FreeFlight Advice: Handling turbulence
January 14, 2014
Q: Fledgling query – beginning to fly in trashy air and finding times when my hang strap goes slack which is unnerving. Given enough altitude, should the default response be pulling in/shifting as much weight forward as possible? What if this happens close to ground?
A: Before I could fully answer this, we need to clarify on the term “turbulence”. There are two types of turbulence: thermal and mechanical. Thermal turbulence is cause by thermic heating… differences in temperature/pressure causing air to mix, some air to rise and other air to sink to take it’s place. There are some variables here as to HOW rough thermal turbulence will be on a given day, and it’s entirely predictable (if you know what to look at and how to interpret).
The other kind of turbulence is mechanical turbulence. This turbulence is cause by the air getting “mechanically” mixed by flowing over or around stuff. The classic example is landing downwind of a tall tree line on a windy day. But mechanical turbulence isn’t always so easy to identify or predict. Air can get jumbled just from flowing over what appear to be pretty small rolling hills and valleys… and that jumbled air can stay that way for a looong time. Harder still is that mechanical turbulence can be caused from air flowing over OTHER air! If one air mass is sliding past another, there is going to be friction between the two… and where these air masses “touch” it can get pretty interesting (in a need a change of underwear kind of way).
I’d like to talk a little about turbulence forecasting BEFORE getting into some ways to handle turbulence when you encounter it. The best way to handle it is to avoid it when possible! Thermal turbulence IS predictable. The reason for that is that the way the air mixes and behaves is based on the laws of physics. The nature of the air can be very different from one day to the next, but there are things we look for to detect these changes. The first is air pressure. Standard pressure at sea level is 29.92 inches of mercury (inHg). Pressure decreases with altitude, so the “standard pressure” for your location is likely lower than 29.92 inHg. There’s a complex formula for figuring it out for a given altitude, but luckily we don’t really care. We are concerned with the characteristics of the air at a given pressure, which is constant regardless of where you live.
The characteristics of “low pressure” is that the air is unstable. In a low pressure system thermals tend to form well, getting big and strong. This does not mean thermals will be abundant, they might still be spread far apart… just strong. Flying into one of these monster thermals can feel like getting punched in the face, as you go from descending at 200 fpm to climbing at 800+ fpm. These thermals are powerful, which means they can suck you in just as easy as spit you out. In general, the lower the pressure the bigger and stronger the thermals will be. For us hang glider pilots, we walk a delicate balance of wanting lower-pressure instability so that there is good lift, and being safe. The lower pressure it gets, the bigger and more powerful the thermals will be. From a soaring perspective… conditions get better and better right up until they get too good and the movement of the air is stronger and more powerful than our weight-shift control and we can lose control of our gliders, getting steered into terrain or even tumbled.
High pressure days are just the opposite; stable air mass and thermals do not form well at all. Do not mistake this for meaning that the air isn’t still moving and mixing due to solar heating… it just means rather than warm air sticking together and forming a monster thermal, lots of little shreds of warmer air rise and mix up all the air. Thermals can still form on higher pressure days, but they tend to rough, with broken lift and sharp edges on the thermals.
There are other factors that affect the traits of the air around you, but let’s keep it simple and leave it with air pressure being the biggest factor and easiest to identify (any weather site will tell you the day’s current air pressure). Obviously the further off the current pressure is from the standard 29.92 inHg determines how much the air will behave like a “high pressure” or “low pressure” day.
The reason I’m touching on this is because the type of air typically described as “trashy” is usually either the rough and broken lift of a high pressure day, or due to mechanical turbulence. The force of air increases with the square of velocity, which is a fancy way of saying that the stronger the wind is the rougher it will get… and the turbulence does not increase at the same rate of wind speed. If the wind speed doubles, mechanical turbulence can be 4x as bad. What this means is that, at most mountain sites, a little wind is good, a little more wind is better, and more wind still is BAD! Every flying site is downwind of something… maybe it’s mountains, maybe it’s a lake, maybe it’s open desert. As I said earlier, mechanical turbulence can be hard to predict… and that is because the air upwind of your site might not be flowing over a big visible object and getting all twisted up… but it might be flowing over a cooler air mass over a large body of water and getting just as twisted. That body of water might be far enough away that the air doesn’t get as messed up when it’s not as windy… or at least it has more time to settle down before it reaches you…
What I’d really like to say, before saying how to handle turbulence, is that by far the safest way to “handle” turbulence is to really understand it- where will it be, what will it feel like? And the biggest question of all- are my skills up to launching, flying, and landing in this air?
