Glide ratio question

[YISkyDive 0]

Hey guys, i have a basic physics question on my mind that i can't seem to figure out.

When a pilot downsizes canopies, in the same make/model, does the glide ratio increase or decrease. When i saw video of a diablo 170, against a diablo 135, the glide ratio was a horrendous decrease.. but also the diablo 135 was about .15 - .2 over manufactures recommend loading.

How does an aspect ration play into that as well? Like why is the Katana so ground hungry and the Crossfire2 have so much better a glide, but relitively same performance?

One last question: What determines the F-riser pressure on canopies. Why do some canopies have feather light inputs, and other have body builder work out programs desgined into them?

Note: I am not looking at jumping any of these canopies for a long time, but just watching people fly had made this a point of intreste to myself.

Just wondering,

Dave.

Edit because I was focusing on the "Ration" - fixed allowance of food, etc and not "Ratio" - relationship between two amounts.

[crewkeith 0]

wow those are all good questions..........anyone???buler?? buler??


.

The skies are no longer safe

I'm back

[johnny1488 1]

I am gonna go out on a limb and say (with no input, mind you) that all of the canopies that I have jumped have about the same glide ratio.
Safires from 209-135
Crossfires from 169-109
VX 114-104
Now the rate of desent is way different, but thats not glide.

I think the xfire vs the katana (glide) is a matter of trim.

I personally think f riser pressure is also a matter of trim and partly from aspect ratio and over internal air pressure, but a designer (Brian, are you listening?) would have a lot better info than me on that.

Johnny
--"This ain't no book club, we're all gonna die!"
Mike Rome

[YISkyDive 0]

thanks for the quick reply!

um, this brought one more question to mind.. that actually i've been wonering about. Why do certain companies (areodyne, PD) choose a steep qlide for there swoop/ performance canopes, while others choose a flatter glide/ trim.

and, yah, the diablo had a faster rate of decent, but the glide also significantly changed. maybe other canopies don't do that, but the diablo went from a glide of 25 degrees (estiment) to 45 degrees. It was insane diffrence. This is also full flight characterstics.

But thanks for the reply, its cool to know other canopies do not change their angles.

-dave

[ScottMcC 0]

I thought I read somewhere once that the higher performance canopies had higher glide ratios than the lower performance ones, but I could be mistaken.

http://www.performancedesigns.com/docs/stiletto.pdf

(they have similar articles on all of their canopies by the way, I just chose that one to illustrate the point)

[johnny1488 1]

I forgot to mention the difference is usually constant between 7 and 9 cells also. ie a 7 cell will not usually have the same glide ratio as a 9 cell.

I think the performance canopy thing has to do with the other characteristics the manufacturer wants to get out of a canopy ie dive, recovery arc ect. They are usually made to build speed and this is probably easier to do with a steeper glide.

All canopies also have a limit to their wingloading and Im sure when you go over it glide will suffer (which may have been the case with the diablo.

Johnny
--"This ain't no book club, we're all gonna die!"
Mike Rome

[billvon 2,391]

>When a pilot downsizes canopies, in the same make/model, does the
>glide ration increase or decrease.

A complex question.

Here's the simplest case. Let's say a jumper has a certain wingloading and increases it by adding lead. His glide ratio will not change, but his speed will increase. Competitive sailplane pilots will often carry ballast to increase their speed without affecting their glide ratio.

Now, if you increase your loading by getting really fat, or by having a larger jumper jump the canopy, you are also adding drag due to the extra surface area. So the glide ratio (also called L/D, or lift over drag) would tend to decline a bit.

If you just downsize to a different canopy, all bets are off. Parachutes do not scale like you would expect them to. A Pilot 117 loaded at 1.5 to 1 does not fly like a Pilot 168 loaded at 1.5 to 1. If you just take the next size down, and adjust the weight so that the loading stays the same, many other parameters of the canopy (including glide) may change.

>How does an aspect ration play into that as well? Like why is the Katana
>so ground hungry and the Crossfire2 have so much better a glide, but
>relitively same performance?

In _general_ higher aspect ratio wings have higher L/D's. Paragliders, for example, are remarkably high aspect ratio wings, and they can have 8:1 glides (around 7 degrees.) This can be completely negated by airfoil or trim angle though.

>Why do certain companies (areodyne, PD) choose a steep qlide for there
>swoop/ performance canopes, while others choose a flatter glide/ trim.

Because a steeper trim often changes the recovery arc and the speed of the canopy; speed translates to flare power. Thus if swooping is all you want to do, a steeper trim may be a good idea.

>but the diablo went from a glide of 25 degrees (estiment) to 45 degrees.

No modern canopy has a 1:1 glide (45 degrees.) If it did, and you opened at 2500 feet, you'd be able to cover less than half a mile before landing.

