Does adverse yaw strongly decrease the turn rate in flex-wing hang gliders?
August 8, 2007 edition
* In Uncoordinated roll inputs in a Schleicher Ka-6 sailplane, we describe a case where adverse yaw from the ailerons completely prevents the flight path from curving when we give a roll input with no rudder input.
* A hang glider is a flying-wing aircraft with no fuselage or fixed vertical tail. Therefore when a sideways component in the airflow impacts against the various surfaces of the aircraft, relatively little aerodynamic sideforce will be created. This means that when the pilot gives a left roll input and the nose of the glider adverse-yaws to point to the right of the actual direction of the flight path and relative wind, the left-to-right airflow component over the aircraft will generate relatively little aerodynamic sideforce to the right, because the airflow is not impacting against a fuselage, fixed vertical fin, etc. This means that there will be relatively little decrease in the turn rate.
* As the nose adverse-yaws in the "wrong" direction, the pilot might perceive a greater lag in the turn than actually exists. The flight path might be curving in the desired direction even as the nose yaws in the "wrong" direction with respect to flight path and relative wind, and even as the nose briefly moves the "wrong" direction with respect to the external world. However if the flight path really is curving in the desired direction, obviously the nose will soon have to start tracking around the horizon in the desired direction, even if the nose remains yawed or displaced to the side in relation to the actual direction of the flight path and relative wind at any given moment. If the nose moves in the "wrong" direction with respect to the external world, and then freezes on the new heading, obviously the flight path cannot curve very much. If the pilot sees that the nose is refusing to move in the right direction for many seconds, then the flight path cannot be curving as desired. But it is possible for the nose to briefly move from left to right in relation to the external world even as the flight path is beginning to curve to the left. For example if a given glider tends to show ten degrees of sideslip during a constant-bank turn, when the pilot initiates a left turn we might see the nose of the glider move 5 degrees to the right even as the flight path curves 5 degrees to the left. At this point the nose is pointing 10 degrees to the right of the actual direction of the flight path and relative wind, and as the flight path continues to curve to the left the nose can track from right to left around the horizon at a steady rate while continuing to point 10 degrees to the right of the actual direction of the flight path and relative wind at any given moment. (This is an oversimplified example-- in reality there will typically be much more slip, from adverse yaw, while the glider is rolling into the bank, than after the glider is reached the desired bank angle. The amount of slip seen once the bank angle is constant is typically quite small.)
* The fact that hang glider pilots generally prefer not to augment the yaw stability of their aircraft with a fixed vertical fin on the keel, suggests that adverse yaw is not very objectionable in hang gliders, at least in comparison to other penalties that may be incurred by flying with a fixed vertical fin.
* However the fact that hang glider designers have gone to considerable effort to find tip airfoils etc that minimize adverse yaw suggests that adverse yaw is somewhat objectionable in hang gliders!
* When I see my hang glider adverse-yaw, my overall sense is that this does indeed have some small negative effect on the turn rate!
* One way that adverse yaw creates an unfavorable effect is that it increases the speed of one wingtip and decreases the speed of the other wingtip in a way that creates an unfavorable roll torque. This effect will always be present and is independent of any considerations involving the wing's effective anhedral or dihedral geometry, or the effect of aerodynamic sideforce on the turn rate. However this effect is never so strong as to create a "wrong-way" roll response before the glider finally begins to roll the right direction. In fact the "twist" in the relative wind arising from the rolling motion itself may be the main cause of adverse yaw. The asymmetrical aerodynamic loading of the wing is another cause of adverse yaw.
* Caution: when we "mix and match" anhedral or dihedral effects, which depend on a sideways airflow over the wing, with effects arising from the difference in wingtip airspeed, which will be present whenever the nose is yawing from right to left or left to right in relation to the outside world, things get a bit complex. For example let's revisit the case we gave earlier where the nose of the aircraft swung briefly to the right even as the flight path began to curve to the left. In this example, the nose of the aircraft was pointing to the right of the actual direction of the flight path and airflow throughout the whole timeline of the example. More specifically, the sideslip angle steadily rose to a "plateau" of 10 degrees (left slip or right yaw) and then remained constant. If the aircraft had effective anhedral, the anhedral would have created a left (favorable) roll torque that steadily rose to a "plateau" value and then became constant. If the aircraft had effective dihedral, the dihedral would have created a right (unfavorable) roll torque that steadily rose to a "plateau" value and then became constant. However, the difference in airspeed between the two wingtips would have created a right (unfavorable) roll torque for just a few seconds as the nose of the aircraft initially swung to the right, and then would have created a left (favorable) roll torque which would have rose to a "plateau" value and then remained constant as the aircraft's yaw rotation to the left became constant. In other words we might expect to the difference in wingtip airspeeds to create an unfavorable roll torque in one direction as an aircraft first adverse-yaws, and then to create a favorable roll torque in the other direction as the nose begins moving in the "right" direction, even if the aircraft is sideslipping throughout the whole process. In other words the initial unfavorable roll torque created by the difference in airspeed between the two wingtips as an aircraft first adverse-yaws, is likely to be more transitory than the roll torque created by anhedral or dihedral as the aircraft sideslips. The entire adverse-yaw (sideslip) process is likely to last much longer than the initial wrong-way swing of the nose. We expect to see some sideslip from adverse yaw for as long as the aircraft's bank angle continues to increase.
* Any aircraft--even one with no fuselage--will be somewhat less efficient when flying sideways (unless the aircraft has a circular wing!)
* In "Experimental results and interpretation: yaw experiments with a controllable rudder and wingtip-mounted drogue chutes on flex-wing hang gliders" we describe observations of a slip-skid ball while forcing a hang glider to yaw by means of a controllable rudder and also by means of a wingtip-mounted drogue chute. For a given yaw or slip angle as indicated by the yaw string, the displacement of the ball was small, indicating that only a small aerodynamic sideforce was being created by the impact of the sideways component of the airflow against the various surfaces of the glider.
* This may have been less true in older gliders with lots of billow: if you drag an old billow-cruiser sideways through the air it would seem this would have to generate some significant amount of aerodynamic sideforce.
* See John K Northup's 1947 Wright Memorial Lecture for another reference to the low aerodynamic sideforce arising from slipping flight in flying-wing aircraft, and also to the small drag penalty arising from slipping flight in flying-wing aircraft.
* Adverse yaw may be more objectionable on the ground than in the air. In the air, as long as the flight path curves in the intended direction, it may not matter if the nose briefly swings a few degrees in the wrong direction. When the pilot's feet are still on the ground, if the nose swings the "wrong" direction in response to a roll input, this may make the glider harder to control in the wind.