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# Canopy Downsizing Chart by Brian Germain

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Also, I was reading about the non-linear nature of scale effect. Your book says that aerodynamics don't scale, and I can't understand why they wouldn't. Having taught aerodynamics for airplanes and

They "don't scale" in that not everything scales equally when changing scale. If someone goes from their present canopy to one that is 20% smaller, things don't scale perfectly because the lines aren't 20% narrower, and the person won't have 20% less drag. Or, if they switched to a jumper of 20% less weight, they still wouldn't have 20% less drag, as the weight changes faster than the drag (in proportion to the cube of the dimensions rather than the square of the dimensions).

All that will change the proportion of drag of the canopy vs. lines vs. jumper, which are significant forces being applied in different locations, so that'll affect the way the canopy is pitched in flight and the glide angle.

Aircraft companies using models need to take scale effects into account too -- halving the linear size of a model makes the weight one eighth, and the wing area one quarter. So the aerodynamic effects vs. the inertial effects are out of whack with each other. Not to mention changing aerodynamics from Reynolds number effects that are significant between a model and a full scale aircraft.

So aircraft companies can use models; they just have to be very very careful about what they are measuring and how the data is interpreted & modified to represent their full sized aircraft.

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All that will change the proportion of drag of the canopy vs. lines vs. jumper, which are significant forces being applied in different locations, so that'll affect the way the canopy is pitched in flight and the glide angle.

a very visual example of that is the Xaos21-21 video. (the example might be exagerated, but it shows clearly Jeffro's drag )
scissors beat paper, paper beat rock, rock beat wingsuit - KarlM

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The reason I ask is the label that all canopies 150 square ft and below are considered high performance. We have a jumper at our DZ that wants to buy some gear. She probally wieghs less than 125 out the door. So should she really start with a 170 so she can fly backwards on landings in our Oklahoma winds?

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BG,
I have 125 jumps and fly a Heatwave 190 loaded at 1.4. I have been working on 90 degree landings and carving. I have read your book, down sizing chart, and Novack's checklist. I am wondering at what point can a jumper disregard the jump numbers for down sizing and use the ability of the pilot. I would like to get a recomendation from you for a canopy coach in the midwest " Oklahoma Area" that could evaluate me on my ability and not the jump numbers. I would like someones honest opinion weather I could be safe on a smaller canopy.

Also, I was reading about the non-linear nature of scale effect. Your book says that aerodynamics don't scale, and I can't understand why they wouldn't. Having taught aerodynamics for airplanes and helicopters I thought I knew this subject pretty well. Airplane manufactures in the past used scale models to determine stall speeds, stability characteristics, and drag ratios prior to ever building a prototype. If it works on a rigid airfoil how does that differfrom a nonrigid canopy? Are there any theoretical differences between a canoy and an airplane wing?

I understand that some folks get special advanced training and have natural talent that improves their chances of survival even if they downsize quickly. The question I would ask is, what is the hurry? You may be able to get someone to give you th go-ahead to exceed the recommendations of the charts, but why? Is having a smaller parachute going to make you happier? What is the rush? ust a thought.

Regarding the non-linear scale effect, that is a fairly long story. There are several aspects to address, but the most significant one is the relationship of volume to area. When a canopy is scaled, it is based on area, which is a square function. The scale factor that each dimension is multiplied by makes the parachute larger or smaller in every respect, which results in the internal volume, the effective drag of the wing, increase or decrease by a cube function. This means that the volume is changing faster than the area. This results in larger canopies performing in a more docile manner than the median, and smaller canopies much more radically. The glide is flatter on large canopies because of the higher drag value, and the recovery arc is shorter. This effect is further enhanced by longer lines on the larger canopies, higher stitch-density increasing the permeability of the fabric of smaller canopies, greater effects of inertia due to greater mass effecting changes of direction, and several other relevant variables.

These effects are significant, even when the wing loading is the same. In other words, a 120 lb person under a 120 square foot canopy will have a notiibly steeper glide ratio and longer recovery arc than a 220 lb person under a 220. Size is much more important than wing loading.

The real proof is in the subjective analysis; you just need to jump all the sizes and see what I mean. Even if you wear weights when you fly
the big wings, you will see that larger parachutes are not the same thing as little ones.

