Inside Story -- March 2004 - RC Groups

Inside Story -- March 2004

This month's inside story covers a quick look at micro flight BEFORE lipoly, pager motors turned power plants, and then Bit CharG car motors/electronics into microflight (complete with plans!), some crazy new birds, and the E-Tec 90mah LiPoly cell.

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The First Bit

In an effort to save time and get this issue of IS out some time this year, I have not changed the title of the preamble from that of my template file. I hope it proves to be amusing enough for you to forget how long it has been since the last installment. In the classic style of the column writer, I have to confess I have been rather busy of late with a variety of commitments, the deadline for my PhD looms and funding has stopped so I have to try and earn my crust. Still, all is OK because good folk like Gordon Johnson and Michael Henriksen have kept the home fires burning and present articles on the wonder of pager motors and models made from tiny RC cars.

What the heck is a pager motor? and how did we get here?

Gordon will tell you all about them in a moment, but before that I would like to remind us all how lucky we are. A few years ago when I began flying micro RC, the Aeronutz to which I belong, had a project called 2C2C. Invented and organized by Mark Denham, 2C2C stood for two-cells, two-channels. The two cells were normally two 50mAh NiCads and the two channels were rudder and throttle. We struggled at first but later succeeded in making really nice scale models that could fly on a mere 2.4v of "juice". By using foam such as wallfoam (a 2mm thick expanded polystyrene used to back wallpaper for insulation) or Depron, the models could be built to 25g or less without much difficulty, building on the experience of the freeflight and larger RC models made previously by the Aeronutz clan.

Only 2.4v, that is TWO point FOUR! Gordon Brown's Bristol had and still has an astounding climb rate, my own Se5 was similar (when it was in trim and the wings stayed on) and Ray Holiday had an array of fighters and biplanes with him at every meeting. All genuine flyers and all could ROG. Why is 2.4v so amazing? Well it isn't really but when you compare it to what we have now it is a wonder we could fly at all back then (and were weren't the only ones). The Kokam 145 and later other Lithium Polymer (or LiPoly/LiPo) cells have 4.2v per cell, which is almost double what we had for two cells. But wait, a 145mAh LiPoly is almost half the weight of a two cell 50mAh Nicad pack. Now you see where I am coming from, the LiPoly revolution has brought us more volts and hence more power with less weight and that is ignoring the increased capacity. We have all this power to do with as we please, either to be more aerobatic, fly slower, fly smaller or just fly longer. It also allows us to do crazy things (more on that later).

But what has all this reminiscing got to do with pager motors? Well pager motor powered models more than any other area of microflight benefit from LiPolys, and due to there weight they ARE the immediate future of Micro Flight. Not the final frontier perhaps but maybe as far as many need or want to go. Why does the LiPoly affect the usefulness of the pager so much? Well it isn't the weight of the cell really but rather the voltage. Anyone who has flown with coreless motors in larger models will probably know that they like their volts. But at the same time they are only small so the models they fly must be suitably small as well (or at least light). A single NiCad weighs around 3.5g (if I am wrong sue me, I no longer care how much they weigh) and so that is probably the limit a single motored pager model can carry. One NiCad is only 1.2v, no chance!! Enter the dc-dc, Team JMP developed a model well ahead of it's time called the "Moins que rien" (less than nothing), this used a single (lightened) NiCad and a dc-dc to provide more volts to the motor. Coupled with a super high gear ratio they flew at a weight only dreamed about previously (9.8g.) Later Chris Fouweather (Dr Chris) began making dc-dc converters to power small models on a single cell, but this time for scale and near scale models. His converters pushed the cells and converter technology to the limit. You see, you never get something for nothing. Power is volts x amps so if you put less volts in to the converter then you get out, you must put in more current. At this stage the high peak currents needed very carefully selected inductors (coils), not just for electrical reasons but for weight! Chris got usable converters down to about 1g and made several models that flew using them.

Then came the LiPoly, more volts to go into the converter so less current was required into the converter. Smaller inductors could then be used. The increased capacity of the LiPoly cell and the lower amp draw being taken from it also increased duration. Both Dr Chris and I simultaneously developed almost identical DC-DC converters to take advantage of this with weights of around 0.2 to 0.3g. At that weight and with easily available components, scale and scalish pager powered models became a real possibility. The first model I made, within a month of the 145 Kokam coming out, was a profile Spitfire that was grossly overpowered. It had a weight of 12g and a 10" wingspan. I never trained it to fly reliably but may have another go after reading Peter Frosick's excellent article in the last IS on trimming low wingers.

I know we were not alone in playing with pagers and dc-dc converters and certainly not alone with playing with pager motors but this is not supposed to be a history lesson, I have a point.... honest! With a high enough gear ratio the 4.2v of the LiPoly was enough to get some models flying without a DC-DC, especially those that were super light or very efficient. Didel had already brought out a range of gears and gearboxes, as well as the pagers themselves and these came alive with the LiPoly technology. So things are good, but then what happens? Didel brings out a new motor that is "just right" as Goldilocks would have said, Gordon will tell you all about that too if you read on. The main point is that this motor is not so "hot" that is burns out but also not so "cold" as to require a large voltage or high gear ratio to get models flying. Most of us generally want to fly sports and scale models and huge props don't suit so a motor that will work on a 4:1 to 9:1 gear ratio is much more sensible. Gordon's Quick Junior plan last issue was for a 15g model using this motor, loads of power, plenty to spare. Gordon's model also takes advantage of another step forward: the Etec 90mAh LiPoly. This is 2g and can provide 500mA, more than enough for a pager. That is another 2g saved, so FM radio (JMP, RFFS) rather than IR or wideband AM radio is OK as long as you don't build too small. Gordon presents info on this cell at the end of this issue.

