TechNFun - Fun with technology
Electronic gadgets reviewed and modified
Reviewed on:
22nd November 2006
Last update:
26th October 2008
Walkera 5#4 "Mini Lama" coaxial helicopter
Things that fly are interesting.
Despite mother nature's best attempts at discouraging human curiosity toward anything that's capable of flying, we remain highly fascinated by the phenomenon. So much so that, as children, many of us like to recreate it with toys and lots of imagination.
| There is an ongoing dispute between modelers regarding the designation of radio controlled models of all kinds. Some say all models are toys by definition, as their purpose is to entertain the user. Others say the really silly little ones (mostly meant for children) are toys, while others are models. I tend to agree with the all-toys theory, but I differentiate between "toy" toys and "serious model" toys. Roughly speaking, if buying one causes you to worry about making rent, it's a serious model; otherwise, it's a toy. |
Back then, the answer was no.
Well, ok, there were toys that could fly: paper airplanes and rubber band powered ones. But I wanted something that could stay airborne for more than a few seconds at a time.
You could get a radio controlled plane, but none of them were toys. They were serious glow-powered models that required lots of work just to build and tune, let alone fly and maintain. They also required you (or your parents) to have a considerable amount of disposable income.
In the middle, there was nothing.
Frustrated that I couldn't get into the hobby in an easier way, I spent some time watching serious RC pilots as they flew their planes out in the fields. And then, other things came and my interests shifted.
But all these years I never lost hope. Secretly, I always harbored the desire of having, one day, a toy that could fly.
At last, that day has come.
This is a Walkera model 5 revision 4 coaxial helicopter, known to its friends as the 5#4 "Mini Lama" (from the name of the real helicopter it's modelled after, the Eurocopter Lama, a derivative of the Aerospatiale Alouette).
Compared to the serious, expensive, complicated airplanes and helicopters serious modelers buy, it is most definitely a toy. It's also relatively cheap, and it comes already assembled. Most importantly, it actually flies.
It requires a lot less commitment than most other flying things you can buy, too. You don't even have to cart it to the nearest park; it's meant to be flown indoors, and while its ideal flying territory is a gym, warehouse or any other open empty space, it can be flown in your living room.
If you're thinking the 5#4 has something unusual when compared to real helicopters, you're right. Most real life helicopters don't have two large rotors. Rather, they have a large one that provides lift, roll and pitch control, and a smaller tail rotor whose purpose is to avoid the helicopter spinning uncontrollably on its axis and crashing. It also provides yaw control.
If you were careful in physics class, you'll remember something that goes "for every action, there's an equal and opposite reaction". It's Newton's third law, and helicopter pilots are very aware of its influence.
I'll leave the exact explaining of how the law works to good ol' Wikipedia, but the lowdown is this: if you have a motor that's spinning something, the motor itself will tend to spin in the opposite direction. This means that if you were to run a portable fan on the International Space Station, the fan itself would start spinning in the opposite direction of the propeller. It also means that anything that's spinning a rotor fast enough to keep itself airborne would, without anything to avoid it, furiously spin as well.
You can probably understand why people aboard a vehicle that follows the same principles would rather like something that'll keep it from furiously spinning. That something is, in most helicopter designs, the tail rotor.
The tail rotor exerts a force that's equal and opposite to the torque of the main rotor, thus keeping the helicopter steady. Increasing or decreasing the power of the tail rotor will also cause a helicopter to spin (yaw) in one or the other direction. But this method has several disadvantages.
First, it robs power from the engine. It has been estimated that as much as 30% of the entire power generated by the helicopter's engine(s) is eaten by the tail rotor.
Second, it's dangerous. A tail rotor failure almost certainly means the whole helicopter will crash.
Third, and this is what concerns us the most, it's hard to control. In addition to exerting a spinning force on the helicopter, the tail rotor also causes a slight lateral drift that has to be corrected. More importantly, the design not inherently stable. The final position of the helicopter determined by many different forces acting on different parts of its frame, so careful, constant monitoring of those forces is needed to keep the aircraft steady in the air.
There's another solution to this problem: coaxial rotors. In a coaxial design a helicopter has two counter-spinning rotors that cancel each other's torque out, so no additional torque source is required in order to keep the helicopter steady.
This method requires a more complicated transmission and rotor design, but doesn't have the disadvantages of the tail rotor principle. All the power is employed to generate lift, there's no small, delicate, protruding part to damage and all the forces that act on the helicopter's airframe are concentrated on one axis, making it much easier to control. Basically, the frame "hangs" from the twin rotors, and is much less susceptible to sudden, undesirable changes of direction.
In real life, there's only one major helicopter manufacturer that's chosen to go this way: Kamov. The helicopter that best shows off Kamov's philosophy is the KA-50 Hokum. Also, if you've seen the movie "No escape", the helicopters were Kamov KA-27.
Now that the "Helicopter physics 101" class is over, let's see why we care about all of this.
