Looking for the slipping turn while hang gliding--overview

Aerial experiments: Looking for the "slipping" turn while hang gliding--overview

August 6, 2007 edition
Steve Seibel
steve at aeroexperiments.org
www.aeroexperiments.org

 

In hang gliding and triking circles, there is a widespread view that if a pilot enters a turn without adequately moving the control bar forward to increase the wing's angle-of-attack and "coordinate" the turn, the glider or trike will "slip" or "sideslip", moving sideways through the air as it slides earthwards.  If a pilot rapidly pulls in the control bar to decrease the wing's angle-of-attack while the glider or trike is banked, this is also widely thought to cause a sideslip. 

 

Soon after I learned to hang glide, I carried out some experiments to examine this idea.  I mounted a yaw string or telltale (i.e. a length of yarn) on the forward end of a dowel rod projecting forward from the center of the base bar of my glider.  (See photo).  Unlike the telltales that hang glider pilots often place high on the front side wires, I could easily view this yaw string in flight. 

 

I found that as I was rolling my glider into a left bank, the yaw string deflected toward the right.  This showed that the nose of the glider was yawing to the right in relation to the actual direction of the flight path and airflow (relative wind) at any given moment.  In other words, the glider was slipping.  The amount of deflection of the yaw string--i.e. the amount of sideslip--was strongly related to the roll rate.  The faster I rolled the glider into the turn, the more the yaw string deflected to the side.  I saw roughly the same amount of deflection of the yaw string--i.e. roughly the same amount of sideslip--regardless of whether I moved the control bar far enough to hold the airspeed constant as I rolled the glider into the turn, or I made no movement of the control bar in the fore-and-aft-direction (which caused the flight path to curve downward and the nose to drop and the airspeed to rise), or I pulled the control bar in as I rolled the glider into the turn (which caused the flight path to curve sharply downward and the nose to drop sharply and the airspeed to rise rapidly). 

 

In other words, I found nothing that supported the popular idea that a hang glider will "sideslip" excessively if the pilot fails to make an adequate pitch "coordination" input as he rolls the glider into a turn.

 

The "adverse yaw" effect that I observed as I rolled the glider into a turn was very similar to the "adverse yaw" effect that I've seen while rolling into turns in sailplanes and airplanes. The lack of any noticeable correlation between the quality of my pitch control inputs--i.e. whether I allowed the nose to drop or rise abruptly, or kept the glider's pitch attitude and airspeed roughly constant to produce a smooth flight path--and the amount of sideslip that occurred as I rolled the glider into the turn was also very similar to what I've observed in sailplanes and airplanes.

 

In a constant-banked left turn, I found that the yaw string deflected slightly toward the right of the centered position.  This showed that the nose of the glider was yawed to point slightly to the right of the actual direction of the flight path and airflow (relative wind) at any given moment.  In other words, the glider was showing just a few degrees of sideslip--much less than occurred while I rolled the glider into the turn.   I saw roughly the same deflection of the yaw string--i.e. roughly the same amount of sideslip--regardless of whether I was making pitch inputs that caused the glider to fly at a slow, constant, airspeed, or caused the glider to fly at a fast, constant, airspeed, or caused the glider to pitch up and rapidly bleed off airspeed, or caused the glider to pitch down and rapidly gain airspeed.

 

In other words, I found that a sustained, constant-speed, diving turn involved no more sideslip than a low-speed, efficient turn. I also found that an accelerating, diving turn involved no more sideslip than a low-speed, efficient turn.

 

Again, these dynamics are similar to what I've seen in sailplanes and airplanes, where a touch of inside rudder is generally needed during a constant-banked turn to eliminate the slight sideslip that would normally be present, and center the ball or yaw string. In sailplanes and airplanes this slight sideslip is usually much more pronounced in a low-airspeed constant-banked turn than in a high-airspeed constant banked turn--sailplane pilots are very familiar with the need to apply a rather significant amount of inside rudder to center the yaw string or ball during low-airspeed, thermalling turns. In high-speed aircraft this effect is often negligible. I wouldn't be surprised if the same correlation between airspeed and amount of sideslip in a constant-banked turn also exists in hang gliders in trikes, but I haven't been able to detect it in the yaw string experiments that I've conducted to date.

 

I found that as I was rolling my glider from a left bank back to wings-level, the yaw string deflected toward the left.  This showed that the nose of the glider was yawing to the left in relation to the actual direction of the flight path and airflow (relative wind) at any given moment.  In other words, the glider was skidding.  The amount of deflection of the yaw string--i.e. the amount of skid--was strongly related to the roll rate.  The faster I rolled the glider out of the turn, the more the yaw string deflected to the side.  I saw roughly the same amount of deflection of the yaw string--i.e. roughly the same amount of skid--regardless of whether I moved the control bar aft enough to hold the airspeed constant as I rolled the glider back to wings-level, or I made no movement of the bar in the fore-and-aft-direction (which caused the flight path to curve upward and the nose to rise and the airspeed to drop), or I pushed the control bar forward as I rolled the glider out of the turn (which caused the flight path to curve sharply upward and the nose to rise sharply and the airspeed to bleed away rapidly). 

 

I found nothing that supported the popular idea that a hang glider is much more prone to slipping than to skidding during dynamic maneuvers where the pitch attitude, angle-of-attack, and/or bank angle are rapidly changing. 

