Pete Rieden

Unfortunately this isn't what is going on. Firstly a motor does not behave as a simple fixed resistance. Certainly the windings have a resistance, but a few quick sums will show that this isn't the whole story. Let's look art an example - the Axi 2820/10 (very popular "600 size" outrunner). This has a winding resistance of around 40mohms (0.04ohms). So one would expect that if it was connected to a 12v battery the current would be as stated by ohm's law: I = V / R = 12 / 0.04 = 300 amps! In reality we expect to use currents in the 25-40A region for this motor, so why? It is due to the fact that motors and generators are the same thing. When a motor starts turning the effect of the magnets on the windings actually generate a voltage, and the voltage generated is always in the oposite direction to the voltage that causes it to move - this is called the "Back EMF". The size of the back-EMF is determined by the speed of rotation; the more speed the higher the volts. So when you connect a motor to a battery it initially draws a huge current, but as soon as it starts moving the voltage AS SEEN BY THE WINDINGS reduces. So our equation becomes: I = (Applied Voltage - Back EMF) / R We can turn this around to tell us the size of the Back EMF: Back EMF = Applied Voltage - (I*R) So for our Axi 2820/10 running from 12volts at 40amps we can see that the back emf must be: BE = 12 - (40*.04) = 10.4v So the windings are ionly seeing 1.6v. If we load the motor for lower currents the back EMF becomes even higher (11.2v at 20A, 11.6v at 10A etc). The back EMF rises until the power developed in the windings is just sufficient to turn the load. It is the rate at which the back EMF rises with rpm that determines the "motor constant" Kv. Secondlty speed controllers are not, of course, variable resistances. They chop the voltage on and off. At the sort of currents we're talking about the use of resistive current regulators would result in vast amounts of power being dissipated, which would be downright dangerous as well as inefficient! For example if a 12v motor was run at half throttle and 20A the speed controller would be dissipating 6volts and 20A, which works out to 120watts - the power of a jumbo soldering iron! So the controllers use switched volts rather than resistance. PDR

A better solution would be to use the seperate power supplies before they get to the reciever. Plug one ESC into the receiver and use it to power the receiver and half the servos. Then construct a wiring harness that takes the positive wire from the other ESC and connects it to the positive wires from the other half of the servos (as for a sperate battery supply to a retract servo). Then the current load is shared between the two receivers. If you want to be a bit clever, and look at it from a safety engineering perspective then connect each ESC via a schottky diode to the receiver, but tap off the servo power *before* the diodes. Divide your servos into two groups and power half from each of these tappings. The clever bit is in deciding which servo to put in each group - if you put the left aileron in one group abnd the right aileron in the other then you will still have aileron power if one ESC or battery dies. Do the same with twin elevator servos and you have a redundant system. If you want to be REALLY clever then ensure the aileron and elevator from one group are in the propwash of the motor powered by the OTHER group's ESC - then you will have maximum control authority if one side fails. This is probably a bit OTT for small models, but it should be a basic design consideration in the systems installation of large ones... PDR

If the ESCs are optos then you don't have to do *anything*. All of this advice relates to using multiple BEC regulators in parallel. Opto ESCs don't power the receiver, and from the receiver's perspective they are purely passive devices. So you don't need to disconnect any wires at all. PDR

...having comprehensively cooked your motor, ESC and batteries... PDR

Static thrust tells you nothing other than what the performance will be like during prop-hanging. Any attempt to optimise props for static thrust simply favours lower pitch props (in the same way that your car will always have more traction in the lower gears). If your prop pitch is too low then you'll have to run higher revs for a given airspeed, and higher revs means more frictional losses in the motor and more aerodynamic losses in the prop, so as a measure the static thrust figure is best ignored. If you want to optimise efficiency then you need to do it under dynamic conditions, and the only real way of doing this is to fit a datalogger which monitors airspeed, current, RPM (or throttle setting) and altitude. You can buy model-type systems that do this, but be advised that getting meaningful data from airspeed and altitude sensors is extremely difficult - googling "pitot position error" should give an inkling of just how difficult this is even in the full size world where a lot of analytical and wind-tunnel data is available. The errors I'm talking about here are not small ones - they can easily be of the order of +200/-80%, that being the difference in calibration factor required for a probe depending on yaw and pitch angles alone... PDR

