Using the rudder as a roll control in 3-axis aircraft

Using the rudder as a roll control in 3-axis aircraft

August 8, 2007 edition
Steve Seibel
www.aeroexperiments.org

The rudder can be used as a roll control in many 3-axis aircraft. Often the pilot can trim the aircraft and the remove his hands from the stick and use the rudder as the sole means of control.

When a pilot removes his hand from the control stick and uses the rudder as the sole means of control, he will find that rapid increases in the bank angle are best avoided as they cause extreme downward curvatures in the flight path, leading to a very rapid gain in airspeed. Likewise, the pilot will find that rapid decreases in the bank angle are best avoided as they cause extreme upward curvatures in the flight path, leading to a very rapid loss in airspeed (and in extreme cases, possibly causing a tailslide). When the pilot is not making any pitch inputs, a given change in bank angle can be accomplished much more smoothly, and with much less downward or upward curvature in the flight path, by keeping the roll rate low.

This sheds some light on why we usually make a nose-up pitch "coordination" input to temporarily increase the angle-of-attack as we roll from wings-level into a turn, and why we usually make a nose-down pitch "coordination" input to temporarily decrease the angle-of-attack as we roll from a turn back to wings-level, especially when we are using a rapid roll rate, even if we don't have in mind any particular airspeed or angle-of-attack that we want the aircraft to end up at. We are "managing" the wing's total lift force so that the aircraft stays close to an equilibrium state, with the vertical component of lift remaining nearly equal to the aircraft weight, so that there is no abrupt upward or downward curvature in the flight path and no abrupt change in airspeed. The fundamental reason that the nose tends to rise rather dramatically whenever the bank angle is rapidly decreasing is that the excess airspeed and lift-force (G-load) from the steep bank do not bleed away quickly enough to prevent the flight path from curving upward. Likewise, the fundamental reason that the nose tends to drop rather abruptly whenever the bank angle is rapidly increasing is that the airspeed and lift-force (G-load) do not rise fast enough, in relation to the increasing bank angle, to prevent the flight path from curving downward. If there were no lag in the rise or loss of airspeed to accommodate changes in bank angle, these marked, temporary changes in the pitch attitude and flight path would not occur, and the aircraft would always be in an equilibrium state, even if we make rapid changes in bank with no matching changes in the angle-of-attack.

The rudder works well as a roll control in the Schleicher Ka-6 sailplane: the sailplane can be flown without using the stick at all as long as the bank angle is not allowed to get too steep and only gentle rates of roll are used. Note that as described in "Uncoordinated roll inputs in a Schleicher Ka-6 sailplane", it can be difficult to use the ailerons as the sole roll control in this glider: with no rudder "coordination" inputs, it's possible for the glider to "stick" in a banked, non-turning sideslip. This is not surprising. The same strong dihedral effect that allows a rudder input to create a strong roll torque, also means that it may be difficult to use the ailerons alone for roll control, because any adverse yaw from the ailerons will be converted into an unfavorable roll torque.

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