![]() Not to be outdone, Eurocopter has created the X3 (pronounced “X Cubed”) technology demonstrator, which combines the fuselage, engines and main rotor of an EC 155 with two high-efficiency propellers mounted on short wings astride the fuselage. Sikorsky, meanwhile, has created a wildly advanced machine called the X2 that relies on rigid, twin-coaxial rotors and a rear facing five-blade propeller that are designed to propel the craft beyond the 250-knot barrier. Based on engineering work pioneered by Boeing and Bell Helicopter, the AW609 is a commercial tiltrotor aimed at government and corporate VIP markets. The approach is conservative if only because it’s proven. (In case you’re wondering, that describes the propulsion system of a $240 million, VTOL-capable Lockheed Martin F-35B Lightning.)ĪgustaWestland has been focused for several years on perhaps the most conservative of fast-cruise-speed VTOL designs (as if that adjective even fits here) with the AW609 civil tiltrotor. Here we have three dramatically different approaches to speedy rotorcraft design, and yet each has the same goal: greatly increased cruise speed while maintaining VTOL (vertical takeoff and landing) characteristics without taking the impractical leap of, say, adding a 43,000-pound-thrust, fully afterburning Pratt & Whitney F135 turbofan engine to the equation. Rotor blades are designed to compensate for this in many helicopter designs by a combination of flapping and blade feathering in such a way as to reduce lift on the advancing blade and increase lift on the retreating blade. This causes a phenomenon known as dissymmetry of lift. But the air velocity over the retreating blade is decreased by the same amount, meaning it’s traveling at an effective velocity of only 200 knots. This means that the total effective velocity at the tip of the advancing blade is now 400 knots. Now you enter forward flight and accelerate to a speed of 100 knots. This means that the tip of the retreating blade must also be traveling at 300 knots. Let’s say for argument that the tip of the advancing blade of a helicopter you are flying is moving through the air at 300 knots in a no-wind hover. But what happens when a helicopter transitions from a hover to cruise flight? In a hover with no wind, this isn’t a problem because the airflow velocity over the advancing blade and the retreating blade is equal. Yet for a helicopter to remain in equilibrium, both sides of the spinning rotor must produce about the same amount of lift. It stands to reason, then, that the slower a rotor spins, the less drag it creates. The drag, in fact, is proportional to the cube of rotor rpm. ![]() Without going into the nitty-gritty details of why helicopters are limited by how fast they can fly, what you should understand is that a spinning rotor creates a lot of drag. ![]()
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