In model-world, the main difference between tail-rotor and coaxial designs is that a newbie attempting his first flights will be crashing a lot less on a coaxial. Movement is much less sudden, and (due to the way the upper rotor is designed) tends to stop the moment you let go of the command on the transmitter. The inherent stability of a coaxial model does wonders when you're struggling to coordinate the movement of your thumbs on the transmitter with those of a twitchy, wobbly thing struggling to hover a few feet from you.
Most experienced flyers consider coaxials to be boring, because their inherent stability and fixed pitch (more on this later) make them much less agile than comparable tail-rotor helicopters.
How much less? Well, a coaxial can hover, move up or down, left or right, and spin. A tail-rotor design, in contrast, can... well, it's probably better if you see it for yourself.
Pretty impressive, huh?
But doing that sort of stuff, even if you're interested in it (which I'm not), requires years of training, lots of crashes and generous amounts of money. If all you want is a toy you can hover around your living room, a coaxial is your best choice.
There are also many tail-rotor micro-helicopters, generally of the size of the Mini Lama, that can be hovered in small spaces, but you probably don't want one. They suffer from the difficult controls of all tail-rotor models and they are very light. This means they are extremely twitchy and sensitive and, using a technical term, real bitches to fly. Definitely not good for newbies.
The main parts of the Mini Lama are the motors, the servos, the rotors, the electronics and the battery.
In this picture you can see one of the motors (the gray one with the diode and capacitors inbetween the terminals), the servos (the two blue rectangular things) and the swashplate and head of the lower rotor.
Sorry about all the dust. I don't vacuum my house as often as I should.
The motors are two generic "180" brushed units. This is good because they are cheap (a pair of replacement motors goes for €13), but it's bad because they wear out.
Without going into the details, brushed motors work by having two fixed carbon brushes in constant contact with the moving rotor. A lot of friction is involved in this design, and it eventually causes the brushes to degrade enough to significantly impact performance. When that happens to large, expensive motors you open them up and replace the brushes. When it happens to small cheap ones you toss them whole and replace them.
To all the cheapskates: yes, it is theoretically possible to replace the brushes in a small brushed motor. No, I have no idea where to get the replacement brushes or how much they'd cost. I also don't even want to think about how much swearing would be involved in the course of the operation.
In contrast, brushless motors (such as the ones in computer fans, hard disks and most CD-ROM drives) don't have electrical contacts between rotating and fixed parts. This makes them more expensive, and they require separate circuitry to work. But it also makes them more efficient and they last, compared to brushed motors, forever.
It is possible to replace the motors in a coaxial helicopter with brushless ones and either adapt or replace the electronics, but such a modification is likely to cost more than the helicopter itself, so most people are content with just replacing the motors every once in a while.
The 180 motors in the 5#4 are generally thought to have a working life of roughly 20 hours. That's the general ballpark, though; if you're careful about not letting them overheat they can last considerably more, whereas if you always run them for as long as the battery lasts you may end up killing them after only a few flights. Luck, as in all things, also has an important part in how long your motors last.
It is generally accepted that if you want an acceptable motor life you should take breaks every 5 minutes of flying to allow them to cool down.
The servos are bog standard Walkera low torque ones. They are likely to last for ages. They are also utterly unremarkable, so there's nothing very interesting I can say about them.
The lower rotor is controlled by the swashplate, on which the servos act.
The upper rotor is instead controlled by the flybar...
...which, thanks to gyroscopic effect, tends to keep it as horizontal as it can. This means that to move the helicopter horizontally the lower rotor has to constantly fight the upper one. Thus, when you let go of the lever on the transmitter and the servos stop acting on the swashplate, the upper rotor stabilizes the helicopter and makes it go back to hovering.
All real-life helicopters, and many models as well, use variable pitch (also known as "collective pitch") rotors. They spin at a more or less constant speed, and variations in thrust and torque are made by changing the actual angle (pitch) of the blades.
Some models, instead, use fixed pitch: the angle of the blades doesn't change, and variations in thrust are made by varying the speed at which they spin. This makes fixed pitch helicopters cheaper, simpler and easier to control.
How much simpler?
Compare and contrast.
This picture, shamelessly lifted from Google Images, portrays the rotor head from a non-coaxial collective pitch T-Rex.
See what I mean?
As mentioned before, there are no variable pitch coaxials on sale at all. I can only think that the double swashplate quad-servo design that would be necessary for such a model would cost way too much. Some badass modelers have managed to scratch-build variable pitch coaxials, but I don't even want to know how much they must have spent on them.
A variable pitch coaxial would also, probably, be able to do all those wild aerobatic maneuvers I linked to earlier. There is no law of physics that prevents coaxials from doing them, but aerobatics absolutely require variable pitch. Fixed pitch models, including fixed-pitch tail-rotors, can't do them at all.
The 5#4's blades pivot on their joints. At first this is unsettling, especially if you don't know they are supposed to do so. The first thing that crossed my mind when I took the heli out of its box and saw the blades all at different angles was that it was broken.
