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) those used by the British armed forces, were grounded due to cracking discovered in the landing gear of a British helicopter during routine inspection. In May 2000, several Chinook helicopters airlifted British and European Union citizens out of Freetown in Sierra Leone in an evacuation due to regional instability. In September 2000 Chinooks were being used to evacuate casualties from fighting in Freetown to RFA "Sir Percivale", a support ship docked there. In July 2006, 3 Chinook helicopters of No. 27 Squadron deployed to RAF Akrotiri in Cyprus to evacuate British citizens from Lebanon; the squadron also flew the EU