As a follow-up to John LeBlanc’s previous article, ‘5 Wing Loading Misconceptions’, he explains some more advanced concepts about wing loading…
Test your beliefs about canopy wing loading. Which of these general statements are true and which are false?
True or False?
- Two jumpers with widely different exit weights will get the same performance if they have the same model of canopy, sized for the same wing loading. True/False?
- It’s possible for two jumpers with widely different exit weights to get the same performance, but the lighter jumper must load their canopy a little more heavily. True/False?
- It’s possible for two jumpers with widely different exit weights to get the same performance, but the lighter jumper must load their canopy a little more lightly. True/False?
Regarding two jumpers flying a Sabre 135 and 230, with the same wing loading of 1.0 lbs/sqft, which of the following statements are true and which are false?
- The jumpers will have the same glide angle and speed. True/False?
- The canopies will have similar handling and responsiveness in turns. True/False?
- The canopies will have similar stall characteristics. True/False?
The above statements are all false. Let me explain why…
Do equal wing loadings on a given design give the same performance?
Though many canopy models may come in several different sizes that are usually scaled geometrically from one to another, they don’t fly the same at equivalent wing loadings. Why? Because the physical and aerodynamic laws that the parachute must obey don’t scale up and down with the parachute.
Most jumpers do not normally consider the above point. They assume that a change in performance when downsizing is due purely to a change in wing loading. Since they normally don’t change their exit weight very much when jumping a particular canopy, they don’t ever load that canopy at wing loadings similar to what they would if they downsized.
However, the change in handling from one size to another is due to much more than a change in wing loading. This can be proven by jumping a small canopy at your normal exit weight, and then jumping a much larger, scaled canopy, with enough extra ballast so that the wing loading is the same. The difference is dramatic. The larger canopy certainly goes faster when carrying the additional weight, about the same speed as the smaller canopy flown at the same wing loading. However the handling still feels very similar to when it was lightly loaded.
Some reasons for differences in performance between large and small canopies
In general, the smaller the canopy, the shorter the lines. It’s true that two proportionally scaled canopies may have similar full glide speed at the same wing loadings, but they turn very differently. The big canopy, with its long lines, has a relatively sluggish reaction to the controls, and the small canopy has a relatively quick reaction to the controls. This is due to the fact that the large canopy must move the jumper a much greater distance out to the side, in order to achieve a certain bank angle needed for any given turn rate. This means that the smaller version of a canopy will feel much more agile, even if flown at the same wing loading.
A dynamic flare is one that results in a change in flight direction from a descending path to one parallel with the ground. This requires a change in the angle of the canopy, when viewed from the side. (Initially, the amount of the angle change is about equal to the angular change of the flight path.) This angle change is accomplished using a toggle movement that causes the jumper to move out in front of his natural hanging place under the canopy. The long lines of a big canopy require the jumper to be moved a greater distance out in front to achieve the same angle. Shorter lines mean that the jumper doesn’t need to move as great a distance forward to achieve the same angle. This means the toggle movement must be more aggressive on the larger canopy, compared to the smaller canopy, even at the same wing loading.
For similar reasons to those above, the smaller canopy will also dive more in hard turns, when compared to a larger scaled canopy, even when flown at the same wing loading.
Applications of these ideas in today’s environment
Lighter jumpers face a greater challenge than heavier jumpers do when jumping typical first-jump student gear, due to very slow flight and sluggish controls
Lighter students usually start at lighter wing loadings and very slow flight speed, compared to larger students, because they often use the same canopy.
The instructors are less likely to give a lighter student the attention to canopy control they deserve. It’s easier for an instructor to notice a slight problem with the larger student’s performance, and they are more likely to correct it. The instructor is likely to be less concerned about the lighter student, because the canopy is so slow that the landing is likely to be soft, even with poor technique, and the possibility of injury is remote.
Lighter student graduates face considerably different challenges when they start downsizing
Lighter students are normally encouraged to be more aggressive when downsizing, making much larger changes in wing loading than their heavier counterparts. Though the resulting speed may be similar or even less than that of a heavier novice, they’ve experienced a much bigger change in speed compared to what they started with.
Due to the fact that the smaller jumper will likely be transitioning to a smaller canopy than the larger jumper will, they’ll get quicker responsiveness from that canopy. Again, this is true even though their wing loading is probably lower.
There is a bigger difference between what a lighter jumper is told and what they are experiencing. Though the lighter novice may be jumping a fairly small canopy with fairly responsive controls, they may be incorrectly told that they are still being very conservative, because their wing loading is low compared to a heavier novice jumper. The heavier jumper, on the larger but more heavily loaded canopy, will still have more sluggish response from the canopy, which dampens the effect of his incorrect control inputs.
We have been erroneously attributing the lighter jumper’s general preference for lower wing loading to the wrong reasons
Looking at the entire skydiving population, females are generally lighter, on the average, than the males. The fact that female jumpers generally settle on a lower wing loading is usually attributed to the fact that society considers them to be somewhat conservative and less aggressive, whether this is justified or not.
I believe that experienced jumpers, whether light or heavy, are probably looking for a certain degree of responsiveness from their canopy. The lighter jumpers will get that responsiveness at a lighter wing loading. The heavier jumpers will get that responsiveness at a heavier wing loading. This is the main reason for canopy wing loading differences.
We have been attributing the lighter jumper’s preference to more conservative models of canopies to the wrong reasons.
The typical experienced jumper’s choice in a style or model of canopy may also be examined in the same way. A smaller jumper gets plenty of responsiveness from their smaller canopy. Therefore, it is likely that they will be less attracted to the quick responses of a highly elliptical canopy, and instead will prefer a more conservative model of canopy. This will allow them to go quite small on the canopy to get the speed they may want, but without getting handling that is too “twitchy”.
Highly experienced lighter jumpers should be very careful in the logic they use to justify their downsizing process.
Until very recently, very small canopies were not available to the smaller jumpers who wanted to experiment with very high wing loadings. Now that they are becoming more common, lighter jumpers are often encouraged to downsize to similar wing loadings as their heavier counterparts. They are often pressured to downsize against their own judgment
We need to consider the fact that at equal wing loadings, the smaller jumper has a much more responsive canopy than the heavier jumper has ever had to deal with. This responsiveness of the small canopy tends to magnify small errors in technique. A larger jumper, at a similar wing loading, is flying a larger canopy, which will tend to minimize the effect of small errors in technique.
We must be very careful when discussing canopies with those who are downsizing. Do not recommend specific canopies to individuals without spending some time to learn about the jumper’s particular frame of reference. Assist others in making small changes in canopy size from what they are used to, and only if they’re ready to deal with the higher speeds and more agile handling. Have fun, but be careful!
- Article adapted from John Le Blanc’s seminar notes on the PD Blog – he first gave this seminar 20 years ago, but the laws of physics do not change with time!
- Make sure you read the first article on the subject, 5 Wing Loading Misconceptions!