"Canted Tabs" Attach To Ailerons
Engineers at Lehigh University say they have designed and
successfully flight-tested a new control device that a pilot can
use to tailor the lateral stability of aircraft.
Joachim Grenestedt, associate professor of mechanical
engineering and mechanics, designed "canted tabs" that are attached
to the ailerons.
Grenestedt, an aeronautical enthusiast who owns and flies his
own two-seater, said the tabs measure a few inches in length and
width. The tabs rotate around an aluminum tube that is inserted
into the aileron.
Three tabs were mounted on each aileron for the test flights,
which took place at the National Test Pilot School (NTPS) in
Mojave, CA. The tabs used in these tests could be adjusted in
flight by as much as 30 degrees.
When tested on an Aermacchi AM-3 "Bosbok"
observation-reconnaissance plane with 8-foot-long ailerons, the
tabs made the laterally unstable aircraft stable, Grenestedt
"We took an unstable aircraft, fitted it with the canted tabs
and made it stable. When the plane started to side slip, the tabs
applied force to the ailerons, causing the plane to bank, or roll,
and regain lateral stability."
The Aermacchi does not possess the ability, required of civilian
planes, to be rolled upright using the rudder alone.
"Normally, you roll an airplane upright using ailerons," said
Grenestedt. "If you lose the primary roll control, then you have to
use the rudder." The Aermacchi lacks this back-up feature.
Russ Stewart, a test pilot instructor with NTPS, said the canted
tabs enabled him to roll the Aermacchi using rudders only.
"The canted tabs actually made the plane handle properly," said
Stewart, who has more than three decades of test-pilot experience.
"When they started to deploy to about a third of their range, they
turned stick forces to the proper direction."
Stability is the tendency of an airplane to fly straight, level
and in an upright position. Positive lateral stability means an
airplane has the tendency, if disturbed, to return to its original
stable position without corrective action by the pilot. Neutral
stability means the restoring forces are absent and the plane
neither returns from its disturbed position nor moves further
Negative stability means the plane will tend to move further
from its original trimmed position, when disturbed.
In the case of the Aermacchi (file photo of type, below), the
canted tabs were used to increase lateral stability. However,
Grenestedt said the canted tabs could also be used to reduce
lateral stability and thus decrease Dutch Roll tendencies and stick
forces during cross wind takeoffs and landings. Dutch Roll is an
oscillatory motion, combining roll, slip and yaw. These
oscillations typically have a short period that can be a challenge
for the pilot to overcome. A moderate amount of Dutch Roll, while
not fatal, tends to provoke nausea, especially in passengers.
Grenestedt and Lehigh research scientist Bill Maroun modified a
set of conventional ailerons for the Aermacchi, and fitted them
with the canted tabs.
"We did not know exactly how the Aermacchi would perform," said
Grenestedt, who did all the design calculations with pencil and
paper. "But the canted tabs showed that they can fix lateral
"The largest benefit of the canted tabs may be to serve as a
cure for lateral stability deficiencies in existing aircraft,
reducing the need for extensive airframe modifications."
Stewart took the modified Aermacchi on two solo flights to
verify its safety and flutter resistance. He and Kent Nelson, an
NTPS flight test engineer instructor, completed four more flights
with instruments to measure linear and angular accelerations,
rudder and aileron deflections, stick force, air speed and
To measure lateral stability, the pilots used various maneuvers,
including steady heading side slips, rudder releases, bank-to-bank
rolls, and spiral stability flights. In particular, the steady
heading side slips showed the effect of the canted tabs. "Deploying
half rudder," said Stewart, "the canted tabs changed stick forces
from 1 or 2 pounds negative [implying negative lateral stability]
to up to 10 pounds positive."
Each test flight lasted about an hour and reached an altitude of
Grenestedt said the idea of mounting canted tabs on ailerons was
proposed by his friend Sven-Olof Ridder, a fluid dynamics expert in
Sweden who is well-known for his airplane and yacht designs.