Fixing some questions/answers on the USHPA written exams

Last updated July 1, 2014

The following two questions and answers from the USHPA Hang 2 exam are misleading and confusing, and should be discarded:

12. As compared to a coordinated turn, a slipping turn:
a. is flown with a lower angle of attack
b. is flown with a higher angle of attack
c. should never be used by hang glider pilots

As a H4 pilot, I have no idea how to answer this question. I know that hang gliders slip primarily while rolling to a steeper bank angle, due to adverse yaw. I know that we get about the same amount of slip for any given roll rate, whether the angle-of-attack is low or high.

I also know that most hang glider pilots, even very advanced pilots, have no idea when a hang glider is slipping and when it is not. Hang gliders give very little cue as to when they are slipping, unless a pilot chooses to fly with a yaw string mounted where he can easily see it in flight. (Telltales on the sidewires don't cut it-- they are either too far to the side, or too high overhead, for a prone pilot to easily view in flight.)

I also know that there is really no practical reason for a hang glider pilot to concern himself with whether or not a hang glider happens to be flying with a bit of sideslip at any given moment.

Therefore there's really no way to ask a practical, sensible question about sideslip on a H2 exam-- or a H4 exam for that matter.

If by "a slipping turn" we mean "a diving accelerating turn", or clearer yet "a turn where the nose is dropping and the airspeed is rising", it's better to say that explicitly. Neither of these phrases are accurate descriptions of a slipping turn. But when people speak about "sideslip" in hang gliders, this often seems to be what they mean.

If by "a coordinated turn" we mean "a turn with a constant airspeed and pitch attitude", it's better to say that explicitly.

A possible replacement question might go something like this:

Q: What control inputs will make a hang glider accelerate and dive while entering a turn?
a) Not moving the bar forward while rolling into the turn, especially if the roll rate is high.
b) Moving the bar aft while rolling into the turn.
c) Either a) or b) above.

The correct answer is c.

The other problematic question:

13. To change a slipping turn into a coordinated turn you should:
a. either reduce your bank angle (roll out) or increase your angle of attack (push out)
b. either increase your bank angle (roll in) or decrease your angle of attack (pull in)
c. reduce your bank angle (roll out) and decrease your angle of attack (pull in)

Again, as a H4 pilot, I have no idea how to answer this question. Hang gliders slip mainly while rolling-- it's a transitory phenomenon. While turning with a constant bank angle, pushing out does nothing to reduce the small sideslip that we see in a constant-banked turn. Reducing the bank angle does reduce (but not eliminate) sideslip-- but only a very small amount of sideslip is present in the first place in a constant-banked turn, even if the bank angle is rather steep. Therefore c) would technically be the closest to a correct answer-- but we're kind of splitting hairs here, because a constant-banked turn involves very little slip regardless of the bank angle. One gets the impression that the author of the question is trying to describe a situation where the sink rate is higher than desired, or the airspeed is increasing and the flight path is curving downward because the vertical component of lift (plus the vertical component of drag) is less than weight. It's not clear which of these two situations the author has in mind, but a) would be the correct answer in either case.

A possible replacement question might go something like this:

Q: While turning, if the nose is dropping and the airspeed is rising, what control inputs will stabilize the glider's pitch attitude and airspeed?

The answers could be the same as given in the present question 13, with a) being the correct answer.

