{"id":4568,"date":"2021-06-19T20:26:44","date_gmt":"2021-06-20T01:26:44","guid":{"rendered":"http:\/\/www.orlandobuzzards.org\/?page_id=4568"},"modified":"2022-12-30T21:08:38","modified_gmt":"2022-12-31T02:08:38","slug":"the-physics-of-an-f3j-launch","status":"publish","type":"page","link":"http:\/\/www.orlandobuzzards.org\/?page_id=4568","title":{"rendered":"The Physics of an F3J Launch"},"content":{"rendered":"\n

The Physics of an F3J Launch<\/strong>                                                         by\nDennis L Harvey<\/p>\n\n\n\n

This\narticle first appeared in the January 2021 issue of Model Aviation<\/strong>, the official publication of the Academy of Model\nAeronautics and is reprinted here with permission.  This is the longer version of the article\nwhich appeared online with a section that included extra detail and graphs.<\/em><\/p>\n\n\n\n

I had been an avid RC sailplane flier during\nthe 1970s but because of my engineering career, I took a very long break.  I returned to the sport in 2017 when I moved from\nthe Midwest to central Florida.  I was\nwowed by the innovations over those 35 years; from the competitiveness of full\nhouse controls, to miniaturization of electronics, to the extensive use of\ncarbon fiber.  <\/p>\n\n\n\n

But when I saw my first F3J event, I was\nparticularly fascinated with the launches. \nThey seemed truly brutal.  For\none, the F3J winch line is much more elastic than the standard twine thermal\nduration winch lines I used in the past. \nThe dynamics were clearly different. \nIf you haven\u2019t seen an F3J launch, look it up on YouTube.  <\/p>\n\n\n\n

In an F3J launch, the goal is to get as\nmuch altitude as you need (often up to 250 meters) while staying on the winch line\nas short a time as possible (usually less than 5 seconds).  Every second you are on the winch line is a\npoint you can\u2019t make up during the flight or with landing points.  As an engineer, I wanted to understand what\nwas happening physically during launch. \nAs I befriended some of the best pilots in my area and did some digging,\nI was surprised to find there is little accessible research on the topic.  It didn\u2019t take much arm twisting to put\ntogether a top notch team to take a more scientific look at the subject.<\/p>\n\n\n\n

The\nTeam<\/strong><\/p>\n\n\n\n

For this experiment, we had some of the\nbest fliers participate by piloting instrumented sailplanes.  Three team members have been part of Team\nSoaring USA, representing the United States on the international F3J circuit.  Jody Miller took second place at the last\ninternational competition in 2018.  Jeff\nDuval is currently the number one USA pilot for the next competition (which was\npostponed this year because of the pandemic). \nGordon Buckland has been on the team numerous times and is a\nquintessential organizer for the sport. Ed Dumas, who drove down from Tennessee\nfor our January 2020 flight weekend, provided invaluable expertise on flying\nand instrumentation.  Though I didn\u2019t\nhave the flying credentials of the others, I was quite at home with\nexperimental design as it was often part of my career.<\/p>\n\n\n\n

The\nApproach<\/strong><\/p>\n\n\n\n

At the core of the experimental design\nwas to measure the sailplane\u2019s energy state as it progressed through the\nlaunch.  The total energy of the\nsailplane is the sum of its potential energy (proportional to altitude) and its\nkinetic energy (proportional to speed squared). \nAdditionally, we wanted to video record the launch from onboard in order\nto determine the wing load (based on wing flex) and the angle of the winch line\nand the horizon.  Where ever possible, we\ntried to confirm our findings through a secondary measurement or calculation. 
\n<\/strong><\/p>\n\n\n\n

A Representative Launch Example<\/strong><\/p>\n\n\n\n

To keep this article simple, I will show\nonly one of 26 launches.  The launch is\nrepresentative of a high performance F3J launch.  Jody Miller was the pilot flying a carbon\nfiber 3.8 meter wing span Xplorer3 F3J model. \nThis is a very common sailplane at F3J events.  The temperature was around 80\u00b0 F, our ground\nlevel was approximately 60 feet ASL, and the winds were less than 4 mph.  Jody\u2019s launch attained an altitude of 251\nmeters with his total time on the winch line of 4.8 seconds.  Flaps were initially set at 15 mm down (12\ndegrees) until the \u201cBucket\u201d phase when they were set 3 mm up for speed.  The winch was a standard competition winch with1.32\nmm diameter winch line.<\/p>\n\n\n\n

The\nInstrumentation<\/strong>  <\/p>\n\n\n\n

The key to getting a potential energy\ndetermination was to have an accurate altimeter.  Onboard the sailplane, Ed offered up a\nHexpert altimeter which could easily record every tenth of a second.  For kinetic energy, we used a FrSky airspeed\nsensor.  Unfortunately, its data sampling\nrate was no better than 0.2 seconds. As a result, when graphed every tenth of a\nsecond, the data has a \u201csaw tooth\u201d look to it compared to the Hexpert data.  Still, the data was accurate enough to give us\ninsight and could be corroborated by other collected information.<\/p>\n\n\n\n

These instruments were mounted\nexternally to the plane as pods shown in Figure 1.  The instruments were controlled by a separate\nreceiver so that someone other than the pilot could operate a second\ntransmitter for telemetry, data acquisition. \nAll the pilot had to do was fly the launch.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Figure 1.  Detachable instrumentation pods.
<\/p>\n\n\n\n

Another critical instrument was the onboard video.\u00a0 We chose a camera that is typically used in small drones.\u00a0 Figure 2 shows a Caddx Tarsier installation that is mounted on a carbon fiber blade and sandwiched between the wing panels.\u00a0 The camera looks towards the fuselage to monitor angle of the horizon, angle of the winch line, and wing flex under load.\u00a0 Data was recorded on an SD card which was removed and downloaded every several flights. The video was analyzed one frame at a time and measurements were made on a large screen monitor. Here is a short video of a launch:<\/p>\n\n\n\n

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