Thankfully, that isn't the case: when the rotors spin up, centrifugal force causes the blades to position themselves correctly. And if you crash you'll be very happy the blades aren't fixed, as the pivoting joint allows them to "retreat" when they hit something and not break.
The electronics are all concentrated in a little circuit board, known as the 4-in-1...
...which you see here as it appears when you remove the helicopter's canopy. It's a nice little package containing the main controller chip, receiver, crystal and an actual piezo gyroscope.
There are two things that can cause trouble in the 4-in-1: the mosfets and the gyro.
If you want to know what a mosfet is in detail, I'll (as usual) point you to the relevant Wikipedia article. If you're happy with the lowdown, think of them as valves for electricity. One side of them is connected to the battery, the other side to the motors (one mosfet each), and a control signal tells them how much energy they should let flow from one side to the other. They are the "gas pedal" of the model.
The problem in our 4-in-1 is that the mosfets are small. Very small. With no heat sinking.
No electronic device is 100% efficient, and mosfets are no exception; of all the energy that goes into them most goes to the motors, but some is wasted as heat. The more energy that passes through them, the more heat they generate. Overload them and they die, often with flames and smoke.
This is usually not a problem for model helicopters of this size, because the current required during normal operation is not enough to overload the mosfets. I've tested the 5#4 with a multimeter, and at full throttle it wants a little more than six amps, divided pretty much equally across the two mosfets. This isn't enough to cause any trouble. The problems come when one or both rotors lock up.
Normally, you stop applying throttle to the helicopter as soon as you crash, so even if the rotors get snagged into something the motors won't keep trying to spin them back up. But if you stubbornly try to apply power in the hope that the rotors will break free, or if the helicopter loses radio contact with the transmitter (because the transmitter's batteries die, because it goes out of range, because you're using a power cord and it gets yanked...), picks up radio interference, goes crazy and crashes, there's a good chance the motors will try to spin up while stalled. A stalled motor sucks a lot more energy than it usually does; in our case, more than enough to blow the mosfets.
Now, assuming the mosfets have the decency to die without setting fire to the rest of the model, it is possible to replace them. You either use the correct replacements or you solder on standard non-surface-mount mosfets (the result is ugly, but functional). Doing so, however, requires decent soldering iron skills, lots of patience, a steady hand and some time. Most people just prefer to toss the 4-in-1 and buy another one, despite new 4-in-1s costing as much as €40.
This isn't a critical problem; it's not like the helicopter will burn a mosfet on its own accord and drop from the sky during normal operation. But it would have still been a better solution to use larger mosfets with some heat sinking, or just to have fuses inbetween the motors and the circuit board (which, by the way, is a modification you can do yourself).
The piezo gyro's failure mode is much less spectacular and complicated. Basically, if the helicopter crashes too hard, or otherwise experiences a strong physical shock, the gyro can get stuck.
This happened to me once; I lost radio transmission, crashed bad, broke a blade and the rotors violently hit each other. When I replaced the blade and reconnected the battery, the servos worked but the motors didn't. After a lengthy diagnose, with lots of help from people in various forums, I singled out the piezo gyro as the source of my problem.
I went to the local electronics store, but they wanted the ridiculous sum of €45 for the one tiny itsy bitsy component, so in the end I just bit the bullet and ordered another 4-in-1. And of course, according to the great tradition of Murphy's law, I only later learned that you can open up the gyro and fix it.
If you're interested, I made macro pictures of the bare board while I was working on it. Get them here and here. The mosfets are the two black chips in the upper right of the first picture; the piezo gyro is the large vertical component in the middle that's reflecting all the light from the flash.
You may be surprised to know that this unassuming little thing is in fact the most dangerous part of the helicopter. It's a two cell, 7.4 volt, 800mah Lithium polymer battery, which you charge with the provided wall wart-style charger (the charge lasts about an hour and a half). Its life in the 5#4 is stated at 15 minutes, but 10 to 12 is a safer bet.
You can also use a larger one, if you want. Its additional weight will make the helicopter more stable, and its higher capacity will give you longer run times (but remember to take breaks every 5 minutes). Going higher than 1100mah is inadvisable, though, as higher capacity batteries are too heavy, and the added strain on the motors would be very bad for them.
It's thanks to these that we have micro-helicopters in the first place; other battery chemistries don't offer enough energy in a light enough package to be useful for powering helicopters this small. The problem is that the lithium polymer chemistry is unstable. Overcharge a battery and it catches fire. Overdischarge it, then attempt to recharge and use it, and it catches fire. Puncture it and it catches fire. Short it and it catches fire. Hit it too hard and... you guessed it.
Should you want to scare yourself with tales of other people's misfortunes with LiPo batteries, read this.