Gordon's Quick Junior (or a model based on that power plant and AUW) gives a hassle-free start to pager models and here are a few more working pager models to give you all ideas. They are all made by Ray "30 model building hours in the day" Holiday, a seasoned Aeronut who has been putting the new Didel pager through its paces. Here are some of his models:

From left to right:

  • Corsair: Details and more photos below. Blue foam construction.
  • Next three are from 2mm Wallfoam, Ztron IR for control.
  • High winger: (I forgot to ask what exactly), 8.7g, 7:1 Didel gearbox and 4.5ohm pager, Etec90 LiPoly cell.
  • Twin: (someone shoot this reporter), 12g, 12"ish span, Falcon PO3 (5.25:1, 4.5ohm Didel pager), 145mAh Kokam LiPoly cell.
  • Polyhedral: That isn't what it is called, Ray hasn't got time for names, 8.7g, 10"span, 7:1 Didel gearbox and 4.5ohm pager, Etec90 LiPoly cell.

You will notice that Ray has four bladed props on all of these models. It is scale on the Corsair but it is also a nice little solution to the problem of prop size. "High" gear ratios tend to like larger props and these 4 bladers give a bit more prop for your Peso. I asked Ray about them, thinking, "I bet the readers will be interested to know about these". Ray's response was typical, "they weren't made to any particular size or design but I tried to get each blade in at the same angle." The angle? That is anyone’s guess but some experimentation and a few yogurt pots (the high sided type) will get you something that will work.

A word on performance, well there is loads of it. I saw the little polyhedral winged model climb out vertically and Ray’s continual attempts and occasional success in doing a loop without an elevator were most amusing. They actually have more performance than most (if not all) M20 based models we see. That is not surprising, look at the weights!!! The Corsair being a bit of a fatty (as Ray didn't lighten it as much as he says he could) is the most sedate but still ROGs and flies very well.

Crazy Things

I mentioned crazy things. Well, here are two 6.9g CRAZY THINGS!

These little marvels were revealed to the public on the same day, exactly 100 years after the Wright Brother's historic first flight. For more details go here. Both use the new pager motor and LiPoly batteries (what else). They simply would not be possible (at this size) without these new developments. It is indeed a good time to be building micro and indoor models, so much more power and so much less weight to use as you see fit. Helicopters, ornithopters, autogyros and other strange machines become more and more realistic at small sizes. Or you can stay with more normal models and go smaller or stay the same size and just enjoy slower and longer flights. Perhaps you are into scale or aerobatics, you can use the power to offset high drag in bipes or perform aerobatics. The low weight obviously helps with the aerobatics but can also allow more scale detail. So go on, use the power, feel the force!!

One point I must re-emphasize. If you start to go smaller .... trim the model not the controls. It is always likely that there will be a slight tendency to turn etc that can be offset by a little trim on the controls and that is fine but really concentrate on getting the model to fly right on it's own. A glide can check the CG, a powered glide the thrust line, and beyond that there are many other hints and kinks that can be picked up from the pros. When Peter Frostick tells me how his spitfire flies like a trainer it brings the point home to me. Micro models are not hard they just need some respect, build light, build right and trim.

Just when you thought I would shut up:

Kokam are now selling a 20mAh cell. Some have used the 40mAh (wafer thin mint) cell successfully but it does have its drawbacks. The main problem is the weight, the package is great for the slim camera it was designed for, but of course has a large surface area and so the packaging weight becomes a considerable percentage. It is also quite large so had to be hidden in the wing, folded (don't recommend) or left hanging in the wind for many applications. The new 20mAh cell is in a different package -- much more compact and with a weight of 0.7g -- they are a whole new world. They can provide 60mA which is not huge but small models don't need much power (Gordon will talk about that too). For more details about this cell please have a look at this thread. Also in the thread the CEO of Kokam tells us he is prepared to make a 60mAh cell, possibly with 7C discharge, let's hope so. We are just SO spoiled!

The Car's the Star:

Michael Henriksen or epilot as he is known on the forum is a real character and his witty remarks and knowledge are always welcomed in the threads. Between doing up motorbikes, cars, building steam engines, moving house and having a life, he likes to build small models and does so at an alarming rate. The model and plan he presents here was designed especially for beginners wanting to build something simple for very little money. Built from foam in an hour or two and using the RC gear out of a cheap radio controlled car, it is certainly at the ultra cheap end of the microflight market. Having said that, there is nothing to stop you installing the finest RC gear money can buy or scaling it smaller for pager motors. Michael recommends bought actuators for convenience and I would go along with that but for completeness let me point you here to the micro forum index thread where you will find loads of extra info on actuator building, coil winding and other approaches with these tiny cars.

And Finally:

After some discussion in a long thread on gearbox making, Robert Guillot showed us a nicely constructed test gearbox to check his construction techniques and meshing. I made a cheeky comment about it looking like mouse furniture. That wasn't enough for Carl Martin, the pictures say enough :)

Right, I really will shut up now. The preamble has really ambled, but there is still much in micro that is new, I'll try and use this section to tell you about it in later issues. For now I am all ranted out.

Feature Article - Pocket Guide to 6mm Pager Motors, (or how I came to love a 4.5ohm pager).

By Gordon Johnson

For some time now there has been a lot of interest in pager motor powered micro planes. There is much confusion about how to put these motors to use and what the differences between motors are. In this article I take a look at many of the commonly available 6mm pager and Bit Car motors. I also examine the Didel 4.5 ohm pager in some detail as well as gears, props, and gearboxes.

As usual, several friends provided useful technical help and advice as I proceeded with the tests for this article. Roger Carignan did parallel tests of the 4.5 ohm Didel pager during initial tests when we were trying to figure out exactly what it was capable of. He and Matt Keennon were also very useful sounding boards for preliminary results and great sources of advice. Carl Martin made gearboxes that allowed motors to be easily swapped and thus many different motors to be tested. Some also allowed testing at several different gear ratios.

