Archive for November, 2010

Eurocopter’s quest for speed

Monday, November 22nd, 2010

In the 1980s, Bell and Boeing Helicopters began developing a twin-turbo shaft military tilt rotor aircraft called the V22 Osprey. Bell then teamed with AgustaWestland to develop a commercial version known as the BA609 and it achieved its first flight in March 2003. During this time the helicopter industry was excited about VTOL aircraft reaching higher speeds. However, Eurocopter was quiet about its plans only saying it had no plans to develop a tilt-rotor aircraft.

On September 6, Eurocopter began test flights of its high-speed, long-range hybrid helicopter concept, which combines vertical takeoff and landing capabilities with fast cruise speeds of more than 220 knots. Called the X3, it is equipped with two turbo shaft engines that power a five-blade main rotor system and two propellers installed on short-span fixed wings. The engines are RTM322s, which power the company’s NH90 military transport. The main rotor gear box is a derivative of the yet-to-be certified EC175 medium size twin helicopter with a modification of two output drives for the propellers.

In cruise flight the rotor pitch is reduced to provide minimal drag and the small wings provide lift. Thrust comes from the propellers. There is no tail rotor so yaw and anti-torque are controlled by a standard pedal configuration that varies the thrust on each propeller separately. The aircraft can be flown like a traditional helicopter until 80 knots, then the main rotor pitch is reduced as the propeller thrust is increased.

According to Eurocopter, the hybrid aircraft will cost about 25 percent more per hour to operate than a conventional helicopter. However, with the increased speed the company points out that when measured in a per passenger/mile basis the operating costs will drop 20 percent. The X3 is currently a technology demonstrator, but Eurocopter says the concepts could be ready for production models in less than a decade.

Tail-boom strikes

Thursday, November 4th, 2010

Some helicopters, like the Robinson R66, have tall masts putting the rotor system high above the tail boom; others, like the MD500, have a more compact design. Engineers take into account flight characteristics of a design when considering the distance between the tail boom and the rotor disk. Even so, tail-boom strikes can and do happen.

One of the more common scenarios is when a helicopter makes a hard landing following an autorotation. Touching down too nose-high on the aft part of the skids can cause a nose-down pitch that instinctively causes the pilot to pull back on the cyclic to counteract it. This action in combination with the fact that the blades are still moving downward can result in the blades contacting the tail boom. An in-flight entry to an autorotation will also cause a nose-down pitch because of the advancing blade seeing a greater reduction in lift than the retreating blade. A pilot who overreacts with sudden aft cyclic will cause the rotor system to flap back while the tail boom is still rising, which can lead to the blades to come in contact with the tail boom. In-flight blade-to-tail boom strikes are normally fatal.

Strong wind gusts can also create a problem. A tail-boom strike can happen as rotor rpm gets lower and the centrifugal force holding the rotor stiff drops. A helicopter that is starting up or shutting down in high winds or near another hovering helicopter is particularly vulnerable. Manufacturers have used droop stops in teetering rotor systems to support the blades at slow speeds. One design uses spring-loaded droop stops with weights that pull them out of the way when the rotor speed–and related centrifugal force–gets high enough. Even so, many manufacturers and operators have maximum wind speeds for start up and shut down.