Does Your Gliding Parachute Really Glide
How far does your gliding parachute really glide? “I don’t know but it goes really fast!” might be the response you get from some canopy pilots. What is the difference between speed and glide? Is there a difference? (Yes.) How can we tell? Does it matter? These are some of the questions we will tackle here.
First, let’s get on the same page and define the aspects of both speed and glide. The first thought that comes to most peoples’ minds is that each can be substituted for the other. Speed can be converted into lift with a flare or application of brakes. Does this lift translate into glide? Is there a trade off in reduced glide as we add brakes? We don’t know and we can’t really tell during flight. Glide can certainly be translated into speed by simply pulling front risers. This pulls the nose of the canopy down, steepening the angle of attack and increasing speed. It is worthy of note at this point that the mode of propulsion is “gravity”. We become lawn darts. This is not a bad thing by any means as speed may be converted back into lift and if done properly – glide, the all-important component. Does this all matter? It certainly does if you take your canopy flying seriously. Your choice of canopy is based upon the type of flying you do and your comfort zone.
Modern ram-air parachutes offer a variety of flavors and like all mature aeronautic designs there are trade-offs, the primary trade-off being Glide/Lift vs. Speed. When we test jump a canopy we have a perception of the speed by the wind in our faces and the sound of the air rushing past our ears but we have no sense of glide because of the greatness of distance from the ground and the fact that we are being affected by the air mass. Therefore we have to ask science for guidance. It would seem that with the new hand held GPS’s we should be able to measure glide or at least see a profile of the jump. We sure can and we have! But how do we compensate for wind, as what the GPS is measuring for distance is ground distance, and we are flying in a moving air mass?
Where better to look than to sailplanes, where it is all about gliding! That’s what they do, after all, is glide. So how do sailplane designers and pilots measure glide? They use polar curves, a kind of graph that shows the rate of sink of an aircraft in relation to its horizontal speed. Wikipedia has an excellent entry about “P” Curves to measure Glide. A “P” curve is a polar curve and a second order polynomial trend line. It is developed on a graph that has as its “X” axis Speed in feet per second (fps) and as the “Y” axis Rate of Decent (ROD) in fps. Data points are plotted on this graph and the second order polynomial is applied. A straight line is then drawn from the Origin (0.0) to a point tangential to the polar curve. This line is then the glide slope of the vehicle. In the case of canopies we would want to title the chart with the canopy type and pound per square foot loading (see hypothetical below).
Different loadings might produce different results. It would be interesting to see if performance across a given product line will have similar performance at similar loadings. Speed has a direct translation to toggle deflection and should be documented during data acquisition. An understanding of the chart by the pilot will tell them exactly how much toggle need be applied to get back to that faraway DZ after a long spot and low pull. They can know this on the first jump on any given canopy (and not suffer the embarrassment of the long walk back from an “out jump” until they learn that canopy). Additionally, this data would provide information to manufacturers as to the optimal deployment brake setting. This would be especially useful in the case of reserves where you want minimum rate of descent and minimum glide for the case of unattended landings. It is assumed they do it today by trial and error using the current test jumper no matter what they weigh. Brake settings could even be revised to reflect lbs/sq. ft. loading.
Such data would act as information to potential buyers of new canopies. WOW, will the manufacturers hate that. Fact instead of Smack! A camera jumper who is tired of having to pull high and not film tandem openings so he/she can make it back to the DZ to film the landings (or is it better to get the opening and miss the landing?), will have the information to define the difference in canopies and maybe then they can get both. It’s assumed that most jumpers would want to have the canopy that would get them back from a long spot. Seemingly the better the canopy glides, the better it would land. Some canopy pilots believe that because the canopy goes fast it will have a better flare. This is misguided as shallow trimmed canopies may be front risered to get speeds equal to steep trimmed canopies but steep trimmed canopies cannot, even with rear risers, attain the flat glide of a high glide canopy.
OK, where to we get such manna? The manufacturers would probably not give it to us, if they even had it. How do we go about taking this data in the first place? Well, we already have “smart” altimeters which record altitude against a time line. That will give us our Rate of Descent. Next we need a recording anemometer; even a hand held one would work, that we could mount out in front of us somewhere. The anemometer readings could be recorded with a video camera. The two data sets can be synchronized by backing up from landing ground contact. A graph can be created, which we would upload to a central parachuting social media site where other acquisitions could also be uploaded and compared. “Dropzone.com” already has a listing of some canopies by angle of attack, on a forum about canopy glide; this is a beginning. If this becomes a reality then all of the questions posed here would be answered. We would know exactly how much toggle to apply to get the maximum glide out of a given canopy at a given pound per square foot loading.