Wing Loading Effects

Understanding wing loading

By Dennis Pagen
Originally published in USHPA Pilot, November/December 2019

We learn in ground school or by smacking the ground how important wing loading is to our favorite light form of flight. Most of us know that added poundage on our wings increases speeds and sink rate. But, that’s often as far as the understanding goes in many cases. But fact is, it’s a bit more complicated than that. And in fact, if you (and I) wish to get the maximum performance out of the equipment we can afford, it is in our best interest to scratch below the surface of this wing loading thing.

For all aircraft, wing loading changes the airspeeds that the aircraft achieves at a given angle of attack. We normally expect the speed to go up if we increase wing loading, and vice versa. However, with our flexible wings matters aren’t quite as straight forward, and intuition can lead us down a crooked path. In fact, even hang gliders and paragliders have difference responses to wing loading, so we will cover their wing-loading details separately.

Hang Gliding Wing Loading

We can affect our wing loading by two main methods: We fly a different size glider or we add ballast. (of course, porking out at McDonalds is a fairly easy way to add ballast, but the payback is long-term and dire). Normally we change wing loading by flying different size gliders or change weight long-term, of course.

In general, hang gliding wing-loading changes do not affect safety, unless we fly a glider too large for us in strong winds (penetration and control problems) or a glider too small for us in light wind on shallow takeoffs (can’t get airborne in the allotted space). However, wing loading can affect our performance and comfort.

Here’s the main technical part we need to understand to understand wing loading effects (Note, if you start hyperventilating when reading this technical material you can skip to the next paragraph, but you’ll only vaguely understand wing loading): As we change weight we change all airspeeds by the square root of the weight change. Or, put another way, to change the airspeed a given amount, we have to change the weight by the square of the airspeed change. Here are some examples: Let’s say your all-up weight (total of you and glider) is 250 pounds (a glider of 60 lbs., a harness of 25 lbs. and a body weight of 165 lbs.). If you add 10 pounds to this all-up weight, you have increased the weight by 4%. But you increase the airspeed by the square root of that or 2%. So, if you stall at 20 mph, your stall airspeed at this new weight is 20.4 mph. If you have the bar position where it would have you flying at 30 mph with the original wing loading, you would be flying at 30.6mph. If your minimum sink rate was 200 feet per minute, your sink rate with the heavier wing loading would be 204 fpm. Those are not big changes, but it should be clear that there may be a difference of 50 pounds or more between pilots flying the same glider. A 50-pound addition increases airspeed by 9.5%, and a 20mph stall speed would increase 1.9 mph to become nearly 22 mph. We can produce similar changes in the opposite direction by reducing weight or increasing the size of the glider.

The previous analysis makes the big assumption that the glider maintains its exact shape and configuration despite what weight we put on it. Generally, this assumption is true for all aircraft, including an airplane, a rigid-wing hang glider or a paraglider, but not true for a normal flex-wing hang glider. The reason is, a typical hang glider changes shape as the weight on it changes. And the effects of these changes are the main point of this article, for the results can be perplexing and have an effect on our flying.

The first change we’ll mention is the glider’s airfoil. The airfoil is the shape of the upper and lower surface taken together as if it were cut by a plane parallel to the keel. About 2/3 of the lift force on a wing is on the upper surface, with the remaining 1/3 pushing up the lower surface. Our upper surface is typically given its shape by battens inserted in pockets sewn to it. But these battens are flexible, with some of them (especially on older gliders) being very flexible. It was often speculated and experience seemed to bear out that when heavier pilots flew such gliders, the upper surface would distort, become more curved, thus making the airfoil able to slow a bit more and reduce the sink rate. In other words, instead of increasing sink rate with more weight, the glider would maintain the same sink rate or even reduce it. Of course, there is a limit to this effect and it was very hard to measure. In addition, modern gliders with tighter sails, internal shear webs and stiffer battens do not alter nearly so much in this manner, but this effect is an illustration of how the results of wing loading changes may be more complex than originally expected.

The next wing loading effect is more important. When a heavier load is put on a glider it bows the leading edges inward and upward more, which results in more twist (washout) in the sail. This effect is shown in figure 1. The result is, a heavier pilot will typically cause the glider to actually slow down as the lifting force becomes more concentrated in the inboard part of the glider, which is forward of the hook-in point. This effect is counterintuitive. Many think that adding weight will make the glider go faster and a heavier pilot has to move the hang point back in order to get the glider to fly hands-off at minimum-sink airspeed. But actually the opposite is true and if a pilot doesn’t know this, he or she may struggle to figure out how to get the glider to fly in trim.

