Starting My Second Year

Believe it or not, this column marks the beginning of my second year as a monthly columnist for the E-Zone.

In celebration of this momentous occasion, I looked back at the my first column, from a year ago. At that time, I described my current airplane fleet:

The Neon just passed 300 flights, and is on its fourth drive train. The UHU, Skyvolt, and Rocket are all flyable, but presently are missing some equipment. Actually, the Skyvolt has not been flown in the last year, and only recently I have re-installed the motor control for it.

A year ago I also described what I expected to build between then and now:

What I actually built in the last year was:

My Simprop Fashion, to be detailed next month

OK, this time I will not even bother to predict what I am going to build in the next year. If I can maintain the current rate of 3 to 4 planes a year, I will be pretty happy - and pretty crowded. At that rate it will take about a decade to clear out my current backlog of kits - without even considering the collection of plans!

How many planes a year do you build? How do you plan future projects? What do you do with them all? Please contact me at skranish(at)

An AC Power Supply for a 12V Input Battery Charger

Most of the 'serious' fast battery chargers used by electric flight enthusiasts are powered from (approximately) 12Volts DC (direct current). This is fine if you always charge your batteries at the field, using a car battery as a power source, and have the time to do so. But sometimes it is very useful to be able to charge your batteries at home (or at work, if you are into lunchtime flying!), and having to connect the charger to a car battery can be pretty inconvenient (unless your workshop is in your garage!).

My Robbe 'Power Peak Infinity' charger specifies that it can be operated from either:

My Robbe Infinity hooked up to a power supply for AC charging

What you need to operate the charger away from a car battery is an AC powered 13.8V DC power supply. This may seem like a pretty simple item, until you look at the input current requirements of a typical high performance charger. Charging a 7 cell pack at 5 Amps requires a 13.8V, 5A power supply. Charging a higher cell count pack at the same charge current requires an even higher input current, because the additional current is used by a current to voltage inverter to get a higher output voltage.

The first thing to consider is whether you really need the full output current from the power supply. The suggested power supply for the Infinity can output 207 Watts (13.8V * 15A), which is a pretty substantial power supply. If you do not expect to use your charger for its maximum cell count or charge rate, you may be able to use a smaller power supply. For 7 or more cells, an approximate rule of thumb is :

Power (watts) = (Cell Count * 1.6 Volts) * Charge Current * 1.2

I have used 1.6V per cell because that is the typical terminal voltage for a cell under charge, at the peak of the charge. This may vary a bit with cell type and charge current.

The value of 1.2 is just a fudge factor to account for the less than 100% efficiency of conversion of input current to output voltage.

There are two general types of DC power supplies: linear and switching.

Linear power supplies use a transformer to convert line voltage into the lower voltage required for the power supply output, and any excess power that comes out of the transformer is dissipated as heat. Linear supplies are typically described as having 'cleaner' or 'flatter' or 'quieter' outputs, all of which are usually true. They also weigh a ton (or more), get hot, and these days are hard to find, and are expensive, unless you can find one at a surplus dealer.

This picture shows a 12V 2.7A linear power supply that I have used to operate my Infinity charger. It will actually output about 3A, weighs about 6lbs (2.7Kg), gets very warm (which is why I added the 12V DC fan on top), and must be more than 20 years old. I bought it from the Salvage Department at my first job.

Switching power supplies use a more sophisticated, but more complex inductive circuit to reduce the input line voltage to the required output voltage, with relatively little wasted energy. Switchers may be 'noisier', but they are also compact, lightweight, inexpensive, and relatively easy to find.

This picture shows a 15V 10A switching power supply that I purchased from Jameco Electronics [] [info(at)]. It is Jameco part number 123431, Mean Well product number S-150-15, rated at 15V, 10A, 150W. It will handle input voltages of 90 to 130VAC and 180 to 260VAC (selected by a slide switch), so it should be usable anywhere in the world. There is an output voltage adjustment, so I reduced the output voltage to 13.8V. This power supply weighs 1.8lbs (about 0.8Kg) and cost US$59.95

The Mean Well (great name, huh?) power supply does not come with any useful documentation, so you need to know a few details about how to hook it up. You will need an AC line cord (you do cut them off old appliances before you throw them away, don't you?), cable to connect the power supply to your charger, and probably some type of connector for the cable to the charger. I will say right here that I do not recommend using any type of battery connector (such as Anderson / Sermos / Litespeed or Astro Flight) for this connector, because it will be just one more thing to connect incorrectly. I use banana jacks, because they are not widely used in model applications, and are readily available.

The terminals on the left, marked L (line) and N (neutral) are the AC line inputs. BE CAREFUL. Do not ever touch these terminals. Better yet, after you have your wiring connected, cover the terminals with caulking or glue, so you cannot touch them.

