Re: [sillie1111] Vented Canopies?
Standard Disclaimer: The following are my opinions only. Ask lots of different people, read everything you can, and then decide for yourself what think.
A modern ram air canopy has two functions, which it performs in sequence.
First, it acts as an aerodynamic decelerator (a big air brake) to stop the jumper's fall. In this it performs the same function as a round parachute (this is pretty much the only function of a round parachute, although drive vents complicate things a little).
Second, it acts as a wing. Once it has finished decelerating the jumper, it transitions into an airfoil, which flies the jumper to the landing area, and allows a nice flared (and soft) landing.
For purposes of this discussion, the interesting stuff all happens during the transition from the first point to the second--while the parachute is changing from a pure aerodynamic decelerator into a flying wing.
Unvented Slider Down:
First lets talk about the opening sequence of an unvented slider down canopy. I'll start at line stretch (because that's where the difference between vented and unvented starts to matter).
Bottom Skin Expansion: After the canopy reaches line stretch, air hits the bottom skin and causes it to expand. [The tailgate promotes a nose first expansion, but let's leave that aside for now, because it's going to work in a roughly similar fashion on vented and unvented canopies.] The bottom skin expands until it's roughly rectangular (something approximately the shape it will be once the canopy is inflated).
Cell Inflation: After the bottom skin expands, the cells fill with air. This happens for many reasons, but a prime motivator is that the canopy will slide or hop forward (almost in the same fashion as an old style tracking jumper--by deflecting air off the bottom skin). This forward motion is not strictly "flight" (the cells are not yet inflated and the canopy is not yet formed into an airfoil shape), but rather more "deflection." This motion results in air rushing in through the nose of the canopy, pressurizing the cells (and creating the "wing" that you will fly to the landing area).
Flight: The now inflated wing begins to fly.
Vented Slider Down:
So, what do the vents do? Basically, they the Bottom Skin Expansion with the Cell Inflation, so the process looks more like this:
Bottom Skin Expansion and Cell Inflation: As the bottom skin expands, air passes through the vents and pressurizes the cells almost simultaneously. The canopy moves directly into the "flight" stage without needing to "hop" or "slide" forward to pressurize the cells.
Flight: The now inflated wing begins to fly.
Slider up:
Interestingly, the slider works in a different fashion, to the same end. The slider also synchronizes bottom skin expansion with cell inflation. However, instead of accelerating the cell inflation to match the bottom skin expansion, it operates more by slowing the bottom skin expansion down to the rate of cell inflation. So, with a slider, the process looks like this:
Bottom Skin Expansion and Cell Inflation: As the bottom skin begins to expand, air rushes into the cells. The slider slows the bottom skin expansion such that the cell inflation is almost synchronized with the bottom skin expansion.
Flight: The now inflated wing begins to fly.
It's been my observation that the slider basically overrides the vents (or they work together, if you prefer) such that there is virtually no difference between vented and unvented cell inflation slider up.
So, what about the brakes?
With properly set deep brakes, the canopy should transition into a wing with the minimum possible forward speed. Some forward speed is necessary for the wing to fly (otherwise it's stalled, and it will drop the jumper out of the sky very quickly). The goal of setting your deep brakes (which ought to be done custom to your body weight) is to find the ideal point where the wing has the absolute minimum forward speed to still function as a wing (flying forward) and to be controlled (respond to riser input). [If you are a "toggles first" kind of jumper, you can pretty much ignore that second part, because you'll give the canopy more forward speed by popping the brakes, pulling it back off the stall point as you initiate your turn.]
What happens if your brakes are too deep (on an unvented canopy slider down)?
If they are way too deep, you'll find the wing opens in a stall, and never gets flying. The canopy will simply drop out of the sky until you pop the toggles. This may be because the pulled down tail (from the brakes) is essentially counteracting the forward "slide" necessary to inflate the cells, and the cells are never inflating. It might also be because the cells do inflate, but the resulting wing never begins to fly, and instead stalls and drops (or slides backward). For ease of reference, let's refer to this phenomenon (stall during initial inflation, or failure to inflate at all, followed by backsliding or straight dropping) as Deployment Stall.
What happens if your brakes are too deep (on a vented canopy slider down)?
This is where you can see opening backsurge start to develop. As the air rushes in through the bottom skin vents, it surges upward, strikes the topskin of the canopy (inside the cells) and then is deflected (both forward and backward). Some of the deflected air is expelled through the nose of the canopy, pushing it backward. This creates a noticeable backward movement of the canopy. I've seen this backward movement stop (and the canopy stabilize in a nearly parked flight mode, sinking almost straight down), and I've also seen it turn into a full on stall (with the canopy losing altitude fast until the jumper pops the toggles and the canopy recovers by diving forward to gain speed).
What does the wind do?
In general, during the transition phase when the canopy is changing from its aerodynamic decelerator mode to its flying wing mode, it is still susceptible to wind effects. However, the effects of the wind on this transition may seem counter intuitive.
Tailwind: As the canopy starts to transition, the tailwind essentially blows wind up the canopy from tail to nose. The canopy "perceives" this wind (in combination with the inflation forces) as "slowing" the rate at which the wind blows into the nose (or if you prefer, the rate at which the canopy moves forward). This means that the back surge air (the air that is flowing in through the vents and striking the topskin) is "stronger" by comparison, and hence more likely to cause a backsurge. The tailwind effectively deepens the brake setting during this step, and also results in a slightly slower inflation (i.e. a greater time from PC pitch to flying wing overhead).
Headwind: During the transition phase, a headwind pretty much acts in the opposite way of a tailwind. The canopy "perceives" the headwind as increasing the force into the nose, and hence reducing backsurge (and effectively making the brakes more shallow for this step). The headwind effectively makes the brakes shallower, and also results in slightly faster inflation (i.e. less time from PC pitch to flying wing overhead).
In reply to:
From what I understand a vented canopy is not good for a beginner because the canopy is more likely to surge either forward or backward upon opening? Is it not possible to use an ideal brake setting on a vented canopy? Or is it just vented canopies are more susceptible to this behavior then a non vented canopy therefore shouldn't be used by beginners?
In general, I'd say that the vents aren't for a beginner, because the vents are most noticeable on lowish slider down, and especially underhung (where you want the wing flying to avoid the object coming out below you), solid objects. Beginners shouldn't really be jumping those types of objects, so why waste the money buying a (pricey) bit of technology that won't help you on the objects you are jumping? I think it's better to wait and get them on your next canopy, which you're more likely to be taking to such sites.
In reply to:
Another question I had was regarding forward speed of a vented vs non vented canopy initially after opening. If a vented canopy is designed to pressurize faster then an un vented canopy does this also mean in the event of a 180 you will begin flying towards the object you just jumped from sooner giving you less time to react?
No. Faster cell pressurization does not automatically yield more forward speed at opening.
Tom Aiello
Tom@SnakeRiverBASE.com
Snake River BASE Academy
