Some notes on sideslip in an Icaro Laminar R-12 hang glider
August 8, 2007 edition
In a related article on this website entitled "Looking for the "slipping" turn while hang gliding--overview", I describe experiments I've carried out in a wide variety of hang gliders to look at sideslip. Most of these experiments involved a yaw string (telltale) mounted on a dowel rod projecting forward several feet from the centerline of the control bar, so that the yaw string was in the pilot's normal field of vision during flight. In general I found that the main cause of sideslip appeared to be adverse yaw during rolling motions: while rolling from wings-level into a bank, the yaw string deflected toward the rising wing, indicating a sideslip. While rolling from a bank back to wings-level, the yaw string again deflected toward the rising wing, which in this case indicates a skid. Pulling in the bar while rolling the glider into a turn did not create any more sideslip than I saw when I let the bar out to hold the airspeed constant as I rolled the glider into the turn. During a constant-banked turn the yaw string was slightly deflected toward the high wing, showing a slight sideslip, and this sideslip was not more pronounced when the airspeed was high and constant than when the airspeed was low and constant. Also, abruptly pulling in the control bar to make the airspeed rise while holding the glider in a constant-banked turn did not create a sideslip.
On my Icaro Laminar R-12 I've found the dynamics to be as described above, except for one exception. I have found that when I start from wings-level flight at a low airspeed (high angle-of-attack), and then I pull in the control bar abruptly while rolling the glider briskly into steep turn, this will create a very pronounced sideslip-- more so than if I hold the bar position constant in the pitch direction as I rolled the glider into the turn, or if I let out the bar to hold the airspeed roughly constant as I rolled the glider into the turn.
It appears to me that this pronounced sideslip is also accompanied by a much higher than usual roll rate.
I did not notice this tendency to slip when pulling in the bar while rolling into a turn while flying another high-performance flex-wing hang glider: an Aeros Stealth KPL.
Here is what I believe is happening during this maneuver: as described elsewhere in this website, on a wing with both sweep and anhedral, the wing's dihedral-like characteristics will be strongest at high angles-of-attack and the wing's anhedral-like characteristics will be strongest at low angles-of-attack. In other words at low angles-of-attack (with the bar well pulled in) the wing behaves as if it has more anhedral than at high angles-of-attack (with the bar let out). Also as described elsewhere in this website, on a wing with effective anhedral rather than effective dihedral, adverse yaw actually creates a helpful or "proverse" roll torque that tends to increase the roll rate. Also as described elsewhere in this website, any rolling motion tends to create adverse yaw due to resulting "twist" in the direction of the relative wind. I believe that when I pulled in the bar to decrease the wing's angle-of-attack at the same time that I rolled the glider into a bank, I created a feedback loop: the wing's effective anhedral was increased to the point where the adverse yaw from the rolling motion created a significant proverse roll torque, which boosted the roll rate, which created still more adverse yaw, which created still more proverse roll torque, which further boosted the roll rate, and so on and so forth.
I believe that the unusually high roll rate was the fundamental cause of the increased sideslip during this maneuver. I believe that the increased effective anhedral associated with the low angle-of-attack strongly contributed to the unusually high roll rate.
I'm not sure whether or not we see the same dynamics when we start the rolling motion with control bar already well pulled in. Perhaps there is less adverse yaw in this situation.
It's clearly not the case that a hang glider pilot's pitch inputs never have any influence on sideslip. I still feel that in most cases with most hang gliders, our pitch inputs do have little or no detectable influence on sideslip, especially if we are talking about a situation where the bank angle is being held constant or is being allowed to increase at some constant rate. The idea that a glider will "slip" if the airspeed is allowed to rise as the glider is rolled from wings-level into a turn, or as the glider is flown in a constant-banked turn, dates back to the earliest days of hang gliding and is not something that arose with the only advent of gliders with a great deal of airframe anhedral, like the Icaro Laminar. It appears to me that most hang glider pilots have difficulty distinguishing whether or not a glider is actually slipping sideways through the air, and tend to assume that a marked sideslip must be taking place whenever the glider is banked, the G-loading is "too low" for the bank angle, the flight path is curving downward, and the airspeed is rising. I think it's a misleading practice to use the phrase "slipping turn" to describe any turn where the airspeed is rising and the flight path is curving downward because the pilot has not made the usual pitch "coordination" input.
In the particular case of gliders like the Icaro Laminar where pulling in the bar while rolling can create a marked increase in sideslip, I don't really feel that this sideslip can be used in any practical way (e.g. to increase drag to steepen the glider path for a spot landing or to avoid "cloud suck"). A flying-wing aircraft will experience only a relatively small increase in drag during a sideslip. The main reason the flight path can be quite steep during this maneuver is that the glider is exchanging potential energy for kinetic energy, not because the glider is losing a lot of energy to drag as it accelerates and/or sideslips. If I wanted to maximize the drag of my glider I would bank it steeply and keep the airspeed high, rather than trying to take advantage of repeated "slips" or repeated rolling maneuvers or repeated accelerations from a low airspeed to a high airspeed or repeated maneuvers involving an aftwards movement of the control bar.