Over the past few years Eurocopter has been tooting the hell out of their own horn, the X-3. It's a cool-ish machine, but we can do (and have done) better. Allow me to shed some light on the history that led to the Eurocopter X-3 aircraft. I'm mildly annoyed that everyone seems to be freaking out over the "amazing" and "revolutionary" X-3. Has the aviation world forgotten it’s history? We haven’t only done the “compound helicopter” thing before, but we’ve gone FASTER than the X-3! Almost 45 years ago!
Let me start by saying that yes, the X-3 is a helicopter. Many people seem to think that just because it generates thrust from something other than it’s main rotor, it is not a helicopter. That is simply not the case. This type of helicopter has been around for decades and is called a “Compound Helicopter” (http://usmilitary.about.com/od/glossaryter…). This definition is accepted by the American Helicopter Society (AHS) as well as the NASA Aeromechanics Branch and the US Army Rotorcraft Directorate. I’d say it’s pretty official, so we should accept it and move forward.
Here are a few other notable compound helicopters that helped pave the road for the Eurocopter:
McDonnell XV-1 Convertiplane
Lockheed AH-56 Cheyenne
Bell Model 533
Sikorsky S-69 Advancing Blade Concept (ABC)
Sikorsky S-72 Rotor Systems Research Aircraft (RSRA)
Of course, there are fast-ish standard helicopters such as the CH-47 Chinook and Westland Lynx, but they are typically limited to around 200 knots in perfect conditions. Why? Glad you asked. Lift symmetry and the speed of sound. As you fly faster in a helicopter, you subject your advancing side blades (pilots right hand in American single rotor machines) to a higher relative airspeed and the retreating side blades to a slower relative air speed. At some speed, the retreating side relative airspeed drops to zero and it’s damn hard to make lift with no relative air speed. Also, as you go faster, you move that advancing side closer to a relative air speed of Mach 1 and the machine hates that too for various reasons. The internet can explain the limiting factors of helicopter speed way better than I can, so I’ll stop here.
So what was the fastest helicopter? The Bell Model 533 was clocked at 275 knots in 1969. Hell, even the Lockheed XH-51 could do the same 263 knots the Eurocopter can do. And the Lockheed did that back in 1967!
Just for fun, watch this (not the fastest, but still cool to see):