Notes on sideslip in steady turns in flex-wing hang gliders
August 8, 2007 edition
Elsewhere in this section of the Aeroexperiments website, we note that flex-wing hang gliders slip while rolling toward a steeper bank angle and skid while rolling toward a shallower bank angle. These slips and skid are due to adverse yaw. See for example "Aerial experiments: Looking for the "slipping" turn while hang gliding--overview". In many gliders the degree of sideslip caused by adverse yaw during a rolling motion is not strongly affected by the pilot's pitch control inputs-- it's a myth that all hang gliders always experience more sideslip if they are allowed to dive and accelerate as they roll to a steeper bank angle, than if the pilot makes a pitch "coordination" input to hold the pitch attitude or airspeed roughly constant.
Once a hang glider is established in a steady, constant-bank turn at a constant airspeed, will it still be slipping? Might it be skidding?
In a rudderless aircraft we would expect the increased airspeed, and increased drag, experienced by the outboard wingtip during turning flight to cause the nose of the aircraft to point slightly toward the outside or high side of the turn. This is a slip. In most aircraft with rudders, the pilot will use a touch of inside rudder to counteract this effect and keep the nose of the aircraft aligned with the actual direction of the flight path and relative wind at any given moment. This is especially true in long-spanned aircraft flying at low airspeeds, where the difference in airspeed between the two wingtips will be most pronounced.
In hang gliders I've noticed that a yaw string mounted several feet in front of the base bar always blows slightly toward the outside or high side of a constant-banked turn. This is consistent with the idea that the glider is slipping.
In a turn, the relative wind curves to follow the circumference of the turn. This is a result of the fact that the aircraft is rotating as well as translating, and both the rotational motion and the translational motion affect the relative wind experienced by any given part of the aircraft. (For an extreme example, imagine a glider rotating like a pinwheel about it's CG with no forward motion--the relative wind would blow in a circle centered around the CG of the aircraft!)
Since the relative wind curves to follow the circumference of the turn, if we see a yaw string indicating some sideslip (relative wind blowing from the low wingtip toward the high wingtip) at the nose of the aircraft, we know there will be slightly less sideslip (relative wind blowing from the low wingtip toward the high wingtip) at the further aft portions of the aircraft.
To get a sense of the importance of this effect, we can draw a set of concentric circles and place a scale drawing of a hang glider on these circles. The distance from the hang glider to the center of the concentric circles should represent a realistic, scaled, turning radius. We can look at the way the curving arcs of the circle cross the hang glider and see how much curvature in the relative wind we expect to see across the physical dimensions of the hang glider (root chord, etc).
Is it possible that while a yaw string at the nose of a hang glider experiences a slipping component in a constant-banked turn, the majority of the wing experiences little or no slipping component in the airflow, due to curvature in the relative wind?
Here is the strongest evidence that the whole wing of a typical flex-wing hang glider experiences a slipping airflow during a constant-banked, constant-speed turn: with any flex-wing hang glider, increasing the length of the flying wires to decrease the airframe anhedral (or to create some airframe dihedral) makes the glider less spirally unstable. Likewise, decreasing the length of the flying wires to increase the airframe anhedral makes the glider more spirally unstable. The same is true when we change anhedral by adjusting "eccentric" fittings built into the leading edge. Clearly, anhedral contributes to spiral instability in a flex-wing hang glider and dihedral contributes to spiral stability, even during a constant-banked, constant speed turn. A wing's anhedral or dihedral geometry will not create any roll torque if the wing is meeting the airflow head-on with no slip or skid. The results described above only are possible if the wing as a whole is feeling a slipping airflow component (blowing from the low wing toward the high wing) during a turn, even when the turn involves a constant airspeed and bank angle.
We see the same dynamics in "conventional" 3-axis aircraft: so long as the pilot is not using any inside rudder to eliminate the slip that is normally is present during a turn, dihedral will create a stabilizing effect and anhedral will create a destabilizing effect. If the pilot applies just enough inside rudder to keep the wing as a whole aligned with the true direction of the relative wind, so that there is no slip or skid, then aircraft's dihedral or anhedral geometry cannot create any roll torque, and cannot influence the aircraft's spiral stability. If the pilot applies more inside rudder, so that the aircraft skids rather than slips, then any dihedral that is present in the wing will actually create a destabilizing roll torque that acts to increase the bank angle, and any anhedral that is present in the wing will actually create a stabilizing roll torque that acts to decrease the bank angle!
The fact that increasing the anhedral that is present in the wing of flex-wing hang glider increases the glider's spiral instability, demonstrates that the glider as a whole is feeling a "slipping" airflow during turning flight: there is a sideways component in the relative wind, blowing from the low wingtip toward the high wingtip. In other words, the aircraft as a whole is flying in a slightly "yawed" attitude, pointing slightly toward the outside or high side of the turn, in relation to the actual direction of the flight path and relative wind at any given moment.
Some hang glider pilots believe that when they need to make a strong rolling-out ("high-siding") control input, they are creating an adverse-yaw torque that makes the nose of the glider point toward the inside or low side of the turn (this would be a skid), which they believe allows the glider to turn "flatter" and more efficiently. I suspect that nearly all the links in this chain of logic are faulty. There may or may not be a very slight performance benefit arising from configuring a hang glider to require high-siding in a turn, but the glider is surely not skidding, and if a skidding turn were possible, it would surely not be efficient. The skid would only increase the drag and sink rate, for any given radius of turn.