Too many variables. The most accurate one would be: Time of flight = end of flight - start of flight But this isn't very useful. Your shortest possible flight time (motor run time) is simply: Flight time (minutes) = 60 * Capacity (in mAh) / Full Throttle Current (in mA)  Or to look at it another way, use the "C" rating. If you're running your setup at a full-throttle rating of 10C then your full-throttle run time is 1/10 of an hour (6 minutes), and if your at 20C then its 1/20 of an hour (3 minutes). For each model and its pilot's flying style the *actual* flight time will vary depending on throttle use. For example I know that my Formosa has a full-throttle current equivilent to just under 10C, but I generally get flight times of 12-15 minutes of continuous aerobatics. So for general aerobatic flying I use a gerenal guide: (1/c-rate) * 2 * 60 minutes For my Trex 450 I have a 2200mAh battery and a hover current of around 8A, so I get: 2200 / 8000 * 60 =around 16 minutes of hovber practice But I also know that I can easily draw over 25A in "enthusiastic aerobatics", which goves me less than 6 minutes from the same battery. HTH, PDR

It probably is less efficient, but I'd try a 12-6 first and measure the current. It really depends on the characteristics of the motor and battery. Be advised that wooden props are extremely fragile and can easily become a "1 per flight" consumable on a small taildragger flown from a grass field. PDR

From the likes of Giancod, BRC etc you could easily buy 4 servos for (say) £20, and a suitable motor & controller for (say) £25. £20 for a single lipo would give you a fair range to choose from (although I'd normally suggest buying 2 or 3 to allow for continuous flying). So we're up to £65, and have £85 left. Well one thing you could do would be buy Alastair Sutherland's plan and build your own trainer. That would cost you around £25 in wood and hardware... PDR

So if you can't find test results for a motor then looks for one with a similar current limit and Kv and these will be a very good guide. Modelmotors publish a comprehensive range of test results for their motors on their website which are a very useful reference source!   Finally the no-load current - this number is a general build-quality indicator. As explained in the link I gave above, the no-load current is a measure of the magnetic and friction losses in a motor. So if you have two motors with similar Kv and similar current limit but different Io then this is telling you that one has better bearings and finer gaps than the other, and so will be more efficient (lower is better). So there are your three numbers, and they provide your good guide. To choose a motor for a given application first choose your power, then look at test results to decide what sort of combinations of Kv and voltage will turn the size props you have in mind, then choose a motor and battery with these numbers. Never consider a motor in isolation – always consider motor/battery combinations. For any given application there will be more than one way of achieving it – for example you can turn a 14” prop to tow up a large electric soarer either with a low-Kv motor and moderate-to-high voltage, or with a moderate Kv motor and lower voltage. In general it will be more efficient to use the higher voltages, but this isn’t an EU directive and there's no need to get hung up over it - sometimes the convenience or cost will more than offset a couple of percent in efficiency.   Finally, if the supplier doesn’t publish the information then walk away and find one that does. There are plenty of sharks selling rubbish in this business. £0.02 supplied, PDR