Blue Skies,
Brian
Keynote Speaking:www.TranscendingFEAR.com
Canopies and Courses:www.BIGAIRSPORTZ.com

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Thanks for taking the time for such an in depth answer.

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The reason I ask is the label that all canopies 150 square ft and below are considered high performance. We have a jumper at our DZ that wants to buy some gear. She probally wieghs less than 125 out the door. So should she really start with a 170 so she can fly backwards on landings in our Oklahoma winds?

So what is the answer to this question? Is it really more safe to land backwards in the wind just to keep it big enough?
I had this issue and argument regarding my 112 pound girlfriend a few years back. After several consistent very risky backwards landings in less than 10 knot winds she went against the advice of the S&TA and went smaller. Her landings improved ten fold and in my opinion were much more safe.

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Is it really more safe to land backwards in the wind just to keep it big enough?

No, it's much safer to NOT JUMP in high winds.

Small and unexperienced girls on small canopies are frankly dangerous. I've seen a few who never really learned to land as they started out at 120 or 135 sqft for their first canopy, with wingloadings of .9. TOO SMALL.
Some got scared/nervous of landing their canopies (jumping was supposed to be FUN?), some started to bellyfly with lots of lead leading to an even zippier canopy, I don't think any had consistent pretty landings. And most of them had a jumping boyfriend advising that small canopy... I also know a few girls with exit weights of 150 lbs or less who stuck to 190/170 canopies for hundreds of jumps. And still jumped loads, did competitions on windy islands, etc etc.

A light/featherweight jumper? This is a sensible progression: jump a 170 until 100 jumps, then get a 150 for at least 200, after that stick to at least a 135 for a while.

ciel bleu,

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No, it's much safer to NOT JUMP in high winds.

Hmmm under 10 is high winds? What's the point in even skydiving if that's the case?
I'm not talking about high winds (as defined by the USPA as above 14 mph).
In fact I'm talking about 8 mph.
Any canopy under a 150 though is considered high performance regardless of the wing loading so it's a fine line in this situation.

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What about light jumper lightly loading small canopy (150ft), but with certain amount of jumps (350-500), almost all of them being on a large accuracy canopy? Does jump number count in that case? With my weight, Brian's chart says I can jump a 135ft canopy, but my jump number doesn't really count, right?

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What about light jumper lightly loading small canopy (150ft), but with certain amount of jumps (350-500), almost all of them being on a large accuracy canopy? Does jump number count in that case? With my weight, Brian's chart says I can jump a 135ft canopy, but my jump number doesn't really count, right?

Than why do you bother? Just move on

???

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Mind if I ask for an explanation?

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No, it's much safer to NOT JUMP in high winds.

Hmmm under 10 is high winds?

What's the point in even skydiving if that's the case?
I'm not talking about high winds (as defined by the USPA as above 14 mph).
In fact I'm talking about 8 mph.

That's not enough wind to be landing backwards and something doesn't add up. Maybe your wind speed indicator is mis-calibrated or the woman is scared of landing and hanging on her brake toggles.

I've personally jumped my Fox 245 seven cell out of airplanes loaded at .8 pounds/square foot in winds up to 20 MPH where AFF students + A license holders were grounded and they were using tandem catchers and did not land backwards (only slightly forwards). I did fly my down-wind legs in brakes facing into the wind with very short base-legs to simplify accuracy. My experience was not inconsistent with Eiff's 22 MPH claimed forward speed for their Classic at .7 pounds/square foot.

More modern designs are trimmed flatter and have less drag for more forward speed. Paraflite claims 28 mph of forward speed at 200 pounds load under their Intruder 300 military canopy but don't specify whether weight is total suspended (.6) or payload (the harness/container is 17 pounds, add maybe 25 pounds for main and reserve, so you'd be at .8).

The effects of going smaller also aren't that pronounced in terms of airspeed, with the textbook effects of wing loading being speed proportional to its square root or a 12% gain going from a 170 to a 135. Paraflite claims a 2/2.8 MPH speed increase going from their 360 to 300 intruder at 200 and 300 pounds.

