Notes for new hang glider pilots-- on turn “coordination”-- shorter version
Last updated July 1, 2014
What do we mean when we talk about turn "coordination" or pitch "coordination" in hang gliders and other aircraft?
When we quickly roll the glider from wings-level into a steep turn, it’s much as if we have suddenly pulled the control bar aft. In either case, the vertical component of lift (plus the vertical component of drag) has suddenly become much less than weight. So the glider “falls”-- the flight path curves downward.
More specifically, the flight path curves downward to an angle that is steeper than the “normal” glide path for the bar position and bank angle. The sink rate “spikes” at an unusually high value, and the airspeed rises and “overshoots” the normal value for the bar position and bank angle.
We can prevent these dynamics if we move the bar forward to "hold the nose up" as we roll into the turn. That's what we mean by pitch "coordination". We simply ease the bar aft to slowly gain some extra airspeed before we enter the turn, and then as we roll into the turn, we ease the bar forward again to hold the airspeed roughly constant as the bank angle is increasing. The faster we roll into the turn, the faster we'll need to move the bar forward if we want to "hold the nose up" and keep the airspeed roughly constant.
The reason that the airspeed is staying roughly constant is that we're making a substantial increase in the wing's angle-of-attack, and this keeps the vertical component of lift (plus the vertical component of drag) roughly equal to weight, even as the bank angle is increasing. So the glider doesn't "fall" towards the earth.
The same pitch "coordination" input that holds the airspeed nearly constant, also boosts the turn rate. As we're boosting the vertical component of lift to keep it nearly "in balance" with weight, so too are we boosting the horizontal component of lift that drives the turn. We're promptly "loading up" the wing with the appropriate G-loading or lift force for the bank angle. When we hold the nose up and keep the airspeed nearly constant with a good pitch "coordination" input, we "carve" out a much nicer, quicker turn.
The dynamics are most important when we are making a large, rapid increase in bank angle. It's when we rapidly roll the glider through a large increase in bank angle without making any pitch "coordination" input, that we really cause a large imbalance between the vertical component lift (plus the vertical component of drag) and weight. This is like pulling the bar briskly aft from trim to well pulled-in. The flight path curves sharply downward, the sink rate "spikes", the turn rate is lower than it ought to be, and the airspeed builds rapidly and then tends to "overshoot" the "normal" value for the bank angle and bar position.
All these dynamics play out "in reverse" when we roll toward wings-level. If we rapidly roll from a steep-banked turn to wings-level, the glider retains excess airspeed and kinetic energy from the turn. As the bank angle decreases, the vertical component of lift (plus the vertical component of drag) becomes larger than weight, and the nose rises as the flight path curves upward into a "zoom" climb. Then the airspeed bleeds off and hits a low point that is well below the "normal" airspeed for the bar position and bank angle. In an extreme case, the glider may run out of airspeed in a nose-high attitude, and be at risk of a tailslide, whipstall, or tumble. We can limit all this simply by pulling the bar aft as we roll toward wings-level, so that the airspeed remains constant or bleeds off only gradually. We're controlling the exchange between kinetic energy (airspeed) and potential energy (altitude).
If you are flying in a steep turn with the bar well pulled-in, never begin an aggressive roll toward wings-level! You won't have enough room to pull the bar further in to keep the nose from rising dramatically. Conversely, in any situation where the nose is rising rapidly and the airspeed is bleeding away, you can use roll as well as pitch to bring the nose down. Shift your weight toward the low wingtip (or toward either wingtip if unbanked), as you pull in the bar.
The kind of turn "coordination" we've been discussing so far deals with keeping the glider "in balance", or nearly so, while the bank angle is changing. There's another aspect of pitch "coordination" that deals not so much with transitions, but with stabilized flight. The bar trims further aft in a banked turn than in wings-level flight. Not only that, but a given bar position commands a lower angle-of-attack in a banked turn than in wings-level flight.
The reason for this is that the flight path is curved in the turn, and so is the relative wind or airflow. This curving airflow "pushes up" against the rearmost parts of the glider, which pitches the nose down, so that the average angle-of-attack of the wing is reduced.