Ok- so you’re flying in trashy air… what to do?! First, keep your airspeed up! Increased airspeed means increased control, but also means the sweep and twist of your wing is able to create more nose-up force. Simply put, additional airspeed means you’re less likely to get pitched over and tumble. Certified gliders should list a Va, or velocity that should not be exceeded in rough air. This is because, as you increase airspeed, the forces acting on the wing also increase. Driving your car over a speed bump at 40 is a lot rougher on the car than crawling over it at 4 mph. Going too fast in turbulent air can actually result in structural failure of the wing. So, fly faster than trim/min-sink… but slower than Va.
Another “trick” for turbulent conditions is to be constantly banked and turning. In physics terms, turning is actually accelerating laterally… and when we accelerate we “pull G’s”. Turning increases wing loading, which means more control since we steer through weight shift (if you weigh more, you have more control). Being banked has another advantage… the likely hood of a tuck/tumble situation is much less. Our gliders are strong enough to withstand the turbulence we encounter on a rational flying day… but what CAN happen is getting pitched into “unusual angles”. If you’re flying level and get pitched over, you’re going to fall into the glider. While the glider can handle the turbulence, it can’t handle your 200+lb body slamming into it. If you’re banked, you might get banked steeper, you might get banked less, you might get pitched nose up or down… but you’re far less likely to fall into the glider.
In any kind of “unusual angle” the glider might get pitched into, it is important to get your weight forward. Pitch stability is directly related to CG, so the farther forward the CG of the wing is the more pitch-stable it becomes. What few talk about, but you should also be ready for, is the alarming airspeed you might end up with during one of these “oh shit” recoveries. You are not flying faster than Va, and plenty fast to positively fail the glider if you let the bar return to trim too quickly. Be as smooth as you can as you let the bar back out. Turning can also help, so that you do not end up climbing in a more vulnerable nose-up attitude.
In your original question, you asked how to handle “side wire slappers” where you are momentarily weightless. In this situation, pulling in does nothing. Sure it changes bar position… but if you weigh nothing you have no weight to shift, so you are not altering the CG of the wing, nor the pitch stability. The best thing you could do is HOLD ON. You need to stay glued to that base tube, so that your body can not slam into a downtube or the wing. Downtubes are under compression in flight, and anyone that’s ever broke a downtube could tell you how easily they fail when pushed out of column. Having that happen in-flight could be catastrophic.
Here is a great video of top pilot Wolfi hitting the mother of all side wire slappers while on glide… yikes!
The thing to think about if you’re flying in air like that… it’s not like your body jumped up toward the wing. The wing was actually pushed down toward your body faster than the inertia of your body could change due to gravity. The wing isn’t “falling” onto you… there’s a pretty alarming downward force pushing it toward you! And as you can see from the Wolfi video above, there was no change in pitch. No warning. CAT, or clear-air-turbulence has brought down the biggest of aircraft on occasion. Encountering something like this near terrain, whether it is the hill-side or the LZ, could spell disaster no matter how you react. The power of the air we fly in demands our respect. “Trashy air” demands our respect.
As a parting thought, I’ll leave you with the idea that some very well respected people in this sport believe there be dragons out there; That there can be turbulence strong enough to tumble a glider and it is completely unpredictable and could be anywhere, anytime. Personally, I wouldn’t be able to bring myself to launch if I believed that. I believe that the laws of nature and physics govern the way the air behaves, and the only way to stay safe and avoid these “dragons” is to be a student of the sky, to give it the respect it warrants, and to be as selective as is reasonable about the conditions we choose to fly in.
Be safe, have fun, live to fly another day!