As a result people are constantly asking "why can't they just label these electric motors as 40-size, 60-size etc". I suppose they could, indeed some suppliers have done just this. But it amuses me that I have never seen anyone making the same complaint about turbine engines!  There are three numbers which tell you most of what you need to know about a motor, and which the supplier should state in their adverts (if they don't then walk away, because the supplier doesn't understand the products and will be prone to B/S in any attempt to support them). These numbers are: The current limit (in amps)The motor Constant “Kv” (in rpm/v)The no-load current “Io” (in amps)The current limit tells you how much power you will be able to develop from a given battery. It is constant, and does not vary with voltage, and yes - this DOES mean that the motor does NOT have an overall "power limit" (for more on this see the bit where I start "Electric motors 101" here). So you compare the current limit to your desired power/weight ratio (100w/lb for aeros, 200w/lb for 3D, 60w/lb for slow flyers etc) at your prefered voltage/cell count.  The motor constant gives you an indication whether the motor is a screamer or a slogger - whether it turns big or small props. Note that the same motor will turn big props on lower voltages and smaller props on higher voltages for very different applications. This isn't a problem - it's the very flexibility that is the core of electric systems. For a given current you can compare motors of similar Kv directly (not 100% accurate, but near enough for our purposes). [continued in part 3]

Unfortunately there is no standard nomenclature because the manufacturers like to do their own thing. Even those that *are* standardised don't really tell you much that is useful. The "Modelmotors" convention (used initially for their Axi range, and copied by others) tells you some physical things about the motor - the diameter and length of the stator plus the number of turns per pole on the windings. So an Axi 2820/10 has a stator which is 28mm in diameter, 20mm long and has 10 turns per pole. Sounds good, but you can't actually do much with the information, so it's best ignored. The "Mega" convention is similar, except that they put in an extra stroke with "16/17/2" (16mm dia by 17mm long and 2 turns per pole), and is equally useless! With IC motors we are used to being able to assume that pretty well all motors of the same physical size will perform roughly the same job, so all 40s will be more or less interchangeable. We then qualify that by saying that (for example) the OS40LA is "lazy" and so won't quite deliver the grunt, the Irvine 40Q is "torquey" and so delivers lots of power at low revs, whilst the Rossi 40R26 is  "a screamer" that delivers loads of power, but only on 9" props at more than 17,000 revs. But aside from this the motors are all pretty similar because there is a relatively small range of variations in a two stroke or four stroke engine that actually work at all. We've got rather used to this and assume all power sources are the same, but electrics aren't. Electric setups are far more flexible, and two motors of the same physical dimensions can have widely differing properties. For that matter two similar motors connected to different size batteries can also exhibit massively different behaviour.  [continued in part 2]

This will probably get me moderated again for being too technical, but those "weight saving holes" always cause me to gnash my teeth! These kinds of holes are great in metalic structures with flanged frames, but in a flat sheet of fibrous material like wood they are utterly pointless because they remove meaterial from the precise place where the load-paths should run. The force the stresses to adopt tortuous routes around the holes and these convoluted load-paths are always dubious. If the material was too strong then use a thinner piece, possibly stabilised with cap-strips or stringer stiffners, for the same strength at a much lower weight. These structures with their pretty rows of neatly cut holes are just copying features of full-sized metal structures on a "monkey see, monkey do" basis without understanding the why or the how. PDR

OK, well here's what was in my mind. When you first connect a battery to an ESC there is an initial current surge to charge the input capacitors of the ESC - this is often seen as a small spark at the connectors are touched together. This kind of arc can do "bad stuff" for unprotected electronics, and is much bigger for higher voltages than lower ones. So on all my setups above 12v (3s/10-cell) I incorporate a "charging lead" in the positive wire. The lead is simply a short piece of wire soldered into the back of the ESC connector, with a 10 ohm resistor on the end (the whole thing being covered in heatshrink to support/protect it). When connecting the battery I connect the negative lead, and then before connecting the positive lead I touch the end of the resistor to the positive battery connector for a couple of seconds before plugging the connectors together. This limits the charge current into the ESC capacitors and eliminates the arc. I have been doing this for years simply because I regard it as "good practice", and I know of several people operating the larger size electric helis (Trex 600 etc) who have adopted the idea after suffering gyro failures which they assumed were caused by the surge. I have no experience of 2.4GHz equipment*, but if this surge is causing grief then this kind of approach should reduce or eliminate it. PDR  * My personal view is that 2.4GHz will become unusable for model flying in a few years because the band will be swamped with wireless WAN signals, so I'm reluctant to invest in the equipment, especially since there are no decently capable transmitters available at the moment.

