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spinglebout

Parachute Performance Factors

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I used to have a Paraconcepts RDS. I was going to get another one before I found the Chupacabra. I really like the solid metal grommets though. I think they have an option for metal rings now but I had an older one with plastic around the grommets. I always had an issue with the grommets sliding up the risers after rotating and coming out of the corner and would wind up with a handful of grommet when I would go for the rears. We added this mod to each of the rear risers like a barb on a fish hook. It worked well so I just left it in place. B|

Also unkulunkulu the biggest danger from routing your risers over the flaps is a toggle coming unstowed before deployment which would almost definitely result in a cut away under a highly loaded x-braced canopy. Take a look at the 2nd attachment. If I have a total mal or pilot chute in tow there's no reason the reserve pilot chute shouldn't launch normally. Granted I have stable body position.

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This thread has been a gold mine of information.

Though now i'm a bit confused on the effect (if any) of wing loading on glide ratio. Two of the links seem to contradict each other.

http://www.aerodecelerator.org/PDF/Lingard.pdf page 22 says "Consequently, in still air conditions, a
given parachute will travel the same distance for a given height loss whatever the altitude or wing loading, but
velocity down the glide path will increase with increasing altitude and wing loading."

http://skysurfer.com.au/hosted/highperf.pdf page 41 says "This increase in wing loading provides still greater thrust and con-
sequently greater airspeed, which in turn contributes to the manueverability
and glide ratio of these canopies."

Are they both correct and maybe talking about something slightly different from each other? Have i just gone mad and completely misunderstood them?

Lingard seems to be basing that statement on the assumption that lift and drag increase in the same ratio as the velocity increases. Seems perfectly plausible, but also not immediately obvious that it must be true. Is this a well known fact, or was it shown earlier in the paper and i missed it?

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drop-bear

This thread has been a gold mine of information.

Though now i'm a bit confused on the effect (if any) of wing loading on glide ratio. Two of the links seem to contradict each other.

http://www.aerodecelerator.org/PDF/Lingard.pdf page 22 says "Consequently, in still air conditions, a
given parachute will travel the same distance for a given height loss whatever the altitude or wing loading, but
velocity down the glide path will increase with increasing altitude and wing loading."

http://skysurfer.com.au/hosted/highperf.pdf page 41 says "This increase in wing loading provides still greater thrust and con-
sequently greater airspeed, which in turn contributes to the manueverability
and glide ratio of these canopies."

Are they both correct and maybe talking about something slightly different from each other? Have i just gone mad and completely misunderstood them?

Lingard seems to be basing that statement on the assumption that lift and drag increase in the same ratio as the velocity increases. Seems perfectly plausible, but also not immediately obvious that it must be true. Is this a well known fact, or was it shown earlier in the paper and i missed it?



I can’t give you an answer without trying to bull shit my through. Give a couple of days to see if I can come up with something that will help. When I was on the job I used to tell the rookies that I didn’t have all the answers but I’ve been around long enough where I know where to look. :P

Sparky
My idea of a fair fight is clubbing baby seals

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Including a few caveats such as the AoA staying the same and no change in area, then both lift and drag change with the square of the air speed. So, yes, they stay in proportion. If you put a weight belt on a jumper that does not increase his drag then the canopy will fly faster but at the same glide angle. On the other hand down sizing to a smaller canopy will not necessarily do the same thing. Not every thing scales. The lines don't generally get thinner. The risers don't get smaller. The body doesn't get that much smaller. And oddly enough the canopy it self doesn't seem to work as well. I'd have to think about it a bit to try to generate a reason but big canopies are noticeably more efficient at the same wing loadings.

On the other hand you will see some strange things written because most people don't look at it from such a strictly technical point of view. The experience in the real world is different. Let's say there is a wind, 10 mph, not exactly strong but a good percentage of the forward speed of the canopy. facing into the wind a more highly loaded canopy will be able to cover noticeably more distance over the ground. Is this glide angle? Most people would call it that. On the other hand a canopy with a low wing loading will have a lower sink rate and facing down wind, as when coming back from a long spot, will be carried much farther by the wind then a heavier loaded canopy. It's even more noticeable if you hang in breaks or a bit of rear riser to further decrease the sink rate. Again this is not glide angle but it is distance over the ground.

I'm afraid with some of these things you have to kind of read between the lines. Some of the things that have been written come from well meaning people with a great deal of experience but limited technical back ground.


Lee
Lee
[email protected]
www.velocitysportswear.com

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Thanks for taking the time to clarify that for me Lee. Now that i look at the Lingard paper again, it does on that very page up the top have equations (when rearranged) showing lift squares with velocity, as does drag. Face palm, it was on that very page. And the coefficients of lift and drag mainly only vary with the angle of attack as you mentioned (equations 5 and 6).

