served straight up
The Archimedes screw took the flying machine
in an entirely new direction.
By Jeffrey Winters, Associate Editor
Its skeletal frame sat chained
up in a Connecticut field, looking like a beast that had been left to
starve. Indeed, everything about the VS-300 was bare-boned: The cockpit
was just a seat in front of the exposed 75-horsepower engine; belts and
pulleys drove the blades; the vertical rotor spun at the end of an awkward
spar.
But as unlikely as it looked, the VS-300 was a flying machine. In the
fall of 1939, it shook and sputtered and finally lifted off the ground
in a controlled flight. (The machine was tethered to keep the thing from
zooming off, had the calculations of its lift been off by many factors.)
And at the controls was a man in a fedora, Igor Sikorsky.
 |
| Igor Sikorsky took the controls of the VS-300 during one of its first tests. Sikorsky opted for his trademark fedora instead of a crash helmet. |
Sikorsky's ungainly machine was an attempt to perfect the helicopter,
which he and others believed would be the aircraft that would bring flight
to the masses. The vision was one of backyard and rooftop helipads, with
commuters taking to the air rather than the road. But engineers had yet
to perfect the helicopter. The prototypes that had made it off the ground
proved to be too cumbersome for regular service.
The VS-300 promised to change all that. If it worked.
It's surely a cliché that the dream of flight was inspired by soaring
birds. But what of helicopter flight? Although hummingbirds and dragonflies
share some of their flight characteristics—especially the ability
to hover or even fly backward—helicopters have no true animal analogue.
And even the maple seedpod, which does corkscrew through the air, uses
the extra lift simply to glide.
Screwed-Up Idea
The idea of the helicopter, in fact, was inspired not by nature but by
a simple machine, the Archimedes screw. A screw pump can push water up
an incline, or a propeller screw can push against water to move a ship
forward. Couldn't a large enough screw pull a machine into the
air?
Leonardo da Vinci, in his studies of potential human flight—from
parachutes to strap-on wings—hit upon this solution in 1483. His
sketch shows a flamboyant fabric helix 13 feet across attached to a central
axle; four people standing on a suspended platform would turn the axle
to create lift. (A scale model was built for the Boston Museum of Science.
Tests on the model showed that the craft could not create enough lift
to fly.)
Almost 300 years later, the French mathematician J.P. Paucton designed
a flying machine with two helices, one for lift, the other for propulsion,
though the design proved unworkable. And it was still based on the notion
of boring through the air like a screw through wood. Modern aeronautical
ideas weren't applied to helicopter designs until the English engineer
George Cayley took up the problem in the early 19th century. Cayley, who
is justly famous for building a piloted glider in 1852, began noodling
with rotating-wing aircraft in the 1790s, and later designed a craft that
relied on four rotors for lift and twin propellers for propulsion. (The
passenger compartment was based on a small boat, with a bird's
head on the bowsprit.)
Unfortunately for Cayley and other would-be innovators, the steam engines
available were too heavy to fly, and internal combustion engines with
enough power wouldn't appear until the end of the century. Experiments
with unpiloted models continued, with contraptions powered by springs
or rubber bands or toy steam engines. But since such flights were short
and uncontrolled, very little could be learned. Meanwhile, kite and glider
flights led to steady advances in airplanes, leading up to the Wright
brothers' flight in 1903.
Quit Stalling
Early airplanes were difficult, even dangerous, to fly. One of the biggest
hazards was stalling: a loss of lift due to pitching the wings at too
sharp an angle of attack. From a high enough altitude, a pilot has a fighting
chance to pull out of a stall, but closer to the ground stalls invariably
led to crashes.
In 1919, the Spanish aircraft engineer Juan de la Cierva y Cordonia began
to study stall after a biplane he designed stalled and crashed, with the
pilot barely escaping with his life. He felt the solution lay in finding
some passive means of providing lift, a fail-safe if pilot or mechanical
error erased lift from the wing surface.
After a number of experiments, Cierva stumbled across the freewheeling
rotor: As a propeller pulled a plane down the runway or through the air,
the rotor would turn, producing lift. Even if the engine failed, the rotor
would continue to turn, providing enough lift to enable a slow, controlled
descent.
Cierva called his creation an "autogiro," and in the course
of refining the design, discovered a fatal flaw—the machine rolled
uncontrollably once it left the ground. Experimenting with various design
permutations, however, he found that the longer and more flexible the
blades, the more stable the flight.
It was a matter of adjusting the lift. Over the course of a rotation,
the amount of lift produced by the rotor blades varied due to the effect
of the airflow. Letting the rotor "flap" would instantaneously
change the angle of attack—and thus the lift.