6mm Motor Overview

Pager motors are a class of inexpensive motors originally used to power a vibrating alert in pagers and cell phones. They are often coreless, but can also use traditional cored technology. One question we might have about a particular motor is how to determine exactly which motor it is. The easiest way is to measure the resistance in ohms across the two terminals or leads. To measure the resistance first connect the motor leads to a multimeter. Put a pinion on the motor shaft and then insert it into a short balsa stick with a hole in the middle that can be pressed over the pinion. The stick makes it easier to precisely position the motor shaft. Measure the resistance, then rotate the shaft slightly and measure again. Repeat this several times. The motor shaft must be completely at rest each time or the reading will not be valid. Use the highest measure because lower readings may occur if the motor shaft is positioned such that the brushes are straddling the commutator for two sets of windings rather than one. Finally, the resistance readings can differ by as much as +/- 1 ohms for identical motors from the same source. With the resistance of the motor in hand and pictures of the motors in this article or knowing the motor’s source, you should be able to determine approximately which motor you have and how to use it.

Connection methods fall into three main categories. Pager motors generally come with wire leads exiting the rear, but occasionally on the side near the front. Bit Car motors generally make their ground connection through the motor case and their positive connection through a small terminal on one side of the back plate, or through a diamond shaped connection in the center. This makes it more difficult to solder up the Bit Car motors. The Didel pager motors with resistances less than 10 ohms are essentially Bit Car motors with short insulated wire leads. This makes them easier to use.

Most of the 6mm motors available are coreless. The top panel in the picture below shows the Didel pager motor after it has been taken apart. There is a steel can that has the stationary magnet permanently attached to the inside of the front bell. A set of coreless windings fit over this magnet and a small plastic plate attaches the windings to the commutator. A shaft passes through the commutator, through bushes within the stationary magnet, and out the front bell. Essentially the windings are a drum that rotates around the magnet.

The bottom panel in the picture below shows a Bit Char-G motor after it has been taken apart. The steel can has curved ferrite magnets inside it. The stator is simply a very small version of a traditional cored stator. In fact, this one measures just 4.2mm in diameter. And, finally there is a plastic end bell with the brushes. The Mabuchi 6mm motor that Toytronics sells looks very similar inside to the Bit Char-G motor.

The motors in these tests include most of the 6mm Didel motors, several of the Bit Char-G motors, several of the Bit Clone motors, and a few of what I am calling surplus pager motors. I do not have all the Bit Char-G or Bit Clone motors so present results for those I do have. The "Snow Globe" Bit Char-G Clone motors were provided by Billy Stiltner. The ZipZap Bit Clone motors I purchased at Radio Shack in a hop-up kit. The Bit Char-G motors I obtained nearly two years ago, early on in the Bit Car craze, from a hobby shop in Hong Kong in genuine Bit Char-G packaging. I don’t know if Bit Char-G cars still come with these cored type motors. I’ve also included the 7mm Didel pager for comparison as people have often been curious how this larger pager motor performs.

Focus of Tests & Methodology

In this set of tests I simplified the problem of evaluating different motors by using just one gear ratio and prop. I chose a 5-inch prop as much if much larger the prop would either look funny or would hit the ground for typical scale or stick planes. I chose the 6.7:1 gearing as the higher ratio from the Didel 81t spur gear is more difficult to fit inside a scale cowl, unless the plane had a radial engine, and would also require a larger prop. I’ve also focused on single stage gearing. Thus, this set of tests is really an exercise in finding the best motor for a given prop and gearing. It is possible that for a different prop and gearing a different motor might do better. However, if one motor outperforms another motor by a sufficient margin, then it is likely to also do so for other props and gear ratios.

Test Results

Table 1 presents results from tests of all motors with 6.7:1 gearing and GWS 5x3 prop. Within the Didel pager motors group we can see that the lower ohm motors generate more thrust, and also have a higher amp draw. The 3.2 ohm motor has an amp draw of 0.58, which would be too high for the E-Tec 90mAh cell’s 0.5 amp max discharge rate. More importantly this motor runs hot and its static thrust quickly declines. This prop and gearing is clearly too much for this motor. The 4.5 ohm motor, however, has a substantially lower amp draw of 0.33, well under the E-Tec 90’s max amp draw. Moreover it does not run hot and generates virtually the same 14.7g thrust as the hotter wind 3.2 ohm motor. Moving to the 6.0 ohm motor the amp draw is almost the same as the 4.5 ohm motor, but its thrust is substantially lower at 10.5g. The higher 8 and 10 ohm motors have substantially lower amp draws of 0.21 and 0.18 amps respectively. But, their thrust is substantially lower at about 7.7g and 7.5g. Clearly within the Didel pagers the 4.5 ohm motor is in the sweet spot in terms of maximum thrust without overheating or exceeding the E-Tec 90’s amp limit. The 10 ohm motor is also clearly best where lower thrust is required but low amp draw is important. Finally, the 7mm pager develops roughly the same thrust as the 10 ohm motor, but pulls the same amps and weighs nearly twice as much. It just doesn’t measure up compared to its smaller siblings.

Overall the Bit Clone motors do not perform as well as the Didel motors. The white bell Snow Globe motor has the same 4.5 ohm resistance as Didel 4.5 ohm motor. However, it pulls 0.44 amps compared to the Didel’s 0.33 amps. And, it develops a gram less thrust and runs hot. Although I’ve tested several of the Didel 4.5 ohm pagers with equivalent results, this is the only Bit Clone motor I have with this resistance. However, other people seem to be finding similar results for the Didel motor outperforming the Bit Clone motors. The hotter wind 3.7, 3.3, and 2.6 ohm Bit Clone motors ran very hot and burned out within a minute. The similar resistance Didel 3.5 ohm motor runs warm and does not immediately burn out. Again, the Bit Clone motors do not seem to perform as well as the Didel motors. However, if the Bit Clone motors are equipped with smaller props they might last longer, although developing less thrust. Overall, the 3-motor ZipZap hop-up package does not seem that useful to me. Two of the motors have too hot of winds and quickly burn out and the third doesn’t have a hot enough wind and develops less thrust than the Didel 4.5 ohm pager.

