Suddenly there’s a red-hot competition under way to develop rotorcraft capable of flying at sustained speeds well above the theoretical limits of conventional helicopters.
To meet the challenge, engineers at some of the top helicopter manufacturers are turning out radical concepts that look downright bizarre. But the big surprise is these odd birds actually fly beautifully, and, more to the point, they are attaining speeds that are attracting notice among influential customers, especially at the Pentagon.
If your pilot certificate doesn’t say the word Rotorcraft on it, you may have wondered at some point why it is that even the fastest helicopters in the world — we’re talking modern military marvels like the British-made Westland Lynx or Boeing’s AH-64 Apache — can’t outrun a lowly Beech Bonanza. Shouldn’t some bright engineering mind have figured out by now how to build a helicopter that can cruise at speeds above 200 knots? Or even 300 knots?
Well, here’s the reality that even early helicopter pioneers like Igor Sikorsky and Arthur Young knew all too well: The rotor blades of a helicopter do just fine when they’re slicing through the air in a steady hover. It’s when the pilot eases the cyclic stick forward and starts to gather forward speed that the physics get complicated. And above 200 knots, they get really complicated.
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. The drag, in fact, is proportional to the cube of rotor rpm. It stands to reason, then, that the slower a rotor spins, the less drag it creates. Yet for a helicopter to remain in equilibrium, both sides of the spinning rotor must produce about the same amount of lift. 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. But what happens when a helicopter transitions from a hover to cruise flight?
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. This means that the tip of the retreating blade must also be traveling at 300 knots. Now you enter forward flight and accelerate to a speed of 100 knots. This means that the total effective velocity at the tip of the advancing blade is now 400 knots. 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 causes a phenomenon known as dissymmetry of lift. 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.
You can see where this is all headed: If you were to accelerate a helicopter to, say, 300 knots, the advancing blade would have an airflow velocity of 600 knots (approaching the speed of sound) while the airspeed of the retreating blade would effectively be zero. For the blade to produce lift it must have some airflow over it, and in this case the retreating blade would stall (hence the term “retreating blade stall”).
Practically speaking, a pure helicopter is limited to a top sustained cruise speed of around 175 to maybe 225 knots — and most current production helicopters fly slower than that. While it might be possible to fly faster with a helicopter if you had big enough engines for propulsion and the right rotor system, the drag penalties would usually cancel out the benefits. After all, the more drag you encounter, the more fuel you’ll burn, and that means you’ll have to carry more fuel, and the vicious performance-eroding cycle comes full circle.
But what would happen if you mounted not one but two rigid counterrotating rotors on a helicopter to solve the problems associated with dissymmetry of lift? Or, taking a slightly less radical approach, you instead bolted an airplane propeller on each side of your helicopter and connected both to the engines, or you designed a system whereby you tilted the entire rotor system 90 degrees midflight to transform your helicopter into a turboprop?