Well yes, but only in one of the lesser events (some kind of circus act IIRC - not proper flying). In the premier event you'll find the race pilots are mostly mode II fliers... More seriously - I've flown both. I learned on mode 2 and was happy with that. When I started flying pattern (what is now called FAI-F3A) in the early 80s I was advised to try mode I as it was suggested that it was easier to keep a constant roll rate if the ailerons were on a different stick to the elevator & rudder. I flew this way for a year, and they were right - for pattern flying it was probably better. But even at that time I only flew the pattern models on mode 1, and my sport models, gliders and helis were flown on mode II. I got bored with pattern flying after 18 months (something to do with being cinstantly distracted by the grass growing on the airfield) and dropped mode I for a while. Then in the late 80s I took up proper flying, and in the first few races I tried flying mode I again. This was partly because I had convinced myself that I flew more precisely in mode I, but mainly because the best race pilot of the day (Barrie Lever) flew mode I and I thought he must have a reason. But it didn't feel as comfortable largely because a neckstrap can be a serious liability in pylon and mode II was easier to hold the Tx withour one, so I switched back to mode II. My times improved, not that this proves anything at all. Possibly the biggest benefit was when I switched to the Phelan (High compression, run rich) engine setup, because this requires an in-flight mixture control. It is quite easy to use an IFM with mode II, but much more difficult with mode I, so for me mode II was the best choice.  But having flown both for extended periods (and indeed having often taken mode I and Mode II models to the field on the same day, because I'm a masochist) I would say that in my experience there is nothing to choose between them. For certain specialist applications one might have a slight edge, but for general flying there ain't much in it. PDR

On a further (perhaps pedantic) point, the statement that a regulator comprises "a transistor and a zener diode" is a little far from the truth. The internal circuit of a 7805 regulator looks something like this: As you can see, it contains around 20 transistors, a zener and assorted resistances. This is, of course, a simplified version of what is actually on the IC... PDR

I would disagree with that last bit. Lipo C-ratings are a useful indication of the quality of the particular battery (in general the higher the C-rating the better the cells), but I size ALL my lipos for a full-throttle draw of 10C or at the most about 12C. Anything less just gives rediculously short flights. A typical aerobatic model with the pack sized for 10C at full throttle gives 10-15 minutes of general aerobatic flying on a charge, but using a 20C pack at 20C only gives around 5 minutes. PDR

Would this be the old Eric Coates plan? (Aeromodeller, some time around 1973ish)? The one with laminated balsa outlines for the tailplane, fin and wing tips?  The scale Puss Moth has no dihederal, but the FF rubber model needed a smidge to stop it spiral-diving. Lovely flier though - mine was built when the plan came out and flew regularly for a year or so. It then got hooked in a thermal and drifted out-of-sight on chobham common, never to be found. PDR

[part 2] A few weeks ago I was travelling up to a business appointment, driving up on the sunday for an early monday meeting. At one point I saw models flying in the distance, so I headed off to look for them and found a local club field (I won't mention the area because I don't want to embarrass anyone). I parked up and walked out to watch for a while, and saw that this club flew nothing but ARTF/RTF models. While I watched a club member arrived and got a 90-4stroke powered Extra out of his car. He was immediately surrounded by a "posse" and a huge argument ensued. Listening from the sidelines I gathered that this model had been crashed the week before, and the fuselage had essentially broken in two just behind the wing. The argument  was over the fact that the owner had actually *repaired* the fuselage rather than ordering a new one as a spare part. The vocal committee members were heard to say (several times) that it really didn't matter how *well* it was repaired; no repair could be to the original design standard and so repairs were (by definition) a "bodge". They then went on to claim that "repaired" aircraft would obviously be uninsured for this reason. I couldn't keep out of it, so I approached one of the committee members and asked how this prinicple was applied to kit-built, or even own-design aircraft. He replied that they didn't allow home-brewed aircraft in this club for "safety reasons" as they could never be as strong as "proper professionally designed models", and that in any event as far as he knew all kit built aircraft were subject to the "inspection scheme". After a while it dawned on me that he was talking about the LMA scheme for over-20kg models. This chap had a BMFA examiner's badge on his cap, and I could feel my hackles rising, so I left before getting drawn into someone else's argument! But are there REALLY places that are this stupid? Are there any real modellers left out there? Am I just a crusty old carmudgeon whose 47 years places me on a different planet? Mutter, mutter - nostalgia just ain't a patch on the proper nostalgia we had in MY day, I can tell you... PDR