This doesn't fundamentally change anything about the pitch "coordination" inputs we make while rolling-- it just makes them more important.
Once we're established in a turn-- no matter how we got there-- we'll need to have the base bar further forward than we'd have it in wings-level flight at the same angle-of-attack. For example, let's say we've trimmed our glider to fly hands-off at the min-sink angle-of-attack in wings-level flight. If we are executing a ridge-soaring turn or a thermal circle in reasonably smooth air, we may wish to keep the glider near the min. sink angle-of-attack in the turn. Before entering the turn, we'll pull in for some extra airspeed. As we roll into the turn, we'll move the bar forward to keep the nose from dropping and hold the airspeed roughly constant-- we're "coordinating" the turn entry. And where should the bar end up once we're established in the turn? Several inches forward of its trim position in wings-level flight. Left to its own devices, the bar will tend to do the opposite-- it will trim several inches aft of the wings-level trim position. So we'll need to exert some forward pressure on the bar if we want to keep the glider at the min. sink angle-of-attack in the turn. We'll end up with a bit more airspeed than we had when wings-level at the same angle-of-attack, because the glider is carrying more load in the turn. To verify that we're getting it right and keeping the wing near the min. sink angle-of-attack, we can watch the shadows of the telltales on the top surface of the wing, about a third of the way out on the trailing edge. When they start to reverse, we're near the min. sink angle-of-attack.
To be on the conservative side, some instructors advise newer students not to exert any forward pressure on the bar while turning. In this case, you can still "coordinate" your turns-- you can still pull in for some speed, and then let the bar come back out to stop the nose from dropping excessively as you roll into the turn-- but you'll end up flying at a lower angle-of-attack than you had in wings-level flight at trim. Of course, in truly rowdy air, a pilot might choose to thermal with the bar well pulled-in, for protection against tucking or tumbling.
Don't get bogged down in trying to define the difference between a "coordinated" and an "uncoordinated" turn. There's no universally accepted definition of these terms-- they mean different things to different people in different situations. Ultimately, in some sense we're "coordinating" our pitch and roll inputs if the glider is responding in the way we want it to. But I would suggest that any turn entry, constant-banked turn, or turn exit where the airspeed is constant or changing only gradually, should be called "coordinated". To me, an "uncoordinated" turn entry or constant-banked turn is one where the airspeed is rapidly increasing. An "uncoordinated" roll-out is one where the glider has "ballooned" into a "zoom climb" and the airspeed is rapidly decreasing.
Spelled out in this much detail, these ideas may seem impossibly complicated. There’s really not that much to it-- we usually move the bar forward when we are increasing the bank angle, and we usually move the bar aft as we are decreasing the bank angle. This avoids rapid changes in the airspeed, and yields a smooth flight path and a good turn rate. For many pilots, these inputs are so intuitive that they are not even really aware that they are making them at all. That’s one reason why you’ll get a lot of different opinions if you ask people about turn “coordination”.
A memory aid taught by some instructors is "LAST"--
* Look-- where you are going to turn
* Airspeed-- pull in for airspeed
* Shift-- shift your weight to the side
* Trim-- let the bar come out toward trim as the glider rolls into the turn
The resulting motion of the bar is somewhat of a "J" shape-- after pulling in, the pilot initially shifts mainly sideways. As the bank angle increases, the pilot stays to the side but moves the bar progressively further forward, to stop the nose from dropping. As the glider reaches the target bank angle, the pilot lets himself come back to the glider centerline, while keeping the bar at whatever fore-and-aft position keeps the glider at the desired airspeed. As noted above, to keep the glider anywhere near the wings-level trim angle-of-attack, the pilot will have to exert some forward pressure on the bar in the turn, enough to keep the bar forward of the wings-level trim position.
Please note that in an "uncoordinated", diving, accelerating turn, the glider is not necessarily "slipping" sideways through the air toward the low wingtip. Most of what you will read and hear about "sideslip" in hang gliders is not accurate. "Coordinated" and "slipping" are not opposites, in the hang gliding context. You don't need to worry about preventing the glider from "slipping" sideways through the air as you maneuver around the sky.
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