IEEE Spectrum September, 2010 - 38

we decided in the 1970s that we wanted to build a really
fast helicopter. The goal was to reach 480 km/h without sacrificing the vehicle's other strengths. Almost 40
years later, Sikorsky is now close to meeting that goal. In
August 2010, a technology demonstrator, known as the
X2, reached 435 km/h, unofficially breaking the helicopter speed record. A few hurdles remain before Sikorsky
can claim the official record, which is maintained by the
Fédération Aéronautique Internationale (FAI), the world's
air sports organization. Having gotten this far, we anticipate that Sikorsky will soon begin producing commercial
helicopters using X2 technologies.
To take a crack at the speed record, we had to make some fundamental changes to conventional helicopter design. The reason why becomes
clear when you consider the difference between how helicopters and
airplanes fly.
When an airplane barrels down a runway, air flows over the wings
to produce lift. At a certain speed, the pilot pitches the nose up slightly,
increasing the angle of the wing to the air. That creates enough extra lift
for the airplane to take off. Once a plane is airborne, its speed is limited
only by the amount of thrust its engines can provide.
A helicopter generates lift quite differently. It manipulates the air
flowing over its spinning rotor blades, allowing the body of the aircraft to hover. The lift generated by the rotor blades can be angled
using the helicopter's flight controls, allowing it to fly sideways, pivot,
or even backward.
Compared with the fixed wings
of an airplane, a helicopter's rotating blades make for a much more
complicated design. Each blade
must withstand the forces of rotation, which can amount to many
times the weight of the aircraft on
each blade. A helicopter also needs
a powerful engine and a large transmission to reduce the engine's rotation rate to something appropriate
for the large rotors. For example,
a U.S. Army UH-60 Black Hawk
engine's output of 20 900 revolutions per minute turns the main
rotor only 258 times per minute, a
ratio of 81 to 1.
But here's the catch. When a
helicopter flies forward, the rotor
blades experience a dramatic variForays into
ation in airspeed. That's easy to see
Flexible
if you imagine a miniature version
of yourself perched on the tip of a
high‑speed
helicopter rotor blade. If the heliFlight
copter were hovering, you'd feel a
constant 800-km/h wind in your

face as the rotor spun around. If the helicopter were
to fly forward, you would note that the wind was
stronger on what's called the advancing side, when
the rotor was moving in the same direction as the
helicopter, but that it would be noticeably weaker
when the rotor was on the retreating side. By the
time the helicopter reached 150 km/h, you would
feel a wind speed of 950 km/h on the advancing
side, versus 650 km/h on the retreating side. The
relative speed of the wind on the retreating side
gets lower and lower the faster the aircraft flies. At
300 km/h, the wind on the advancing side would
reach 1100 km/h, while the wind on the opposite
side would be 500 km/h.
Eventually, the helicopter would reach a point at
which the difference between the lift on the advancing and retreating sides of the rotor could not be balanced and the vehicle wouldn't be able to maintain
level flight. To complicate matters further, portions
of the tip of a fast-flying helicopter's advancing
blade can exceed the speed of sound, producing
shock waves that cause large vibrations and generate considerable noise. For these reasons, most helicopters just don't like to go fast.
i n t h e 1 9 5 0 s , a i r c r a f t d e s i g n e r s began to look
at other configurations to achieve vertical takeoff and landing and reach forward speeds greater
than 450 km/h. One approach was to design an

CLOCKWISE FROM LEFT: SIKORSKy AIRCRAFT CORp.; BELL HELICOpTER; J. R. EyERMAn/TIME LIFE pICTuRES/GETTy IMAGES;
KLAuS pETERS; ExpRESS/GETTy IMAGES; DASSAuLT AvIATIOn; pIERRE vERDy/AFp/GETTy IMAGES

a t s i k o r s k y a i r c r a f t c o r p. , i n s t r a t f o r d , c o n n . ,

the

wacky

world
of

Vtol

NA * iEEE SpEctrum * SEptEmbEr 2010

Coaxial rotorS: to achieve high speeds, as well
as vertical takeoff and landing (VtOL), the Sikorsky X2
uses two rotors spinning in opposite directions.

spectrum.ieee.org

http://spectrum.ieee.org

Table of Contents for the Digital Edition of IEEE Spectrum September, 2010