Airlocks & Turb.

[riddler 0]

I won't propose to be an expert on any characteristics of flight, but here's my two cents. Stalling can happen to any canopy - it's a characteristic of anything that moves forward in the air. AOA/forward velocity limits exceeded and you will stall. I do believe, however, that air-locks canopies recover faster from a stall than no airlocked canopies. In a stall situation, an non-airlocked canopy will deflate. An air-locked one will deflate MUCH LESS (yes, airlocks do deflate, but the rate of deflation is less). If a stall occurs, forward movement stops and the canopy will tend to deflate. It will deflate at a much slower rate if it has airlocks. At some point (hopefully quickly and before you hit the ground), the turbulence will stop, your weight under canopy will position it at a good AOA, and you will start moving forward again. At this point, you either have a snivel that needs to reinflate (requiring precious altitude) or a canopy that is already inflated (i.e. and airlock) that will get you moving forward again. The altitude that you did not lose due to the airlock could be the difference between landing and landing really hard.

One thing to consider if you want to buy an airlock - if you cutaway, the canopy tends to stay inflated and tends to float away. You are more likely to lose your airlock canopy if you cutaway.

Trapped on the surface of a sphere. XKCD

[billvon 2,435]

>I do believe, however, that air-locks canopies recover faster from a
> stall than no airlocked canopies. In a stall situation, an non-
>airlocked canopy will deflate. An air-locked one will deflate MUCH
> LESS.

I don't think I believe this. Brian Germain did a test a while back where he removed the outer airlocks from a canopy, keeping the center ones. If the airlock theory held (i.e. that it's a big stiff air mattress) that would eliminate (or at least greatly reduce) the added stiffness, since air could now escape via crossports out the outer cells. However, when he jumped that canopy, he didn't notice a significant difference in stability from a fully airlocked one.

It has been my experience that airlocks contribute slightly to a canopy's stability. I believe that much of the effect comes from the cross-bracing of the nose that the canopy affords, rather than from any "air mattress" effect. While there is such an effect, it is minor compared to the forces that turbulent air can put on a canopy.

[billvon 2,435]

>Well, by my perverse logic, let's suppose you are landing into the
> wind. A wind which is inconsistent, gusting and varying it's direction.
> If you are in part brakes and the wind decides to lull on you, your
> canopy is going to surge forward, correct?

If the wind changes such that your airspeed decreases, then yes, the canopy will want to recover its speed by diving (unless you stop it with more brake.)

> But as you are already into the brakes, you are giving up control
>range of the canopy. I could be wrong in my assesment here.

Yes, I think that's exactly what can happen, and that's the tradeoff. I think that a small amount of brakes will make your canopy more resistant to collapse, but will also slow you such that you have less flare. However, on most modern canopies, going from, say, 1/4 brakes to full brakes will give you a perfectly acceptable landing, although it might not be pretty. If you have a loss of airspeed near the ground, it will be even less pretty.

So it's a question of what concerns you. If your concern is that you want the best possible chance of avoiding a collapse and serious injury, but you are willing to risk a biff, some brake might be a good idea. If you want to maximize your chances of a great landing, with some small increase in the chance of a serious injury from a collapse, then full flight might make more sense. Personally, for me, it would depend on conditions. In sorta bumpy conditions (i.e. a typical afternoon at Perris) I would fly at full flight, adding brakes as needed if the canopy wants to surge. If it were a survival condition (i.e. under a Diablo 120 at Quincy one year as a thunderstorm gust front hit) I would ride the brakes.