 

The "adverse yaw" effect that I observed as I rolled the glider out of a turn was simply the mirror image of the "adverse yaw" effect that I saw while rolling the glider into the turn, and was undoubtedly driven by the same physical forces. Again, this is very similar to the "adverse yaw" that I've seen while rolling out of turns in sailplanes and airplanes. In these aircraft, just as an ample amount of inside rudder is needed to keep the yaw string or ball centered and prevent a slip as the aircraft is rolling rapidly from wings-level into a turn, so too is an ample amount of outside rudder needed to keep the yaw string or ball centered and prevent a skid as the aircraft is rolling rapidly from a banked attitude back to wings-level. And again, the lack of any noticeable correlation between the quality of my pitch control inputs and the amount of adverse yaw that occurred as I rolled the glider into the turn was also very similar to what I've observed in sailplanes and airplanes.

 

Some of these experiments involved large aftward movements of the control bar, which unloaded the wing to a very low angle-of-attack, dramatically reducing the normal G-load on the pilot.  This was done while the glider was steeply banked.  In most cases this did not create a visible sideslip.  In some experiments involving exaggerated wingovers, where the airspeed was brought far below the normal 1-G stall speed as the glider "floated" over the top of the maneuver in a very steep bank with almost no G-loading, the yaw string did show some sideslip.  I think that in these extreme cases, the airspeed was so low that the aircraft's inherent yaw stability or "weathervane effect" became relatively ineffective and failed to keep the nose of the aircraft aligned with the actual direction of the flight path and airflow (relative wind).  But in most cases, moving the control bar sharply aft did not create a noticeable sideslip, even when the glider was steeply banked. 

 

These experiments were originally carried out in a Wills Wing Spectrum.  I've since observed very similar behavior in my Airborne Blade, both with the VG loose and with the VG tight, as well as in nearly all the other hang gliders that I've flown.   More recent tests were conducted in a Stealth KPL topless glider, where the results continued to be as described above, and in my Icaro Laminar R-12 kingposted glider, where the results were slightly different in some particular maneuvers (see the related article on this website entitled "Some notes on sideslip in an Icaro Laminar R-12 hang glider").

 

I submit that by moving the control bar forward while rolling into a turn, a hang glider or trike pilot prevents the flight path from curving downward into an accelerating dive.  I submit that--contrary to popular belief--this pitch "coordination" input has nothing to do with preventing a sideslip.  I suggest that adverse yaw is the primary cause of sideslip as a hang glider or trike enters a turn.   I also submit that hang glider pilots often mistake the sensations of a diving, accelerating turn--which involves a lower G-loading than would normally be associated with the glider's bank angle--for the sensations of a sideslip.  But simply reducing the G-loading will not create the sensation of a sideslip. In other words, simply reducing the G-loading will not make a pilot fall toward the low side of the cockpit or control bar, if the aircraft is not actually slipping sideways through the airmass. It's a misconception that whenever the G-load is "too low" for the bank angle, the force of gravity will "pull" the pilot toward the low side of the cockpit or control frame. For more on these ideas, see the related articles on this website entitled "You can't "feel" gravity!" and "Complete analysis of forces: fully balanced turn, turn with inadequate lift or G-load, slipping turn, non-turning slip, and skidding turn". (Links to these articles appear at the bottom of this page). Slips and skids are yaw-axis phenomena--they are caused by nothing more complicated than the nose of the aircraft being out of alignment (in the yaw axis) with the actual direction of the relative wind, i.e. with the actual direction of the aircraft's flight path through the airmass.

 

A few more words on adverse yaw: adverse yaw is known to have several different causes. In most aircraft, one of the most important causes of adverse yaw is the fact that the descending wing and the ascending wing each experience a twist in the local direction of the relative wind, compared to the direction of the relative wind experienced by the aircraft as a whole. This change in the direction of the local relative wind is caused by the rolling motion of the aircraft--because the descending wing is descending, its relative wind vector has more of an upward component than does the overall relative wind experienced by the whole aircraft. This change in the local direction of relative wind twists the descending wing's lift vector forward, creating a thrust component in relation to the reference frame of the whole aircraft. Conversely, because the ascending wing is rising, its relative wind has more of a downward component (or less of an upward component) than does the overall relative wind experienced by the whole aircraft. This change in the local direction of the relative wind twists the ascending wing's lift vector aftwards, creating a drag component in the relation to the reference frame of the whole aircraft. Naturally, these thrust and drag components create a yaw torque. Other significant causes of adverse yaw in most aircraft include the physical difference in the shape of the descending and ascending wings that arises (due to the deflection of the ailerons, or in a flex-wing aircraft, due to billow shift) when the pilot makes a roll control input. In a weight-shift controlled hang glider, we also have an additional cause of adverse yaw: whenever the pilot is making a weight-shift roll control input, we can visualize that not only does a difference arise in the cambered airfoil shape of the two wings, but one wing also becomes physically longer than the other wing, in relation to the CG of the whole system (including the pilot's body). The longer wing will tend to experience more drag than the shorter wing. Another effect that is not negligible is an aircraft's rotational inertia in the yaw axis. If an aircraft had a control system that was designed to create zero yaw torque (no adverse or "proverse" yaw) when the pilot made a roll input, the pilot would still see some slip while rolling into a turn and some skid while rolling out a turn, due to the aircraft's rotational inertia in the yaw axis. (This last point is only true for bank angles up to 60 degrees-as the bank angle is increased beyond 60 degrees, the yaw rotation rate needs to decrease, rather than increase, so the aircraft's yaw rotational inertia will tend to create a skid rather than a slip).

 

It is very easy to carry out the experiments that I've described here, and I strongly encourage any hang glider or trike pilot who is interested in these dynamics to conduct their own explorations of slips and skids while rolling into turns and out of turns, while making various kinds of pitch inputs. Seeing something for oneself is much more meaningful than reading about it! For more specific directions on how to construct and use a base-bar mounted yaw string probe to duplicate these observations, click here.

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