not by the seat of his pants
By Kevin Kochersberger
On Dec. 17, 2003, spectators
celebrating the centennial of flight at Kitty Hawk will watch a 750-pound
aircraft take off under its own power on a controlled flight. The pilot
on board will be exercising an entirely different type of control from
that of other aircraft.
As one of four pilots in training for the centennial celebration on December
17, I've flown the Wright Experience 1902 glider in preparation
for powered flights on the 1903 reproduction machine. The glider's
controls are similar to those of the powered Wright Flyer. A hip cradle
operates the wing warping system and connects with the rudder, while a
hand lever moves the canard.
The design established the 1902 glider as the first three-axis controlled
machine and formed the basis of the Wrights' aircraft patent.
Lying prone, a pilot must arch his back to see forward, a position that
rapidly grows uncomfortable.
My first few flights under tow revealed this aircraft's lack of
control harmony. Little balance exists between the sensitivity of the
longitudinal control—the canard—and the sensitivity of the
lateral control—the wing warping.
The canard almost twitches under a pilot's input, partly due to
the 1:1 ratio between hand rotation and the rotation of the 15-sq.-ft.
surface. Care is needed not to over-control the aircraft in pitch, a problem
that the aircraft's neutral-to-unstable pitch characteristics exacerbate.
The glider has very little pitch inertia as well, making changes in pitch
attitude rapid and at times unexpected.
The hip cradle that controls wing warping challenges the coordination
of the pilot, who must struggle to maintain the aircraft's pitch
at the same time. Compared to the craft's pitch control, the roll
rate is much more predictable and stable, about 11 degrees per second
at full deflection.
This rate is respectable for an aircraft that flies at 24 mph. In even
the lightest winds, though, roll power can suddenly disappear. At high
angles of attack, yaw power diminishes to the point that coordinated turns
without slipping sideways are impossible.
 |
| The author, like the Wrights, must learn to glide before he can fly. |
A 4-mph gust made the towed glider hard to control in pitch and roll.
This airplane exhibits less stability at low angles of attack than it
does at high angles of attack, making recovery from pitch-down excursions
difficult. Variable stability is uncommon in most aircraft, but shows
up in all of the Wright biplane, canard-configured designs. The 1903 machine
is no exception. Since it flies at lower angles of attack than the glider,
it tends to have a constant level of instability over its operating range.
Over the years, this instability has grabbed the attention of the flight
dynamics experts, who have published several predictions on the performance
and flight handling characteristics of the Flyer. Flying and handling
qualities can be related to the industry-standard Cooper-Harper scale,
which allows pilots to rate an aircraft's behavior on a numerical scale.
Recent predictions of the Flyer place it at 9.5 on a scale of 1 to 10—almost
unflyable.
Yet, the Wright aircraft were not designed to accommodate just any pilot.
With over 1,000 glider flights completed by the time they were ready to
fly the powered machine, the Wrights knew how to handle unstable aircraft
equipped with hip cradles for wing warping. Their designs connected the
pilot to the control system seamlessly; the airframes formed a physical
extension of the pilot. An evaluation of the Flyer must consider the pilot-in-the-loop,
and this makes an unstable airplane controllable.
The Wright brothers separated themselves from their peers in the quest
for powered flight by focusing on control as a necessary subsystem of
a successful flying machine. The brothers saw beyond the lifting surfaces
of Lilienthal and Chanute, which were controlled only through pilot weight
shift, to three-axis, fully controllable aircraft.
They defined, for the first time, the role of the pilot as a manipulator
of aircraft. They added pilot training to the mix.
Today, pilot training teaches the aviator what to expect from most airplane
designs. These stable aircraft fly hands off, predictably, and controllably
in turbulence. They have adequate power for maneuvering in variable winds.
The 1903 machine demands a different approach, with aircraft-specific
training that includes low power, ground effect, and instability. For
an airplane designed to fly a few hundred feet, the requirements are not
insurmountable.
Author Kevin Kochersberger, an ASME member and associate
professor of mechanical engineering at Rochester Institute of Technology,
designed the wind tunnel support and power systems for the Wright Experience
1903 Flyer replica, and ran the wind tunnel tests on the two glider reproductions.