The Bit Char-G and Mabuchi motors are not coreless. The Mabuchi pulls slightly more amps and develops less thrust than the equivalent 10 ohm Didel. The higher resistance Bit 2.2 motor pulls more amps and develops less thrust than the Didel 4.5 ohm motor. Clearly the coreless motors have the advantage here.

Within the general surplus motors the Namiki shines and develops equivalent thrust to the 10 ohm Didel. This motor has been known for a while to be a good pager motor and this confirms it. Still, it does not develop the thrust that the 4.5 ohm Didel does. The brown bell surplus motors are the ones often available from general electronics surplus houses. In addition to being heavy, and having a non-standard 6.1mm case, they don’t perform well either. They have relatively higher amp draws for their resistance and develop only typical thrust.

One last way to characterize all these motors is by some simple figures of merit. What we see is that the Didel 4.5 ohm pager has a very high thrust/weight ratio and also a high (thrust/weight)/amps ratio. The other motors with higher values on this measure, which considers both weight and amp draw, have substantially lower levels of thrust. Since the E-Tec 90mAh cell can easily handle the amp draw of the 4.5 ohm Didel this motor overall looks like the best choice for a 6.7:1 gear ratio and 5x3 prop.

Exploring the 4.5 Ohm Didel 6mm Motor

Since I first flew my Quick Junior with the 4.5 ohm pager motor in August of 2003 it is quickly becoming “the” pager motor to use. Properly geared and propped, it generates good power without getting hot.

Let’s start with a quick review of the this motor’s characteristics. Early on in my use of this motor, before it was widely available, I sent one to my friend Roger Carignan. He was interested in using it direct drive in a super micro 10g version of his pusher configuration Pinky, while I was more interested in using it geared in my Quick Junior. Roger enhanced Joachim Bergmeyer’s formulae for finding motor constants for a given motor (see the February 2003 Inside Story article). Roger’s version improves the calculation of rotational power loss based on two measurements of no load conditions, rather than one. I don’t want to go into more detail here. It’s enough to say that Roger measured motor constants for this motor and then I did the same with my equipment and got virtually the same results.

The graph below shows predicted motor efficiency and power, in Watts of output, for the 4.5 ohm pager at various RPM’s and at 3.6 volts. These predicted measures were obtained using Joachim Bergmeyer’s techniques referenced above. The dotted red line shows power output which peaks at 0.64W and 25,519 RPM. The solid blue line shows efficiency which peaks at 52% and 33,818 RPM.

Table 2 below shows a variety of static tests for the 4.5 ohm pager. The first two rows give the predicted measures for max efficiency and max power and the corresponding motor RPM. These are useful for interpreting the results of the various static tests. In general we will want to choose prop and gear combinations that have a motor RPM that lies between the max power and max efficiency RPMs. If the ET-90 LiPoly cell is used we have plenty of capacity, so efficiency will not really be an issue, and we should try to find prop and gearing choices where the motor RPM is closer to the max power RPM.

For each test the amps, watts, thrust, and prop and motor RPM are given under the static measurements heading. For each test the predicted amps, efficiency and power are also given under the predicted motor measures heading. And, for the efficiency and power the percentage of the max efficiency and max power at the top of the table are also given.

The Falcon PU04 propulsion set now comes with the Didel 4.5 ohm motor. It has a gear ratio of 6.5:1, and will soon come with a folding KP00 96mm prop. So a limited set of tests for this gearing are included. However, since the 6.5:1 and 6.7:1 ratios are so close, I present more tests for the 6.7:1 gearing, but they really apply to the PU04 with its 6.5:1 gearing as well.

Let’s work through an example for the 6.7:1 gearing with a 5x3 prop. This combination has a motor RPM about midway between the max efficiency and max power RPM’s. Its 0.34 amp draw is well within the 0.50 amp max discharge capability of the ET-90 LiPoly cell. The predicted motor measures columns show a predicted amp draw of 0.36 amps, which is very close to the actual 0.34 measured amp draw. Predicted efficiency is 47%, which is 90% of the 52% max efficiency for this motor. Predicted power is 0.61 Watts, which is 96% of the max power of 0.64 Watts for this motor. Overall, this is a very good combination for this motor. In fact it is probably the best combination if prop diameter is not a constraint. The higher pitch 5x4.3 prop has a higher amp draw with the 6.7:1 gear ratio but develops less thrust. Although the motor RPM is closer to the max power RPM, it is below that RPM. So, it would seem to be moderately over propped. Overall we find that just about the best combination for this motor is either the 6.5:1 or 6.7:1 gearing and a 5x3 prop.

If you are building a semi-scale plane with a 10 to 12 inch or so wing span, the 5-inch prop is probably not an option. A prop the size of the U80 or maybe a slightly larger 3.5-inch prop is probably a better choice. From the table we can see that the U80 is a good match with 4:1 gearing. It develops about 9g thrust with high percentages of max efficiency and max power. If a bit more thrust is needed the larger diameter and higher pitch 3.5x2.7 prop delivers 11g thrust, but efficiency drops.

One way to save weight on the lightest models might be to eliminate the gearbox, which typically weigh about 0.7g, and run the motor direct drive. To explore this possibility I tested the motor DD with a GWS 2.5x0.8 prop. Then, I trimmed the prop to a smaller diameter, tested it, trimmed it again, tested, and so on. The results are shown near the top of Table 2 along with the prop diameter in millimeters after trimming. The first thing worth noting is that the untrimmed prop is clearly overloading the motor. The motor RPM is far below the max power RPM. Only the 37mm trimmed prop has an RPM close to the max power RPM. The picture below shows the untrimmed and the 37mm trimmed prop, which can best be described as a “nub.” The best prop is probably the 41mm version. The amp draw is on the high side for this motor, and I don’t know how long it will last at a 0.43 amp draw. It ran only slightly warm so motor life might be OK. The other thing to note is these were very crudely trimmed props where the ends were simply chopped off and then sanded thin. It may be possible to get better thrust by trimming this prop to have a slightly larger diameter than 41mm, but with thin tapered blades. If this is attempted the goal should be to get a much nicer trimmed prop that pulls about 0.43 amps, and hopefully with higher thrust.