With the "flying scale" series (inaccurately described in both senses, but never mind) the main ingredient was 1/16"sq strip, and yes as a schoolboy I had built all of the KK and Veron ones by the age of 11, moving on to CL when I saved up for my first engine at 12 and RC when I was 14. I have to say that for me the state of modern modelling is summed up in two respects in the mags - this isn't a dig at RCM&E because it must serve its readership, but it shows how things have "changed": 1. Back in the mid 70s a magazine would do a kit review of a fully-built-up RC scale kit in a four page article. This would cover construction, finishing and flying in some detail, with comments on the quality of the kit and the flying characteristics of the model. Today's magazine will cover the assembly of an ARTF kit in two (and sometimes even three) theree to four page articles split over several issues. I almost see it as "this week's masterclass - how to screw in the left hand wing bolt. Next week - moving to the right-hand one!" I'm continually amazed at how much detail the readership seem to need on basic stuff like installing a servo, or bolting an engine mount to a firewall. 2. When the all-sheet small electric jobs (like those of Hawes and Nijhuis) are published as plans it is felt necessary to cover the schoolboy-simple task of carving/planing a section onto a solid wing with between 10 and 20 column inches of instructions. None of these are beginner's models - they would be in what we used to call the "2nd low-winger" category, and the idea that RC pilots can get to this stage without the basic model-building skills is frankly depressing. [continued in part 2]

The other day when I was talking to Phil about these kits suggested that even if they don't list a model in one of the scales (like in this case, where they don't list a 1/12 scale one) this company will often do one in the new scale for a "normal" price. PDR

The potential for interference is identical from both - that is to say that it would depend on the length and position of each wire rather than whether the wire was on the battery or motor side of the ESC. The current on the motor side most certainly isn't "smoother" - I can only assume that whoever made this assertion was under the impression that the ESC outputs a 3-phase, sinusoidal current (which it most certainly doesn't). PDR

...and just to complicate matters further: The vast majority of the 36" stripwood wood sold on model shops as "spruce" is actually Ramin, and has the structyral properties of slightly stale cheese. If you want proper sitka spruce you have to go to a full-size home-build supplier like skysport or the specialist free-flight suppliers. PDR

There are some very nice kits done by "Arizona Model Aircrafters" - laser/CNC kits with full hardware (including fittings, cockpit interior, dummy engine, guns etc). These are done in a range of sizes - 1/6th, 1/4th and 1/3rd, and sometimes 1/12th as well. I believe you can get the plan rather than the kit if you prefer. The Albatros DVa kit is listed here: http://www.arizonamodels.com/product_info.php/cPath/40/products_id/101  In the UK they are imported/distributed by Phil Clark of Fighteraces (www.fighteraces.co.uk) PDR

Gold embrittlement is a known issue in the electronics industry, as the googling shows. Soldering to gold is generally regarded as "bad practice", but we seem to get away with it. I'd certainly rather see model flyers using soldered gold connections than attempting to make effective 50+Amp crimp connections because the sort of crimp connectors that handle these kinds of currents require calibrated crimp tools which cost upwards of £200, and require training to use correctly.  PDR