Thread: Mini-Review Rimfire motors
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Old Sep 08, 2007, 04:28 PM
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Mini-Review
Rimfire motors

Objective analysis of some Rimfire motors.

Through the generosity of Bing Sheng and “e-guru” at Great Planes/Electrifly, I received for test purposes, four Rimfire outrunners (28-26-1000, 28-30-950, 35-30-1250 and 35-36-1000) and one inrunner – an Ammo 20-40-3500Kv, along with a Hyperflow 56mm EDF unit.

The motors are all well-packaged in “blister-packs’, with tech specs on the backing card, and some test data inside (more on those later). Inside there is a very informative brochure (appropriate to the size of the motor) which covers, in some detail, diverse topics such as batteries, motors, propeller adapters, ESCs, propellers, power system requirements, assembly and installation instructions, and precautions.

The 28mm motors have their tri-armed back-mounts already screwed in place. Four 3mm cross-head mounting screws and two O-rings are also provided. These motors come with long thin 2mm male bullet connectors fitted to the ?18ga tri-color leads - females (for the ESC) are also provided.

The 35mm motors have a four armed X-mount already fitted, and a radial mount prop adapter, rather than prop saver and O-rings etc. Naturally these larger motors have heavier gauge leads (16ga) and 3.5mm bullet connectors.

The Ammo 20-40-3500 is similarly packaged and documented – it has 18ga tri-color leads with the long 2mm bullets, and two 2.5mm cross head mounting screws are supplied.

I have assembled the fan unit, but I need to modify my stand to test it out [so, more on that later]. So far I have tested both the two 28mm and the two 35mm motors – so here are my impressions.

I used my usual MkIII Pusher Test Stand, with an Ohaus CS2000 Digital Scale, Zurich DS-304M Power Supply, Medusa Analyzer Plus meter, Futaba 6YG Tx, FMA Extreme 8 Rx, Phoenix 25 and 45 ESCs, Hobbico Digital MiniTach and RayTek IR temperature gun.

The fact that my test stand is in Pusher configuration means that there is really no cooling, from the prop blast, for test motors on my stand. However, even if it were in Tractor mode, I’m not convinced that there would be much air being directed through the motor anyway – though I admit that there would be some over the outside of the bell. The prop hub essentially blocks the front of a 28mm motor, and on top of that, there is surely very little airflow rearward from the very root of the blades.

A note on motor temperatures: Electrifly, in their testing, continuously record the temperature of the outside of the bell, while the motor is running. I don’t have equipment set up to do that, and my temperature readings are taken from the windings, immediately after a test series has been completed. Since the windings are the source of the waste heat, my measurements will inevitably be higher than those recorded by Electrifly. On occasion, I also recorded the external bell temperature, and, as expected, found it to be significantly lower (as much as 25%) than that of the windings.

Recent threads have highlighted the occasional, but potentially dangerous, tendency of the smaller 28mm Rimfires to lose the shaft (and prop) when they are used in tractor, non-propsaver mode. [This condition can occur if the propsaver screws are not tight, and/or the shaft is a loose press fit in the bell]. The minor modification of reversing the shaft and repositioning a circlip to the rear of the bell would seem to solve that problem – and it is to be hoped that Great Planes will address this with a tech bulletin.

I examined the 28-26-1000 and the 28-30-950, both of which have the problematic propsaver configuration. Since my test stand is set-up for pusher operation I had no need to undertake the reverse shaft/circlip modification, but I did check the prop saver screws for tightness and the shaft for security of the interference fit. In both motors I found that the propsaver screws were barely more than finger tight, but countermanding that, the shafts were so well-fitted that there seemed no way they could come loose! I attempted to press out the shafts on both motors and managed to get them to move only with very considerable pressure. I am convinced that under normal operation the shafts [on my two motors anyway] would be absolutely secure!

I removed the bells from both motors to look at the internal workmanship. The bearings seemed fine, and the windings looked neat enough, but in the 28-26-1000 I was a little surprised to see blobs of blue magnet-holding glue (some form of epoxy) some of which had run to one side of the bell (see photo). I have since learned that this is the method for dynamically balancing the bell (and certainly in testing everything seemed okay). One minor complaint is the quality of the shaft circlips – they appear to be made from very cheap steel, with virtually none of the vital “spring” they need to perform their task. I used high-quality circlip pliers to expand and remove these clips, but both were deformed during the operation – the fact that I was able to reshape them with just a pair of surgical forceps indicates the absence of “spring steel”.

