james_h wrote on 03/11/10 at 10:44:45:


Your conservative approach is probably a good one because most of the 'damage' suffered by lipos occurs as you discharge the last 30% of available energy.  Another way of looking at it is that five tows followed by a charge of, say, 30 minutes and then another 5 tows is just as good as ten tows followed by a charge of one hour but with the advantage that you're not stressing your cells as much.

Mike

James

I am not a great fan of calc spreadsheets once past the "general pointer" stage. I prefer to get out and do some real testing. With the help of Ivan Goodchild we optimised our set up by fine tuning on the flying field. I have lost count of the number of different props we tried but you will be amazed at the difference just changing the make of prop makes. (A big thank you to Dave Wilshire at Motors and Rotors for lending us a box of props). Both the Ampley and the 1/4 scale Cub that we have  now run a similar set up drawing 60 amps at takeoff but as soon as the combination is off the ground we have found we can "throttle back" to 55 amps on the tow up without any loss of performance. With this set up we get the 5 tows to 1000 ft at a climb rate of 1000ft per min towing a glider not exceeding 12 kg. The Cub does not quite match the Ampley climb rate due to its inbuilt drag and the fact that it is a little bigger so we tend to use it more for scale towing and towing low and slow in circuits and figure eights. Great practice for both the tug and glider pilot when there are no thermals.

John

Mike

you are right re the props hgaving to be optomised for eachmodel but after all the testing we have ended up with the G sonic 20 x 10 on both the Cub and Ampley. This may be more due to the wide paddle type blade on the prop working well whilst towing. A lot of the other props we tried were APC types which have a thin blade which "may" not move the air so well with the weight and drag of the glider on the back.

John