airdolphin

Hi: I heard some well-respected in the industry people expressing their opinion that Crossfire is more prone to a collapse due to its partially enclosed nose, esp. at lighter loading and more turbulent conditions. Would be interested in people's thoughts and opinions. Thanks, Pavel.

In the other thread, this article was also offered as a reference: http://www.bpa.org.uk/doc_uploads/ellipticalleblanc.pdf

My post was in response to other posts which questioned whether elliptical planform makes the canopy turn faster. I think that elliptical canopies are more efficient wings, capable of generating greater lift. The reason why they respond to control inputs so much more readily is because they are usually smaller and tend to have shorter lines and be flown at a higher wing loading. In fact, the length of lines will probably be the greatest factor affecting maneuverability of a ram-air parachute. Thus a famous PD question - having the same two canopies 210sq.ft. and 120sq.ft. and loading them 1:1, will they be flown similarly? The answer is no. Even at the same design and wing loading the smaller canopy will be more agile (mostly due to shorter lines and to the fact that air molecules don't scale, as parachutes do). Thus another dilemma - what parachute to put small light people (small women) under? Putting them under large canopies will create a very lightly loaded system, putting them on smaller canopies creates a hazard for them...

I found an interesting article re wing planform (elliptical, tapered) by the late Ian Bellis: http://www.flyaerodyne.com/download/planformfactor.pdf Also, googling "elliptical planform" yields lots of articles: http://en.wikipedia.org/wiki/Elliptical_wing http://www.faatest.com/books/FLT/Chapter17/WingPlanform.htm http://www.onemetre.net/design/downwash/liftline/liftline.htm The main point seems to be that "An elliptical wing is a wing planform shape that minimizes induced drag" and "An elliptical wing planform has the interesting property of yielding tip vortices which are the least "concentrated", that is, the downwash they yield is spread most evenly along the wingspan"

If your canopy allows to pull the toggles all the way down without stalling it, then you can just put your thumbs through the hip rings (if available) and your arms won't be as tired. Also, when using the rear risers you may choose to push them aside rather than pulling them down. This way you achieve two things: "effective shortening of rear lines" and thus higher angle of attack and more lift; and your canopy is better spread out (flattened out) and thus more effective as well.

I now understand why you are posting and pushing this. I am not sure what you are insinuating by your post, but my post was mostly a result of the conversation with someone going through the FJC. He knew nothing about parachute aerodynamics and when we discussed a little bit of, light bulbs seemed to light up in his head. His exact words were - "so, now I understand why when I was flaring slowly I still hit the ground relatively hard". I then simply compared two disciplines - helicopter flight training and skydiving training and questioned why it is mandatory to pass aerodynamics in the first case and the FJC students don't even get their feet wet in the second. I believe that knowledge is power (though not a substitute for practice), and it may help people stay safe rather than try to fly "by feel only". Considering the other point that the majority of accidents and fatalities occur under functioning parachutes (and frequently due to ignorance), it only made sense to raise the question. But now I am repeating myself.

Bill: is that because when the canopy tries to yaw it starts pulling on the lines and so the suspended weight (jumper) is being swung to the opposite side causing the parachute to bank?

Perhaps some of the responders are correct, but then another solution is to make a more in-depth course like that mandatory for the B license. Problem is that once people get out of the FJC, it's a choice for them to learn more or not. Many don't even like flying parachutes and use them only as the necessary evil to get down on the ground. So, although I am not too big on enforcement, I'd be willing to enforce such training here. It may save many bones and lives. I recently took Brian Germain's course at the Ranch and what was very illustrative is that the only people at the course were jumpers with very few jumps (under or slightly above 100). I was the only one with the number of jumps over 1,000. Perhaps partially it may have been poorly advertised, but I'd expect to see people around 500-800 jumps - one of the most dangerous "phases"... So, OK, I'll concede to the responders that having students study aerodynamics for the A license may overload them, but why wouldn't this be done for B-D licenses?! It would only make sense.

I would disagree: if we look at statistics (for the last 10 years), it seems that it's quite skewed, and according to it the most dangerous part of skydiving is landing, not dealing with mals or fewer points turned during the freefall or bad body position during the opening. So, I'd make a case that it's much more important to teach parachute flying and landing! And once again (as in the other post), helicopter primary students deal with the "information overload" and most of them pass their tests and do learn about the retreating blade stall and effective translational lift. So, I believe that the FJC students can spend a bit more time and still study. There is no need to dumb them down, esp. if they realized that their life and health really depends on it!