Both these 28mm Rimfires are designed to be affixed to a firewall using a three-armed back-mount. Such a mount does not suit my test stand, but I found that I could use an aluminum Cobri outrunner stick mount to which each motor could be attached with two 3mm x 8mm socket head machine screws (see photo). I do not like using propsavers on test motors (especially those capable of >100W) so I oriented the motors on my stand such that I could use a collet adapter (MJ 8031) on the 3mm shaft for prop attachment.

The 28-26-1000 is said to be good for 12A (135W) continuous and 15A surge with APC SF props ranging from 9x3.8 to 11x4.7. On the stand the motor ran very smoothly, but it became obvious that 12A was pushing it and 15A was probably rather optimistic even for short bursts – with the larger props the winding temperature rose rather too fast for comfort. The motor clearly struggled to spin the recommended 11x4.7 APC SF [and this “struggle”, even at only 12.7A, is evidenced by the fact that at WOT on 10.8v the prop rpm (5340rpm) was below 50% of Kv x V (10800)]. I would suggest that a 10x6 GWS HD is a good prop for this motor – ~24oz @ ~40mph from a readily sustainable 10.35A. An 8x4 GWS HD on 4s is also a possibility: 24oz @ 42mph from just 8.55A (125W).

The 28-30-950 is said to be good for 14A (155W) continuous and 20A surge with APC SF props ranging from 9x3.8 to 12x6. The motor seemed happy enough even at close to that 14A, but once beyond that it clearly struggled to maintain rpm, and temperatures rose accordingly. Though I did not hold it at WOT for longer than necessary to record the numbers, this motor survived a brief burst with the 9x3.8 APC SF on 4s (226W). Using an APC SF prop at this level cannot be recommended since the 9x3.8 APC SF was well outside its safety range – 9450rpm calculates to 85000rpm/diameter – and the maximum limit is only 65000rpm/diameter! At the end of the testing I subjected the motor to a final indignity by trying the recommended 12x6 APC SF – initially pulling over 21A, the motor could not sustain that long enough for me to reliably record all the parameters – and after scarcely 15 seconds at this level the temperature rose from around 30C to 85C! Again the 10x6 GWS HD seems a good, if conservative, prop – 27oz @ 42mph from just 11.20A. For greater thrust at lower pitch speed it can spin the 10x4.7 APC SF – 30oz @ 30mph from 13.80A.

Now to some puzzling data.
Inside each package “card” there is a chart headed “Rimfire Motor System Configuration” and all the various models of the particular size motor (28mm/35mm etc) are listed, with entries for “Prop/RPM/Current/Watts” at the nominal 7.4v and 11.1v voltages (rarely 14.8v too).

In using my Power Supply, I set the voltage before each run at the 7v, 8v, 9v, 10v, 11v, 14.8v mark. Inevitably there is a small drop in voltage as the load increases. I have learned that Electrifly runs their tests by adjusting the voltage to the nominal 7.4v, 11.1v, 14.8v figure under load and so their test data and mine may not be exactly directly comparable. I suppose, if I had several extra hands, I could do that too, but it is not easy to adjust my Zurich to a specific voltage while a test is being run – the dial is very sensitive and I would need to make each motor-run a lot longer than my normal 15 second maximum, while I fiddled with the voltage level! I find it easier to run at, say, 6.9v or 10.8v… whatever…. and extrapolate to nominal voltages if need be.

The entries for the 28-26-1000 for the “11x4.7A” [I assume this means 11x4.7 APC SF] read:
7.4v, 4880rpm @ 12A, 89W

Now the problem I see is that my test stand numbers, with this very same prop, at the same voltage, are seriously at odds with these ones!
7.4v, 4470rpm @ 8.55A, 63W

With what they call the 9x3.5 APC SF, their given data are:
11.1v, 8470rpm @ 11.7A, 130W

But I measured:
10.9v, 7350rpm @ 9.40A, 102W

The entries for the 28-30-950 show slightly better correlation.