All sorts of gadgets (mobile phones, PDAs, mp3 players...) use high density lithium polymer cells, but the reason you don't see people suddenly reaching for their pocket, getting a terrified look on their faces and then bursting into flames is because low-discharge cells such as those in portable devices are much less chemically stressed, and always have protection circuitry built in. The circuits keep voltage and current under control, monitor cell temperature and generally speaking make sure your portable appliance doesn't unexpectedly become a flaming firework.
(They aren't always succesful, though.)
Lithium polymer batteries meant for R/C use have none of that circuitry. They are just cells and wire. The reason for this is the discharge current.
A digicam can have a peak draw of an amp or so, maybe an amp and a half, though normal use is likely to require much less energy. Circuitry that has to control one amp of current can be relatively small and efficient.
But controlling a cell that has to routinely be able to deliver six amps, and possibly peaks of rather more, is a much harder task. Run six amps through pretty much any sort of control circuitry and it's inevitable that a lot of energy will be wasted as heat. This would shorten battery life considerably, and the additional electronics and their heatsinks would add weight to the aircraft. Thus, unprotected cells.
In all fairness, LiPo cells aren't that dangerous. Yes, if you leave them unattended as they get charged, run them absolutely flat every time and generally don't respect them, you're far more likely to one day be awoken by a flaming inferno in your house. But if you follow basic safety procedures, charge them in metal containers or concrete blocks, treat them with care and dispose of them if they get damaged, the worst that can happen is that you get a faceful of toxic smoke (which wouldn't be pleasant, but which also wouldn't be toxic enough to threaten your life).
One last thing about the battery: I suggest you get a spare. They are maybe €10 plus shipping on eBay (UPDATE: rather less on DealExtreme... and why not get a decent dedicated charger while you're at it?), and it'll double the chance you'll have one charged and ready for when you want to fly now NOW NOW!
The transmitter's got the normal two sticks, the antenna and a serial port on its back that's meant to be used with a computer to run simulations, but which I haven't managed to get working yet due to my unwillingness to pay €15 + shipping for a basic connector cable. The two switches are unused; the same body is used by Walkera for their 6-channel transmitters, and they don't bother removing the switches from the 4ch. They aren't connected to anything inside.
The transmitter comes set to mode 1. That means you have throttle and roll (sideways movement) control on the right stick, and pitch (forward/backward) and yaw (horizontal rotation) on the left one.
The other one is mode 2, where you have throttle and yaw on the left stick and pitch and roll on the right one. I prefer mode 2, because this way once you set the throttle to hover you can keep the helicopter steady using only the right hand, which allows you to use the left one to grab it out of the air, shoo the cat away or move stuff around.
To switch mode you have to open up the transmitter, swap the appropriate cables around and swap a spring and a metal doodad as well.
In the picture it's in mode 1. With my 1337 Paint skillz, I marked in red the parts you have to swap around to change mode.
It doesn't take a genius to modify it, but many people shy away from opening stuff and leave their transmitter forever in mode 1.
So, now that we've finally covered all the theory... how does it fly?
This thing is fun! Really, really fun!
It's just what I used to dream about when I was a little kid and playing with toy helicopters that couldn't, by themselves, do anything.
To start, you charge the battery and insert it in the holder under the helicopter. Then you don't plug in the connector. The helicopter has no on-off switch; whenever the connector is plugged in, it's on. And if it's on it can pick up random radio interference, which can very realistically cause it to fly off on its own accord and smash itself to pieces against something.
Instead, you put batteries in the transmitter (8 AAs, but it can use an external pack or power supply), screw in the antenna, extend it, switch the transmitter on and put all the trimmers to the middle position except the throttle one, which should be in the lowest position. You also check that the throttle stick (the one that doesn't have a spring pushing it back to the middle position) is on the minimum setting. If I need to tell you why you have to do this, you shouldn't be playing with a RC helicopter.
Then, finally, you plug the battery connector in and watch as a green LED lights up and flashes while the servos move a bit as the circuitry runs self tests and stabilizes. You are to hold the helicopter still and on the ground while the LED is flashing; if you move it the circuitry controlling the gyroscopic sensor will get confused and you'll have lots of trouble holding the thing in the air.
The worst mistake you can do at this point is to ignore the part of your brain that's telling you to take it easy and punch the throttle. If you do, you'll be quite surprised as your helicopter shoots up in the air, hits the ceiling, breaks a blade or two, drops like a stone and smashes itself in a myriad of tiny little pieces.
Or maybe you manage to control the surprise enough to kill the throttle in time, but not enough to remember to apply it again before the thing plummets back to the ground. Again, expensive plastic all over the floor. In any case, I'm pretty sure all your excitedness will have vanished in a heartbeat.
If, however, you've done your homework by watching videos and reading about RC flying, and/or if you trust the part of yourself that tells you to be careful, you'll apply barely enough throttle to get it to lift off... and then lose control and smash the blades against some furniture.
I'm not kidding.
