Advanced rotor designs break
conventional helicopter speed restrictions
A fast future awaits for rotorcraft with coaxial rotor systems and pusher propellers.
Rotational tip speed in this figure is 300 kts. If the helicopter flies at 100 kts the resultant relative wind on the advancing blade is 400 kts and only 200 kts on the retreating blade. The difference in lift across the rotor disc is called dissymmetry of lift.
The larger fuselage has more drag and weight, which allows for more cargo space but again adds weight to the aircraft.
AVX Aircraft Pres & Chief Engineer Troy Gaffey says tandem rotor systems are inefficient for high-speed flight and have a high amount of download on the fuselage. High download, structure, support, weight and drag are all penalties that act against going fast, and while the CH47 is impressive, it will not be able to achieve speeds the military is searching for in FVL.
Coaxial rotor systems have been around for a while and are used on the Russian Kamov KA50 as well as other types from the same company. The KA50 has 2 stacked coaxial rotor systems turning in opposite directions. This provides the increased lift of 2 rotor systems without taking up a lot of space like a tandem system.
The main advantage of opposing rotation is that the aircraft no longer needs an antitorque system. One of Newton's laws of motion states that for every action there is an equal and opposite reaction. This means that, if the rotor system on a conventional helicopter turns to the right, the fuselage wants to turn to the left.
To combat this tendency, conventional helicopters are equipped with tail rotors to keep the fuselage straight in all phases of flight. On the KA50 the rotors are fully articulating and they lead, lag and flap, which means they are moving around a lot and must be spaced well apart from each other—thus the very tall mast and long push/pull tubes, which increase drag.
Unfortunately, the coaxial rotor system on the Kamov still experiences retreating blade stall, and so doesn't completely solve helicopter high-speed regime issues.
Intermeshing opposing rotor systems, as seen here on the Kaman K-Max, use 2 rotor heads with only 1 transmission. This system reduces weight and does not require a tail rotor system.
A variation of both coaxial and tandem rotor systems is the intermeshing rotors of the Kaman K-Max. One version is currently operated for the US Marines in Afghanistan as an unmanned external load aircraft.
Kaman Business Development Mgr George Schafer says the advantage of the intermeshing rotor system is that it forms a dual rotor system without the large size of the tandem system. Since they share a single transmission, he explains, the 2 rotor systems do not require a large fuselage and the K-Max has very little parasitic fuselage drag.
The transmission, engines and external loads are directly beneath the rotor system, making for an excellent external load aircraft. The intermeshing rotors turn in opposite directions, so no tail rotor is required—this allows all power to be used for lifting.
Blade pitch is controlled by Kaman's servo flap control, in which control tubes go up through the mast and out the inside of each blade, which operates a flap and changes the blade pitch.
Using this system decreases the parasitic drag of external pitch pull tubes and negates the need for a hydraulic system. Lower aircraft weight translates into decreased operating costs and a higher power-to-weight ratio for external load operations.
Sikorsky X2 and S97 Raider
Exceeding 250 kts in test trials, Sikorsky X2's sleek design and innovative ideas won Sikorsky the Collier Trophy for its successful program.
Sikorsky's X2 helicopter has 2 rigid rotor systems stacked relatively close together with very stiff blades. Kevin Bredenbeck, chief test pilot of the X2, says that having stiff blades and 2 rotors stacked close together creates less drag and gives excellent performance in high-G maneuvers.
Hoping to expand the optional envelope of rotorcraft while retaining vertical lift ability, Sikorsky took lessons learned from the advancing blade concept (ABC) it developed earlier and applied them to the X2 program. X2 Chief Engineer Steve Weiner notes that the coaxial system on the X2 is nothing like the system on Kamov's helicopters. The closely stacked dual rotor system acts as a "deswirler," which increases hover performance and decreases drag.