Don't you think that "dumbing down" FJC students works against them in the long run (it might be better to study it during the course and not during the boring time of lying in a hospital bed)? My original point was that those who aspire to be fixed/rotor-wing pilots must study aerodynamics and then pass an array of tests to prove that they didn't simply forget it. We are also pilots (although, luckily, not as much regulated - and I hope it will remain so). So, why wouldn't we also be taught at the same level? It won't make things worse - on the contrary, understanding of flexible-wing aerodynamics will make people safer. Yes, there are some people who start sweating profusely when they see a lift formula, but they all get through their ground courses. I think FJC students are no dumber than primary heli students and can eventually conquer one formula and a few diagrams.

Kind of pertaining to the other thread (Parachute Aerodynamics): can someone please help me resolve the following conundrum: when a rear riser is pulled down (say on the left), that effectively shortens the rear lines (C-D groups) on the left. Such action increases the angle of attack on the left. Therefore, according to the principles of aerodynamics, the parachute should bank to the right (left wing up, right wing low) and begin turning to the right. However, as we all know, pulling on the left rear riser, the canopy turns to the left. Why? Are the reasons in the fact that the drag is also increased on the left (due to the "step" between C and B lines)?.. I still seem to be unable to wrap my head around it, and find a solid scientific explanation to the fact easily supported by practice. Thanks, Pavel.

I would say that most even experienced jumpers don't know what the front risers do and how they work... They may say that if you pull on the right front riser, the parachute will turn to the right and spiral down while accelerating. But few will describe this as: pulling on the front riser effectively shortens the front group of lines (A-B) and thus on that side the angle of incidence (and thus angle of attack) is decreased. Thus the lift on that side decreases as well, so that canopy banks and therefore turns. A more interesting conversation arises when discussing what happens when that riser is let go. In that case the angle of incidence is restored and therefore the angle of attack is increased, so the canopy's lift increases and that causes (in some cases) the parachute to plane out. There is pitching action that happens in that case, but it's vastly different from the pitching action as a result of a toggle movement. Another example is what Brian Germain was explaining in his recent workshop - coordinated toggle turns. Most people think that pulling on a toggle is enough, but in reality it causes an uncoordinated turn, and this can be easily seen by looking at the pilot chute trailing behind. If it stays right in the middle, the turn is coordinated. But if it shifts, the parachute is not just turning - it's also slipping. Such turn is uncoordinated. To make a proper coordinated turn one needs to bank the canopy with the harness input in the same direction first, and then add toggle input. In such case the turn will be coordinated...

Bill: with all due respect, 10 mins is too short. When I teach helicopter aerodynamics, it takes probably several full days to do so... Flexible wing is a bit simpler than things like retreating blade stall and autorotation, but it's still quite complicated. My point precisely is that it should be in the USPA curriculum and tests for licenses. So, that people don't forget and study. I kid you not, but a friend of mine made a 90 degree front riser turn at 50 feet while landing out at a small back yard because he heard somewhere that front risers are safer than toggles... Luckily he lived and he is down to "7 lives" now... Besides, how many people pull their toggles slowly on landing, without getting any pitching action, and then are surprised why they hit hard. That's pure ignorance about how parachutes flare, what makes them pitch, etc...

It's been a while since my AFF course and recently I had a chat with someone who is going through his IAF course. It was a surprise to learn that the course's curriculum hasn't been updated with a treatise on parachute's aerodynamics. As a commercial pilot and instructor I can assure you that one cannot get any kind of rating without knowing what angle of attack, pitch/yaw/bank, lift, drag, etc. are. So how is it that we, the pilots of flexible wings are not taught what makes them fly, stall, turn and flare? Especially considering the recent wave of accidents under perfectly good parachutes? Some of those accidents are accidents, but many are due to ignorance and could've been prevented with some training and knowledge! Some people (such as Brian Germain) have attempted to travel the globe and teach people the aerodynamics of the flexible wing (which is even more complex than that of a solid wing). But not to teach students and require them solid knowledge of this during the IAF course is a serious flaw, IMHO. Any thoughts on this? Pavel.

What recommendations do you have for selling/buying gear through skydiving classifieds? How can both parties protect themselves from fraud, being cheated or treated unfairly? How can payments be done (money orders, PayPal, etc.)? Thanks, Pavel.