Maxing out the performance of a given canopy (W/L vs aerodynamics)

[SkydiveNFlorida 0]

I am trying to understand more about aerodynamics... specifically of a parachute. I have read that once you load past a certain point on a canopy, it will actually lose performance. Can someone explain to me why this would be true on a crossbraced canopy? I would assume deformation of the wing would likely be the culprit sooner than other factors would come into play on a non-crossbraced.

Is it due to the velocity of a highly loaded wing favoring parasite drag rather than a balance between parasite drag & induced drag? Does the angle of attack remain as it is designed to be past the manufacturers recommendations (ie, do all points where the lines connect to the parachute have even weight distribution), or is your angle of attack no longer ideal?

I also notice that ellipticals seem to be able to be loaded more highly before experiencing this decline in performance. Is this due to the tapered wing having creating less downwash and therefore less induced drag? Or, why is this?

I'm a little lost. Trying to get through this book with an actual understanding of how aerodynamics work, rather than just reading through it and having a bunch of new terms to show for it. So, any help is most appreciated. Thanks.
Angela.

[AggieDave 6]

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Does the angle of attack remain

Angle of attack is a constanst, think of it as a slope, the higher the wingloading, the faster you go down the slope, but the angle is the same.

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I also notice that ellipticals seem to be able to be loaded more highly before experiencing this decline in performance.

That's more to the fact that most canopy pilots go elliptical and high wingloading instead of otherwise. You can still load a Sabre1 up at 1.5-1.7 and have good swoops (I've seen it quite a bit, or used to atleast).


Canopy performance is a couple of things factored together and it all really depends on your end goal. If you're goal is speed, lots of speed and not HUGE swoops, then you're wingloading will most likely be higher to achieve that speed. If you're goal is really long swoops, then you're wingloading will be a bit different. Since you're average jumper is all about speed and a long swoop, they generally stick to the "sweet spot" wingloading on their make of canopy. Generally speaking, from what I've seen on Stilletos its about 1.6, Crossfire2 its about 1.8-1.9, on Velocities and VXs its about 2.1...but that's some of my experience and some of what I've seen, no true fact there.

The reason a X-braced can be loaded up higher then a non-Xbraced and still maintain its performance is due to the wing's rigid nature (since its X-braced). You have much less top skin distortion that way, which distorts the wingshape and hurts overall performance (lift).


That's as I know it, guys like Chuck Blue and Derek V. please jump in and correct anything you see that I'm wrong (that'd be good since it'll help me learn things better and others).

Hope this helps you out.

--"When I die, may I be surrounded by scattered chrome and burning gasoline."

[SkydiveNFlorida 0]

So, why, in general, are the max loading for non-ellipticals lower than for ellipticals of the same size? Not taking into account that a jumper of a certain skill level wants an elliptical,... only taking into account that the manufacturer states that such a w/l is most appropriate for some canopy.

I see that you note two types of performance, speed & distance... what is it that the manufacturers are recommending for? Is the intersection of those curves deemed "max performance"? And, how is it determined?

There are so many confusing concepts that at times seem to counter one another, and are not intuitive.

Here are just a few things off the top of my head :

increased wing span = greater total drag, but less downwash therefore less induced drag. I assume this doesn't much apply to parachutes because the aspect ratio is likely the same as you downsize, therefore increased wingspan by a certain % is the same as the %increase in total area?

smaller wing = increased velocity = increased parasite drag and decreased induced drag. Is this still less total drag?

ellipticals create less downwash due to decreased vortices and therefore the induced drag is less.

deformation of a non-crossbraced wing = possible disturbance of the laminar boundary layer which would prematurely go turbulant.

As velocity increases, pressure decreases, skin friction increases, but would a decrease in pressure
also mean a decrease in decreased pressure differential, or is the decrease in pressure uniform about the wing?

As you can see, I need help. Are there performance curves where parachutes of different types are compared?

Angela.

[AggieDave 6]

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what is it that the manufacturers are recommending for?

Liabilty. They don't want to get sued, etc.


Same thing for your first question.

In my experience, the performance peak on a Sabre1 is about 1.6-1.7, about the same for a Stiletto. In the hands of a capapble canopy pilot, both will swoop just about as far at the same wingloading.

Aspect ratios are generally the same through a canopy model, regardless of size.

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Are there performance curves where parachutes of different types are compared?

Not that I've seen. Generally manufactures tend to just leave that to the jumper. The factory pilots figure out what works best to win comps (thats why you'll see 2 sizes used, a speed size and a distance size) and they stick to it...side note, if you haven't noticed, the major names in swooping have upsized a bit in the past 2 years.

