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Title: Iyer’s tech talk - Is L/D 40 possible? Post by: K K Iyer on March 26, 2019, 09:24:26 PM Hi everyoe,
The next Tech Talk after a long time. Five years ago I built the Indore RC TLG. When I tried to launch it like a discus or tip launch glider, this is what happened https://m.youtube.com/watch?v=0tI2lGnxYqQ Title: Re: Iyer’s tech talk - Is L/D 40 possible? Post by: K K Iyer on March 26, 2019, 09:29:46 PM Then I test glided it like a towline glider
This was the test glide https://m.youtube.com/watch?v=ie2thgI193A This went 60meters, or 200ft, from a 5ft launch. That is an L/D of 40. This appears impossible. My calculations show that an L/D of over 10-12 is unlikely. (Will share if anyone interested) So how come it went 200’ from a 5’ launch? Appreciate responses. Regards Edit Proof of 200’ from 5’ launch is there. The question is ‘how was this possible’ SAE High Lift competitors may like to tackle this... Title: Re: Iyer’s tech talk - Is L/D 40 possible? Post by: pnkyjs on March 27, 2019, 05:58:18 PM I think there was a tail wind just after launch which swept the glider forward smoothly . If sir you launched it in the opposite direction then the result might be a much shorter softer glide. Even a cross wind is bound to lift the glider as it seems to happen two times. So maybe there was a gust of wind . Also, I see you launching like a DLG but if there were enough height or if you had been on top of a hill i am sure this glider would have got the time to righten itself and glide nicely.
Title: Re: Iyer’s tech talk - Is L/D 40 possible? Post by: K K Iyer on March 28, 2019, 10:01:45 PM @pnkyjs,
Nice response. Thanks. However you may recall that test glides are, as a rule, flown into the wind, and not downwind... Regards So, as it is not downwind, what could account for such a high L/D? Looking forward to responses especially from SAE contestants... Edit: Or, to putit differently, what best could we expect? Title: Re: Iyer’s tech talk - Is L/D 40 possible? Post by: Aeroresurrect on March 31, 2019, 11:58:45 AM Good glide video !
At 0.08 sec, either a gust or momentary stall-like situation disturbed its normal glide path. Daresay a 100'-120' distance would have been more normal. Title: Re: Iyer’s tech talk - Is L/D 40 possible? Post by: K K Iyer on March 31, 2019, 07:43:54 PM Thanks, Aeroresurrect.
The funny thing is that calculations using the speed, wing chord, aspect ratio, Reynolds Number and the polar for the AG03 airfoil show a theoretical L/D of only about 12.5. Or say ~10 in practice, more like one would expect from a model of this size. So one would only expect say 50-60 ft. Responses on RC Groups suggested something I had not thought of. That the glide was improved significantly by being in ground effect. Best would be to repeat the experiment in still air. That I fear has to wait, as: 1. The wind speed here in Indore is 20km or more till it gets dark ;D 2. A year or two ago, the model hit power lines. The metal towhook snagged on one line, and the fuselage boom touched another line. The boom was of carbon fibre, so there was a big spark, and the balsa stabilizer caught fire. Fortunately the towhook melted and vanished instantaneously. And that released the model! Repairs are still pending... |