Peter Griffiths 1

I think I've discovered how to upload images to the forum. Here's a picture of my 2M esoarer to test the procedure.

Here's my third attempt at posting the graph (it may show a web link rather than an image): http://s1290.photobucket.com/user/pvg47/media/Odysseysink-ratetest_zps1ec46b4d.jpg.html

  Here is the graph from my previous post (assuming I've fathomed the upload procedure).   <img src = "http://s1290.photobucket.com/user/pvg47/media/Odysseysink-ratetest_zps1ec46b4d.jpg.html">       Edited By Peter Griffiths 1 on 13/08/2013 20:48:19

I reckon the sink rate for my Odyssey 2 metre esoarer to be around 2 ft/sec. This figure is derived from data provided by an onboard altimeter from rc-electronics.org (available from esoaringgadgets.co.uk). The altimeter device I have only records height and time, not speed or distance travelled, hence its performance metric is sink rate. To calculate glide angle or glide ratio would necessitate flights being measured along a fixed course on the ground, such as a triangle between pylons, or, more accurately, by an onboard GPS module by which to determine distance travelled from motor cut-off to landing. The problem about obtaining reliable test figures for glide performance is the difficulty of controlling atmospheric conditions. Clearly, lift and sink should be avoided, but other variables such as wind speed, humidity, temperature and launch height have not been standardized when measuring model characteristics. The best one can do is pick a time of day at a flat site when the air is judged as stable. Also, the way the model is flown affects how it will descend. During a flying test, control inputs should be minimal as moving aileron, flap, rudder and elevator surfaces away from their neutral positions creates drag. Although a straight line flight test over level ground would be ideal in order to establish glide performance, practically, we have to opt for launch and landing points to be the same and to fly using lazy circuits as much as vision will allow. The graph below shows my Odyssey 2M tested in such a manner on a summer’s evening close to sunset when the air was calm and dry and thermal activity had ceased. I set the height limiter switch to 650 ft (200 metres) in keeping with esoaring competition rules. The logger showed the throttle switching off at 652 ft with a climb time of 29 secs. The glide duration was 5 mins 49 secs. I left the glider’s controls largely alone and did not use flap or spoiler on landing. The logger gave the average sink rate over the flight as 1.84 ft/sec. Dividing height loss by duration (i.e. 652 ft/349 secs) gives a similar value of 1.87 ft/sec. The perfect theoretical sink-rate graph is a straight line from start to finish of the glide and what I obtained came close. A graph showing deviations indicates zooming or diving caused by turbulent air or pilot input and must be discarded for the purposes of testing the efficiency of the model. A previous posting questioned why maximizing the glide ratio matters. In glider design, this is the holy grail and what has driven improvements in design of full-sized and model gliders over the decades! In duration or distance competitions, all other factors being equal, the glider with the highest distance to descent ratio, i.e. the lowest sink rate, will be the winner. A glider is powered by the the steady force of gravity and is always sinking in the surrounding air, even in ascending wave or thermal. As gravity cannot be altered, all the glider designer can do to is experiment with weight, aerofoils, streamlining and materials in a continuing quest to improve lift and reduce drag.