With a 12x6 APC SF they get:
7.4v, 4080rpm @ 14.8v, 110W

I measured:
7.4v, 3960rpm @ 13.45A, 98W

With a 10x3.8 APC SF they got:
11.1v, 7200rpm @ 15.1A, 168W

I measured:
11.1v, 7020rpm, @ 13.85A, 152W

How does Great Planes get their motors to draw so much more current, and generate higher rpm than I can manage. They used SS-12 and SS-25 ESCs, and I can’t imagine that those units would allow significantly more current flow than did my Phoenix 25. Electrifly is providing me with an SS- 45 ESC, so I will re-run some of these tests to see whether the type of ESC and the settings make a significant difference.

The motors I have tested so far are nicely machined, and they run smoothly and well. However, I can find no substantive evidence for the claims on the packaging which suggest that due to their “innovative housing” which “optimizes cooling” Rimfire motors “produce 50% more performance power than out-runners of similar size”.

One could make endless comparisons (with other “outrunners of similar size”), in an attempt to validate (or disprove) this “50% more power” claim. Since I have immediately at hand some data from some random motors which seem sufficiently “comparable” in size and Kv… let me quote just a few sets of “same prop/same voltage” figures from the same test stand.

The Rimfire 28-20-1000 (41g) and the Komodo KH2208-9 (1020Kv, 41g) seem close enough don’t they?

9x5 GWS HD:
Rimfire: 10.9v, 8.40A, 91W, 7740rpm, 36.6mph, 597g, 21.02oz, 6.56g/W, 85rpm/W
Komodo: 10.9v, 10.25A, 111W, 8340rpm, 39.5mph, 719g, 25.32oz, 6.48g/W, 75rpm/W

The Rimfire 28-26-950 (54g) and the Axi 2212/26 (920Kv, 57g) are close.

10x6 GWS HD:
Rimfire: 10.9v, 11.20A, 121W, 7380rpm, 41.9mph, 760g, 26.76oz, 6.28 g/W, 61rpm/W
Axi: 10.9v, 13.20A, 143W, 7770rpm, 44.1mph, 812g, 28.59oz, 5.68g/W, 54rpm/W

The “efficiency” of the Rimfires, as measured by g/W or rpm/watt, is indeed better, but that is largely due to the fact that these other motors generated somewhat higher rpm and greater thrust: it follows that as power-in/rpm/thrust increase, g/W and rpm/W, of necessity, decrease anyway. I just don’t see 50% more “power” or “rpm” or “thrust”……


Now to the performance of the 35mm motors: Rimfire 35-30-1250 and Rimfire 35-36-1000.

The Rimfire 35-30-1250 (71g) is said to be good for 30A (333W) continuous and 35A max. surge. The only recommended prop is the 10x7 APC E.

I began with a 10x6 GWS HD just to get a feel for the motor (and remain within the limits of my Power Supply). The motor ran smoothly and maxed out at 23.6A (251W) with that prop. That generated 1226g (43.17oz) @ 54mph. I then tried a 9x5 GWS HD, at voltages all the way to 4s - my Power Supply is said to be good for 30A burst, and, though inevitably the voltage drop was significant (to 14.3v), I was delighted to find that it somehow was able to deliver 31.40A. At this power input the 9x5 GWS HD was obviously over-taxed – at 12150rpm it is well-over its rpm limit (and showed it) - and so too much reliance should not be placed on the 1484g thrust figure. However, this run did indicate that, at least for a short burst at 450W, the motor did not get excessively hot.

I then tried the suggested 10x7 APC E using a 3s2p TP 4200 ProLite as a power source in the hopes of maintaining 11.1v so that I could compare my numbers with those supplied by GP for the same voltage. At 11.2v the motor peaked at 29.70A (330W) with this prop – this generated 1340g (47.18oz) @ 60.5mph. As the pack voltage dropped to 11.0v, the amp draw diminished a little to 28.40A (312W). The GP data indicated a 27A draw (300W) so we are not too far away – however, while I was recording >9000rpm at this power level, strangely, the GP figure is only 8250rpm. Hmmm?

Increasing the prop size a little to test performance at the 35A level might be worth a try, but since the motor was already getting a bit too hot for comfort (65C) after just a brief burst with the 10x7 APC E at <30A there may not be much point in pushing the envelope just to see what happens.

