turbine (RAT) is another way to supply emergency power. This is a propeller-driven device mounted on an aircraft surface (at the fuselage underbelly) that operates when an aircraft is in motion. A RAT is retractable. Figure 15.25 shows the schematic layout.