In addition to the above, the bowing up of the wings also creates more effective dihedral in the glider. Added dihedral increases roll stability, but slows or makes roll control harder. The glider is “stiffer.” Now here’s the problem we run into: Typically a designer has to set a given size glider up for the average-weight pilot flying it. If a heavier pilot flies it, the hang point may have to be adjusted forward, but more importantly he may find the glider a bit stiffer to roll due to the increased dihedral. However, his added weight may mask the roll effect. But going the other way, a much lighter pilot may find the glider has so much anhedral (the opposite of dihedral) that the glider feels “squirrely.” It wants to constantly roll to one side or the other in textured air. The glider in this situation may be easy to initiate into a roll, but the constant corrections required will be fatiguing and may affect safety in turbulence near the ground.

The problem has been very apparent in the not too distant past, and probably still shows up in gliders out there in the flock. Many gliders intended for light pilots exhibited this problem because manufacturers did not have a 100-pound test pilot to help them get the setup right for very light pilots. I have personally helped many female pilots of high caliber skills set their gliders up to take care of these trim problems (both CG or hang point and dihedral adjustments). It should be clear that most gliders will accommodate a wide range of pilot weights with no problems, and it is only at the extremes of their wing-loading range that some of these effects are felt. In addition, most manufacturers are aware of the situation now and can provide help to very small or very large pilots. But all pilots should be aware that gliders can and should be trimmed for maximum comfort, safety and performance.

Paragliding Wing Loading

Paragliding wing loading is a different story than the above—-it can be a major safety factor. The technical aspects still apply (the airspeed changes with the square root of the wing loading change), but the wing doesn’t really distort because of all the lines, stiffeners, shear webs (both vertical and diagonal) and battens in some cases. So the main effect of changing wing loading is to alter the airspeeds plus alter the glider’s dynamic pressurization.

Dynamic pressurization occurs because the canopy moving forward causes air to move into the inside of the canopy through the front openings until it fills the canopy, then maintains enough pressure to keep it inflated. On a modern glider the canopy openings are placed precisely where the air hits the airfoil at a right angle (this is known as the stagnation point and is the place with the highest positive pressure). Because the canopy varies its angle of attack a bit, the opening is large enough to allow the stagnation point to change a bit and still maintain good pressurization. (It should be noted how much smaller the openings of today’s gliders are compared to the past. This design change is due to designers figuring out the ideal placement of the opening, and it certainly has led to better performance by reducing drag.)

There is still a potential problem with the pressurization of a canopy, and that occurs when the glider is flown very fast, with a low angle of attack, especially when on the speed bar. The speed bar pulls the front part of the canopy down (lowers the angle of attack) and it may move the opening below the best point for the air to flow into the openings and thus invite a collapse. Collapses are not a severe problem for experienced pilots as long as there is altitude for the glider to recover. For that reason, the CIVL (body governing competition) has implemented many rules to prevent pilots from racing to goal close to the ground. In the earlier days without such rules we had some very good pilots suffer some very bad accidents racing into goal.

But a more difficult problem was the glider size availability. Not so long ago, competition gliders came in only two or three sizes. Often what was left out was the smallest possible gliders. Partially this was economics. Human weights follow a normal curve where most everyone falls in the middle with a few at the extremes of very small or very big. So, because of the high cost of glider development and certification, the manufacturers would not produce gliders for the extremes. As a result, very light pilots had to fly gliders too big for them (to have a chance to be competitive). Even using quite a bit of ballast, these glider would be too big for the pilot. The result was that pilots too light for a glider would not pressurize the canopy enough and it was very vulnerable to serious collapses in thermal turbulence. I remember one European Championships not too long ago where we had three serious accidents and all three were smaller female pilots flying the intermediate size gliders—their only choice.

The above problem was dealt with by requiring manufacturers who hope to certify a competition glider to produce at least four different sizes, accommodating pilots of all weights. This ploy has seemed to solve the problem for the time being.

The takeaway from this little discussion is for all paragliding pilots to realize the effects of wing loading are very important to safety. And even though the wing-loading problem showed up in competition gliders, all pilots should be aware that flying any glider with too light a wing loading will make it prone to collapses. Even though today’s gliders are more resistant than ever to such problems, the only safe approach is to follow the manufacturer’s guidelines on glider sizes and thus wing loading.

Wing loading affects our flying no matter which of the two sports we follow. Some of the effects are unexpected, but with a little learning we avoid the unexpected. That makes us safer, better and happier!