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Aviation History
1981
1981 - 0091.PDF
FLIGHT International, 10 January I9BI 85 l^iSM *B ; , l^f : : • This view of the Twin Squirrel shows the optional lower window in the door and the ground handling wheels ment, i.e., limiting the consequences of fire. The company's philosophy on this point is that when fire occurs in the main gearbox compartment the pilot must be warned and the helicopter must be able to continue flying for five minutes. This obviously requires the installation of a fire detection system, but various other modifica tions have been implemented to pro tect flying controls, the airframe and adjacent areas from fire. Aerospatiale has conducted a successful full-scale fire test. The anti-vibration systems on the Single and Twin Squirrels are identi cal, a device in the rotor head can celling out a large part of the main rotor vibration at source. The system consists of a mass located in the rotor head above the rotor head star and pivoted below the level of the blades. Three springs behind each blade root maintain the mass central when the rotor is stationary. When running, the mass oscillates relative to the rotor head and in the opposite direction to any horizontal vibration, thereby damping it. Vertical vibration varies along the horizontal axis of the aircraft with nodes (zero vibration points) and anti- nodes (high vibration points) occur ring at particular locations. Unfortu nately, the cabin of a helicopter often sits on an antinode. Aerospatiale has incorporated cabin resonators under the pilot's and copilot's seats. These are similar in design to the rotor- head anti-vibration system and con sist of simple sprung masses which vibrate vertically and have a reson ance identical to three times the normal rotor r.p.m. of 394. This reduces the vibration and also moves the node under the pilots' seats. The key to keeping vibration levels low in the first place is to isolate them at source. Twin Squirrel's main gear box is attached to the airframe by four inclined rigid bars, bolted to the top of the gearbox, which transfer lift to the aircraft. The bottom of the gearbox is secured by a crossbar which is fastened to the main structure by cylindrical laminated pads consisting of a stack of thin rubber and light alloy discs bonded together (see cut away). These pads are rigid in com pression and thus transfer torque to the airframe, but being flexible in shear they reduce vertical and hori zontal vibration transmission to the airframe. The average three per rev frequency vibration level measured on Twin Squirrel during flight-tests at fast cruise is 0 • 07g on the pilots' seats and 0-06 on the rear seats. The airframe primary structure con sists of four basic sections—bottom, body, rear and tail boom. All parts receive anti-corrosion treatment dur ing assembly. External final paint (polyurethane) is selected by the cus tomer from a standard range of schemes and colours. The bottom structure consists of a floor resting on two main longerons with cross beams, while the body structure houses the transmission deck with the centrally mounted flexible main gear box mounting already mentioned. Two longitudinal bulkheads are located in alignment with the bottom structure longerons and two stiffened lateral bulkheads complete the main frame. Two further stiffened lateral bulkheads, a common ceiling and two press-formed base pieces, together with the longitudinal bulkheads, form the integral fuel tanks. Two side bag gage compartments are provided behind the fuel tanks. The rear structure is a truncated conical frame covered with an alu minium alloy skin. It includes the powerplant support deck, made of press-formed stainless steel held by two main frames and a rear frame providing the tail boom link. A third baggage compartment is accessible through a door on the left-hand side. The tail boom is also a truncated conical shape, the front frame being bolted to the structure below the engines. Three main frames at the rear of the tail boom carry the tail rotor gear box, fin and tail skid. Inter mediate frames provide rigid support for the drive shaft bearings. Secondary structures include the cabin, main gearbox and engine cowl ings, and power fairings. To facilitate pre-flight inspection and maintenance, the main gearbox and engine cowlings are provided with quick-release fasteners. They open up wards and are held open by stay-rods. Other removable cowlings include the main gearbox and engine upper cowl ings, the air intake mounted on the cabin roof, the cooling system cowling, the tail rotor drive shaft cowlings and the tail gearbox fairing. Streamlining of the lower part of the body structure is achieved with two fairings attached with quick-release fasteners. When removed, the forward fairing located under the front seats allows access to the pilot's and copilot's flight con trols, the auxiliary controls connected to the quadrant and the electrical wiring in the pedestal. The rear fairing, positioned under the fuel tank, gives access to the fuel tank, fuel pump and fuel level sensor. The main rotor control mixing unit and main electrical wiring is also accessible with this fairing removed. The cabin consists of two heat-
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