PC Extraction and Inflation

Including while influenced by wingsuit turbulence.

Matt G, Will Kitto, Martin Tilley

Recently we spent some time in a large wind tunnel at the University of Ontario Institute of Technology, Canada, testing pilot chutes of all sizes.

During our careers as BASE jumpers, we have developed preferences and techniques based on what we feel works best for us. The pilot chute sizes, bridle lengths, and pilot chute type (materials, venting, shape, handle type, etc) that we use are not random – these are decisions we have arrived at after many years in the field and a lot of video analysis and aggregated feedback from experienced pilots.

But we still have questions. Things like “How much does a PC really pull when it is in wingsuit wake turbulence?” Or, “Is a 10’ bridle better than an 8’ bridle, and if so how much” And, “How does the inflation snatch force vary from PC to PC, really?” And finally, “Can we really measure this stuff predictably?”

The truth is that these factors are very difficult to test, and it’s impossible to draw strong conclusions from the data we have. To say that pilot chutes, used with or without wingsuits, present a wide range of variables is a major understatement. Unfortunately, this article does not contain any groundbreaking answers. What it does contain is a series of graphs that will give you a rough idea of the drag force and stability of common pilot chute types. And some videos with neat images. Take from it what you like.

Strong Throw vs. Weak Throw

No major surprises here. We all know that weak throws suck. In the video above, we can see that a weak throw is a good way to cause your PC to interact with the bridle downstream. By tossing weakly, slowly, or dropping the PC, we allow more bridle to pass downstream of the PC before it is released or carried away. When the PC is upstream of a significant amount of bridle, it increases the likelihood that it will interact with the loops of bridle waiting for it. This may be one way that bridle knots and bridle/PC entanglements occur; the PC collides with a waiting loop of bridle (other possible causes are the PC and bridle mixing together in the “burble” on the top surface of the wing in wake turbulence, and packing methods).

A stronger throw helps to keep the PC clear of the bridle from start to finish. Conventional wisdom here states that you throw the PC as fast and far as you can, without totally ruining your body position, heading, and symmetry.

S-Folded Bridle vs. Standard (bridle stowed in mesh)

For wingsuit BASE, Matt G has been mostly packing his PC using the S-Fold method described here since 2010 and it is not a new concept in skydiving or BASE jumping – the theory is that this stages the bridle and helps to prevent it from dumping out into the airstream together with the PC.

Based on what we saw in the video captured from this session, we are unable to say that we have gathered further evidence that the S-Fold method is better. A far more important factor seems to be how well the PC is deployed (thrown).

As the PC is extracted and the first section of bridle is exposed to the airflow, the bridle is carried downstream. In the video, we can see that the airflow on the bridle alone can be enough to extract the remaining bridle from the BOC and even push it downstream together with the PC, which damages the “staging” theory of this packing technique. Stowing the bridle inside of a secondary smaller/tighter pocket inside the BOC (as some BASE rigs allow for) may help ensure staging here, but increasing bridle payout resistance is then a new issue.

We have seen some other possible bridle stow techniques being tested in the community, for instance, by James Yaru. Watch this space, our sport is evolving.

Reverse Handle?

Martin Tilley recommended that we test a reverse handle design: what if we put the handle on the bottom of the PC? So we did. The deployment using this modified design is labeled in the video. It appears to function. But, there is still a handle, there is still a bridle, and those things can still interact. More testing is needed.

Drag Force: Toroidal Arc vs. Standard Double Disc designs

There has been some debate about whether a toroidal arc PC, like the Snatch, pulls with the same force as a traditional PC. During the Snatch development, we stuck prototypes in a skydiving wind tunnel, attached to a simple scale, and pull force was comparable to traditional PCs across sizes tested (36”-42”). But as any tunnel ninja will tell you, first and second generation vertical wind tunnels are far from smooth. The drag force we measured backed up field testing, but the turbulence was not a benefit.

The UOIT facility, with its long recirculation loop and wind-conditioning devices, provided us with an opportunity to measure again with a more accurate load cell and smoother wind. The dimensions and wind quality of this tunnel allowed us to test drag force and also deployments, inflation, and stability. The following graphs show drag force of each PC tested at four different airspeeds per PC.

For the 36” and 38” sizes, this test is mainly an illustration of stability. We have three different pilot chutes here that would be used for a typical high airspeed BASE jump:

  1. A traditional 38” with an apex vent.
  2. A traditional 36” design with a hexagonal vent at mid-skirt (the Asylum Toxic).
  3. A toroidal arc design (the Squirrel Snatch).

What we are seeing here is similar average drag force but some variation in stability. The spikes on the chart are variations in load, caused by PC oscillation. Fewer spikes means more stability. Stability, as we know, is a crucial factor in PC performance. We can see that the toroidal arc PC (Snatch) is most stable, followed by the mid-skirt vent (Asylum Toxic), with the top vent PC (Brand X) being the least stable – indicated by the spikes in the graph.

Martin Tilley: “This data, combined with what we have seen in the field since 2014, clearly shows that the Snatch is a very stable pilot chute. The stability is a result of the design, and the 3D shaping of the panels overpowers all other factors. Even an imperfectly trimmed Snatch will most likely outperform a symmetrically trimmed double-disc PC design. We have also seen that there are significant variations in stability of double-disc designs. Symmetry and trimming are important factors that can be controlled during the manufacturing process.”

Will Kitto: It is important to note that measuring a Snatch is done differently from normal pilot chutes. Standard double disc pilot chute diameters are measured flat. A 42” PC has a 42” diameter when flat on the table. Because a Snatch or any toroidal arc PC design contains 3D shaping, it will not lay completely flat. Therefore, the size of a Snatch is calculated in terms of projected surface area.

The following video and graph show several PCs tested at ascending airspeeds. Again, we tested three common 42” pilot chutes on the market today: top vent (Brand X), top vent (Asylum), and the toroidal arc (Snatch). The graphs show that stability was comparable across three types with the highest average drag force exerted by the Snatch.