Notes for new hang glider pilots-- on turn “coordination”
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.
Eventually the glider will find its way back to equilibrium at the “correct” glide path, airspeed, and sink rate for the bar position and bank angle. Alternatively, with a well-timed pitch input to lift the nose and pull the glider out of the steep dive, we can “freeze” the airspeed just as it hits the “right” value, and prevent the “overshoot”.
But it's much smoother to avoid the dive entirely. 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" sharply earthwards.
It's ok if the glider accelerates a little bit as it is rolling. We just get a much smoother flight path, without the excessive downward curvature and the "spike" in sink rate, if we gain airspeed gradually rather than rapidly. With a gradual acceleration, the glider stays much closer to a steady-state or "balanced" condition, flying at the "normal" glide path, sink rate, and airspeed for the bar position and bank angle at any given instant in time. The point of our pitch "coordination" input is to make the glider change airspeed gradually, not rapidly.
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. In contrast, when we let the nose drop and the flight path curve downward, the G-load or lift force doesn't build until the airspeed rises. As long as the flight path is curving downward, the G-load or lift force is lower than "normal" for the airspeed and 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.
We're not suggesting that the pilot needs to worry about controlling the turn rate independently of controlling the glider's pitch attitude and airspeed and bank angle. As long nose isn't dropping sharply and the airspeed isn't rapidly increasing, the glider is appropriately "loaded" for the bank angle, and the turn will proceed at the “normal” rate for the bank angle and airspeed.
The dynamics are most important when we are making a large, rapid increase in bank angle. If we keep the roll rate low, we can make a large increase in bank angle without throwing the glider "out of balance". This is like very slowly easing the control bar from trim to well aft, in wings-level flight. The glider will pick up speed, but gradually. There won't be any excessive "spike" in the sink rate or excessive downward curve in the flight path, and there won't be any tendency for the airspeed to "overshoot" the "normal" value for the bar position and bank angle. The flight path will smoothly curve down to the "normal" glide path for the bar position and bank angle, and the airspeed will smoothly rise to the "normal" value for the bar position and bank angle. With a gradual roll or pitch input, we give the airspeed enough time to change that the vertical component of lift (plus the vertical component of drag) stays nearly equal to weight. So we get a smooth, controlled flight path even if we don't make any pitch "coordination" input. Similarly, if we only increase the bank angle by 5 or 10 degrees, it's like pulling the control bar only an inch or two aft. No matter how abruptly we carry out the maneuver, the glider isn't thrown very far "off balance". 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.
Of course, if we pull in the bar at the same time that we roll briskly from wings-level to a step turn, we create an even larger imbalance between the vertical component of lift (plus the vertical component of drag), and weight. Now we'll really get an extreme downward curvature in the flight path, followed by an extreme "spike" in the sink rate, followed by an extreme, temporary "overshoot" in airspeed to a value that is far above the "normal", high airspeed for the bar position and bank angle. During the initial part of the dive, the turn rate will be much lower than it "ought" to be for the airspeed at any given instant. Since drag varies with airspeed squared, this glider loses a lot of energy during this maneuver, which is ultimately expressed as a loss of height. The point where the glider is losing energy most rapidly is not the point where the sink rate is highest-- here the glider is mainly just converting potential energy (altitude) into kinetic energy (airspeed). The glider is losing energy most rapidly when the airspeed and G-loading are peaking, even though the glide path is flatter and the sink rate is lower at this point.
How can we descend most rapidly? For any given maximum G-loading or airspeed that the pilot cares to endure, my experience is that simply keeping the bar fully "stuffed" to the full reach of the pilot's arms while also keeping the glider in as steep a bank as seems prudent (not more than 60 degrees) yields a higher average sink rate in the long run than any series of maneuvers involving frequent changes in bank angle and bar position, even though the airspeed, G-loading, and sink rate will soon stabilize at their "normal" or steady-state values for the bank angle and bar position. If the pilot can unzip his (pod) harness, loosen the shoulder-limiter line, rock his body head-down, and pull his knees over the bar, he'll be able to sustain an even higher airspeed and possibly an even higher sink rate, even if he simply leaves the wings level (unbanked). (I've not found it comfortable to try sustain a banked turn while flying in this posture.) As an added bonus, the glider is very roll-stable at this extremely low angle-of-attack. Yaw-roll oscillations are not an issue, and the glider tends to return to wings-level on its own after any disturbance. The outboard parts of the wings are generating negative (downward) lift, so the lower side wires may go slack.