One problem with the questions as written is that they give misleading ideas about what causes sideslip in hang gliders. The other problem with the questions as written is that they suggest that a "coordinated" turn is the opposite of a "slipping" turn. "Coordination" is kind of a squirrelly concept in hang gliding. Our pitch "coordination" inputs have nothing to do with preventing sideslips. Rather, they relate to control of airspeed and G-loading (lift). We "coordinate" our roll inputs with pitch inputs to make the glider do what we want it to do. But there's no absolute and universal criteria for saying that one particular maneuver is "coordinated", while another maneuver is "uncoordinated". If we pull in for extra airspeed, and then move the bar forward enough to hold the airspeed exactly constant as we roll in to a turn, is that a "coordinated" application of pitch and roll? Most pilots would say "yes, of course". If we let the nose drop and the airspeed rise as we roll into a turn, rather than moving the control bar forward as we roll to "load up" the wing and hold the airspeed constant, is that a "coordinated" maneuver? That's more debatable-- perhaps it depends on what our intentions were, and what happens next. As we roll into a turn, if we let the airspeed rapidly build and then adjust the bar to "freeze" the airspeed exactly as it reaches some target value, that's not what we usually call a "coordinated" turn entry--we'll see a "spike" in the sink rate as the glider is rapidly accelerating-- but maybe the maneuver is in fact a fine example of "coordination" between pitch and roll inputs, if the pilot wanted to gain some airspeed quickly. What if we just yank the bar in to our waist and hold it there as we roll from wings-level into a turn? Now we can be sure that the sink rate and airspeed will both "spike" at some very high value before finally settling down at the "correct" or steady-state value for the bank angle and bar position. Most pilots would not call this a "coordinated" maneuver. Still, there's no absolute, widely accepted criteria for saying that any given maneuver is perfectly "coordinated". If we want to define such a criteria, the most logical choice would be to say that a perfectly "coordinated" turn entry is one where we move the bar forward as we roll into the turn, at just the right rate such that the airspeed stays exactly constant. This also means that we are "loading up" the wing in the "correct" relation to the increasing bank angle, rather than allowing the lift force (G-loading) to stay low until the airspeed rises. But if we allow a slight increase in airspeed as we roll into the turn, does the maneuver suddenly become "uncoordinated"? That's a bit of a stretch. And here's another source of confusion-- imagine that a glider is flying at a steep, constant bank angle, with the bar well pulled-in, and the airspeed and sink rate both quite high, but constant. Is this a "coordinated" turn? I would say so-- the airspeed is constant, which means that the wing must be "loaded up" with the correct G-load for the bank angle-- but some pilots would say "no". In fact, it seems that in questions 12 and 13 above, the author may have been using "slipping" versus "coordinated" simply to mean "higher sink rate" versus "lower sink rate", never mind the nuances of whether the airspeed is constant or rising, and the G-loading is "correct" for the bank angle or a bit too low. I can't quite tell from the questions. In short, talking about a "coordinated turn" may seem to be a handy shortcut, but it really just opens up a whole barrel of confusion. We can talk about "coordinating" roll inputs with pitch inputs, but we really shouldn't be asking pilots to judge whether any particular maneuver is "coordinated" or "uncoordinated" in some absolute sense, especially in the context of a multiple-choice question on a written exam, where we won't be giving a lot of background explanation and we want things to be as clear as possible. There shouldn't be questions about "coordinated turns" on the written exams.

It's true that in aviation, "coordination" is often used specifically to mean "the absence of sideslip". But that's not the way we use it in hang gliding. That kind of "coordination" requires a rudder. Our kind of "coordination" deals with the pitch axis-- this is the kind of "coordination" that an airplane pilot accomplishes by moving the control stick or yoke forward or aft, to control the airspeed and G-loading and make the aircraft do what he wants it to do. When we try to compare and contrast "coordinated turns" with "slipping turns", we perpetuate the old misconception that our pitch control inputs have something to do with preventing sideslips. They don't. Sideslip isn't cause by a "shortage of lift" or a "lack of G-loading". Sideslip is caused mainly by adverse yaw generated by rolling.

It's a misconception that sideslip in hang gliders is associated with flight at high airspeeds or low angles-of-attack. It's a misconception that hang gliders show an increased amount of sideslip if we pull in the bar while turning, or while rolling in to a turn. If we pull in the bar while holding the bank angle constant, we see little or no increase in the very slight amount of sideslip that is always present in a constant-banked turn. If we pull in the bar while rolling in to a turn at some given constant roll rate, we see little or no increase in the adverse yaw and sideslip caused by the rolling motion. If we pull in the bar while rolling in to a turn with some constant amount of sideways displacement of our body (weight shift), the decrease in angle-of-attack does tend to cause a boost in roll rate, which does increase sideslip. While banked, if we start with the bar at trim and then pull the bar briskly aft, and we don't make any weight-shift roll input in either direction, we'll actually make the glider roll toward wings-level and on into a turn in the opposite direction. (We won't take the space to explain here why this is so-- but try it sometime!) As the glider is rolling toward wings-level, it is not slipping at all-- it is actually skidding, or moving slightly sideways through the air in the direction of the high, but descending, wingtip. Again this is the result of adverse yaw associated with a rolling motion-- while rolling, the glider flies through the air in a yawed attitude, so that the rising wingtip is also the "trailing" or "downwind" wingtip, and the "descending" wingtip is also the "leading" or "upwind" wingtip. This is true regardless of whether we are rolling toward a steeper bank angle (causing a slip toward the low, descending wingtip), or rolling toward a shallower bank angle (causing a skid toward the high, descending wingtip).

Some of this content is probably new to most readers-- including some very experienced, competent hang glider pilots. Obviously one doesn't need to completely understand sideslip dynamics in order to fly a hang glider well. In a rudderless aircraft that is able to turn just fine even while experiencing some sideslip, there's little practical application of this knowledge. That's why there's no reason to put a question about "sideslips" on the written exams. But if we are bound and determined to have one, how about this one:

Sideslip in hang gliders is:
a. something that happens whenever the vertical component of lift plus the vertical component of drag is less than weight
b. something that happens whenever the lift force (G-loading) is "too low" for the bank angle
c. something that we can control by making the right pitch "coordination" inputs
d. a, b, and c above
e. a transitory byproduct of rolling motions, that is difficult to detect without instrumentation and that we don't need to worry about

The only correct answer is "e".

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