[grega 0]

first of all, don't compare canopies with planes (sailplanes), because we don't have a tailwing and the whole thing is different.
if the glide ratio changes by increasing weight? hmm some say it does, some say it doesnt. ask any canopy designer to calculate you the angle of attack for some specific canopy with someone specific under it, and he will go , it's a huge think to calculate...
and as far as i know there weren't much practical tests done under canopies to confirm this.

but one thing is true, drag plays a big role, so if you just downsize, like it was said, all bets are off, because the wing is smaller in proportion to your body than a bigger wing...

it's not just aspect ratio. most of everything is in airfoil (profile), including friser pressure.

some canopies have center of lift more to the front of the canopy, some more to the back. so obvious the ones with the center of lift to the front will have big friser pressure... and vice versa...

this post could go on forever, canopy has so many variables that all change the way of flight... not to mention the line attachment points, lenght of lines, trim, wing design (crossbraced, zbraced, normal,...) 9cell/7cell, tapered (front, back) not tapered, material,...

"George just lucky i guess!"

[murps2000 86]

I'm sure many of the engineers will chime in, and I won't try to debate them and their figures. It's an excercise in futility. I can tell you that when I went to Brian Germain's seminar last year, I tried to pick his brains about these very subjects, and what he told me was very interesting, and not what I expected. I would urge you to try and contact him to get the straight answers you want. There's too much to explain here. I will say that what you may know about rigid airfoils cannot accurately translate directly to canopies. There are far more variables involved.

An example of this that I can provide is a story Brian told us that I'll never forget. He said that he was once among some designers that were drop testing a stiletto 120 with about 500 lbs. suspended load. They wanted to see what would happen with it loaded over 4:1. He said it didn't do what they expected in that the glide ratio actually improved. What they deduced was that the canopy, loaded as it was, finally was flying fast enough to achieve laminar flow. That is, a smooth boundary layer of air was attached to the top of the wing, nose to tail, increasing the efficency of the wing. I was surprised to learn that normally loaded canopies didn't achieve this. I figured it was essential to flight. But he said that in normal flight modes they don't.(Maybe Luis Cani's does, now)

Now most engineers will tell you that a flight vehicle's glide ratio will remain constant regardless of it weight. I don't dispute this with regard to rigid wing aircraft, but with canopies, it may not be so.

Many people will tell you that recovery arc is solely a function of a canopy's line trim, but it is also affected by line length, and wing loading. Brian told us it is also affected by how much drag your jumpsuit produces.

Some will point out that front riser pressure is determined by how far forward or aft the center of lift is on the canopy, but it can be affected by something as subtle as changing the trim between the A & B lines (which has the effect of "skewing" the cascade points froward or aft). This can affect stability, as well.

Go Big Air's website and contact Brian, I'm sure he can provide you with the insight you seek.

[HCnorway 0]

You can find some good answers in these two articles about Wingsuit flying and Basic Aerodynamics 1 and 2. ( http://www.phoenix-fly.com/articles.htm ) They speak of glide ratio of flying object in general.

They gave me a couple of a-ha's.

Good luck in the exploration of aerodynamics.

[crazydiver 0]

glide RATIO. THat was bothering me.

Cheers,
Travis

[dsbbreck 0]

Hey travis, is that our hybrid from this weekend?

David

"Socrates wasn't killed because he had the answer.......he was killed because he asked the question."

[crazydiver 0]

It sure is. It works great for a profile pic, but its tiny otherwise because it was just taken from video. Great jumpin with ya though. THat was a fun saturday.

Cheers,
Travis

[BrianSGermain 1]

Quote

First Question:
"When a pilot downsizes canopies, in the same make/model, does the glide ration increase or decrease."

The glide ratio decreases with square footage, even when the wingloading is the same. This is due to the change in balance of drag between the pilot and the parachute. With a smaller canopy, the jumper's drag is greater in proportion to that of the canopy, so the suspended weight sits further back. This alters the angle of attack, and diminishes the glide.

Next question:

"How does an aspect ratio play into that as well?"

Yes, aspect ratio has a significant effect on glide. In general, the higher the aspect ratio (Span vs chord), the better the lift to drag ratio (L/D). This due to the fact that the wing has more lifting area for roughly the same amount of drag.

One last question:

"What determines the F-riser pressure on canopies. Why do some canopies have feather light inputs, and other have body builder work out programs desgined into them?"

