Helicopter rotor of gravity fore-aft. However, it requires the expense of two large rotors rather than the more common one large main rotor and a much smaller tail rotor. The Boeing CH-47 Chinook is the most common tandem rotor helicopter. Coaxial rotors are a pair of rotors mounted one above the other on the same shaft and turning in opposite directions. The advantage of the coaxial rotor is that, in forward flight, the lift provided by the advancing halves of each rotor compensates for the retreating half of the other, eliminating one of the key effects of dissymmetry of lift: retreating blade
Boeing Chinook (UK variants) was to be based on the CH-47B but the order was cancelled in a review of defence spending in November 1967. UK Chinook procurement ambitions were revived in 1978 with an announced requirement for a new heavy-lift helicopter to replace the Westland Wessex. Thirty Chinooks were ordered at a price of US$200 million. These helicopters, comparable to the CH-47C with Lycoming T55-L-11E engines, were again designated Chinook HC Mk1, and entered service in December 1980. Eight more Mk1s were delivered from 1984 to 1986 with the CH-47D's Lycoming T55-L-712 turboshafts. The replacement of the Mk1's metal rotor blades with aluminium