Back to turn "coordination"-- when we roll towards wings-level, all the dynamics that we've been discussing here play out "in reverse". 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.
It's worth noting that the pitch dynamics we're talking about here play out over a longer timescale than the changes in bank angle. When we roll briskly into a turn without moving the bar forward, the steepest point in the resulting dive may not happen until we've stopped increasing the bank angle, and the peak of the airspeed "overshoot" will happen later still. Which means that even if we didn't move the bar forward as we were rolling into the turn, we can still take action to limit the full extent of the dive, and the spike in the sink rate, and airspeed "overshoot" that follows. Likewise, when we roll quickly from a steep turn to wings-level without pulling the bar aft, the nose may still be rising even after we've arrived at the wings-level attitude.
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. By making pitch "coordination" inputs as needed to keep the nose from falling as we increase the bank angle, or to keep the nose from rising as we decrease the bank angle, we smooth out the flight path and help the glider change airspeed in a smooth, gradual way.
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 exacerbates all the dynamics that we've been discussing here. When we roll to a steeper bank angle without moving the bar forward, it's actually much like we're moving the bar further aft as we roll-- the wing's angle-of-attack will be reduced. Making no pitch "coordination" input is actually much like making an "anti-coordination" input! To hold the nose up with a good pitch "coordination" input, we need to move the bar far enough forward to substantially increase the wing's angle-of-attack, especially if we're using a high roll rate. Likewise, if we roll to wings-level without pulling the bar aft, it's actually much like we're moving the bar forward as we roll. In either case, the wing's angle-of-attack will be increased. 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.
As long as you don't overdo it, the wing is actually further from the stall angle-of-attack when you are exerting some forward pressure in the bar in a banked turn, than when you are flying at trim with the wings level. Keeping the rate of change of bar position low (don't do a landing flare in mid-air!), see how far forward your arms have to reach to make the glider start to "mush" near stall when flying wings-level, than versus when flying at a 30-degree or 45-degree bank angle! It's hard to reach the stall angle-of-attack in a steep turn.
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. Some advanced pilots even feel that by not exerting any forward pressure on the bar while thermalling, they gain more in having a light "touch" on the bar-- they can better feel what the air is doing-- than they give up in sink rate. The higher airspeed also affords a better roll response, and the ability to do some “dynamic” maneuvering where the pilot applies a little extra G-loading when the lift is strongest. 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". The glider is in a "balanced" state, with the vertical component of lift equal to the vertical component of drag, or nearly so. 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.
Some closing notes:
* Much of what we’ve written here applies to more “conventional” airplanes and sailplanes as well. Just substitute “moving the control yoke or stick aft” for “moving the control bar forward”, etc..
* To really appreciate the importance of pitch “coordination”, try programming a radio-controlled model sailplane or airplane so that you can drop the pitch servo rate to zero with the flip of a switch on the transmitter! See how quickly rolling the aircraft through a large change in bank angle causes the nose to pitch down or up dramatically, as the aircraft exchanges altitude for airspeed or vice versa. See how slowly rolling the aircraft through the same change in bank angle causes much less disturbance in the flight path.
* The way that rapid changes in bank angle tend to cause an aircraft to dive or climb, is one reason why pilots flying in clouds on instruments restrict themselves to shallow bank angles wherever practical.
* We’ve taken a few shortcuts in our discussion of the balance of forces on a glider as it rolls into a turn. For a more rigorous discussion of these forces, and the pitch “phugoid“ that results from an “uncoordinated“ turn entry, see the related article on this website entitled "When the airspeed is changing... some thoughts on the real meaning of pitch "coordination"".
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