Postby Simon Wright » February 26th, 2010, 8:34 pm
Hi Terry,
I assume that you are considering a fixed pylon approximately on the CG position with batteries in the fuselage. The critical feature on powerpack selection with a pylon will be prop type and diameter. It is generally considered best to use as large a diameter as possible with fairly course pitch turning at low rpm. At 1/4 scale the pylon would probably be 10" or so allowing reasonably large 18" diameter props on a geared inrunner or low Kv outrunner. Folding props are preferred for drag reduction but the concern would be of fouling on the pylon on start up.
A decision needs to be made on how you wish to fly your model too. Aiming for 50 w per lb of aircraft weight would give a reasonable climb to altitude (11 lbs =550watts) running on 4 Lipo cells would give (550/14.8=37.8Amps)a fairly low current. If you discharged at a maximum of 20c you could use a 1900mah pack (37.8/20=1.89a/h) to give 3 minutes of full power climb. On a model of that size I would probably fit 5000mah cells to give just under 8 minutes of full power climbout.
Selection is a trade off between cost, duration, power etc. It may be worthwhile to look at a few catalogues and brochures for similar size models with electric power to find a good starting point.
Several on line motor calcuators are available, Drivecalc.de is very good for producing thrust figures and power curves but you have to figure out how it will work on your model in flight. Motocalc is available as a free trial version and this can be fed with your aircraft data to give in flight estimates of performance.
hope this helps
Si
LMA2345 now that should be easy to remember!