The canopy's responses are due to a pendulum effect (this is over simplified), generally speaking smaller canopies have shorter lineset and the pendilum swings quicker, so its "quicker" in the response.
The performance difference you are talking about here is really pretty significant. Sseveral drag factors contribute to a higher airspeed on the smaller canopy (less drag = higher airspeed): shorter lines, smaller jumper, thinner airfoil, smaller span. Also because the entire system is physically smaller and faster it turns quicker (more responsive). The combination of higher speed and increased responsiveness make the handling difference fairly substantial, possibly critical for a relatively inexperienced jumper.
quade (D 22635)
Nov 26, 2004, 7:03 PM
Post #8 of 12
I won't claim to be an expert, but as an aerospace engineer, hopefully I can explain a few reasons why the smaller canopy will seem to have "more" performance...
If you are flying strait, the difference is probably very small... the canopy will fly the same speed, decend at the same rate, ect. This is because the airfoil is the same. When you introduce turns into the equation, though, there is more than the airfoil at work.
First, you've got less mass, so there is less force required to move things around. this allows things to happen more quickly. Similarly, there is less inertia (the higher the inertia, the more force it takes to make something spin). The combination of less mass and shorter lines dramatically reduces the inertia, so you can start spinning a lot faster.
The last thing, and something that I've seen a LOT of people overlook is that on a 200 sq ft canopy, you have to move the toggle a lot further than you would on a 100 sq ft canopy to get the equivalent deflection of the airfoil. So if you make a small mistake like flaring 2 inches further on one side than the other, you might not notice on a big canopy, or only think you encountered a "crosswind"... but if you flare 2 inches apart on a really small version of the same canopy, you could find yourself in a serious turn. it makes the canopy feel a lot more sensitive or "twitchy."
When you combine lower mass, lower inertia, and more sensitivity, well... it all adds up to a lot more performance. If you want a parallel, think about aircraft. Try to imagine a 747 performing like acrobatic plane. doesn't make sense, does it? if that's too outlandish for you, try to imagine even a small acrobatic plane performing like a tiny R/C plane... still never happen.
billvon (D 16479)
Nov 28, 2004, 8:00 PM
Post #10 of 12
>If you are flying strait, the difference is probably very small... the > canopy will fly the same speed, decend at the same rate, ect. This > is because the airfoil is the same.
Even at the same loading, a Sabre2 120 does not fly the same as a Sabre2 230. It does not have the same forward speed, flare, glide ratio etc. John LeBlanc does an interesting presentation in which he asks about 20 questions about two hypothetical jumpers under identical canopies loaded the same (both 1:1) but one under a 230 and one under a 135. Do they flare the same? Do they have the same glide ratio? The answer, presented at the end of the talk, is always the same - no. Aerodynamics of ram-air canopies doesn't scale that way, as counterintuitive as that is.
What is the drag caused by the jumper, lineset, canopy? The lines of the 120 are shorter than the lines of the 230, arm of the jumper is about the same length. Same amount in brake input on shorter brake lines? As you see the jumper does not scale, so any more question?
It does not have the same forward speed, flare, glide ratio etc.
Sorry, I perhaps did not word what I was saying well. What I intended to say was that the difference in speed and glide ratio in level flight would be small in comparison to the difference in turning/flaring performance. Having never tested this by flying my Sabre 135 in level flight next to a guy 116 pounds heavier under a Sabre 230, I don't know how much difference there really is... perhaps it is more dramatic than I believed.
I am curious if you have any input on whether the difference in performance is itself a function of WL... What I am asking is, would the hypothetical jumpers, both at 1:1 under a 135 and 230 experience as much difference as another set of hypothetical jumpers at 1.5:1 under the same 135 and 230? My guess would be not.
As a final note, something that was stressed in the article you sent a link to was that a large part of the equation is that lighter jumpers have a big problem with "percieved speed" because they start at such a light loading compared to heavier jumpers. I have to wonder how much of the speed increase is real and how much is percieved, at least in level flight (I'm not saying you or PD is wrong, just that this seems an important point to consider).