How to use Table 2

Here’s how to use Table 2. Spec out your plane to figure out the maximum size prop you are willing to have on it. Once you have your prop size, find something close in the table that has a motor RPM close to the max power RPM and has a good thrust compared to other similar sized props. I’ve highlighted one prop in each gearing category that is a good choice, although others may be acceptable too. Next compare the static thrust with some of the rules of thumb about how much thrust to weight you need and compare this to your estimated plane weight. A good one for indoor models is that if your static thrust is at least equal to 1/4 your model weight it will be able to fly, and equal to ½ your model weight it will fly well, and equal to ¾ it will be aerobatic. So, if you have a plane that is not excessively draggy, and it weighs 15g and you choose a prop and gear combination that yields 10g static thrust you will have a great performing plane. Alternately some people prefer a watts/ounce rule that at 1.5 watts/ounce the model will have average performance, 2.0 is about right for a typical sports model, and 2.5 will give aerobatic performance. In this case a 3.5x2.7 prop geared 4:1 would be a good choice. These rules of thumb are thanks to John Worth, Bob Aberle, and Don Srull.

Thrust Over Time

I can almost hear someone reading this thinking “I wonder if I can make a pager plane with this motor that will hover?” The graph below shows the results from a test designed to simulate full throttle over time. All the previous tests were powered by a laboratory power supply straight to the motor, with volts measured at the motor terminals. This test powered the motor via an RFFS-100 receiver, with a single BSD MiniAct hooked up, and powered by a freshly charged ET-90 cell. During the test I operated the actuator back and forth to put an additional load on the cell due to actuator use. I measured volts under load at the battery and also at the motor terminals. Volts at the motor terminals were about 0.15 volts lower than at the battery throughout the test. The test was terminated when volts at the battery dropped below 3.0 volts.

This test represents an extreme test of the propulsion system and the ET-90 cell. Thrust drops steadily during the test from about 14g to 8.5g. Volts under load at the motor also drops steadily from 3.6 to 2.9 volts. During the first three minutes of discharge the average thrust is 11.8g and the average volts is 3.4. Over the remaining 7.5 minutes of discharge the average thrust is 9.8g and average volts is 3.1. These are very encouraging results. Since most of us don’t fly our micro planes at full throttle for the entire flight we will have power for the occasional loop if the plane is relatively light. If we choose to cruise around at partial throttle the flight time will be considerably longer than 10 minutes. Now about that hovering. If someone builds a plane light enough to hover, these results show that the hovering will have to be done in the first couple of minutes of the discharge.

Gears and Gearing

The table below shows the single stage gearing possibilities using Didel or Falcon gears. The Didel gears are 0.3 mod (except for one pair of 0.2 mod gears) and the Falcons are 0.25 mod.. The Didel spur gears have lightening holes and a 12t gear on the front. The also offer a wide variety of gear sizes. The lightest gear set by far at 0.078g is the 5:1 gearing using the Didel 0.2 mod gears. Gear sets that were used in tests for this article are noted with a check mark.

The picture below shows a range of possible gearboxes for pagers. Didel Makes a 6.7:1 gearbox for the 6mm motor that allows the motor to be inserted or removed easily. More recently Falcon has introduced a 6.5:1 (PU04) gearbox which comes with the Didel 4.5 ohm motor glued in place. Falcon also sells their 5.25:1 (PU03) gearbox which comes with the 6x15mm Namiki motor. Also shown is a simple stick gearbox which takes very little time to construct and is simply the motor and an aluminum/nylon bearing tube glued to a stick. It is light and works well. At the far right is one of the laser cut gearboxes Carl Martin made me which allowed easy swapping of motors. Carl also made gearboxes for both Didel and Falcon gears in a wide range of gear ratios not available in gearbox form from either manufacturer. Thanks again Carl!!!


Now a word about props. It would be nice if we had available an assortment of very light props perfect for pager motor gearboxes. But, we don’t. Both the Didel and Falcon gearboxes use 1mm shafts for weight reasons. The U80 prop will press fit on this size shaft. And recently Falcon has added a special version of the folding KP00 96mm folding prop that is also a press fit on a 1mm shaft. But, other than the GWS 2.5x1.0 and 2.5x0.8 props none of the GWS props are a press fit on a 1mm shaft. Bob Selman Designs has just introduced a set of six molded prop adapters that will convert the 4mm bore GWS props to a press fit on three different small shaft sizes: 1mm, 1.5mm, and 0.046 inch. These adapters are a micro necessity as they allow a wide range of GWS props to now easily be used. The 3mm bore GWS props can be used with these adapters if the hole in the prop is carefully reamed or drilled out to 4mm. The famous blue 10cm and 12cm rubber props have about a 1.5mm bore and won’t work with a 1mm shaft. However, I’ve found that by cutting a short length of aluminum tube that is a snug fit (use CA) over a 1mm shaft, I can sleeve the shaft up to a larger size and then I ream out the blue props to fit over this sleeved shaft. All the Didel gearboxes come with a similar sleeve to adapt the shaft to 2mm bore props such as the Westechnik CF props.

Weight in any pager powered plane is always a concern. Weights for these props (without adapters) is given in the table. If you truly want to save weight, molding a CF copy of the GWS 5x3 prop will result in a prop that weighs between 0.55 and 0.65g. That’s nearly a gram of weight savings, which is quite substantial on a plane weighing between 13 and 18 grams. See the June 2003 Inside Story for more information on how to mold your own CF props. And, of course there are other light weight props that can be made. There are the popular yogurt container thin plastic props. Didel also sells a variety of hubs that press on the 12t gear on the front of their spur gears. These allow balsa or plastic prop blades to be glued to CF rod which is then inserted into the hub from each side. These allow easy adjustment of the pitch of the blades.


