杏吧原创

Austin, Texas

IMAGINE a world in which autogyros ferry mail to and from city centres,
landing and taking off from the top of post office buildings. Large models carry
up to five people at a time, while newspapers use smaller versions to rush
photographers to where the news is happening. Sound far fetched? Well it
shouldn鈥檛 do, because this is a scene from America in the 1930s. And, if two
modern-day companies have their way, the scene is about to be replayed.

The autogyro is the strangest of flying machines. A free-spinning rotor
overhead gives it lift, while a propeller鈥攚hich these days is mounted on
the back鈥攄rives it forward. After its brief period of popularity in the
1930s, the craft largely vanished from the public eye in the 1940s after Igor
Sikorsky convinced the US Army to opt for his newly invented helicopter.
Helicopters could hover, which autogyros could not. And while helicopters were
not particularly fast鈥攖heir top speed today is only 400 kilometres per
hour鈥攊f the US Army wanted more speed it could buy aeroplanes instead.
Squeezed from both sides, the autogyro vanished into obscurity, surviving only
as an aviation curiosity and a way for amateur fliers to get their feet off the
ground.

Today, advances in technology and our understanding of aerodynamics are
helping the autogyro to fight back, according to Groen Brothers Aviation of Salt
Lake City, Utah, and CarterCopter of Wichita Falls, Texas. The companies are
working independently to add the final touches to their gyroplanes鈥攁s
autogyros are called today. By borrowing technology from helicopters and planes,
the companies are trying to build hybrid machines that will do everything a
helicopter can do, except hover, at a fraction of the price. They may even
outfly propeller-driven planes.

Helicopters are so highly prized because they can take off vertically, hover
and land vertically. But this aerial agility comes at a cost. The main motor
must provide both lift and propulsion, which creates massive complexity. (The
tail rotor simply ensures that the rotor turns and not the helicopter.)

Shock waves

Relative to the air, the advancing blade of a forward-moving helicopter
travels at a faster speed than a retreating blade, creating uneven lift. To
compensate for this, the pitch of each blade must change as it revolves. In
addition, the pitch of the blades must change continuously to increase lift or
to propel the craft along. The machinery to manage all this needs constant
attention. 鈥淔or every hour of flight of a modern helicopter you鈥檝e probably got
five hours of maintenance over the life of that thing,鈥� says Jay Groen, chairman
of Groen Brothers and an experienced helicopter pilot and engineer.

Helicopters also aren鈥檛 that fast, partly because their engines tend to be
big and heavy. Also, if a helicopter flew fast enough its advancing blades would
reach the speed of sound. Shock waves from local sonic booms would disrupt the
rotor鈥檚 ability to generate lift and could even damage the blades. These effects
place strict limits on helicopter designers.

Compared with helicopters, light aircraft are mechanically simple beasts.
Wings provide lift, and a propeller the thrust. And since they are more
aerodynamic than helicopters, they can fly faster. The only drawback is
that the lift generated by wings depends upon the speed of the aircraft. So to
maintain control, planes cannot fly too slowly and need runways to take off and
land.

Between these two extremes sits the gyroplane, with its unique mode of
flight. 鈥淭he rotor is actually tilted back slightly so that as you move forward
air flows up and through,鈥� says Jay Carter, president of CarterCopter. This flow
of air causes the blades to autorotate. Typically, the rotor is tilted at an
angle of 10 degrees, while the blades themselves are tilted about 3 degrees to
their plane of rotation.

As they sweep through the air, these blades generate lift just as an
aircraft鈥檚 wing does. Yet because the rotor is not powered, there鈥檚 no torque to
counteract, so no need for a tail rotor. This mechanical simplicity makes
gyroplanes easier to maintain than helicopters鈥攁nd hence cheaper to
operate. They are also safer if the engine fails. The free-spinning rotor still
generates lift, so the gyroplane drops to the ground like a sycamore seed.

The problem of balancing the lift of advancing and retreating blades is
solved by letting the rotor hub teeter about so the blades find their own
equilibrium. An advancing blade rises, reducing the lift it generates, while a
retreating blade descends, increasing its lift. Again, this mechanism is simpler
than the helicopter鈥檚. On many modern gyroplanes, the hub also replaces rudder
and ailerons. Simply tilting the hub changes the direction of flight.

Jumping gyros

The most straightforward way to take off in a gyroplane is to speed along a
runway. The rotor blades start to spin until they are turning fast enough to
raise the craft into the air. But another method lets the gyroplane take off
virtually vertically. The principle is to redirect the engine鈥檚 power from the
propeller to the rotor and spin it up so that it is revolving faster than it
would normally turn during flight. This is done with the pitch of the rotor
blades set at zero鈥攌nown as zero collective pitch. At the moment of
takeoff, power is switched away from the rotor and back to the propeller. At the
same time collective pitch is restored to normal. The spinning rotor lifts the
gyroplane into the air as the propeller begins to push the craft along.

While the gyroplane jumps into the air, it still needs some clearance because
it does not climb vertically. Groen, who has flight-tested prototypes of his
gyroplane, says that it climbs at least 17 metres in the first 90 metres of
flight. This is enough, he says, for taking off from the tops of most buildings.
CarterCopter鈥檚 machine (see
picture)
will also be capable of jump takeoffs,
but it hasn鈥檛 been tested yet. The company aims to test fly a prototype this
year.

