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) such as the AgustaWestland Merlin HC.3 of the Royal Navy's Commando Helicopter Force and the RAF's Westland Puma HC.2, based at RNAS Yeovilton and RAF Benson. The RAF has a total of 60 active Chinooks in inventory as of 2015. In 2018 the UK issued a request to the United States to purchase 16 additional aircraft. The Chinook is expected to remain in RAF service until the 2040s. In March 1967 an order was placed for fifteen Chinook HC Mk1s, standing for "Helicopter, Cargo Mark 1", for the Royal Air Force to replace the Bristol Belvedere. This original MK1 variant