The terminal pairs marked -V and +V are the - and + outputs to connect to the charger input. Remember: black is minus (-) and red is plus (+). It does not matter which of the two -V and which of the two +V terminals are used on this power supply, because they are connected together on the PC board. Some other power supplies may have a metal strap between the pair of  (+) terminals, and another between the pair of (-) terminals, because they each have a different purpose - drive and sense, although in a simple application such as ours they may be shorted together at the power supply.

The white trimmer pot under V ADJ is the output voltage adjustment. Rotate this with a PLASTIC screwdriver until the output is 13.8V. You do have a voltmeter, don't you?

There are other possible sources of power supplies, depending on how much (or how little) you want to spend. You could probably modify a computer power supply to put out 12V instead of 5V, but don't ask me for the details of how to do it. (Desktop computer power supplies typically have a 12V output, but it is usually only good for a few amps, so it is not directly useful for our application). You could also check out an electronics surplus/salvage/junk store, or a company that breaks up old electronic equipment for scrap. You could also contact an electronics distributor, such as Digi-Key, but you will probably find the price for a (new) power supply to be positively breathtaking.

I have had no problems with the switcher I purchased from Jameco, and it is compact and light enough to leave in my field box with my charger.

Some electronics dealers, such as Jameco, sell DC power supplies that are specifically meant  to provide power for equipment that is normally operated from a car battery. These would seem like an ideal solution, but they are usually very low grade linear supplies, and one comparable to the above mentioned switcher will generally cost more and weigh a LOT more. Jameco lists part number 138667 as a13.8V, 10A linear power supply, for US$69.95. It weighs 10lbs (4.5Kg)!

It is also worthwhile to get a copy of the Jameco catalog. They carry a lot of hobbyist electronics materials, meters, switches, connectors and the like. Jameco is a good source of electronic components for RC projects.

If all of this seems like too much trouble, you can also purchase a DC power supply from some of the battery charger vendors. Astro Flight [] sells a power supply similar to the above described unit from Jameco, all set up with cables, and with the voltage already adjusted.

How Much Wiring Do You Need, Anyway?

Well, to answer that in a simple fashion: as little as possible, with as few joints or connectors as possible.

I recently met a newcomer to R/C (I won't mention names, OK, Bob?) who had built a Great Planes Spectra, and had it set up with a Kyosho AP-36. I did not get to see it fly, but I did get to see the result, and it occurred to me that part of the problem may have been power losses in the wiring harness.

This harness may have been based on the wiring that is supplied in the kit. The supplied harness is really pretty useless, because it is intended for a servo switch on-off, and the wire gauge is much too thin.

The above photo shows the stock wiring harness supplied in the Great Planes Spectra kit. The PT Electric includes the same harness with a slightly different motor. While Great Planes was well intentioned in including the harness in the kit, it has several obvious problems:

On the positive side, at least the harness includes a fuse, and the interlock switch is soldered into the harness, as is the motor.

Back to my friend's installation. If I remember correctly, the harness in this plane had:

Everything you add to a wiring harness adds resistance, and resistance means power loss. Rather than getting into a discussion fo the resistance of each item and each connection, I will offer a general suggestion: minimize wiring, connections, and components.

I think that all electric flight installations larger than a Speed 400 should have a fuse and an interlock switch, so I will not argue with any of the components listed above. However, I would make the following changes:

The above photo shows an Astro Flight 211 wiring harness, with interlock switch and fuse. The connectors are for the battery and the motor, and the wiring is Astro Flight 13 gauge. The wires are less than 4" long on each side of the controller, and this has been adequate for several different installations. The fuse is actually soldered to the wiring. This is a reliable, low resistance wiring harness.

Source of the Month

Have you ever needed some unusual hardware or fittings? Dubro and Great Planes sell a pretty good collection of hardware for models and can be found at most hobby shops, but they do not carry sizes below #2 screws, and everything is sold in those silly little packages with not enough parts and too big a price. Hardware stores and home centers barely touch #4 screws, and generally have little in the #2 size, and nothing smaller.

I needed some #1 wood screws to hold in place the Volz Zip servos I had purchased from Unbeaten Path Imports. The servos have 4 holes 0.079" diameter, and do not include any mounting hardware. The mounting lugs are so small that I could not see enlarging the holes to fit #2 screws.

Micro Fasteners [] [microf(at)] sells a variety of screws, bolts, blind nuts, and other fasteners in sizes appropriate for model aircraft use, and quantities appropriate for stocking a workshop. Sizes as small as #0 and as large as 1/4" are available, along with some metric sizes. Most items are sold in lots of 20, 50, or 100, depending on the per-piece price, and are packaged in small zipper bags that are easy to fit into workshop storage systems.

They had the #1 x 0.25" brass round head wood screws that I needed in stock at US$3.25 for 100 pieces.

If you would like to recommend a 'Source of the Month', please feel free to contact me at skranish(at)

Another Source of the Month

If you need a large selection of drive train components, you should know by now that the best place to go is New Creations R/C. Kirk stocks pretty much everything that is available (except some of the exotic German gear) and is very knowledgeable about everything he stocks.