It's really hard to control a R/C helicopter, even an easy newbie-friendly one like the 5#4, if you've never done it before. Move the levers on the transmitter too much and it'll fly too fast and crash. Move them too little and it won't seem to do anything much, at which point it'll either crash on whatever it was heading for when you tried to change its direction, or you'll overcompensate and it'll start flying at breakneck speed in the opposite direction. And then, of course, crash.
You can keep overcompensating all the time in the hope of eventually learning how to properly control it, or you can set it down and scratch your head. If you choose the first way make sure you have lots of replacement blades handy and don't do it somewhere where there's a Louis XVI chair.
Me, it took a few tries and a broken blade to understand that that wasn't how I was supposed to be doing it.
What I did and, I learned afterwards, what everybody is supposed to do, is to apply just enough power that the helicopter wants to lift, but not enough that it actually does. This way it'll get light on its skids and you can start to play with the controls without any more expensive surprises. If you see you're losing control just cut the power and it'll settle down on its skids and stop.
You keep doing this for as long as it takes you to be able to keep it still, then you apply some more throttle and marvel at how hard it is to keep it steady in the air despite having mastered it on the ground.
After more training and, probably, more broken blades, you'll be able to hover it almost effortlessly. At this point the hardest part is done; it takes little more practice to have it go where you want, as long as you remember to start very slowly.
There is one last thing you want to learn: nose-in flying. When the helicopter has its nose pointed toward you, the roll and yaw controls are reversed. This can lead to severe frustration for the reasons you can imagine (you know how certain arcade games have a "bad bonus" that inverts your controls?), so it pays to spend some time just hovering it nose-in in front of you, first only correcting its drift, then slowly flying circles. It helps if you think of it this way: move the sticks in the direction the helicopter is going, and it'll compensate and then start going the other way.
When you've learned to do that you can start flying hops from table to bed, from a room to another room, and eventually from one side of the house to the other. You can set up targets to land on, or try to land it on as small a surface as possible. Grab it while it's in the air and let it take off from your hand (or keep holding it and be amazed at how hard it pulls at full throttle). Bend some paperclips into hooks, tape them to some Lego men's backs, scatter them around, tie a string to the helicopter's skids and stage a rescue operation.
Be warned that despite what you may have seen in movies, using it to send a help message to the authorities from a basement where you're being held by kidnappers isn't likely to be an effective strategy.
Another thing you usually see in movies is, instead, surprisingly feasible. I'm talking about strapping a baby camera to the helicopter and either recording a movie or sending the video stream straight to a tv screen on the ground.
The 5#4 has enough power to lift a weight of about 90 grams. You're going to have to keep it almost at full throttle just to hover with that heavy a weight, but you can do it. More realistically, a 50-60 gram weight is easily transportable, and there are some cameras around that are really tiny.
It won't do for wannabe James Bonds, though, mainly because of the noise. Normal micro helicopters are noisy enough, but since this one has two main motors the noise doubles. It isn't deafening, and certainly isn't as annoying as larger glow- or gas-powered models, but but you ain't gonna be spying anyone outside their window without them noticing. Also, the quality of the movies isn't great and the vibrations make them look all weird and wobbly.
But if you abandon any voyeuristic hope you might have of catching your hot neighbor as she's coming out of the shower, and just enjoy the movies, it's very fun: you get the feeling you're flying a real-life helicopter in a world of giants.
Another entertaining thing you can do is to scare your cat witless.
Or not.
Mine is surprisingly unafraid of the thing. She'll carefully watch it as it flies, but not back away unless it gets really close. I say "surprisingly" because she's usually afraid of... well, pretty much everything. Perhaps it has to do with the fact that I let her sniff it abundantly as it was laying unpowered on the floor.
But she should be scared. The 5#4's blades spin fast enough that they will draw blood if you let your bare skin be hit (guess how I know), but she doesn't seem aware of that. I make no guarantee that your own cat won't see the 5#4 as a large insect, attack it and end up with cuts and bruises everywhere. YMMV.
On the subject of being hit: the blades are made of soft plastic and they aren't much of a threat beyond superficial cuts. Crash the helicopter on someone's naked torso and they'll bleed, but they'll want plasters (and possibly a bat), not an ambulance. Just be careful not to blind someone.
The 5#4 can be flown outdoors, but only if there's no wind, and by no wind I mean it really has to be dead calm; it's very sensitive to interfering air currents, to the point where even flying it in a room with an air conditioner or fan on is highly inadvisable. Hit a very light breeze, and it'll wobble and be hard to control. Anything stronger than that will cause it to crash.
If you're flying outside, resist the temptation to see how fast it can go. It's really meant to be a hover and slow movement aircraft; fly it too fast and there's a very real risk of the upper blades hitting the lower ones, or the lower ones hitting the tail. This can be avoided by modifying the flybar, but I leave that to the experts. It's a rare day without wind here, so I never fly it outdoors anyway.
I haven't made other pictures of the helicopter flying, and neither any movies. If you want to see a Mini Lama fly, YouTube has more material than I could ever provide.
