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Wing Loading Options
The reoccurring question arises by all pilots entering the sport. Heavy or light. There is pro and con to both, with advantages and disadvantages depending on conditions. Within the time frame of a typical flight, there are scenarios where itís better to be heavy, and others that favor light loading.
It ultimately is a personal decision based on the type of flying you do mostly.
The math isn't linear, because many of the relationships are exponential, but within the percentage variance were dealing with, we can assume linearity to simplify the discussion.
As wing loading increases, glide angle stays the same, but speed increases, both forward and down. Imagine your on a bike, on a shallow slope with a good surface. What happens if you add 20 pounds of ballast? Will your angel of glide change? Will you go faster? Will you get to the bottom of the hill sooner or later?
In still air, more loading will give you the same glide, but you'll get there sooner, so the question is; do you want to get there sooner or later. I don't usually fly in still air.
Stability increases with wing loading. Since aerodynamics is a dynamic thing, the forces increase with increasing load and speed. By stability, I mean that it's easier to keep the canopy overhead. Normally, we have more available brake travel downward than upward, so we have good control authority to limit forward surges, but limited authority when the canopy wants to rock back. When we're light, the canopy can get further out of position behind us. In my opinion, one of the most important skills in flying is angle of attack control. (see http://paraglide.net/comment/01/asymmetric_aoa.htm )
You often hear that increased loading results in more internal pressure. It should also be noted that lift increases directly with loading. The increased lift results in more span wise tension. It is my suspicion that the increase in span wise tension is the major contributing factor in the canopy feeling more solid.
Larger gliders typically operate at higher wing loading. For example: At the upper end of its certified weight range, a large Nova Carbon has a wing loading of 0.93 lb/ft≤. The small size has a wing loading of 0.85 lb/ft≤. The higher wing loading results in more lift, and more stability.
There is an efficiency of scale. Compared to small gliders, big gliders perform better in glide. Without getting too technical, it has to do with exponential relationships. For example: There are 2 types of drag; induced and parasitic. Induced drag is a byproduct of producing lift, parasitic drag is all the extra stuff that isn't part of lift production, like the pilot. As you scale the package up, the pilots surface area (producing parasitic drag) does not increase as much as functional items, like the airfoil. The parasitic drag becomes a smaller percentage of the total drag, so a larger glider will achieve a better lift to total drag ratio (L/D ~ glide ratio) for the same wing loading. Because of this, most comp pilots fly big gliders and ballast up. I don't like to fly with ballast because it's problematic on the ground, and extra energy needs to be dissipated in hard landings. It's really a sport for couch potatoes. Fat pilots glide better, but hit harder.
Small gliders have advantages. Their pendulum timing is quicker, so they maneuver better, and generally thermal better, especially in the small sharp cores. I suspect they are also better for most aerobatic maneuvers. Big gliders roll slower and there is more span wise asymmetric angle of attack to manage.
Brake pressure increases with wing loading. Carrying heavy tandem passengers on an active day can be a workout. Marketing hype always claims light break pressure. Don't believe it.
Most newer pilots have a limited understanding of micro meteorology, and are content to boat around in mild conditions. If you get to the point where you want to drive around on big days, it's best to load up with some driving horsepower.
© copyright 3/8/02
Their are questions regarding operation outside the tested weight range of a canopy.
Although operation outside the tested weight range may be undocumented, we can make basic reasonable and consistent assumptions about how a canopy will behave when loaded outside the normal operating weight range.
http://www.dhv.de/typo/Home_English.3.0.html Canopies can be tested to a number of different criteria, and historically there have been multiple entities that would certify canopies tested according to their methods and meneuvers. The DHV has evolved to be our industry de facto standard. Gliders are tested by independent contractors who are sanctioned by the DHV. Canopy testing criteria are intended to be objective, but testing is done in real air by real pilots which are both variable. The evaluation of a behavior inherently contains a subjective element despite published objective criteria.
DHV certification does not mean a canopy is "safe". It means that its behavior in specific common configurations as been evaluated by a trained professional based on objective criteria. Not all combinations of potential configurations and dynamic scenarios are tested.
Manufactures have to pay a DHV sanctioned contractor to test their canopies if they want DHV certification. Outside of Germany, certification is optional, but often deemed necessary by the market. It is expensive to test a canopy, and sometimes manufactures choose not to certify all sizes due to the expense.
Canopies are rated on a 5 point scale of 1 to 3 (1, 1-2, 2, 2-3, & 3). A lower rating number indicates more benign behavior requiring less pilot input and skill to deal with the scenario. The overall canopy rating results from the highest score received in any category. The most problematic category on higher performance canopies is usually Accelerated Asymmetric Collapse at maximum weight.
Manufactures naturally perceive marketing benefits in having their products rated docile by DHV testing. They want the most performance out of a canopy with the lowest DHV score. If they can afford it, they might have the canopy certified twice. Once without an accelerator (speed bar), and again with the accelerator. They will then use the unaccelerated result in their marketing hype, but produce and deliver the accelerated version. The upper weight range of a particular size my be dictated by it's behavior in accelerated asymmetric collapses. If the score in the accelerated asymmetric collapse at maximum weight is not acceptable, the weight may be reduced to achieve the desired result.
There are advantages to heavy wing loading. More drive, faster, more tension and stability in rough air. There are also disadvantages. Higher speeds and more kinetic energy can result in sharper breaks with higher amplitude gyrations following leading edge tucks. The canopy can drive out in front more aggressively.
When operating at higher wing loading, pilots need to respond quicker and more aggressively. I personally have lost control of my canopy while operating at high wing loading. On two of those occasions, I was not able to walk away after impact [Walts Point] [Aspen]
The maximum tested weight on a DHV certificate is not a magic number below which the canopy should be considered safe, and above which the canopy could be considered unsafe. The change in behavior is progressive, as wing loading increases, recovery from front tucks can be increasingly more problematic.
Operating too light also has disadvantages. Since the canopy is more prone to rock back in a gust, it is also more prone to getting out of position and stalling. Recovery from parachutage is also less certain because there is less driving force. The canopy will not be tensioned well and can deform easily.
Much like a sailboat hull, canopy airfoils are also designed for an optimal airspeed. Airfoils may not perform as efficiently when airspeeds are above or below the optimal design speed.
© copyright 2/23/04