Front riser pressure is effected by several variables. One is the trim of the canopy. The steeper the trip A-B, the lighter the front riser pressure tends to be. The second significant variable is the location of the fattest point of the airfoil relative to the chord. In other words, we are talking about the center of lift. On canopies that produce their lift way up front, most of the load is carried by the front risers. This means that putting more weight on the front risers will not be a significant change to the flight performance. Your load was there anyway. The third variable is the general recovery arc of the parachute, which is a conglomerate of many things. If the canopy wants to go back to level flight following an airspeed-increasing maneuver, it will tend to increase it's front riser pressure as it does so. The act of pullling out of a dive is generally a front-loading maneuver, which places your load toward the front of the parachute. This is due to the fact that modern parachutes produce their lift toward the leading edge.


Hope this is the information that you were looking for.

Blue Skies,

Brian

Instructional Videos:www.AdventureWisdom.com
Keynote Speaking:www.TranscendingFEAR.com
Canopies and Courses:www.BIGAIRSPORTZ.com

[cobaltdan 0]

im guessing the 120 canopy 500# story was about us.

it was a comp cobalt 120 with continuous lines, not a stilletto. exit weight was 535 pounds and the pilot on all the jumps was john heady and yes the glide ratio did increase however im not sure about your deduction listed. without going into a lengthy discussion, i can tell you the the inflated shape of the cobalt 120 at 5:1 wing loading is different from the canopy at 0-3:1. the is much less spanwise distortion which is the reason for the increased L/D. the reason for the reduced distortion is in brief how the canopy tensioned at such a high wing loading.

these tests formed the basis for our understanding and design techniques of canopies we produce for the u.s. army which fly at wingloadings from 4 to 20 pounds per square foot with glide ratios up to 6.2:1

sincerely,

daniel preston
atairaerospace.com
atairaerodynamics.com

Daniel Preston <><>
atairaerodynamics.com (sport)
atairaerospace.com (military)

[murps2000 86]

I'm very interested in the results and conclusions of your test, and perhaps Brian himself may correct me on this, but I doubt it was the same one that was relayed to me. First, I'm certain that at no time during the entire weekend of the course I took last year was a Cobalt mentioned (at least to me). Second, just to be clear, the deductions weren't mine, but were given as explanation. Third, although this is the only thing that I'm not absolutely sure about, it was my impression that this was a drop test, and involved no pilot. After reading Brian's post above, however, it makes me think that that alone would affect glide ratio.

Quote

without going into a lengthy discussion, i can tell you the the inflated shape of the cobalt 120 at 5:1 wing loading is different from the canopy at 0-3:1. the is much less spanwise distortion which is the reason for the increased L/D. the reason for the reduced distortion is in brief how the canopy tensioned at such a high wing loading.

This statement I find quite intriguing, and a further example of how radically different a canopy is from the rigid wing of an aircraft. I realize you're trying to be brief, but you are saying that the increased tension at that loading actually reduces spanwise distortion? That is another result that I imagine would have been unexpected. I envy the designers their opportunities to make these discoveries.

Would that imply, then, that under a dynamic load, say, an increase in loading as a result of a recovery from a turn, that a canopy at such a high loading would actually be more efficient?

[cobaltdan 0]

murps, canopies are flexible and stretchable membrane surfaces. a canopy at extremely high wingloadings will tension diferently and form a different shape than one at normal loadings. unless you design the canopy specifically to take advantage of this effect the result is typically more negative than positive.

dan<><>

Daniel Preston <><>
atairaerodynamics.com (sport)
atairaerospace.com (military)

[murps2000 86]

I'm sure this is about as detailed a reply as you can give without a great deal more effort than it's worth, so I'll accept that. I'll admit, however, that am a bit perplexed at the allusion to the manifestation of a higher L/D ratio due to the tensioning of the canopy at extreme wing loadings, particularly in light of the fact that you state that the results of such tensioning are more negative than positive in conventional designs (as one might intuitively expect). Perhaps I'll be lucky enough have an opportunity to explore that some other time. Regardless, I'll never know as much as I want to know about canopies so the point is probably moot. Thanks for the reply.

[diablopilot 2]

Quote

When a pilot downsizes canopies, in the same make/model, does the glide ratio increase or decrease.

Same model, same trim, all things else the same, and you'll get the same glide angle. The progress along said glide path will be faster....

Quote

How does an aspect ration play into that as well? Like why is the Katana so ground hungry and the Crossfire2 have so much better a glide, but relitively same performance?

Canopy with a larger aspect ratio (wider span, shorter chord) will have a shallower glide angle, than one that's more square. The reason for the Katana vs. Crossfire difference you mentioned probably has more to do with trim differences.

Quote

One last question: What determines the F-riser pressure on canopies. Why do some canopies have feather light inputs, and other have body builder work out programs desgined into them?

Some has to do with the overall wingloading, but much has to due with how the canopy is trimed and where your body hangs under the wing. Furthur back and the fronts get lighter, furthur foward and they are heavier.

Canopies are one of the most interesting things in skydiving, and some of the biggest mysteries.

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