Aside from the really expensive, complicated modifications such as the aforementioned conversion to brushless motors, there are a number of more modest things you can do to improve your 5#4.
For starters, it's a good idea to cut some holes in the plastic canopy so the 4-in-1 gets better cooling. An overheated 4-in-1 won't die with an explosion, but it will make the helicopter much harder to control. The piezo gyro gets really confused when operating outside its temperature range, with predictable results. I cut holes in the canopy as soon as I got the helicopter; you can see them in the pictures.
Another thing you can do is to replace the tail. The stock one is flimsy and breaks very easily (as you can see from the pictures, mine lacks one of the fake ailerons and a blade of the fake tail rotor). The best solution is to toss it and replace it with one of the various aftermarket carbonfiber tails you can find all over the place on eBay.
I chose to keep the tail as it is. If the tails were sold for anything approaching their actual value I would have probably replaced it, but €15 + shipping for a 20 cm carbonfiber stick is not my idea of a wise investment.
In case you're wondering, you can't just toss the tail. Theoretically a coaxial doesn't really need one, but without it the whole thing shakes like a very shaky thing during an earthquake and becomes incontrollable. Ask me not why. Also, if you fly it farther than a few feet, the tail helps you determine where the helicopter is pointing.
People have been installing small heatsinks intended for computer RAM chips to their helicopter's motors in an attempt to improve cooling. Some think they make a difference, some don't. I don't. There was a thread on some forum that showed that it was actually counter-productive to install them. I've lost the link, though, so you don't get to read it. Sorry.
Another thing some people have done is to replace the 2-cell 7.4 volt LiPo battery with a 3-cell 11.1 volt one. This greatly improves the helicopter's performance, but it's bad for pretty much all of its components.
The motors overheat more easily and die far sooner, the servos are more stressed, the 4-in-1 produces much more heat and woe betide you if you happen to lock up the rotors and hit the throttle. With a 2-cell pack you have a grace period of a few seconds before the 4-in-1 smokes its mosfets; with a 3-cell pack you get maybe a fraction of a second, and then you need a fire extinguisher. Personally, I steer clear from such dangerous mods.
Some people do entire body swaps on coaxial helicopters. The Blade CX body can be adapted to the Lama, and you can find Kamov KA-27 bodies for it and for the Blade CX as well, and also canopies adapted from the bodies of other non-coaxial models.
I fail to see the point in this. I can (just barely) understand those who have very large models and want them to look as similar to the real thing as possible, but you have to consider the 5#4 and its friends are to real helicopters as AirSoft mini guns (no, not AirSoft miniguns) are to real assault rifles.
Then again, to each his own.
Perfectionists also like to balance their helicopters' blades by grinding off excess plastic and/or adding weight with tape. Some also say the helicopters behave better if their lower rotor has the blades slightly shortened.
If you're thinking you've got better things to do with your time, you're not alone. There's some tape on my lower blades, but it's because they developed some cracks after hitting some furniture. I thought I'd fix them with superglue, and in the meantime I'd put some tape on them to keep them from breaking. They're still waiting.
And I think that's it. There are only so many modifications you can do to such a simple model.
If, for some reason, you don't want a 5#4 but still want a coaxial, there are plenty more choices.
Here in Europe the 5#4 is by far the most popular coaxial helicopter available, and you can get it online in umpteen places (the first of which being eBay, of course).
In the US, the most famous coaxial helicopter is the aforementioned Blade CX, which is similar but with fixed blades and a different body.
Its competitor is the Esky Lama 2, which is sold under a different brand, but is otherwise pretty much the same thing as the 5#4.
Then there's the Dynam Vortex. Different plastic body and a slightly different component layout.
There's also the Hirobo XRB, which is more expensive but supposedly of better quality.
Keep in mind that at least one thing changes inbetween all the models: the 4-in-1. They are pretty much identical in how they work, but each brand has its own. The difference is usually just in the position of the electronic components on the circuit board, and they are often interchangeable (although you'll need a Walkera transmitter to use a Walkera 4-in-1; I'm not sure if this applies to the others).
They all fly pretty much the same, too.
Needless to say, there are also a lot of clones and knockoffs. Lots of ebay auctions for RC micro helicopters, coaxial and not, have suspiciously low prices and routinely end at less than €30. Don't be tempted: they are almost always two-channel helicopters.
Two channel means they have no servos. You only get up-down and yaw left/right commands. You can't get them to fly forwards/backwards at will, and neither to roll left and right. They are supposed to slowly move forward when you apply power, but they rarely do. Often it's just the body of a four-channel helicopter with empty holes where its servos should be and a simpler receiver.
Those bodies are supposed to be used for servo-controlled flight, so when used for a two-channel heli there's no telling which direction they'll go once airborne. On some particularly bad designs the weight of the tail can cause them to always go backwards. Putting small weights on the nose helps, but precise maneuvering remains impossible, and then you can't hover them in one place.