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| By 1938, the German-built twin-rotor Focke-Achgelis Fw-61 reached altitudes of more than 11,000 feet, setting a record for helicopters. |
By early 1923, Cierva's autogiro had successfully navigated a closed
course at a height of 100 feet. More experimental machines were built
and, in 1928, one became the first rotorcraft to cross the English Channel.
Soon, hundreds of autogiros of various designs were being constructed,
some of which featured rotors powered at takeoff and tilting rotors for
greater maneuverability.
The performance of some makes was impressive. The Kellett Autogiro KD-1
had a maximum airspeed of 125 miles per hour and a range of 200 miles.
A Kellett KD-1B carried mail between the Philadelphia Post Office and
Camden Airport.
While gyroplanes found a niche in the 1930s, efforts to perfect proper
helicopters continued to sputter. Although helicopters capable of carrying
a pilot flew in France in 1907, just four years after the Wright brothers'
first powered flight, the efforts were marginal successes. One helicopter
remained tethered and rose only 2 feet, while another flew freely, but
for only 20 seconds.
Throughout the 1910s and 1920s, all sorts of experimental designs flew.
The main problems to be tackled were generating enough lift and accounting
for torque. Increasing lift was a matter of refining blade design and
boosting motor power density, a straightforward if non-trivial task. But
accounting for torque—a twisting of the fuselage equal and opposite
to the force applied to the rotor axis—was a real problem.
The solution most designers hit upon was pairs of counter-rotating blades
that would cancel out each other's torque. Within this general idea were
a number of ingenious variations, however. Four, six, even eight powered
rotors sprouted like a forest on airframes. Other designers opted for
coaxial counter-rotating rotors, or rotors that intermeshed like giant
eggbeaters.
European engineers eventually settled on a workable configuration: twin
rotors powered by a single engine, with lateral control through changing
the pitch of the rotors. In 1937, a German helicopter designed by Heinrich
Focke with those specifications set world rotorcraft records for altitude,
endurance, and airspeed. But while such machines were serviceable, they
didn't provide stable, fully three-dimensional flight.
From Rubber Band to Liftoff
By the late 1930s, helicopters attracted the attention of another celebrated
aeronautical engineer. Igor Sikorsky had been enamored of helicopter designs
from the time he was a child. He built a rubber band-powered model when
he was seven, and had constructed two full-scale helicopters before he
was 21. (Neither produced enough lift to carry a pilot.) Sikorsky turned
his attention to fixed-wing aircraft, building bombers for Czarist Russia
and flying boat passenger planes in the United States.
Sikorsky had filed a patent in 1931 for a radically new design of helicopter,
but was never able to develop it into a prototype. In 1938, flying boat
production ground to a halt, and Sikorsky's boss at United Aircraft (now
United Technologies) in Stratford, Conn., allowed him to begin experimenting
with his first love.
Sikorsky took a then-radical approach to helicopter design. In many ways,
he took a cue from the autogiro. Instead of tandem rotors that cancel
out torque, Sikorsky used just one main rotor for lift. This greatly simplified
the mechanism, and made controlling the craft through changing the pitch
of the blades much easier.
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| In the 1930s, autogiros thrilled air show crowds, such as this one at Washington Hoover Airport, but they never broke into mainstream aviation. |
To eliminate torquing, Sikorsky mounted a small rotor on the end of the
fuselage to act as a fan that blew counter to the rotating force of the
main rotor. (A 1920s Dutch design approximated some of this, but thanks
in part to mounting a separate engine to power the tail rotor, the craft
never flew more than a few feet off the ground.)
The first flight of Sikorsky's VS-300 was far from auspicious. On Sept.
14, 1939, the craft cleared the ground by just a few inches—almost
certainly due to the effect of the rotor blowing air downward—and
the whole event lasted 10 seconds. The engineers fixed a glitch that shook
the machine violently when the rotors whirred at speed and, by November,
short one-minute hops were possible.
During the shakedown period, the Sikorsky team made a number of adjustments.
The initial design called for feathering the blades of the rotor to change
the pitch, but this proved to be unworkable. After a crash in December,
two small horizontal rotors were attached to the fuselage to provide lateral
and longitudinal control.
Test flights through the spring and summer of 1940 helped Sikorsky and
his team better understand the aerodynamics of the helicopter. They also
began practicing some of the three-dimensional feats that make helicopters
so useful: landing on a dime, hovering over a single point, even throwing
down a rope ladder for a rescue.
Once the control problems were better understood, Sikorsky and his team
were able to eliminate first one and then both of the horizontal auxiliary
rotors, opting instead for changing the pitch of the main rotor to control
longitudinal and lateral motion. The modern helicopter was born.
The helicopter never achieved Sikorsky's dream of being the aircraft for
everyone. But maybe that's for the best. It's impossible, after all, to
imagine a morning commute with millions of whirlybirds blackening the
sky.