The rudder as a roll control: aircraft with anhedral
This page was last modified on August 2, 2006
In "Oblique and side views of aircraft with anhedral", we saw how an aircraft with anhedral experiences an increase in the angle-of-attack of the "downwind" wing, and a decrease in the angle-of-attack of the "upwind" wing, and a roll torque in the "upwind" direction, whenever there is a sideways component in the relative wind or airflow over the aircraft.
Sequence of events for making a left roll input with the rudder on an aircraft with anhedral:
1) The pilot applies and holds right rudder
2) The nose of the aircraft yaws to the right in relation to the actual direction of the flight path and relative wind. There is little change in the direction of the flight path and relative wind at this point.
3) The relative wind now has a left-to-right component in relation to the aircraft. Due to anhedral, the "downwind" or right wing experiences a higher angle-of-attack than the "upwind" or left wing. Therefore the right wing develops more lift than the left wing. This creates a roll torque to the left.
4) The aircraft begins rolling toward the left.
5) As the bank angle increases, the increasing sideways component in the wing's lift vector causes the flight path to start to curve to the left. The sideways component in the wing's lift vector is the "centripetal force" that makes the flight path curve, just as the "centripetal" pull of the sun's gravity on the earth makes the earth follow a curving path through space.
6) As the bank angle approaches the desired "target" bank angle, the pilot relaxes most of his rudder input. For reasons that we'll explore elsewhere in this section, in an aircraft with anhedral it's likely that the pilot will have to hold a slight amount of rudder against the direction of the turn in order to keep the bank angle from increasing.
7) As the flight path continues to curve, the aircraft's inherent yaw stability or "weathervane effect" ensures that the aircraft's nose begins clocking steadily from right to left around the horizon, so that the nose points in approximately (though not exactly) the same direction that the aircraft is actually moving through the airmass at any given moment, regardless of the fact that the nose was initially yawed in the "wrong" direction, and regardless of the exact direction and amount of rudder input that the pilot needs to apply to keep the bank angle constant.
This aircraft was successfully flown with "backwards" rudder inputs serving as the sole means of roll control. The aircraft has lots of anhedral! The aircraft was quite unstable and I had to exercise vigilance to keep the bank angle rather shallow; sometimes steep banks produced spirals that could not be recovered from without also using the ailerons. Planform view of same aircraft (with wings adjusted to a flatter configuration).
Advance to "Definition of a 'positive coupling between slip (yaw) and roll'"
Skip to "'Conventional' use of the rudder"
Up to the Aerophysics Exploration Pages index
Up to the Aeroexperiments site map