Though one other variable that effects the coefficient of lift is the anhedral angle (which makes sense since as that angle decreases, the lift vectors from the edges of the wing change direction to better align upwards). Could the higher internal cell pressure from an increased velocity flatten out the wing a little and reduce this angle? That's completely wild arsed speculation, and i really don't know. Maybe any change is too small to be relevant, or maybe my thought process is just wrong. In the real world highly loaded canopies do look flatter, but that's more to do with their construction (ie crossbracing).

Think you are right that the comment in Sobieski was probably talking about angle relative to the ground, not the moving air mass. It was more a side comment he made, not the main point he was discussing, so could just be a mistake. Technical writing even, when you really know the subject deeply, can be difficult to make sure every statement is perfectly accurate, and yet as a whole everything remains readable.

You also brought up the scaling of wings. That's something i've just skipped over at the moment, still trying to get a grasp of the basics first. Still would love to hear any thoughts you had on it though.

Thanks again,
-Luke

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I'm not sure I can give you a definitive answer on any of it but here are some thoughts.

Let's say you have an I beam constructed canopy. By that I mean that every other rib is a load bearing rib. The individual half cells want to inflate and bow out wards. That's mostly a ratio of the rib thickness at that point relative to the half cell width that's why the distortion is worse towards the back. This ballooning just is with standard construction. The only thing that might change it would be a significant change in the tip vortices.

With every other rib loaded there is another kind of distortion as the unloaded rib drifts upwards. This is a product of the amount of the lift generated by the canopy vs. the inflating pressure of the canopy. Note the difference. The first is based on ratios in the fundamental geometry of the wing. The second relates to the forces and dynamic pressure. The amount of distortion will depend on the full cell width and the tension out wards at that point and the amount of lift being supported at that point on the wing. It's like a suspension bridge with the lift across the cell being supported by the top skin balanced by the outwards pressure of the cell pulling the skin tight. So discounting the ballooning of the half cell it wants to be a parabola just like a bridge. Because the lift is dependent on the position along the cord you see more of this at the front of the canopy. How bad it is depends on the balance between the lift and the dynamic pressure, basically on the AoA. This distortion can significantly reduce the span of the canopy. Yes, that does reduce the anhedral angle but that does not even come close to making up for the loss of surface area and reduction in AR. Distortion=bad. It's most prominent at high angles of attack and low air speed. Like on flare as you run out of air speed and are at a high angle of attack. Notice how much the canopy can shrink across the front at that point in your landing when you need it the most. You can also see high G induced wing loadings at high air speeds like when pulling out of a dive. If you are digging hard on the breaks even if you are screaming fast trying to pull out of the corner you can be at a high angle of attack.

One of the reasons cross braced canopies perform well in both the situations described above is that they are less dependent on the inflation of the canopy to support the wing. The "parabola" is built into the cross bracing rather then distorting the top skin to support that load. So virtually no distortion at high angles of attack. You don't lose span width or surface area at high angles of attack. So although these designs are associated with super fast canopies what they really are is better low speed wings. Oddly enough it's there low speed performance that allows people to land them at such high wing loadings.

Scaling. It's kind of a mystery to me. Some of it I can explain some of it I can't. Some things are very obvious even in the range of size that you see in skydiving canopies. But it can become even more apparent as you go bigger. The largest canopies I've played with are 1200 sq. ft. not the biggest ever built but still three times larger then a typical tandem canopy and at that point the effects of scaling become very obvious.

One thing that is noticeable even in normal sized canopies is the changes in the dynamic response of a canopy. Even between a 135 and a 120 of the same canopy at the same wing loading. The difference in the length of the line set, even though it's in proportion can noticeable affect the pitch and turn response of the canopy. And just wait till you hit 107 in it. So even though she's a 100 lb. girl that 120 might not be such a good idea.

But what I find most interesting is that as you get bigger how much better the canopy it self flies. I'm referring to the aerodynamics. Same design, same AR, same wing loading and yet they seem to fly so much better. Or should I say that small canopies fly like shit? And the smaller you go the shittier they perform. But lets look at this positively. The bigger you go the better it gets and it doesn't seem to stop. Those 1200 are the best performing canopies I've ever seen. And it's not like it's a supper advanced design. You would think it would be real draggy, 112 suspension lines, non cascaded. And F-111. Even loaded over 1:1 you don't need ZP, and they can load those things at 2:1 and the fly fine. See how long an F-111 120 last at 2:1. I have never come up with a good explanation for this but it's the truth.

Lee
Lee
[email protected]
www.velocitysportswear.com

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