It does make a bigger difference in how much ground you cover (30 MPH of airspeed with a 20 MPH head-wind is 10 MPH ground speed, which is double what you get with 25 MPH of airspeed and 5 MPH of ground speed) but you needn't run into problems there as long as you refuse to get out down-wind from a safe landing area.

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BG,
I have 125 jumps and fly a Heatwave 190 loaded at 1.4. I have been working on 90 degree landings and carving. I have read your book, down sizing chart, and Novack's checklist. I am wondering at what point can a jumper disregard the jump numbers for down sizing and use the ability of the pilot. I would like to get a recomendation from you for a canopy coach in the midwest " Oklahoma Area" that could evaluate me on my ability and not the jump numbers. I would like someones honest opinion weather I could be safe on a smaller canopy.

Also, I was reading about the non-linear nature of scale effect. Your book says that aerodynamics don't scale, and I can't understand why they wouldn't. Having taught aerodynamics for airplanes and helicopters I thought I knew this subject pretty well. Airplane manufactures in the past used scale models to determine stall speeds, stability characteristics, and drag ratios prior to ever building a prototype. If it works on a rigid airfoil how does that differfrom a nonrigid canopy? Are there any theoretical differences between a canoy and an airplane wing?

Lots of very good questions.

As far as when to disregard this or any other chart that describes a reasonable path to safety, my advice is, go slowly and with the approval of people who are further along the path. It can only be our impatient ego that desires for us to skip such steps, and when we follow its desires, we sometimes run into regrets. This is not always the case, and with advanced training and approval of your S&TA or canopy course director, it may turn out to be a good choice. Mostly however, accelerating the downsizing process is a mistake. What is the rush anyway?

As for the nonlinear nature of scaling, there are several things to consider that are distinct from other types of aircraft. The following list of relevant variables is not in order of magnitude.

1) The fabric thickness does not scale with surface area, which means that the fabric is proportionately thicker on smaller canopies

2) The needle perforations represent a larger percentage of the surface area on small canopies. Further, the ribs are closer together, which makes the overall permeability of the fabric greater on smaller canopies.

3) The suspension lines are shorter on small canopies, but the diameter is not usually scaled in proportion to the size of the canopy.

4) The most significant effect has to do with the way in which parachutes are scaled. When we scale a canopy, we quantify the size by the surface area: (span x average chord). We figure out the scale factor, a number by which we multiply each dimension of the canopy to achieve the new size. Since we are using length x width, this is a square function. The result is a the same parachute but smaller, right? Wrong. Here's why...

The area is only one aspect of a parachute's size, while VOLUME is a very significant contributing factor when it comes to performance. A canopy's drag comes from two locations, the canopy itself (D1), and the suspended load (D2). The greater the drag on top (D1), the higher the average angle of attack will be, thereby improving glide ratio and shortening recovery arc. The converse is also true. Based on this, when we consider the effects of scaling the rib height by the same scale factor as the length and width, it is easy to see that the volume is changing by a cube function. This means that as we scale up, the drag in increasing faster than the area, and vice versa.

The consequence of this difference is, large canopies have more D1 and therefore have a better glide and shorter recovery arc than small canopies, EVEN AT THE SAME WINGLOADING. Yes, line length also works toward this same end, but the effect is low order in comparison to the volumetric discrepancies.

To get an idea of the magnitude of this discrepancy, if we were to scale the rib height in a way that would cause the volume to scale at the same rate as the area, we would need to keep the rib height the same for all sizes. This is not practical of course, as putting the same airfoil on a 97 and a 190 would be silly. Some canopy designs do compensate slightly by dropping the airfoil height on larger sizes and increasing it on smaller canopies. This is the exception rather than the rule, by far.

The best answer I can come up with for the future is to get all the manufacturers to agree to switch from using "Square Feet' as our unit of measurement, to "Cubic Feet". This would require a bit of math (area under the curve), but it is possible. This way, we would be comparing apples to apples, and help customers to get a better idea of the performance of a canopy than square feet ever will.

Hope this helps clarify things for you. Thanks for the good questions!

Happy Landings,
Brian
Keynote Speaking:www.TranscendingFEAR.com
Canopies and Courses:www.BIGAIRSPORTZ.com

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This is a great chart and information. Thanks! :)

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