It couldn’t be a better time to build pager powered micro planes. We now have the 4.5 ohm Didel “super pager” that is at a sweet spot in terms of what we need from a pager motor. We have highly suitable LiPoly cells. We have a variety of sources for gears and gearboxes for pager motors, as well as prop adapters that allow a wide variety of props to be used with them.

I have presented data that will allow matching the 4.5 ohm pager with a good gearing and prop combination for the intended application. If you want to get your feet wet in pager powered planes, the Quick Junior plans in the December 2003 Inside Story column is a good plane to get started with. After that an appropriately sized and light semi-scale plane is a possibility. The gym ceiling is now the only limit to what can be done with pager powered planes.

Plan Feature - Lots of fun for only a ”Bit” of money

By Michael Henriksen

By now everyone should be familiar with the micro R/C cars sold under a myriad of different names and in various shapes and packaging. The original was by Tomy and is called Bit Char-G. The clones are often sold in a packaging that looks like a snowglobe and some even refer to them as “Snowglobe” cars. The market is flooded with them and the prices have dropped considerably. On online auctions you can find them for as little as $5.

It was only a matter of time before diehard micro flight enthusiasts (or cheapskates, as in my case) starting eyeing the little cars with the intention of gutting them and using the innards for R/C planes. Attempts have been made to use as much of the car parts as possible; motor, gears, cells, actuators and of course the R/C. Several have done this successfully but I feel that it is more practical to just used the R/C equipment and keep the remaining parts as spares for other cars – at $5 I’m sure you’ll be buying more than one car. When not flying you can hold races with your friends or try to drive the cat nuts. An interesting side effect of running the cars on the dining room table is that they tend to make wives shake their heads and roll their eyes.

Modifying the R/C Equipment


So, what needs to be done to the R/C equipment to make it usable? Not a lot really. The modification I prefer is to switch the controls and use a MOSFET to drive the motor. In stock form the receiver has output transistors that will not pass much more than 150mA. Since I use a motor that draws 800mA something must be changed. The solution is to substitute one of the transistors with a tiny MOSFET that can pass more than 1 amp safely. It just so happens that there is a MOSFET available that is a drop in replacement for the original transistor. It is made by Internal Rectifier and the part number is IRLML2502 (available at digikey, Conrad, Farnell, IF and others). The price is right, app. 50 cents (the money, not the rap artist). The receiver has outputs for motor forward/reverse and left/right steering. The forward/reverse is performed by a so called H-bridge, a circuit that allows the power to the motor to switch polarity. This is useful for driving a single coil actuator so we will be using the forward/reverse for steering the plane. We don’t really need to be able to reverse motor direction on a plane anyway. That leaves us with left/right steering for the motor. These are just plain outputs that can switch on/off. We can use either channel to turn the planes motor on. This is where the MOSFET in needed. The output transistor is removed and the MOSFET added. The easiest way to remove the transistor is to cut the tiny legs with a scalpel and then desolder the leg pieces left on the board. A word about soldering here: You must use a small iron with a narrow tip. Forget about the 100 watt monster you use for plumbing. We are dealing with surface mount parts here and if you are not careful you can end up desoldering other parts by accident. When adding the MOSFET use thin solder (0.5mm) or you can easily bridge traces. The picture shows the location of the MOSFET (red circle).

While we are working on the receiver we might as well do the other needed modification for it. This simply consists of removing the existing aerial and replacing it with app. 30-50cm of magnet wire or other thin/light wire. Final length might need to be adjusted to achieve good range. That is pretty much it for the receiver conversion.


Now for the transmitter. First of all we need to switch the controls. I cannot specify exactly how to do it because there might be variations in transmitter layout. However I can give you an idea of how to do it. Basically you just cut some of the traces to the switches and rewire them. If you look at the picture you will see what I mean. Take your time and follow the traces carefully before you start cutting. You can see a U-shaped piece of wire in the picture; it should not have been there. No price for guessing that I do not practice what I preach.

Some TXs hold two AA cells, others 3 AA cells. Get the 3-cell TX if you can. In order to boost the signal from the TX and get better range you can safely use up to 5 volts on it. I have used four nicads on my TX but then you need some sort of external battery holder. Don’t use four dry cells as they will deliver 6 volts and fry the TX. You might be able to squeeze a 9 volt battery into the battery compartment and hook it up via a 5 volt voltage regulator but I want to keep it simple for now. If you have a bit of electronics knowledge you should have no problem doing this. If you don’t have electronics knowledge I am sure you will prefer the simpler solution.

Last mod to the transmitter is to lengthen the antenna. I simply doubled the length by attaching a piece of music wire with a wheel collar. Make sure to put a bead on the end of the antenna so you don’t poke someones (or your own) eye out with it. Difficult to judge the distance to the walls with only one eye.

The whole antenna lengthening is very unscientific. There are ways to calculate the correct length and formula can be found on the Internet. I can barely use a calculator, so I used the TLAR principle for determining antenna length and then did a range test. I was lucky and had over 50 feet range with the mods. You might want to experiment a bit with antenna lengths to find the optimum size. I found these values for ideal RX antenna lengths quoted on a German Bit site, BitMod. Every time you double the length, you increase sensitivity.

Frequency and Ideal Antenna Length

  • 27 MHz = 17,3cm
  • 35 MHz = 13,4cm
  • 40 MHz = 11,7cm
  • 49 MHz = 9,6cm
  • 57 MHz = 8,2cm

And Now for Power

That takes care of the R/C equipment; now lets find a suitable powerplant and powersource.


