The "Purr" Optics

"How To" by our Pilot Staff
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The "Purr" Optics

Post by Lawndart » Tue Dec 15, 2009 7:50 pm

As most of you know, flying a perfect position often means the pilot has to fly a sight picture far from perfect so that the spectators get to enjoy the proper optics. We do this in several of our maneuvers throughout the show (just like the real Thunderbirds) and cater to the crowd's vantage point. Here's an example:

The Diamond Pass in Review (PIR, a.k.a. the "Purr").

In this screenshot (from VFAT 2009), the wingmen are seen from a ground observer at show center.

In this screenshot (same frame), the wingmen are seen from above and you can see some of the following (read below):
  1. (Leader) has to set the correct bank angle and altitude (i.e. target G is very important as it correlates to bank angle in level flight). In other words the parameters and "line" are extremely important or everyone will appear OOPs (out of position).
  2. (Left wingman) has to fly deeper and further inboard towards the Leader (placing his canopy angled off the bottom of the Leader's missile rail).
  3. (Right wingman) has to fly higher (less stack) and slightly closer towards the Leader (placing the Leader's missile rail on the wing strake below the flag panel).
  4. (Slot) has to fly a deep and right offset, but more importantly also has to move his position throughout the entire pass. He starts underneath the Right wingman's missile rail and slowly moves to slightly right of the Leader's longitudinal axis by the end of the pass (crossing show center his canopy is roughly in line with the Leader's blue stripe/RWR antenna). This technique means that his longitudinal axis will always be in line with the Leader's longitudinal axis as seen from the crowd, regardless of when throughout the pass it is observed. (Similar techniques exists for the Slot pilot in other maneuvers as well, and demand a great deal of attention to detail).
With the target G and corresponding bank angle set and flown the same for the "Purr" every time by the Boss, each wingman can then hone in on their exact positioning by continuous review from the ground observer's perspective (and No. 7). A perfect "Purr" is also a lot harder than it looks because the dynamics involved in the maneuver itself. Not only does our Slot pilot have to "drift" throughout the pass itself between two very specific positions, but we go from level flight into a ~70 degree bank only 4,000 feet from the show line, making the roll-in very dynamic and fluid. The goal is to appear symmetrical from show center, whereas in reality nothing is symmetrical in the air.

Hopefully, you'll appreciate some of the challenges in each maneuver that may not be apparent at a glance to the observer next time you watch an air show from show center, but the pilots always try to cater to the crowd. :wink:
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Post by Robin Hood » Wed Dec 16, 2009 5:55 am

Awesome thread, Lawndart! Great insight on a tremendous job done for the spectators to enjoy a truly beautiful show :)

Once again, Virtual Thunderbirds (not to mention the real TB) are synonymous with professionalism :shock:
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Post by kerdougan » Wed Dec 16, 2009 8:36 am

Great thread LD! Thanks for sharing your little secrets :D
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Post by Luse » Sat Jan 02, 2010 12:59 am

Found a picture showing exactly how far right the slot pilot must fly during the Purr. Happens to be from Townsville!

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