As for your other questions, although I've read a bit about aerodynamics and such, I can't really answer your technical questions. Someone else (I bet Hooknswoop knows the answers) will have to step in with those.

--"When I die, may I be surrounded by scattered chrome and burning gasoline."

[SkydiveNFlorida 0]

Thanks, Dave:)

[nicknitro71 0]

You ask very good questions.

I think the parasite drag for an elliptical the same size would be a bit less but probably not significantly less. I would like to know the stats myself.

A cross-braced canopy must have the less parasite drag mainly due to their very low hight cord and partially enclosed nose.

Remeber that parasite drag goes higher with higher speeds, the opposite is true for induced drag so a X-braced will have the lowest total drag at virtually any speeds.

As for wingloading, well you reach a point where you have good lift only at very high speeds but very low lift at low speeds or alternatevly during the last part of the flare especially in down-wind/no-wind landings. So it comes down to what speed you feel comfortable to start running!

Think about a glider and a a fast aerobatic monowing plane. One needs to produce lift at very low speeds hence the huge wing span; the other needs to manuver quickly without puting too much stress on the frame hence the short wing span for low parasite drag, powerful engine, and crappy landings for the most part! But usually those guys who fly planes like the Extra 300 will land them just fine! In Aviation the concept is quite the same. Small wings, higher speeds, less parasite drag, less lift, higher stall speeds, and you must take off and land at higher speeds. Big wing span, lots of lift, slower take offs and landings, high parasite drag hence a reduction in performance.

In conclusion there is no plane or canopy that fits all.

Memento Audere Semper

903

[psw097 0]

"Angle of attack is a constanst, think of it as a slope, the higher the wingloading, the faster you go down the slope, but the angle is the same."

You have AOA and AOI reversed. Greatly simplified: Angle of attack is the wing to the relative wind and is positive. Angle of incidence is the angle to the pilot, or ground in stable flight, and without an engine better be negative. Both can be changed to a certain extent by yank'n on handles/risers.

[AggieDave 6]

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You have AOA and AOI reversed. Greatly simplified: Angle of attack is the wing to the relative wind and is positive. Angle of incidence is the angle to the pilot, or ground in stable flight

Yup, I did, thanks for correcting.

--"When I die, may I be surrounded by scattered chrome and burning gasoline."

[SkydiveNFlorida 0]

This is interesting stuff! :)

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! In Aviation the concept is quite the same. Small wings, higher speeds, less parasite drag, less lift, higher stall speeds, and you must take off and land at higher speeds. Big wing span, lots of lift, slower take offs and landings, high parasite drag hence a reduction in performance.

Why is it that you must take off and land at high speeds on the wing with less span? And you can take off and land at slower speeds on the wing with greater span? Is this due in any part to ground effect? A greater % of the downwash would be eliminated on the craft with a shorter span, therefore the craft will have less induced drag (which is the largest portion of drag at low speed) and therefore greater speed on landing and takeoff? I assume ground effect plays little or no part in parachute landing, unless you have a nice low swoop? Any insight on that?

AOA & AOI I may also be a little confused on. I understand about AOI being (-), but i'm not sure I can see AOA being positive? So, the relative wind hits the nose from the front/below? I think I need to spend a little more time watching parachutes fly, rather than flying my own.

Thanks everyone!
Angela.

[twnsnd 1]

More wing equals more lift at ANY speed. However, the greater parasitic drag created by a larger wing inhibits high performance.

-We are the Swoophaters. We have travelled back in time to hate on your swoops.-

[nicknitro71 0]

As twnsnd simply put: more wing = more lift = more parasite drag = less performance.

In most ariplane you have flaps that generate more parasite drag, more lift, and reduce the stall speed.
We quite don't have that luxury in our modern canopies although some could argue that flaring itself act as some sort of flaps but not quite true.

There are now some flex hang glider with flaps and most rigid hang gliders have flaps making landings much safer and easier do to the lower stall speed and slower approach speed.

Ground effect on an airplane can only be appreciated significantly on low wing birds although it does not substitute for proper use of flaps, good approach speed, and descent rate.

Memento Audere Semper

903

[psw097 0]

I recommend:

http://www.afn.org/skydive/sta/highperf.pdf or
http://www.skydiveaz.com/resource.htm or
http://www.monmouth.com/~jsd/how/

Generally, ground effect can be felt within 1 span length of the ground. For a given surface area a wing with a higher aspect ratio will produce more lift - this is FM and I cannot remember all the details, has to do with form drag vs. parasitic drag. AOA is the relationship to the relative wind, it will be positive in relation to the relative wind only, its the deflection portion (Newton's contribution) of lift.

Editted for messed up hyperlink.

[SkydiveNFlorida 0]

Thanks everyone! :)

I will read those articles, too.

Angela.