My favorite motor for Bit conversions is the cheap N-20 motor that is available from a number of sources. I bought a large amount from BG Micro and paid 49 cents each. If you just need one or two I suggest you get them from sources like Bob Selman Designs or IndoorFlyer – you probably need to order some cells anyway. My first Bit model had an AUW of 20 grams and it use the N-20 in direct drive with the small GWS 2.5x0.8 prop. With this setup it had a nice slow climb rate and by flying large figure eights I was able to maintain altitude. Depending on you model you might try the GWS 2.5x1 or the U-80 if you need more thrust. Remember you have no throttle so you do not want a model that rockets to the ceiling. Settle for a nice sedate climb. Why not use the gears and motor from the Bit car you ask? It is possible to use it in a very light model if you can lighten the gearbox and find a suitable prop. However Bit clones come with a variety of different motors and some of them might burn out in a matter of seconds. Try the simple and tested direct drive setup I described above and save yourself a lot of hair pulling.


We need some “juice” for the motor. The clones come with one NiCd or NiMh cell mounted. Use it as a paper weight or keep it another year or two, then donate it to a museum. For indoor flight, NiCd and NiMH cells are out, Lithium Polymer is in. My preferred cell is the 145mah Kokam cell. They will endure the ampdraw of the N-20 motor and are inexpensive. One cell is one third the weight of a similar capacity/voltage battery built from NiMH cells. Lithium polymer cells should not be discharged below 3 volt. I was a bit concerned about this at first but I measured the cell voltage just after the model became reluctant to climb and it was still within safe limits. Better land a bit sooner than try to squeeze that extra minute flight time out of the poor cell.

Rudder Actuation

Last piece of kit to think about before we cook a model to mount the Bit bits in, is something that will wiggle the rudder. As I mentioned earlier we are going to use the receivers forward/reverse outputs for this. Actuators have been covered in previous issues of Inside Story so I will not try to re-invent hot water here. The basic idea is that you hook up a coil to the outputs and place a magnet in the middle of the coil. Unless you already have experience with making actuators I strongly suggest you buy a ready made unit from one of the indoor equipment vendors. Since you only need one the cost is not high. Whether you chose a remote type actuator or BIRD type actuator is up to you. Keep in mind that a heavy actuator in the tail of the model could lead to balancing problems. (Gordon's actuator torque article lists most actuators and suppliers for them, GS)

The Actual Model

With all the fancy bits taken care of, let's cook a model. Although not quite a “shake’n’bake” design, I have kept it very simple. Simple designs have a lot going for them: they are easy to build, cost pennies and don’t break your heart when you break them. I have settled for a generic high wing design. It was originally designed to test if EPP could be used for an indestructible micro model. The concept proved successful but since EPP is not the best looking building material in the world and I decided to name the model BUMP. This is a family friendly site and the censors will surely delete the first word when I tell you what BUMP is short for. Therefore I will only say that UMP stands for Ugly Model Plane and leave it up to you, dear reader, to decide what “B” is. You might want to SIT and think about it. No more hints will be given.

Not everyone has access to EPP foam and even if you do, cutting it into thin sheets is not easy. It is also heavier than need be. I chose to build the pictured model from 4mm white beaded foam. It is available here (Germany) in rolls. It is very light and is flexible enough to take some punishment. Do not despair if you cannot get foam like this. Depron, split fanfold or any other thin foam sheet you can get your hands on will do. Thin/light balsa (1-1.5mm) should work as well. If you have a hotwire cutter (or know someone who has one) you can cut your own thin sheet from discarded packaging material.

I will not give a blow by blow building description. The plan and picture should be enough for anyone with a little building experience. There are a few things I would like to point out however. 4mm wall foam is too weak for wings without a spar. A 1mm carbon fiber rod glued to the underside of the wing will strengthen it considerably. Simply gluing it in place along the C og G works fine. I wrapped a bit of copper wire around my soldering iron to form a very fine tip and with the aid of a steel ruler melted a shallow trough in the foam. I then glued the rod into this using foam safe CA. Looks a bit nicer. For a small, light model like this there really is no need for formers other than a nose former. I made the noseformer from two layers of 5mm Depron. There is no template for the former because the size will vary with the thickness of your chosen building material. If building with foam melt a hole for the motor with the soldering iron. Motor is a friction fit so thrust angles can be adjusted. Once you have the settings right, a blob of epoxy will hold the motor in place. Wing is rubber banded on. Use a hole punch to make some doublers from cardboard or thin ply. Glue these onto the fuselage sides and drill holes for the rubber retaining rods. Use 1mm carbon rod or split bamboo skewers. Make sure the rudder hinge is very free moving. Use whatever hinging method you prefer; I use music wire/alu sheet hinges that are glued in place. See sketch.

The model should balance at 1/3 chord (3cm back from the leading edge). Try and shift the RX and cell around to achieve balance rather than adding extra weight to nose or tail. If you plan to build just one model, simply print out the templates and glue them to the building material with a glue stick. Cut out the parts and peel the paper off. I always make cardboard templates because I always end up building several models of the same design. Save those Cornflake cartons.

Aim for an airframe weight below 10 grams. Less is better of course. The 4mm foam model I built has an airframe weight of 5 grams and the AUW will be app. 19 grams. It has 3.42 of wing area so it is a real floater.

Model needs a paint job. Permanent markers are great for this and add almost no extra weight. Build a couple of models while you are at it and have the kids decorate them. If you don’t have kids, make some (this is almost as much fun as building model planes) or borrow some kids from the neighbor.

(Editors note: ensure you get permission before borrowing said children.)

Time to charge the cell and go fly. If built straight the model should fly with “off the board” with a bit of down and right thrust. You might find some trim changes between motor on/off so this can require a bit of tweaking. Also experiment a bit with the C of G. If you get everything just right you will be rewarded with a model that is easy to fly and will have a decent glide. Although the steering is “bang-bang” –full throw or no throw – you can fly wide circles by pulsing the controls. If the throws are mechanically limited it should be safe to hand the model over to even the rawest of beginners. If they get in a jam just tell them to let go of the motor on button and let the model do what it wants. With the low AUW it is more likely to bounce off the wall than break. If it does break, a bit of tape or some foam safe CA will have it flying again in no time.

