Spirit of Sam Power System Decoded

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.

Phil Pearce Spirit of Sam Afterburner Setup
Phil Pearce Spirit of Sam Afterburner Setup

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

Phil Pearce and his 5 ounce Wren Spirit of Sam competition model.
Phil Pearce and his 5 ounce Wren Spirit of Sam competition model.

Equipment is a 4000kv 12mm Afterburner with 1.5mm shaft. Running in a GWS LPS gearbox 4.8:1 ratio.

close up of gws lps gearbox with afterburner motor installed.
Close up of GWS LPS gearbox with Afterburner motor installed.

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

side view motor and rx battery spirit of sam geared brushless power system
Side view motor and RX battery.

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.


Micro Stick Fleet Tryout

Gil Weiss from southeast PA sends a beautiful photo of his Micro Stick. Just test flown. Looks Great Gil!

Gil Weiss Micro Stick from South East PA.

Hi Dave,

Several years ago I built a Micro Stick. The kit went together very nicely. The little plane sat on a shelf until today. I enjoyed the last nice day of calm warm weather here in SE PA and test flew the plane. It was a real Hoot! Flew great and had an unbelievable roll rate. Landed fine. Other than add some more “expo”, no changes were required. I will keep this plane in my active fleet from here on out.

Regards, Gil Weiss


Reluctance Motor In The News

Chrome users may need to refresh the screen to see embedded video.

A reluctance motor is a different breed. It is a form of a brushless motor. However, the brushless motors we use in the RC hobby, the number of stater teeth (each iron segment that is wound with copper wire) will always be 3,6,9,12…; A number always divisable by 3. Magnets will always number 2 or 3,6,9,12 some number divisable by 3. Each magnet is considered a pole. So, a 6 magnet motor is called a 6 pole motor.

The cause of investment into the development of Reluctance motors is communist China produces most of the materials described as rare earth that are used to produce modern high power magnets. China has limited the export of these materials and magnets in order to help China monopolize permanent magnet motor production. The reluctance motor does not need any of these materials and is hence much cheaper to manufacture. Also our ability to produce motors in the USA would not be hindered by the unholy alliance between government and business as is so common in socialist/communist states. Regardless of the type of government, anytime governments stick their fingers into business, supply problems and shortages ensue. Perhaps these developments will stifle the anti-capitalists this time. Sustained monopoly’s are only possible when governments collude with business (or nationalize industry). It’s an interesting side note to mention there have never been any long lived monopoly’s that were not created and maintained by government regulation.

Clean Tech Open 2012 Winner HEVT

According to HEVT the spot price for neodymium increased from $19 a pound in 2010 to $129 a pound in 2011.

HEVT Hybrid Electric Vehicle Technologies

Many times technologies are developed and press releases fly all over the internet. These products offer great promise yet often the creators are just looking for somebody to buy the technology and when they are unsuccessful (most of the time) we never see the great new products. This development strikes a different tone with me, I feel this very well could be the beginning of a new paradigm shift in motor technology. Lets just hope it makes it down to us modelers sooner rather than later.

Gigaom Story: An Electric Motor That’s Ditched The Rare Earth Materials

Reluctant Hero’s (The Economist)

Learn more about Switched and Synchronous variations here: Reluctance Motor Wiki

Many photo’s and drawings: Google Images Reluctance Motors


Converting A Model From Lipo to A123 (LIFE) Cells

Chris from Michigan Asks;

Hello Dave,
I would like to power my Ryan with A123 cells. I have been using a 9s 4p 6000mah Lipo pack to power a Hacker C50 motor.
If I understand things correctly, I believe that I need 4packs 6s 2300mah to equal the same watts and flight duration as the Thunder Power packs that I have been running.
Because of the cost of these A123’s, I want to be sure that what I am ordering is correct!


To match duration of a 6000mah Lipo, you’ll need at least 6000mah in A123 which will be 3P. You’ll actually be a little over as the 2300mah A123 cells actually test most of the time around 2200, at 3P you’ll have 6600mah which should result in slightly more flying time.

I assume your 6000mah lipo is made up of 1500mah Parallel packs. So, 4 1500’s in parallel = 6000mah.

If what you really meant was your running 6000mah cells, 4 in parallel, then your pack size is 24000mah which would be about 11P or 11 A123 in parallel. I am not thinking you meant you had a 24000mah pack.

If your running 6000mah total in the airplane, and are thinking of running A123 in 2P for about 4400mah real world, this may be just fine. I generally only use the top 60% of a Lipo (70% max) on a routine basis. 60% of 6000 3600mah, 70% is 4200mah. I’m more comfortable running A123 cells a little deeper than Lipo’s as the risk of hitting BEC cut off cause me less worry. (Hitting BEC cut off is hard on Lipo’s) So, running a 4400 A123 down 80% is 3200mah. So, a 2P A123 should get close to the Lipo in actual use. However, your not running as much cushion between a solid end of flight habit and the bottom of the battery.

For matching running voltage, you’ll need 10 to 11S A123. I’d probably go to 11. At 12S, you’ll definitely have 3-4 more running volts. It will be like 9.8 Lipo or something like that.

To do conversion at nominal voltage, (# Lipos * 3.7) / 3.3 nominal of A123 = cell count.
To do conversion at full voltage: (# Lipos * 4.2) / 3.6 full voltage of A123 = cell count.

Nominal conversion is: 9 Lipo = 10.09 A123 cells

Full voltage Conversion is: 9 Lipo = 10.5 A123 cells.

Since you do most of your flying between full and nominal voltage I lean towards the full number for this estimation. 11 is the best choice. 10 you might notice a slight decrease in performance by your motors KV * volt reduction of the 10S pack. Right in the front of the pack, the 9S Lipo is 37.8v. Right in the front of a 10S A123 your full voltage will be 36v. So, KV X 1.8 = drop in top rpm. If your running a 500kv motor, that’s 900 rpm.

If you go with 11 cells, you’ll be starting out at 40.4v meaning your over the Lipo voltage by about 2.6v. So, you pick up (with 500kv motor) 1300 prop rpm.

Either choice means to get back to exact performance you had on Lipo you may need to alter the prop slightly, maybe an inch more pitch for the 10S A123 and an Inch less pitch for the 11S A123 or something similar to re balance things back out.

So, on balance, not knowing everything about the model and power system, I’d lean towards 11S. If you go 12S as your proposing, you’ll likely end up way over on RPM and Watts from where you were with the 9S Lipo pack. Important considerations here are if you mind a little more or a little less power (if the ESC minds more amps/voltage) and if you would need to change props, is there a convenient prop up or down that would suit the model and flying preferences. For example, if your running right at the edge of the ESC at this time and didn’t want to upgrade it, a slight decrease in power is acceptable, 10S becomes the obvious choice.

Another consideration not taken into account above is there can be a wide variation in quality of Lipo’s people are using out there. (not picking on Thunder Power, remarks for general readers of this post) Your current pack which may be performing just fine for the application may be worn and not really up to snuff compared to the original new condition. Thus if the current lipo has more voltage depression than it should, an A123 10S pack depressing less by some significant amount, could end up taching and watt metering out higher than the battery you are now using.


“Dave always took the time to talk to me and give advice at Toledo even though I didn’t even buy anything from him. He even stopped what he was doing, got his laptop out and ran motor specs for me for a kit I got at the swap meet. It was a classy thing to do and he is a fantastic ambassador for RC! Keep up the great work Dave!”


March 2009