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.
