The most powerful helicopters in the world max out around at 170 or maybe 180 knots of airspeed. These can be multi-million dollar machines with two, or even three, of the most powerful turbine engines available. Why are the fastest helicopters slower than some of the quickest piston single airplanes?
There is a natural aerodynamic speed limit to a conventional helicopter that is similar to the “sound barrier” in fixed-wing airplanes, but in helicopters the barrier is more solid. The only way to accelerate past the helicopter speed barrier is to adopt alternate technologies such as the tiltrotor or something like Sikorsky’s experimental twin concentric rotor technology demonstrator that is capable of airspeeds well beyond 200 knots indicated.
The fundamental reason that helicopters are airspeed limited is a phenomenon called retreating blade stall. As the main rotor blades complete their trip around the circle, the blades “see” a different angle of attack at each portion of the rotation. The reason is that as the helicopter moves, one side of the rotor disk is advancing into the relative wind of motion, but the other side is retreating from the relative wind.
On one side of the main rotor disk the blade advancing into the relative wind of motion experiences an increasing angle of attack, just like a normal wing increases its “alpha” as airspeed increases. That’s a nice benefit for the helicopter on one side of the rotor disk because it is getting sort of free lift from the forward motion.
But the problem occurs on the other side of the rotor disk where the relative wind of motion is subtracted from the airspeed created by rotation of the blade. On the side of the rotor disk where the blade is retreating from the relative wind the alpha continuously decreases. The rotor system design keeps increasing the pitch of the blade on the retreating side, but at some forward speed the relative wind of motion overcomes the maximum lifting ability of the retreating blade, no matter how much that blade is pitched up. When that happens the helicopter experiences retreating blade stall, just as a conventional wing stalls, and the helicopter rolls toward the retreating blade. At some airspeed the rolling caused by the retreating blade stall becomes uncontrollable and that is what sets the effective maximum airspeed for the helicopter.
An easy way to visualize retreating blade stall is to think of taking off downwind in a conventional airplane. With only a few knots of wind blowing on the tail the takeoff can work. But try it with a 100-knot tailwind and see what happens. That is what the retreating blade in the helicopter main rotor experiences.
So, why not just spin the main rotor faster so the retreating blade moves through the air quicker to defeat the effect of the relative wind? Well, that works to a point, but at some rpm – actually a pretty low rpm compared to propellers – the local airspeed over the outer portions of the rotor nears or even exceeds Mach 1, the speed of sound. When that happens lift creation is disrupted and efficiency goes away. In fact, that “slapping” sound of a Huey that became the trademark audio of the Vietnam War is caused by the blade momentarily reaching Mach 1 during the added load of maneuvering.
There are two pretty obvious ways to overcome the airspeed limitation of the retreating blade. One is the tiltrotor concept, in which the aircraft flies slowly and hovers as a helicopter, but then the rotors rotate 90 degrees to become propellers and pull the aircraft forward using a conventional wing for lift. The other method is to have two main concentric rotors and spin them in opposite directions. With the dual rotor concept there is always an advancing blade on both sides of the rotor disk so the loss of lift from the retreating blade is canceled.
The dual concentric rotor concept has been tried many times over the decades with only limited success. Most often the weight and complexity – not to mention cost – overcomes the airspeed gained. To really increase the airspeed the dual rotor needs a source of forward thrust in supplement that generated by the main rotor, so that is an added complication.
But with new materials and technology it appears that Sikorsky may be on the verge of overcoming the natural obstacles to fast helicopter flight. Sikorsky is using a pusher propeller on the tail for thrust and dual rotors for lift and expects to have an aircraft for the military that can perhaps fly at 250 knots indicated while carrying a reasonable amount of payload. The price will still be high, but undoubtedly less than for a tiltrotor while being less complex than that combination of airplane and helicopter.
Maybe the retreating blade stall airspeed barrier in helicopters is like the sound barrier is to fixed wing airplanes – a technical and cost challenge, but one that can be resolved with the best people and materials. I hope so.