The Rimfire 35-36-1000 (102g) is said to be good for 40A (592W) continuous and 53A max. surge. The 10x5 APC E is recommended for 4s use, and the 11x8 APC E for 3s (I don’t believe that an 11x8 APC E exists – I have an 11x7 APC E and an 11x8.5 APC E – so I tried both of those to approximate GP’s 11x8 data).

My Power Supply is inadequate to cope with much over 25A and 300W-400W. Though I can run 3s series using my Enerland 6000 or 3s2p 4200 TP Pro-lites, I don’t have suitable 4s packs to supply >25A @ 14.8v…. so the data for this large motor are rather limited. Electrifly is working to provide me with a big 4s pack!

I first ran it with a 9x5 GWS HD for a full series, from 7v up to the 4s equivalent of 14.8v, just to get a feel for things. On 14.5v it drew 22.00A (316W) and generated 1471g (51.80oz) @ 56mph.

I could not try the 10x5 APC E at the 14.8v Electrifly had used – but I did run that prop for my standard series on the PS at up to 10.7v, then again using a 3s 6000 Enerland pack at 11.2v. The 11.2v run drew 19.50A (217W) and generated 1189g (41.87oz) @ 42.9mph. The GP data indicate that on 14.8v it will draw 38A (562W) and get 11790rpm from a 10x5 APC E. Someone with a more robust test set-up will be needed to verify that! I tried several other 10”-11” props primarily for comparative purposes with other motors of similar size and Kv – but none of them pushed the motor even close to its rated maximum of 40A/592W continuous). The most I managed was 372W (11.1v @ 33.60A) with an 11x8.5 APC E. How Electrifly managed to draw 40A/444W at that same 11.1v, with an 11x8, is puzzling.

I had hoped to compare the performance of these two 35mm Rimfires, with other brands of motor of similar size and Kv, but I found that for the 35-30 I did not have enough data for meaningful head-to-head comparison. The following data are all from runs with my Power Supply set at 11v.

The only other 35-30 motor I have tested is the Gemini 35-30-C (~1100Kv) – but with its significantly lower Kv, direct comparison proved not to be very useful.

With a 10x6 GWS HD the Rimfire 35-30-1250 drew 23.60A (251W) and generated 1226g/43.17oz @ 54.2mph (9540rpm) – 4.88g/W.

The Gemini 35-30-C drew only 16.40A (177W) and generated 906g (31.90oz) @ 45.3mph – 5.12g/W.

I have tested a Gemini 35-36-C (1000Kv) which one might assume would be directly comparable to the Rimfire 35-36-1000 – it turns out that, with several different props, the Gemini consistently drew a little more but produced only a little more (less efficiently) or actually produced less (obviously less efficiently).

With a 9x5 GWS HD the Gemini 35-36-C drew 16.00A (173W) and generated 954g (33.59oz) @ 45.3mph (9570rpm) – 5.51g/W.

The Rimfire 35-36-1000 drew 13.50A (146W) and generated 898g/31.62 oz @ 44.5mph (9390rpm) – 6.15g/W.
With a 10x6 GWS HD the Gemini 35-36-C drew 20.30A (217W) and generated 1110g (39.09oz) @ 50.3mph (8850rpm) – 5.12g/W.

The Rimfire 35-36-1000 drew 17.55A (189W) and generated 1090g/38.38 oz @ 50.6mph (8910rpm) – 5.77g/W.


With an 11x7 GWS HD the Gemini 35-36-C drew 27.10A (296W) and generated 1422g (50.07oz) @ 51.5mph (7770rpm) – 4.80g/W.

The Rimfire 35-36-1000 drew 26.10A (279W) and generated 1460g/51.41 oz @ 52.7mph (7950rpm) – 5.23g/W.


As far as I can ascertain, these Rimfire motors appear to be well-made, they run smoothly, and they perform pretty much as advertised. Far too many brands of motor come with inadequate or misleading technical specs and information (or none at all!). The documentation which accompanies these Rimfire motors is very detailed and generally accurate. I was given access to in-house program notes which outlined the strict testing regime used by the GP/Electrifly technicians to ensure that their motors do work as they should – and it is fair to say that they have succeeded.


Cheers, Phil
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Last edited by Dr Kiwi; Sep 08, 2007 at 04:33 PM.
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