FreeFlight Advice: Turning efficiently
Posted on FreeFlightAdvice.com October 23, 2013
Q: I wonder whether you could explain the exact mechanisms that are at work when we turn a hang glider? How can we learn to make 100% efficient turns in all situations, and especially avoid having to counter-compensate to keep the wing from turning further and steeper than intended? Also, what can we do to prevent a wing from constantly digging deeper into the turn while thermalling? I fly an Icaro Orbiter and feel that I spend a lot of unnecessary energy on stopping turns because I can’t get them right from the beginning. I’ve tried different strengths and durations of input for making 90 degree turns right/left. It seems like I’m only able to do very slow and careful turns without having to counter-compensate at the end of the turn.
Do you have any suggestions how I could work on this?
Thanks for doing this!!
A: Let’s start with what makes hang gliders turn. Essentially, the same thing that makes all aircraft turn… “ailerons”. It’s a bit more obvious on an airplane though, because the ailerons are visible at the trailing edge of the wing, and you can see them move up and down. In a hang glider, however, the flexible nature of the wing allows the entire wing to act as an aileron.
Shifting your weight to the right, for example, increases the wingloading on the right wing, and decreases the wingloading on the left. The right wing, with higher loading, twists more lowering the angle of attack (trailing edge rises). At the same time, the left wing flattens out because it has less loading, raising the angle of attack and creating more lift. The imbalance of these two forces- one wing making less lift and the other making additional lift- rolls the glider.
It may be obvious to some, but it’s worth specifying that our weight shift inputs ROLL the wing… and rolling is not the same as TURNING the wing. The airfoil makes lift, which pulls us up and forwards. When you roll the wing, up and forwards relative to the wing becomes a curved path, and that is what actually results in direction change.
Another thing to aknowledge is that, when the sail deforms from shifting our weight, that rolling force isn’t the only force created. The unloaded wing, due to the higher angle of attack, also creates more induced drag. The other wing, with it’s now lower angle of attack, has reduced induced drag. The impalance in THESE forces is what leads to adverse yaw. Weight shift right, and the left wing creates more lift and rises, but also slows breifly because of the increased drag. The right wing dips but with less drag accelerates slightly. Since you are asking about efficiency, I’ll limit my discussion on adverse yaw to that topic. Yaw is inefficient because it effectively shortens your wingspan, reducing lift.
Going back to what makes our wings turn I can answer your questions about roll stability- which is a wings tendency to return to level flight if you give no further roll input. Ideally, beginner gliders would be highly roll stable, so that if students stopped giving any input the wing would level and fly straight on it’s own. Unfortunately, though, we are constrained to an aircraft that flies at very low speeds, and limited by how much ‘deflection’ our control surfaces (sail) is capable of. Because of this, designers much walk a fine line finding a balance in roll stability with roll instability. What we end up with is beginner gliders that are very mildly roll stable, or may just be roll neutral. Higher performance wings with smaller, tighter sails further limit sail deflection, so those wings need to be less roll stable to allow for weight shift control.
It is the required instability that makes us have to “high side” to maintain a constant bank angle (some gliders require a lot more than others). If you are flying a glider with VG, pulling more VG further decreases roll stability, requiring even more high siding. Now you might be thinking, if the roll stability decreases with VG, why doesn’t handling improve? The answer to that is because the sail is pulled tighter, which further limits how much ‘deflection’ it provides when you shift your weight.
Now that we’ve talked about how the glider turns, let’s talk turning efficiency. This is a simple answer, although takes much practice to perform: the wing flies most efficiently when you are doing as little as possible. If high-siding is required, being as still as possible allows the wing to be as efficient as possible. If you are continually making roll in and roll out inputs, your wing is making a lot of unnecessary drag to create those roll forces, as well as the inefficiency that comes with the adverse yaw in each roll input.
Knowing exactly how much roll input will result in how much bank depends on a whole slew of variables, and is something every pilot really needs to feel out for themselves. Remember there is a delay between when you shift your weight and when the wing rolls… and again when you return to center and the wing stops rolling. Rolling to a precise bank is a matter of feel and practice and anticipating how your wing will react to your inputs before you ever see any actual reaction. There is no shortcut for practice practice practice… but staying focused on precision will help you learn. When you roll, aim for a specific bank. Learn from if you under or over shot your desired bank angle. Same thing with headings- fly a constant heading, then do a 360 and try to level out exactly on that heading. Practice both sides of course.
Something I remind myself often when trying to thermal efficiently is that hang gliders FLY and pilots PILOT. If I’m wearing myself out making a million inputs I am making it harder for the glider to do what it does- fly. If I do as little as possible and just steer the wing, I climb much much better! It really is a less-is-more kind of thing. And again, not saying it’s easy… but the idea is simple enough, and how well you are able to do it is just how much have you practiced it.
One more thing I’ll mention that may help you out… Although it is not the most aerodynamically efficient way to thermal, there IS a trick for being more physically efficient. It consists of using pitch inputs to change the rate of direction change of your 360. Pushing out just a little will raise your angle of attack, making more lift, and if you remember I clarified that it’s the lift your wing makes that actually changes your direction… this little bit of push out will tighten up your turn radius a little. Now pushing out past trim is both inefficient AND dangerous, so use this very carefully. Much more usable is pulling in, lowering your angle of attack, making less lift, and slowing your direction change. Pulling in to stretch out one side of your 360 is MUCH easier on your arms than rolling out a little and then rolling back into the turn again. Flying fast when trying to climb is NOT aerodynamically efficient, though… so this is a technique best used to conserve energy (for a long XC perhaps) or give yourself a break (lots of thermalling can be very tiring) or to just be lazy (we do fly for FUN, don’t we?!).
I hope this has been helpful, educational, and not completely boring! Happy turning!