Mar 23, 2002, 7:54 PM
Post #1 of 6
Understanding wing loading will help you become a better canopy pilot. It is an important tool in advancing your canopy skills and increasing your knowledge of canopy performance. Wing loading is defined as total exit weight divided by the square footage of the canopy and is expressed as a ratio of how many pounds of suspended weight per square foot of canopy. To find your wing loading, gear up as if you are going to the plane and weigh yourself, (weighing yourself on the way to the plane is the most accurate). Divide your exit weight by the square footage of your canopy. The resulting number is your wing loading. For example, a jumper with a 180 pound exit weight flying a 130 square foot canopy has a wing loading of 1.4:1. What does this number really mean to you?
It is difficult for a canopy pilot to place numbers on canopy performance. Factors such as forward speed, descent rate, turn rate, toggle/riser pressure, and recovery arc are each subject to the individual pilot’s perceptions. One jumper may find a particular canopy sluggish, while another jumper flying the identical canopy feels as if he has been fired out of a cannon. However, wing loading is the only measurement the skydiving community has to predict a canopy you are downsizing or for that matter, upsizing to before actually flying it. If you change the type of canopy, square to elliptical, F-11 to Zero-Porosity, you have to take that into consideration also. Generally, as wing loading increases, the performance factors increase, for the same type of canopy. This general rule does have some limitations.
Canopy manufactures test their products through a wide range of wing loadings. They publish recommended minimum and maximum wing loading for each model canopy. If you fly a canopy at less than the manufacturer’s minimum recommended wing loading, it may not deliver the advertised performance for the canopy.
Canopy performance cannot be defined on a linear scale. A common misconception is to assume that downsizing from a 169 to a 149 will result in the same increase in wing loading as downsizing from a 189 to a 169. This is not true, even though you are decreasing the canopy size by the same amount; the 20 square foot reduction represents a 10.6% of the total canopy square footage of the 189 but 11.8% of the 169. This means that downsizing from the 169 to the 149 would result in a larger increase in wing loading than downsizing from the 189 to the 169. As you downsize canopies, the same decrease in square footage results in a higher increase in wing loading and therefore, performance. For example, downsizing from a 110 to a 90 (the same 20 sq. ft.) represents 18.1% of the total canopy. As the wing loading gets higher, the same increase in wing loading results in a more dramatic increase in speed and performance. For example, an increase of 0.3, from a wing loading of 1.8 to 2.1 will result in a higher performance gain than the same 0.3 increase from a wing loading of 0.8 to 1.1.
A 169 with a wing loading of 1.4 lbs./sq. ft. will not have the same performance as a 109 loaded at 1.4 lbs./ sq. ft. This is due to the reduced drag from the smaller size of the canopy, reduced surface drag of the smaller jumper under the 109 and the shorter lines of the smaller canopy. Also the shorter lines on the 109 will allow the canopy to turn faster because the pilot isn’t swung out from under the canopy as far and less lines mean less drag.
A canopy’s design also affects performance, for example, a cross braced canopy with a wing loading of 1.6 lbs. / sq. ft. will fly radically faster than an F-111 seven cell of the same size loaded at 1.6 lbs. / sq. ft. Other factors that can affect performance are line type, aspect ratio, canopy material (F-111, Zero Porosity) and the shape of the canopy, (elliptical, semi-elliptical, and square). An elliptical canopy will out-perform a square canopy at the same wing loading, all other factors being equal.
Another factor is your exit weight. The heavier you are, the more your wing loading will increase as you downsize. A lighter jumper’s wing loading will not increase as fast as a heavier jumper’s will. This disparity increases as wing loadings increase.
wing loading also affects stall speed and speed. An increase in wing loading will increase the stall speed and increase the cruise and maximum airspeeds your canopy will achieve.
By calculating your wing loading for all the canopies you have jumped, you can put a number to the performance you felt while flying the canopy and use that information to predict how an unfamiliar canopy size will fly. This is a great tool in the decision process of buying a new canopy. Don’t forget to consider any differences in shape and design.
If you are considering a new, smaller canopy or a faster design of the same size canopy, first ask yourself, “Have I learned everything my current canopy has to teach me?” This includes being able to fly the canopy fast, slow, and your landing accuracy under different conditions. If you can honestly answer “Yes”, then the next step is to figure out your current wing loading and what your wing loading would be under the new canopy you are considering. Armed with this information, discuss your options with an Instructor or Safety & Training Advisor. Also discuss your intentions with your rigger. You will need to decide if your new main will fit in your current container or if buying a new container, what size? Also, will your new main be compatable with your reserve? As part of your decision making process you need to honestly ask yourself why you want to downsize. Peer pessure and wanting to be cool are poor reasons to buy a smaller main. Some manufacturers will build a canopy to the exact size you want, allowing you to pick your wing loading and not have to just be close.
Figuring out your current wing loading and tracking your past wing loading as you fly different sizes and types of canopies will help make the decision of what to buy for your next canopy easier. Be cautious combining downsizing and changing to a higher performance type of canopy in a single step. You may be the one feeling like you have been shot out of a cannon.
However, wing loading is the only measurement the skydiving community has to predict a canopy you are downsizing or for that matter, upsizing to before actually flying it.
It may be the only measurement, per se, but like you said, it's not the only quality that can be used to predict flight characteristics. The other things you mentioned, like ZP, elliptical, and cross-braced, are qualities that are equally important in predicting the flight characteristics. I'm intentionally not saying performance.
In reply to:
...results in a higher increase in wing loading and therefore, performance.
I think this has to be argued. Very generally, an increase in wing loading means an increase in "performance," but only to a certain extent. Yes, a person could jump a VX 26 at a wing loading of 8 to 1, and he would fly and turn extremely fast, but would his "performance" increase? It depends on what your definition of performance is. It doesn't seem like the jumpers that have been winning the PPPB meets, and the PSNs, etc. have been jumping canopies loaded at 3.0 and higher. The "performance" they were looking for seemed to be achieved at lower wingloadings. I just think we have to keep in mind that the "performance" envelope is on a curved line that drops off at a certain point when the wing loading is too high, just like when it is not high enough, for any given design. Just a thought.
"The "performance" they were looking for seemed to be achieved at lower wingloadings. I just think we have to keep in mind that the "performance" envelope is on a curved line that drops off at a certain point when the wing loading is too high, just like when it is not high enough, for any given design. Just a thought."
At the competeions, they need a canopy that will do different things, only one event is speed. Distance and accuracy require lighter wingloadings. Classic accuracy canopy wingloading is around .5:1. So competetors comprimise to get the best for each event.
Right now, wing loading is limited by two things, how fast can the pilot run/slide and will the canopy plane out. I jump a high wing loading and haven't experienced or seen anyone else experience a performance drop off. As the wingloading increases, the normal full flight speed, top speed, turn rate, and stall speeds of the canopy will increase until you either cannot run fast enough or if you can run very fast, the canopy cannot generate lift equal to your weight and plane out.
It would be cool to start a new thread and pool our thoughts and define canopy performance.