But where do you go if you want a large selection of airframes? There are only a few good choices, and one of the best is Northeast Sailplane Products (NSP) []. NSP originally sold only sailplanes, but has moved into electrics in a big way. NSP sells a number of proprietary products, and is the primary distributor for a number of small manufacturers. They also import a variety of airframes and drive trains from Europe, especially the Czech Republic.

NSP is run by Sal DeFrancesco, who is always interesting to talk to, and can make the realistic recommendations that you can get only from an experienced flyer. He seems to have flown everything he sells, and then some.

The NSP web page is actually their entire catalog on line, but I highly recommend purchasing a copy of their printed catalog for US$10. It is a real 'wish book' with photos and descriptions of all of the available airplanes and accessories, and a number of articles about building and flying techniques. It makes great bedtime and bathroom reading.

The NSP web page has a Specials section that should change monthly, and a Clearance section which tends to be odds and ends of which there are only a few in stock. Sal has some Trade-Ins, and will take high quality planes in flyable condition, if you want to move on to something new - though don't expect to sell him your beat up Gentle Lady.

The current Specials page offers good deals on the Jeti motor controls and Velcom 'Strontium' motors.  It also lists the Castle Creations 'Sprite' motor controls for less than US$50. Tell Sal I sent you!

Tool of the Month

This month we have a 'Tool of the Month' from Tim McDonough, a guest contributor:

I have a suggestion for your tool of the month section. A year or so ago I was introduced to the PermaGrit sanding and cutting tools manufactured in England. They are distributed in the US by Bob Violet Models (

These tools are available in all sorts of wonderful shapes and sizes in several different grits. Sandpaper isn't even in the same league. They will probably never wear out and are very hard to clog up. I have several different Perma-Grit tools but my favorites are the long sanding bars. I have'nt found anything nicer for preparing leading edges, sanding wing skins, rounding fuselage corners, etc.

I'm not affiliated with Perma-Grit or Violet models, just a contented user of their products.

I agree completely with Tim. Below is a photo of my collection of PermaGrit tools. They are also available from Hobby Lobby [].

If you would like to suggest a 'Tool of the Month', please feel free to contact me at skranish(at)

Another Tool of the Month

I recently discovered a new use for the ancient Pelouze balance that usually sits on a shelf above my workbench. I normally use this balance for weighing very small assemblies and parts to get an idea of how much weight I am adding (or subtracting!) from a model.

I make extensive use of epoxy in my models. This is not to say that I use a lot of epoxy - I don't - but I use small amounts of it in a lot of places. My long-term habit for mixing epoxy has been to put equal length lines of the two parts on a 'bingo card' (these are the direct mail response cards that arrive in my office in large quantities, and are essentially free index cards.) I have had rather inconsistent results with this technique, and it is not usable for laminating epoxy, which is so runny that it must be mixed in a cup. The little plastic mixing cups sold by most hobby dealers work fine for this purpose, but the markings are rather coarse, so I frequently mixed far more epoxy that I needed, because I was using the markings on the cup to measure the epoxy. I have had inconsistent results with this technique, too. Using both methods, I have had batches of epoxy that took days to really harden. The inconsistent results do not appear to be caused by the age of the materials - I have had the same problems with newly purchased and year old epoxy. Some of the problem may be the room temperature, but I have had problems during the summer, when it is certainly not too cold. So the problem may well be the accuracy of the mix. I may be using too much of one part of the glue.

I recently wanted to mix up a small batch of laminating epoxy, and it finally hit me - do it by weight, which can be done very accurately, rather than by approximate volume. The instructions for E-Z Lam epoxy state the mix ratio as:

I zeroed my balance with a mixing cup on each side, and then set the scale to 2.0g. I poured part A into the left cup until it balanced again. Then I set the scale to 2.9g, and added part B until it again balanced. I then mixed the epoxy, and applied it - and it cured about as expected.

I have since taken to using the same procedure with NHP 30 minute epoxy, using 1:1 ratio for the weight, and have been very pleased with the results. This method has resulted in very consistent curing times - which I do not think I have ever seen before.

Small 1g or 2g batches (or even less) can be done on equal size bingo cards (cut 2 together to get them the same size). Very small batches of known weight make it easy to control the added weight - if you only mix 1g, and have about half left when you are done, you have only added 0.5g - or less if some of the glue is still on the spreader. A few grams of carefully placed epoxy can make a big difference in the over durability of a model.

It may be possible to find a balance like this at Edmund Scientific or a school supply company. If your neighborhood still has a 'Head Shop', they probably have them, too.


This document is copyrighted (c) 1997 by Steven Kranish, and may not be copied or used in other forms of publication (electronic or paper) without written permission from the author. I will probably grant permission, but I would like to know about it, so go ahead and ask.


If you have any questions, please feel free to contact me at skranish(at)