Have fun! That is what this hobby is all about. Even if you are seasoned modeler I think you will be able to enjoy the simplicity and low cost of a Bit R/C model. I know I do.

The plan

The plan is in 3 pages in PDF format, all you need is Adobe Acrobat Reader. It should print to the correct size automatically. (I added a scale just in case you want or need to scale it and have made it so all templates are in one piece when printed-- Graham.) Just right click and select "save target as"


page 2 of 3.   2.0 KB
page 3 of 3.   1.7 KB

The E-Tec 90 mAh LiPoly Cell

By Gordon Johnson

Almost since the Kokam 145’s came on the scene and revolutionized micro planes I’ve been perusing LiPoly manufacturers’ web sites looking for a cell between the Kokam 40 and 145. A cell from a Chinese manufacturer looked promising but ended up weighing more than stated and having a discharge capability that was not up to our needs. Finally I found a new cell from the Korean firm E-Tec listed for Bluetooth consumer electronics applications. It was listed as having a 90mAh capacity with a weight of 2.3 grams. Bob Selman carries E-Tec cells and agreed to try and obtain a few to test. After quite a wait we finally obtained three cells to test. Matt Keennon agreed to do constant discharge tests on the cell and as soon as we got the results we knew the cell was what we had been looking for for pager motor based planes. The cells are available now from Bob Selman Designs and other micro vendors.

Constant Discharge Tests

The results of Matt’s discharge tests are shown in the two graphs below. The first graph shows volts over time at several discharge rates. At a constant 5.5C, or 0.5 amp, discharge rate the voltage does not drop below 3 volts until nearly 5 minutes. This 0.5 amp discharge rate probably represents the upper limit for what the ET-90 can do for our purposes. In contrast, the 3.3C, 0.30 amp, discharge rate doesn’t result in the volts dropping below 3 until 14.5 minutes have passed. This is a pretty long flying time for most of our models. So, we will probably want to use this cell in applications that have amp draws of between 0.25 and 0.50 amp.

The next graph shows the capacity in mAh that can be pulled out of the cell at each of the same discharge rates. At the 3.3C discharge rate the cell delivers 80 percent of its stated 90mAh capacity, down to the 3 volt cutoff we normally observe for LiPoly cells. At the 5.5C rate it delivers just 44 percent of the stated capacity, and hence the significantly shorter discharge time. The manufacturer lists this cell as 80mAh minimum and 90mAh typical. These discharge tests show we should use the typical rating.

I’ve had good results with this cell in my 15g Quick Junior with the Didel 4.5 ohm 6mm pager motor. And, I’ll be making other planes soon to take advantage of this and other combinations. Clearly this new cell opens up additional possibilities in terms of the ultra micro planes we can build.

The Last Bit

Don't sit around reading this, go off and build something, go on!

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Mar 16, 2004, 06:34 PM
Alfredo Rubio's Avatar
Congratulations, this is a great effort for share to the micro planes modelers, definetely a guide in the chose of motors, gears reductions, props, plans...etc.
Maybe is the most complete data to start in the micro size planes, with links of many places where we can find the equipments...

Thanks for share, sure that took many time to writte

Alfred Rubio
Mar 17, 2004, 05:24 AM
Registered User


a realy great column with lots of clear ifo , makes me want to get building straight away ,but spring is here in switzerland and I must get my outdoor electrics the cold days I used to build in the winter , now with indoor flying ...uuurrrggghh.
I do hapen to have 4 didel 4.5 moters with gears and I want to build an indoor Lancaster ,long a dream of mine , it is now within reach ...
urs (switzerland)
Mar 20, 2004, 08:43 PM
Registered User
I wish to comment on Gordon's pager motor article.

Gordon, we're all in your debt for such a wealth of useful information. My thanks go to you and Carl Martin and everyone else who contributed to this.

I hope others will publish any test results they have so we can continue to add to this useful data set.

Mar 22, 2004, 09:31 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Glad you liked the pager article. As I come up with new results I will put the tables with existing results and the new results up on my web site
No results are there yet as there is nothing new.

The Falcon PU03 will now be available with either the 6x15mm 10 ohm Namiki motor or with the Didel 6x12mm 4.5 ohm pager. So, those will probably be the the first set of new results. I didn't include many of them in the article since at that time the PU03 (5.25:1 gearing) was not available with the 4.5 ohm pager.

Mar 31, 2005, 07:30 PM
Registered User
Nickerz's Avatar
Hi all!

I'm becoming more and more interested in the micro planes and I'm glad there is an understanding (for now) that there is micro and then there is parkflyer. I'm interested in truly micro planes.

I saw some of Ray Holiday's planes and am interested in his little Corsair. The article I read said it does ROG, which I'm really interested in (a true micro plane that can ROG!). I didn't notice any mention of how I can get my hands on one though. Is there a break down of all parts and where I can get it to put it together?

Thanks in advance and love the articles and effort put into this fast growing craze of micro!

Sep 27, 2008, 03:15 AM
Registered User

Inside Story


I'm becoming more and more interested in the micro planes and I'm glad there is an understanding (for now) that there is micro and then there is parkflyer. I'm interested in truly micro planes.

Thanks in advance and love the articles and effort put into this fast growing craze of micro!

Last edited by AdrianZimmer; Sep 27, 2008 at 03:17 AM. Reason: Inside Story
Dec 26, 2008, 05:37 PM
Registered User

Micro Fairy: Much lower performance with replacement motor

Guys, a year ago I bought a micro Fairy helicopter (PicooZ clone) and after a few months the main motor burned.

So I replaced the main motor but the performance of the new motor
seems to be much lower than with the original motor.

I noticed that the original main motor had a white end plate and the (new) replacement motor had a black end plate (see picture).

Does anybody know, if there is a difference between white and black motors?

Thanks for your help!

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