In 1493, Leonardo da Vinci sketched his idea for an aerial screw that he envisioned climbing vertically into the sky, powered by four men, rotating a canvas helix sail around a vertical shaft. Almost 450 years later, Igor Sikorsky flew what is widely recognized as the first practical helicopter, the VS-300, at Stratford, Connecticut. Since that time, thousands upon thousands of man-hours and many billions of dollars have been spent on improving the design. The goal: to make a vertical takeoff and landing aircraft fly larger payloads farther and faster. The engineering and aerodynamic factors limiting the capabilities of helicopters are complex and numerous, and thus the design approaches over the years have been varied and innovative. But very few have made it to production.
Why the Tilt rotor concept?
While very large, heavy-lifting helicopters such as the CH-47 can carry on the order of 26,000 pounds, most rotary-wing machines are limited to speeds of approximately150 knots at the top end and a range of around 350 nm. One of the most significant factors limiting the maximum forward speed of a helicopter is an aerodynamic phenomenon known as “retreating blade stall.” To balance the lift produced across the disc of a spinning rotor that is moving forward through the air, the advancing blade must reduce its angle of attack to maintain the amount of lift it produces as the helicopter’s forward speed is added to the rotational speed of the blade. The retreating blade must similarly increase its angle of attack, as the machine’s forward speed is removed from the blade’s rotational speed.
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