You're better off avoiding them, with the sole exception of the 4-channel Apache-styled model. You have to be very careful, because the same body is used for both the 2ch and 4ch version, and the auctions often don't explicitly say which version is on sale.
Apparently, the 4 channel version is actually half decent. Its electronics are of a lower grade and if I understand correctly its 4-in-1 doesn't have a piezo gyro, but you do get servos and real direction control. Just don't expect it to be as nice and easy to fly (relatively speaking) as a 5#4.
Also: sometimes these are sold with a 350mah battery made out of 7 2/3AAA NiMH cells. This battery is worthless. 2/3AAAs aren't supposed to ever have to deliver the high current required by a helicopter model, so they won't even give you the almost-half capacity you'd expect by simple mathematics. They'll overheat and their voltage will sag much sooner, so expect maybe three minutes of flight time, if you're lucky.
If you like your toys bigger, there's the Walkera 53, successor of the venerable Walkera 10. It is exactly like the 5#4, but scaled up, and not suitable for use in your average house. It's really meant for large rooms (think office reception) or garages.
There's also the AirScoot. It's larger and definitely butchier-looking than the others, but high price and some poor design choices (such as yaw control through the deviation of the rotors' downwash) have made it unpopular.
If instead you like your toys smaller, there's the Walkera 5#5. It is, again, exactly like the 5#4, but scaled down. I'm not aware of any clones or similar models, but I wouldn't be surprised if there were as many as for the larger helis.
Another smallish coaxial is the "Air Hogs" Helix. It has three channels, but it uses a more direct approach to get forward/backward control: two small fans, each with its own motor, are affixed on the sides of the body and provide horizontal thrust and yaw.
This version obviously flies faster, but many have found it sluggish and hard to maneuver in small spaces, as it tends to move around on its own accord. Also, helicopter purists sometmes dislike it because direct horizontal thrust motors diverge from traditional helicopter principles.
These smaller helicopters are more suited to be used in small spaces, but their lower weight makes them slightly harder to control.
And then there are the really tiny ones, which you could probably fly in a closet: the BladeRunners. These don't have any servos; rather, they use the "ProxFlyer" system to get directional control. The BladeRunners exist in various versions.
The first and simplest one is sold as the "BladeRunner", "Mini BladeRunner" or "Rescue helicopter", only has two channels, and suffers from the same direction problems of its larger two-channel brothers. To me, this makes it worthless.
The second version is sold as the "BladeRunner 2". It has a small tail rotor, but instead of pointing sideways this one points up. It can spin in both directions, and its purpose is to provide a small amount of positive or negative lift to the tip of the tail boom. If the tail is pushed up the nose goes down and the helicopter flies forward. If the tail is pushed down the opposite happens. This is a rather clever way of getting a third channel without using expensive servos, but the flying speed is very slow.
Sometimes the resellers screw up the names and the BladeRunner gets sold as the BladeRunner 2, or vice versa. Be sure to have a look at the helicopter before buying it, or you may get the wrong model.
Another tiny-copter using the same principle is the Micro Mosquito. It's pretty much the same as the BladeRunner 2, but with an improved tail rotor for more consistent pitch control, a slightly different transmission and no plastic body.
There's yet another ProxFlyer project, which has two tail rotors in a "V" configuration. They work together to provide pitch control, but they can also work against each other to tilt the helicopter sideways, thus getting it to roll. However, it seems it hasn't left prototype stage. I've seen no BladeRunner-like helicopter with this configuration on sale anywhere.
A nice thing about the tiny little things is that the tips of their blades are linked by a plastic circle. This decreases the flimsiness of their rotors, and you can freely bump them on walls and flat obstacles without breaking anything.
A not-so-nice thing about them is that their exceedingly small weight forces them to carry very small batteries. Don't expect much more than 6, maybe 7 minutes per charge. Some sites claim 10 minutes, but I'm doubtful.
Silverlit also sell an even tinier helicopter known as "Picoo Z", with a weird 2-channel control mode that makes it fly circles if you spin it in one direction and stop if you spin it in the other. It's very limited in its controls.
If you're looking for something more... unusual... than a helicopter, maybe a Silverlit X-UFO is the thing for you. (I've linked to a reseller's site rather than to Silverlit's because I'm allergic to crappy Flash sites made by 12-year-olds.)
The X-Ufo has four rotors in a square layout and lacks a proper airframe, with all the electronics squeezed together in a small central hub. It's another inherently unstable design, and it's thus rather harder to fly than coaxial helicopters (there are three gyros in it, and they aren't there just for show), but it sure is original, and there are plenty of hacks available to improve its handling.
As usual, eBay is flooded with cheap knockoffs, but these tend to work even worse than the cheap knockoffs of helicopters. A quad-rotor design requires much more electronic help to stay airborne than a coaxial, so supercheap electronics of questionable quality affect X-UFO clones much more.