During landing, a gyroplane imitates another flying machine that does
鈥渘o-roll鈥� touchdowns鈥攁 bird. A gyroplane pilot can descend steeply and,
just before landing, pull up the nose to increase the pitch of the entire rotor.
With the extra drag this creates, the craft can be at a dead stop when it
touches down.

For many modern gyroplanes, flown for fun around the world, all this is
pretty standard stuff. To lift gyroplanes out of the hobby market and make them
competitive with light aircraft and helicopters, something extra is needed. For
Groen Brothers, that something is better control of collective pitch. All
gyroplanes must be able to change collective pitch between two extremes in
order to perform jump takeoffs. But the Groen Brothers craft can alter pitch
smoothly between zero and eight degrees.

This technology, borrowed from helicopters, leads to important improvements.
During takeoff, the pitch of the rotor needs to be steep, to generate a large
amount of lift. But when a gyroplane is cruising, the pitch can be reduced to
minimise drag. This allows Groen鈥檚 machine to fly more efficiently, and so
faster, than other gyroplanes.

Groen expects the top speed of his three-seater to be 260 kilometres per
hour, with a maximum altitude of 5800 metres. Flight tests have been good enough
for the Shanghai Energy and Chemical Company to order 200 gyroplanes for an air
taxi service between China鈥檚 congested cities: a deal worth more than $40
million. Groen hopes to begin limited production this autumn.

At CarterCopter, the goals are more ambitious. The company aims to create a
craft that can outfly any propeller or rotor-driven aircraft. The CarterCopter
gyroplane has a more radical design than the Groen Brothers craft. For a start,
it has wings that do more than just provide stability during flight.

At low speeds the rotor provides most of the lift, as in other gyroplanes.
But as the craft flies faster, things change. 鈥淓ventually almost all of our lift
is provided by the wing and all the forward thrust by the propeller, so that the
rotor blade on top is essentially unloaded,鈥� says Carter. 鈥淚t doesn鈥檛 give any
thrust and almost no lift.鈥�

Plenty of drag

At high speeds, then, the rotor creates little lift but plenty of drag. To
combat this, CarterCopter would like to make the rotor鈥檚 aerodynamic profile as
small as possible. The collective pitch would be turned to zero and the plane of
the rotor set parallel to the direction of flight. This would slow down the
speed of rotation of the rotor and reduce drag. However, the rotor must be kept
rigid and stable, so its spinning cannot be completely halted.

Unloading the rotor in this way during high-speed flight is no easy task.
CarterCopter has a patent pending on a mechanism to change the tilt of the rotor
hub without the need for a cumbersome mechanism. In essence, the device changes
the way the rotor hub sits on its spindle. In the new design, the spindle
extends through the centre of a doughnut-shaped rotor hub and the hub then
鈥渉angs鈥� from it. This allows the plane of the rotor to be tilted or held
parallel with the direction of flight.

Even if the CarterCopter gyroplane can unload the rotor, its wings still
create drag. But here too, its designers have managed to keep it to a minimum.
Because the lift generated by wings increases with speed, and because the
gyroplane鈥檚 wings are needed only at high speed, its wings can afford to be
relatively small鈥攚hich means less drag.

Carter is optimistic that his modifications will allow the gyroplane to
travel at 800 kilometres per hour, and fly higher and climb more steeply than
any helicopter or propeller-driven plane. However, whether the real craft will
live up to Carter鈥檚 expectations is a matter of some speculation among aerospace
engineers. While various pieces of the design have been demonstrated in concept
or practice, getting them all to work together may prove too much.

鈥淲hat Jay Carter is trying to do is to come up with a new set of
trade-offs鈥攖he engine, the rotor, the propeller, the wing, and all
that鈥攖o get a different sort of aircraft,鈥� says Wally Acree, an aerospace
engineer with the rotary wing group at NASA鈥檚 Ames Research Center, California.
鈥淚 don鈥檛 think there鈥檚 anything on that aeroplane that hasn鈥檛 been done before,
at least conceptually . . . The challenge is in what I would call systems
颈苍迟别驳谤补迟颈辞苍.鈥�

Once the gyroplane designs are complete, they are not expected to be any
cheaper to buy than the aircraft they are intended to replace. But they should
score over their rivals in other ways. The CarterCopter machine, for example,
could well squeeze the market for small, business aircraft. It could be
especially attractive to people who travel between city centres.

Likewise, the biggest obstacle for many would-be helicopter owners is the
operating cost, says Groen. He hopes to drastically cut the cost of staying
airborne. Groen has already been inundated with calls from police forces up and
down the US that can鈥檛 afford helicopters or want to cut the cost of aerial
surveillance. 鈥淛ust for patrolling, a gyroplane is ideal,鈥� Groen says. Police
helicopters rarely hover, but tend to circle instead. Gyroplanes can 鈥渓oiter鈥� in
just the same way, he says.

Here, then, is a curious turn of fate. Fifty years ago, the rise of the
helicopter put paid to the gyroplane. Today, the soaring cost of flying a
helicopter could give the gyroplane a new lease of life.