I’ve had a number of inquiries for an alternate Spirit of Sam power system. I worked out a system for Phil Pearce. He’s had a chance to do some run time and amp draw tests. Here are his comments thus far.
Good morning Dave. Here are a few picture of the little 5 oz Wren, built for the Spirit Of Sam event at the Society of Antique Modelers events. The rules call for the power battery be a single 160 mah lipo. The task is endurance, using the second best flight time of two flights. The motor you provided is an “Afterburner High Performance Brushless Motor. The number on the info. sheet says: MR-012-030-4000-1.5 I ended up using a GWS 9/7 prop. As the picture shows, the rear lipo is for radio power. The BEC is disabled. My procedure for maximum flight time is to hand launch with full power of about 1.5 amp draw, than establish a slight climb at minimal power of probably about 1 amp. I level out at about 200 feet and only use enough power to maintain altitude while looking for signs of lift. The motor will start slowing down at somewhere between 8 and 10 minutes with no thermal help. I must immediately start a landing approach while the prop is still spinning and plan on landing in about 15 seconds. Unfortunately a folding prop is not allowed. The drag on a stopped motor is dramatic on this small light model. I hope this is what you wanted. Thanks again for you help. Regards, Phil
P.S. If you have any advice on another prop, please advise. PP
Equipment is a 4000kv 12mm Afterburner with 1.5mm shaft. Running in a GWS LPS gearbox 4.8:1 ratio.
Links to parts supplied by Radical RC:
GWS LPS 5:1 “B” Pinion: GWGER002
GWS LPS 5:1 “B” Blue Spur Gear: GWLPS4B
GWS LPS 5:1 Blue Spur Gear: GWLPS4B
GWS LPS Ball Bearing Set: GWLPS2A
GWS LPS Gearbox Shaft: GWLPS6A
GWS LPS Gearbox Frame: GWLPS5A
Afterburner Motor (Take 1.5mm shaft option): MR-012-030-4000
Screws for AFterburner Motor: MR-SCREW-080-125-2
All parts needed are contained in the system linked below. Replace motor with 12mm Brushless Afterburner. Note, If you buy the brushed motor system to get all the parts, consider buying a pinion. A puller is needed to remove one from the included brushed motor and it can be ticklish. Some skill, minor tools and a drill press (or press of some sort) are required to install a pinion on LPS RXC OR RLC Drive take “B” gear ratio option: GWLPS-RXC or GWLPS-RLC
GWS LPS Gearbox Frame: GWLPS5A
Note, the point of this blog post is to record what we did in a way that will last longer than my memory. If you want to do the same thing, all the links are provided.
Some thoughts that I have based on what I know thus far: An “A” ratio (less reduction) could be considered in order to reduce the pitch or perhaps the disk diameter. That might lead to a 9-4.7 or 9-5 or 8-6 being optimal. I’m tickled by the idea of a 4 or 5″ high rpm setup in order to try and get the model to glide with a stopped prop longer than Phil reports with 9″ props. Not being experienced in the competition, my thoughts might be completely off base here. The higher KV Afterburner motor should be avoided because the higher no-load current. Although it might be more efficient at wide open throttle, the minimum no-load current would kill you in cruise (partial throttle flight). So, the lesson is stick to the 4000kv motor. Anything you can do in this contest to utilize a low non-load current motor would probably be an advantage over most motors with higher no-load currents. The no-load current rating is one of the fixed parameters (motor constants) of every motor. Explaining it fully is beyond the scope of this article.