Superficially similar, but directed to a different audience, is the DraganFlyer. The concept is the same (four rotors, central hub, unstable design made flyable by the electronics), but this one is intended as a semi-professional camera platform for aerial photography, not as a RC toy to fly for fun. Some of the sites that sell it emphasize this by saying you can use it to start an air photography business and flat out state "this is not a RC toy". Not that anybody's going to mistake it for a toy with its $2400 price tag.
What if you're a serious professional modeler and you want a big, glow- or gas-powered coaxial?
Well, if you are, tough. You can't buy any.
The general consensus is that if you're a serious modeler you want a tail-rotor helicopter, because that's what the experts fly. Nobody seems to understand that there are people who aren't necessarily very good at flying helicopters but don't like electric motors and love internal combustion engines. Gas/glow powered coaxials do exist, but they are all built with great care and expenditure by real experts who don't like to be restricted by what's available in already-built form.
If you want one and find someone willing to build it for you, be prepared to shell out a lot of money. If you don't find anyone and decide you know enough to build one yourself, you're probably wrong.
While we're talking about spending, let's talk prices.
The 5#4, Esky Lama, Blade CX and Dynam Vortex have very similar prices, from €120 to €150 shipping included (which translates roughly to USD150 to 190). You'll pay less for a used one, but beware of engine wear; only buy one second-hand if the seller guarantees it's been used very little and if you can trust him.
Bear in mind that if you find a real-life (as in, non-online) shop that stores them (which is a hard task in itself), it'll cost a hell of a lot more. The markup real-life shops add is ludicrous.
22/12/2006 update: the impossible has happened. I've seen 5#4s sold in, wait for it, a perfume and razor shop. For €170. They had PiccoZ on sale as well (for €59). Armageddon is coming, repent your sins while you can!
The Hirobo XRB costs about €200. Some say the difference in quality pays off. Me, I'd rather spend much less and replace something every now and again.
The little brother of the 5#4, the 5#5, costs (as you'd expect) less: about €100, shipping included, from your friendly Hong Kong eBay dealer. If you live in the US, you might find one for a little less from national resellers.
The Air Hogs Helix can be yours from about $70 to $90, depending on the retailer. International shoppers can find them for $65 on ebay, but the final price will be heavily influenced by shipping charges. If I wanted one sent here to pizzaland I'd have to pay €81.
The larger Walkera 53, despite having more powerful engines and, of necessity, a higher capacity battery (1250mah), surprisingly doesn't cost more than the smaller 5#4 (and company). Hong Kong dealers will sell you one for €130 shipping included.
The AirScoot used to cost some $600, but the company is clearing its stock to concentrate on the full-size AirScooter, so it cut its prices first to $440 without transmitter and charger, and now to $220 with all the necessary accessories.
Keep in mind that the model is no longer new and hasn't been updated, so you get some rather anachronistic equipment. The battery, for instance, is an older NiCD 2400mah pack with a rather short run time. Replacing it with a NiMH pack of similar configuration, or even better a LiPo, will do very good to the helicopter but won't be very cheap. In addition to shipping ($35 for the US, a lot more for international buyers) the final price is likely to be significantly higher.
The helicopter really is much bigger, though.
Back to tiny-land, the various ProxFlyer-based models have the following prices:
BladeRunner/MiniBladerunner/Rescue helicopter: around £60 without shipping (€90, $113). It'd probably be much cheaper on eBay, but I can't find any auctions for it.
BladeRunner 2: €40 from eBay, but if you're not in the US you're going to have to pay rather more than the price of the helicopter to have it shipped to where you live. For Italy, that's a whopping €62 (for a total of €102). Not much sense for an Italian to get one when you can get a 5#5 for the same price.
Micro Mosquito: people in the US can get them at $70 from Radio Shack. Others can't, but they are sold on eBay. The price is €150 including shipping for two of them at different frequencies, so you can fly both at the same time. But if you don't want two, there's no way I can see to only have one of them. Still, €75 for each isn't so bad, and you can always resell the other one.
Keep in mind that the ProxFlyers have no servos and their rotors are easily fixed with tape, so it's unlikely you'll have to buy lots of replacement parts (unless you mistakenly sit on them, or something).
The 5#4 is one of the best toys I've ever had. It's cheap (relatively speaking), it doesn't require you to have a true dedication to the hobby nor to spend endless hours to assemble, tune and repair. It's not very dangerous and it genuinely is loads of fun.
You can fly it anytime, anywhere (well, almost), provided you have charged batteries at hand. Just turn it on and off you go, ready to
save the poor Lego men stuck on the box flaming building.
Get one. You know you want it.
The world of micro helicopters is in continuous evolution, though, and newer and/or different models are available all the time. I've listed a few, but there are bound to be more. Your best bet is to head on to RCGroups and have a look at the "Multi rotor helis" section. Any new model that becomes available, any hot piece of news, anything at all will be reported on that forum as soon as it comes out.
Good luck, and good flying.