Confusing terms-- "coordinated", "uncoordinated", and "slipping"

Last updated July 2, 2014

In the hang gliding context, I'd like to suggest that the best definition of a "coordinated" turn entry is one where the bar is moved forward enough to hold the airspeed roughly constant, or to allow the glider to gain airspeed only gradually, as the bank angle increases.

If the bar is not moved forward as the glider is rolled into the turn, the nose will drop and the flight path will curve downward and the sink rate will "spike" and the airspeed will increase rapidly and then tend to "overshoot" the "normal" or steady-state value for the bank angle or bar position at any given moment. A good pitch "coordination" input prevents all this.

A good pitch "coordination" input also "loads up" the wing promptly with the "correct" G-loading for the bank angle, so that the initial turn rate is higher than if we just wait for the glider to increase the G-loading in its own manner-- by increasing the airspeed.

Once the bank angle is constant, I suggest that the best definition of a "coordinated" turn is one where the airspeed is held constant, or is only allowed to increase or decrease gradually. Again, whenever the airspeed is rapidly increasing, the G-loading is "too low" for the bank angle, and the turn rate is low, and the flight path will curve downward and the sink rate will "spike" and the airspeed will then tend to "overshoot" the normal or steady-state value for the bank angle and bar position.

Along the same lines, I'd suggest that a well-coordinated roll-out of a turn is one where the nose is not allowed to "balloon" upwards into a "zoom climb", with a rapid loss of airspeed. When glider is rolled rapidly out of a steep turn, this will require that the bar be pulled aft.

These definitions aren't meant to say anything about where the bar is relative to trim or relative to the min. sink angle-of-attack, at any given moment. We're talking about changes in bar position here, not absolute bar positions.

I think it's best to avoid the practice of referring to any high-speed diving turn as "uncoordinated", even if the airspeed is constant rather than accelerating. Granted, one could argue that any pitch control input that is a routine part of a normal turn might be considered to fall within the realm of pitch "coordination". And our gliders trim to a lower angle-of-attack while banked and turning than while flying wings-level, so we'll often want to exert some steady forward pressure on the bar while turning, so that the angle-of-attack is at least somewhere in the ballpark of the min. sink angle-of-attack. If we're not doing this, is the turn "uncoordinated"? That's a rather subjective question.

Ultimately, one could argue that any time we make the glider do what we want it to do, we're doing a good job of "coordinating" pitch inputs with roll inputs. While it's useful to talk about "coordinating" pitch inputs with roll inputs, it's probably best to try to avoid describing any given maneuver as being "coordinated" or "uncoordinated" in some absolute sense-- the above definitions notwithstanding. This is particularly true in the context of multiple choice tests, etc, where we are striving for clarity and don't have room to give a lot of background explanation. It's better to describe exactly what the glider is doing in terms of airspeed, pitch attitude, etc, than to expect someone to understand what we mean by a "coordinated" turn or an "uncoordinated" turn.

Moving on to sideslip-- it's simply not the case that there's any corelation between diving and accelerating while turning, and sideslip. Sideslip in hang gliders is mainly driven by adverse yaw, which is an aerodynamic byproduct of rolling. When we are rolling-- when we are changing bank angle-- we are slipping or skidding, always. The nose yaws toward the rising wing, so that the descending wing is also "upwind" or "leading" wing, and the rising wing is also the "downwind" or "trailing" wing. When we are rolling to a steeper bank angle, we are slipping. When we are rolling to a shallower bank angle, it's more correct to say that we are "skidding"-- the nose yaws toward the rising wingtip, which is the wingtip on the outside of the turn. Our pitch "coordination" inputs don't minimize adverse yaw and sideslip. Pulling in the bar while turning doesn't make a glider slip sideways through the air. Pulling in the bar while rolling into a turn doesn't cause any extra sideslip, for a given roll rate-- though it may boost the roll rate and thus increase sideslip.

Confusion around this issue goes all the way back through the earliest days of hang gliding. While it's far too late to "nip this in the bud", we should stop perpetuating these misunderstandings. In the hang gliding context, "slipping" is not the opposite of "coordinated".

Be aware that "coordinated" is used in two distinctly different ways in the aviation world. One meaning is that the pilot is using all the available and relevant flight controls in a smooth and harmonized way to produce the intended result. The "intended result" often includes a constant altitude and/ or airspeed, and zero sideslip. The other sense is more narrow and specifically means that the pilot is using the rudder as needed, to prevent sideslip. These two different meanings are easily confused and confounded, to suggest that pitch control inputs may be having an important effect on sideslip. This is rarely the case in actual practice.

On the practical level, one problem with talking about "slipping" turns is that it may give the student the impression that there's some mysterious "other" thing that he or she has to worry about while turning-- some independent variable apart from airspeed, angle-of-attack, pitch attitude, and bank angle. In truth, turn "coordination" is simply all about recognizing that a rapid increase in bank angle will drive a rapid increase in airspeed, along with a marked downward curvature of the flight path and a "spike" in the sink rate and a lower-than-normal turn rate, unless we are also increasing the wing's angle-of-attack to "load up" the wing and increase lift. There's no mysterious "other" thing to worry about. It's basically just pitch control. For example, there's no benefit to flying with a yaw string to see when the glider is or is not slipping-- because we have no control over sideslip anyway, apart from going easy on our roll inputs to keep the roll rate low. Sideslip only imposes a mild penalty on a hang glider in terms of lost turn rate and increased drag, and it's a transitory phenomenon that vanishes whenever the bank angle is constant, except for a very slight residual slip that accompanies every constant-banked turn regardless of airspeed and bar position. It's not something that we benefit from worrying about!

Early in my own learning experience, I know I was overdoing my pitch "coordination" inputs out of fear of the glider failing to turn and "slipping" sideways toward the ground. I would have progressed more quickly toward a smooth technique if I understood that pitch "coordination" is really just about preventing a rapid change in airspeed, and the accompanying high sink rate-- the glider will turn as long as it is banked. It's not like a sailplane, where the wrong control inputs (failure to use the rudder) will create a true sideslip where the turn rate really might be close to zero.

On the less practical level, as long as we have a flawed understanding of what makes a glider slip sideways through the air, we'll continue to see articles in "Hang Gliding" magazine and other publications, attempting to describe the physics of complex subjects like lockout dynamics, the effects of a vertical fin, etc, that will be based on shaky foundations and come to incorrect conclusions.

See the related content on this website for more on these subjects.

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