[aeroexperiments]

http://vimeo.com/39838913

Sideslip due to adverse yaw while rolling.

Watch the smoke and the yaw strings. Regardless of whether the bank angle is increasing or decreasing, the smoke and yaw strings blow toward the rising wing. This means that the glider's nose is yawed to point toward the rising wing, rather than staying precisely aligned with the actual direction of the flight path and airflow at any given moment. This is adverse yaw in action.

One large adverse yaw torque component is created directly by the rolling motion itself, independent of the deformation of the wing. Another adverse yaw torque component is created by the aileron-like deformation of the wing as the pilot makes each roll input.

These are good "coordinated" roll inputs, pitch-wise. Contrary to popular belief, pitch "coordination" inputs do little or nothing to prevent sideslip in hang gliders. Roll rate is the main thing that determines sideslip in hang gliders -- the higher the roll rate, the more the slip. For a given roll rate, pulling in the bar to make the glider dive as it rolls into the turn does not make any extra sideslip.

It is true that pulling in while rolling into the turn can increase the roll rate, and this will create more sideslip than would the same amount of sideways weight shift, but no pull-in.

The sideways airflow in a slip interacts with the anhedral wing geometry to create a helpful (proverse) roll torque, particularly at low angles-of-attack (high airspeeds) where the gliders overall "effective dihedral" (as determined by the combined effects of wing sweep, airframe anhedral, and sail billow) is most strongly negative. This is why pulling in the bar can boost the roll rate for any given weight-shift roll input.

During aerotow-- where the angle-of-attack is typically fairly low-- we can utilize this "negative effective dihedral" to create helpful roll torques by making wrong-way yaw inputs or at least allowing the glider to freely yaw in the "wrong" direction as we weight shift. We should avoid using "bicycle" or "shopping cart"-style roll inputs which inhibit the glider's tendency to adverse-yaw away from the direction of our roll inputs. (Videos to follow).

In free flight we have little power to yaw the glider directly, so we don't worry about yaw and just make whatever body motions shift our weight most effectively to the side --typically "bicycle" or "shopping cart" roll inputs as shown here.

Note that the direction of the yaw/ slip shown here is opposite to what we would predict if the "shopping cart" or "bicycle" steering inputs were really yawing the nose of the glider from side to side. For example, pilot's feet shift left, pilot's body is yawed to right relative to glider centerline, pilot's arms seem to be exerting a strong left-twisting bicycle torque on the control bar, but the yaw strings and smoke trail off to the right-- toward the rising wing-- showing that the nose is pointing to the right of the actual direction of the flight path and relative wind at any given moment. Not the left.

How can this be? Because the pilot's "shopping cart" or "bicycle"-style inputs are really exerting no direct yaw torque on the glider or on the pilot's body (except very briefly as the pilot's body is changing its yaw rotation rate in relation to the outside world). It is like flying an airplane or sailplane without using the rudder at all. Therefore we always see the aerodynamic adverse yaw torque created by our roll inputs, no matter how strongly we twist our body this way or that.

We know that we cannot be exerting a direct yaw torque on the glider, because if we were, the glider would exert the opposite direct yaw torque on our body, and we would start spinning round and round the hang strap at an ever-increasing (accelerating) rate. That would be a bad thing! (Videos to follow!)

How can we twist away on the bar without exerting any net yaw torque on the glider? Consider a left roll input where we "lead with the feet", shifting our feet far to the left. To accomplish this, we twist the bar to the left in a "shopping cart" fashion, pulling with our left hand and pushing with our right hand, which twists our shoulders to the right and our feet to the left. Twisting the bar to the left does indeed exert a left yaw torque component on the glider. However, we are also exerting a right yaw torque on the glider by pushing the bar--which is ahead of the glider's CG-- to the right as we shift our CG to the left. The net yaw torque we transmit to the glider with our arm muscles is zero, just as the net yaw torque that the glider exerts on us through our arm muscles is zero. The main benefit of the "shopping cart" style roll input is that it is an ergonomic, efficient, effective way to shift our CG far to one side. Note that since our feet can swing far to one side without hitting the down tube or rear wires, we can shift our weight further to the side in this fashion, than if we were keeping our body parallel to the glider centerline.

In the video above, note that the sideslips are not accompanied by a "falling" sensation of reduced G-loading. When the G-loading is "too low" for the bank angle and the nose pitches downward and the airspeed rises, this signals that the glider is in a diving, accelerating turn, not a sideslip. If the roll rate is zero, this maneuver will involve little or no sideslip. During a low-G diving turn, the pilot falls with the glider and experiences no tendency to fall toward the low side of the control frame.

For more notes on related dynamics, see the thread 'The towline gives us the power to yaw the glider + slip notes" http://www.hanggliding.org/viewtopic.php?t=25742

Steve

[timmay]

Great lesson, thanks!! i am starting to learn to aerotow soon, so this is critical info for me. During my lessons on the hill, i instinctively thought of a shopping cart when shifting weight.

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