Effect of pitch inputs on yaw coordination in a Schleicher Ka-6 sailplane
August 8, 2007 edition
The Schleicher Ka-6 sailplane has a fairly small fixed vertical fin, and seems to have significantly less inherent yaw stability ("weathervane effect") than most of the other sailplanes and airplanes I'm familiar with (Slingsby Swallow, Schweizer 2-33, Schweizer 2-22, Cessna 152, Cessna 172, Taylorcraft BC-12, etc.) This makes the Ka-6 an interesting subject for experimentation.
I've carried out some tests in a Schleicher Ka-6 sailplane to look at the effects of pitch inputs on yaw coordination. In general the results were as described in "Looking for a connection between pitch inputs and sideslips in sailplanes and airplanes--overview"-- adverse yaw from roll inputs was the main cause of sideslip, as revealed by the position of the yaw string and the slip-skid ball. As described in "Uncoordinated roll inputs in a Schleicher Ka-6 sailplane", in some cases adverse yaw arose from a displacement of the ailerons even when the bank angle was constant. In general, pitching the nose up or down, either while the bank angle was constant or while rolling into or out of a turn, had little effect on the position of the slip-skid ball. In general, pitch inputs could not be used to help center the yaw string and slip-skid ball in any given maneuver, especially if the ailerons were being used to any significant degree.
During exaggerated wing-overs where the glider "floated" over the top in a near-"weightless" condition, with the G-load near zero, in the absence of rudder input from the pilot the slip-skid ball did tend to fall toward the low side of the cockpit and the yaw string did tend to deflect toward the high side of the bank. I've seen this in other aircraft but it was more pronounced in the Ka-6.
In some experiments I put the Ka-6 in a constant-banked turn and then rather abruptly moved the control stick forward to "unload" the wing (making the flight path curve downward and the airspeed rise) or moved the control stick aft to "load up" the wing (making the flight path curve upward and the airspeed decrease). The stick movement did seem to create an immediate, very small (less than one-half diameter), but consistent, movement of the ball: the ball moved toward the low side of the cockpit when I moved the stick forward, and the ball moved toward the high side of the cockpit when I moved the stick aft. Presumably the yaw-string also moved in concert with the slip-skid ball, but my observations were confined to the ball.
I've not been able to detect this direct connection between pitch inputs and slip or skid in any other aircraft. It seems possible that the effect may exist in the other aircraft I've flown, but to a lesser extent than in the Ka-6, because the other aircraft have more yaw stability. Even in the Ka-6 these dynamics were very subtle and did not suggest that a pilot's pitch inputs had any practical effect on the aircraft's yaw coordination.
For reasons described elsewhere in the Aeroexperiments website, I do believe it is inaccurate and misleading to suggest that as a matter of general principle, an aircraft will slip whenever the vertical component of the lift vector is "too small" in relation to the aircraft's weight. Slips are not caused by an "imbalance between gravity and centrifugal force" as many flight training handbooks allege. Slips are caused by adverse yaw and other related factors, and to a lesser extent, in situations where the rate of curvature of the flight path is increasing in the yaw axis, by an aircraft's rotational inertia in the yaw axis, which makes the aircraft's heading "lag" behind the true direction of the flight path at any given moment. When the lift vector is "too small" in relation to the aircraft's weight the flight path will curve downward, and if the aircraft is banked this will contribute to an increasing curvature of the flight path in the yaw axis. However, "unloading" the wing also decreases the turn rate, which contributes to a decreasing curvature of the flight path in the yaw axis. When we consider all these factors together it is not at all obvious from theory alone whether "unloading" the wing of a banked aircraft should cause a slip, a skid, or neither.
Elsewhere in the Aeroexperiments website we present some arguments, based on the rate of change of yaw rotation, as to why we might actually expect to see a slight skid when we force the airspeed to rise during a constant-banked turn, and why we might actually expect to see a slight slip when we force the airspeed to decrease during a constant-banked turn. I've not actually detected these effects in any aircraft; these arguments are based on the final equilibrium condition of the aircraft and do not contradict the idea that we might see a very slight skid immediately after putting an extra load on the wing, or a very slight slip immediately after unloading the wing.
I still believe that it is generally inaccurate to suggest (as per Langeweische's "Stick and Rudder"!) that pitch inputs have a practical effect on yaw coordination, or that rolling an aircraft into a diving accelerating turn creates significantly more sideslip than rolling an aircraft into a turn while holding the airspeed constant. For a given roll rate, in most aircraft adverse yaw is more pronounced at low airspeeds than at high airspeeds.