Is the “conventional” main-rotor-and-tail-rotor helicopter forever limited to a top speed of 250-miles per hour?
By: Ringo Bones
Ever since Igor Sikorsky perfected his main-rotor-and-tail-rotor helicopter configuration during the latter half of the 1930s many overly-optimistic inventors inspired by him have pushed forward to design a generation of helicopters capable of ever higher speeds. But is 250 miles per hour or even 300 miles per hour the ultimate speed limit of the “conventional” main-rotor-and-tail-rotor configuration helicopter?
By the laws of aerodynamics, the relationship between the helicopter’s top speed and its maximum rotor speed obey the “law of threes” – that is the maximum tip speed of the helicopter’s rotor is three times the maximum forward speed of the helicopter. And by the laws of physics, as the main rotor’s tip speed approach and exceed the speed of sound – its lifting efficiency drops off like a rock off a cliff. Remember that 1980s TV series called Airwolf about a helicopter capable of flying faster than the speed of sound? I wonder how many helicopter designers worth their salt had ridiculed the idea due to its physics-defying premise. And despite the mechanical complexity of a “Sikorsky-style” main-rotor-and-tail-rotor helicopter configuration, 97-percent of existing helicopters flying today opt for this design because it is currently the simplest to manufacture from the company’s perspective and most economic to maintain.
During the Vietnam War, Lockheed managed to design a helicopter capable of flying 250 miles per hour or 400 kilometers per hour in level flight called the Lockheed Cheyenne. It consists of a tail rotor that intermeshed with a pusher propeller. In its maximum forward speed, only 25-percent of the main gas turbine engine’s power goes to the main rotor as the rest is diverted to the pusher propeller. Despite being called a “compound helicopter” as opposed to a true helicopter, the Lockheed Cheyenne never went to service in the Vietnam War theatre due to its “mechanical complexity” might cause problems to ground crews and it is somewhat pricier than the next fastest helicopter of the time – the slightly slower Huey Cobra. And despite their popularity with the world’s various military organizations, there are spots cars today priced around 50,000 US dollars that can run faster than the Apache Longbow helicopter gunship – and top of the line 270 mile-per-hour capable supercars from Lotus and Ferrari can easily left the Huey Cobra in the dust in a quarter-mile drag race because the Huey cobra only has a top speed of 230 miles per hour.
During the early 1970s, helicopter aerodynamicist Peter Wilby together with Geoff Byham developed the swept-back paddle helicopter rotor tip. They manage to come up with the design to “wring more speed” from the conventional main-rotor-and-tail-rotor helicopter design by trial and error methods because they don’t trust previous existing mathematical helicopter main rotor analysis that had gone before – despite these being obtained via the most advanced mainframe computers available at the time. But the work of Wilby and Byham later paid off when their then unusual swept-tipped / swept-back paddle helicopter main rotor configuration were tried out on the Westland Lynx back in 1986 that allowed it to achieve the Guinness Book of World Records’ adjudicated world helicopter speed record of 249.1 miles per hour that stood until today as a fastest speed for a true helicopter.
So what is the future of the helicopter when it comes to pushing the top speed envelope? Well, engineers at Sikorsky had recently developed the X-2 experimental helicopter that achieved the unofficial world speed record of 288 miles per hour. But many “helicopter” purist wonder if Sikorsky’s X-2 is still a “conventional” or “true” helicopter because despite of using the contra-rotating main rotor configuration to cancel out torque, it uses a 6-bladed pusher propeller to achieve such speeds. The X-2 uses a fly-by-wire control system and computer assists to help stabilize the craft and also allows it to outmaneuver existing attack helicopters. Ultimately, the Sikorsky X-2 prototype will be the basis of the upcoming Sikorsky S-97 Raider and despite the S-97 Raider still has no working prototype other than the proof-of-concept X-2, the S-97 Raider can easily fly faster than 250 miles per hour when fully loaded with ordnance and can fly backwards at over 100 miles per hour.