Freefall Speed Analysis for Style Jumps
            -- using a ProTrack freefall computer

Summary Originally published (in slightly modified form) on the Canadian Sport Parachuting Association chat list on 9 Nov. 2004.  This writeup looks at the speeds achieved during a Style maneuvers skydive. A number of Style jumps and RW jumps are compared. The speeds achieved during the dive in preparation for turning Style are not as high as is often supposed, and are not "much faster" than when belly-flying.  Data graphs are included. The data was collected only over a few jumps, for a relatively inexperienced competition stylist.

 

I'm starting to believe that the speeds in a style dive are slower than what many think. For an amateur, it is not a case where one gets really fast in the dive, and then slows down during the maneuvers because one is then in a higher drag tuck. That's a completely false impression.

My Protrack records indicate that a style dive, at least as done by an amateur, may result in a speed at the start of the style set that is LESS THAN BELLY TERMINAL, although it is faster than one would have been in another body position. If one had more than 7200' available, and wasn't being conservative with altitude as I was, one could do better.

These conclusions are based on data from 4 judged style jumps I did this year, with head down fall times of 10 seconds or slightly more (while I was counting "thousands" to 10 or 12 but probably doing so a little fast). The corrected style times are usually in the 10 to 11 second range. A more experienced jumper's style dive may be different! I was usually be done with the style set by the 23 second point, at which time I was at a comfortable 3500' or so. Therefore I could have taken the dives a little longer.

Protrack records indicate that vertical acceleration continues as long as 20 seconds, or at least 15 seconds, in typical RW and style jumps. THESE TIMES ARE WELL BEYOND THE "BOOK VALUE" OF 12 SECONDS that we all learn. Whether it is style or RW, turns and movements do get a little crisper later in the dive, long after transitioning off the hill.

The style jumps can be compared to 5 RW jumps I did, ones with 2 to 4 participants (so there was no diving involved). In the RW dives I was averaging about 113 mph SAS (see 'appendix' below for details), once it appeared that the accelerating phase had stopped. (While the speed did wander up and down from that point on, on average it did seem to climb a little more towards the end of the jump, up to 120 mph. This should still have been before tracking started.)

[The following two paragraph are new as of June 2005:]
The following paragraphs discuss how quickly different speeds were reached, for style vs. RW jumps. The speeds are useful for comparison but should not be considered to be perfectly true at the time stated in the dive. This is because the way that the ProTrack smooths data, creates a lag in the reported speed. The speed reported at a particular time in the jump is actually the average of the speed over the previous 6 seconds. This is an undocument characteristic of the ProTrack that I and others have determined to be true from analysis of raw ProTrack data.  

In practice, all this means that while the speed chart is nicely smoothed to avoid problems from air pressure fluctuations, there is a lag in the change in speed over time, usually about 3 seconds. (Yes, a 3 second lag is the way it typically works out for a 6 second averaging.)    

At the ProTrack's 12 second point in the RW jump average, the speed was 95 mph. (Due to the lag mentioned in the above paragraphs, that speed was actually reached probably about 9 seconds after exit.) This 95 mph is 84% of the later 113 mph speed, but provides only 71% of the control force available at 113 mph, because of the relationship of force varying with velocity squared. 

The style jumps quickly got to about 106 mph in 9 1/2 seconds, at which point the RW jumps were only reaching 83 mph. So the absolute speed was slow, but it was reached relatively quickly. The RW jumps didn't reach that same 106 mph until more than 6 seconds later. The style jump speed then plateaued for a few seconds, probably from a non-instant transition on my part from the dive into a tight style tuck. Then came a slow acceleration to about the 20 second point, by which time the speed averaged 120 mph. After 20 seconds, the data wasn't meaningful as the style set was complete on some jumps and jumper behaviour varied.

There is plenty of variation in speeds from jump to jump and within each jump. For example, on the RW jumps during the relatively steady state part of the jump, speeds varied from about 107 to 121 mph, with much of that variation possible during a particular jump, and not just between jumps. In style, the speeds at what appeared to be the end of the style set varied between 115 and 122 mph.

  > View the speed analysis graphs

Some day I'll have to collect data on a style set done with a LONG dive, from some altitude well above 7200'.

Other data & interpretations welcome.

 

 

The 'Appendix' for background information:

-- SAS, "skydiver's air speed", is an airspeed calculated by the ProTrack that is the best for comparing speeds between different ProTracks on different days. The actual velocity towards the planet will typically be higher at high altitude because of less air resistance, and about the same as the SAS value at pull time. That's the quick explanation, not the aeronautical engineering explanation.

-- Most of the RW jumps were 150 lbs body weight (& 6'1" tall) with 6 lbs in a weight vest plus gear, in a tight RW suit.

-- The style jumps were done in a tight RW suit without booties, and no weights.

-- Protracks aren't perfectly accurate, and can be fooled by some body position changes, but the interpretations appear reasonable.

-- Variation in RW graphs during the initial acceleration:
        The graphs for the RW jumps show what appear to be significant timing differences right at the start, i.e., very different speed at, for example, 8 seconds after exit. The question is, is this a true difference in speed due to different exits and randomness, or is it due to variation in the way the ProTrack senses the exact moment of exit?  I considered the latter and tried to compensate by shifting the times for each plot left or right, to match the initial acceleration curves better with each other.
        Doing so did not fundamentally change the results, so such experiments in data reduction have not been shown. In any case, the speeds during the bulk of the RW jumps still vary up and down, while the Style jumps reach a higher airspeeds faster than the RW ones do, no matter whether exit was a second or two earlier or later than the ProTrack thinks.