The rudder as a roll control: aircraft with dihedral
This page was last modified on August 2, 2006
In "Oblique and side views of aircraft with dihedral", we saw how an aircraft with dihedral experiences an increase in the angle-of-attack of the "upwind" wing, and a decrease in the angle-of-attack of the "downwind" wing, and a roll torque in the "downwind" direction, whenever there is a sideways component in the relative wind or airflow over the aircraft.
Sequence of events for making a right roll input with the rudder on an aircraft with dihedral:
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 dihedral, the "upwind" or left wing experiences a higher angle-of-attack than the "downwind" or right wing. Therefore the left wing develops more lift than the right wing. This creates a roll torque to the right.
4) The aircraft begins rolling toward the right.
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 right. 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 lots of dihedral it's likely that the pilot will have to hold a slight amount of rudder in the direction of the turn in order to keep the bank angle from decreasing.
7) As the flight path continues to curve, the aircraft's inherent yaw stability or "weathervane effect" ensures that the aircraft's nose starts clocking steadily from left to right 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 exact direction and amount of rudder input that the pilot needs to apply to keep the bank angle constant.
The upper aircraft in this photo is designed to fly with the rudder as the sole means of roll control. Therefore the aircraft has lots of dihedral.
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