Lithium polymer battery packs are superb sources of energy. When managed properly, they can yield hundreds of charge/discharge cycles throughout their lifespan. But when proper management is ignored, pack degradation or failure will rear its ugly head. Cell balancing is a crucial part of this maintenance. However, in order to balance a pack, it must be wired for it. The good news is that these days more and more packs are "prewired" for balancing. But what if you already own one that isn't? As usual, RC Heli magazine comes to the rescue.
A Li-Po Battery Pack Primer
For those of you already using Li-Po battery packs, let's begin with what I hope is a review. For those who are just venturing into electrics, let this serve as an introduction to Li-Po fundamentals.
Li-Po packs are built from individual Li-Po cells wired together in such a way as to increase voltage, capacity, or both. Each individual cell produces a nominal voltage of 3.7 volts and is available in a wide range of capacities. To power a popular heli such as the Align T-REX 450, you may use a pack consisting of three 2200 mAh cells wired in series (referred to as a 3S pack). This configuration results in an 11.1 volt / 2200 mAh pack.
The "C" rating of the pack will specify its maximum discharge rate. The pack described above with a 20C rating lets you know that you can discharge that pack at 44 amps (20 x 2200 milliamps).
The two most common ways of misusing Li-Po battery packs are by exceeding their C rating and/or depleting them beyond their rated capacity. A good rule of thumb is to only deplete a pack to within 80% of its rated capacity. Using the pack above once again as an example, it should only be discharged to 80% of its 2200 mAh rating (or 1760 mAh). This is a much safer practice than depending on the auto cut-off feature of an ESC to let you know when it's time to land.
Balancing
So where does "balancing" figure into the equation? Consider this...
Li-Po cell manufacturers inform us that running an individual cell below 3.0 volts may damage it. That would equate to 9.0 volts for a 3S pack. Without regular balancing, cells become "unbalanced."
An unbalanced pack can lead to a scenario where you might run the pack as an entity down to 9.2 volts. However, two cells could be at 3.2 volts each, while the third cell is down at 2.8 volts. By just measuring the pack, you might believe that you're safe because it measures out at 9.2 volts. In reality, however, one cell haze already ventured into the danger zone.
How they are charged without balancing
Basic Li-Po battery charging is a two-step process. During the beginning of the charge cycle, a constant current charge is utilized while the pack voltage is monitored. When the pack's voltage reaches approximately 4.2 volts per cell, the charger switches to a constant voltage charge. During this phase of the charge cycle, the current going into the pack is monitored and the charge is terminated when the current decreases to just above zero amps.
As you can no doubt see, unbalanced charging can lead you into the same situation as I just described. This can result in the overcharging of individual cells.
What balancing accomplishes
Packs that are wired for balancing facilitate access to each cell in the pack.* The cells are not charged via the high current discharge leads, but rather through a separate set of leads that allow each cell to be monitored and charged individually. The charger can then ensure that each cell is charged to its maximum capacity.
Balance chargers, as well as in-line balancers, can do their thing during the discharge and/or charge cycle. When discharge balancing, higher voltage cells are discharged until all the cells are at the same voltage as the lowest cell in the pack. Discharging continues in this manner until all the cells reach a pre-set cut-off voltage. The opposite occurs during balance charging. However, in reality it isn't quite that simple. During charging, a pulse-width modulation scheme is used to control the current delivered to individual cells. But for the purpose of this discussion, you have the essence of what's going on.
* Balance packs that contain cells wired in parallel will expose those cells wired in parallel with each other as one cell.
How the batteries are wired
The following diagram details the wiring schematic for a standard non-balance 3S pack:
Only the negative terminal of the first cell and the positive terminal of the last cell are brought out of the pack. Both charging and discharging are performed through these leads.
The schematic for the same pack with balancing leads would look like this:
As you can see, by virtue of the balance leads, you are now capable of accessing each cell individually. Looking at the balance leads, the black and white leads are the negative and positive terminals, respectively, of the first cell. The white and blue leads are the negative and positive terminals of the second cell. Finally, the blue and orange (or red) leads are the terminals of the last cell.
Exception to the rule
Be aware that there are battery packs on the market that should not be modified for balancing without extensive modification. These packs are recognized by the presence of a separate two-lead charge plug. Packs of this style contain internal circuitry designed to safeguard the cells during the charge process.
Wiring a battery for balance charging/discharging
IMPORTANT: Before attempting to modify any Li-Po battery pack, one must be aware of two things. First and most important: Be very careful that you do not short any of the wires or cell tabs. Shorting will generate a very high current, which in turn produces extremely high heat. Cell damage and fire may result. Second: Performing a modification on a pack will almost certainly void its warranty.
To begin the modification, you'll have to remove the pack's protective heat shrink covering. Use a sharp razor knife and be very careful not to cut into the cells or their associated wiring. Save the label that contains the battery specifications in order to transfer them on to the pack when it's re-covered.
Once the covering is removed, the individual cells will be exposed. The inter-connections between the cells and leads are on one end of the pack. You may have to remove an insulator to expose the solder pads.
The balance leads must be soldered to the individual cell tabs. There are different style connectors available and the one you use is a matter of personal choice. For this example, I'll use a Thunder Power balance connector.
Looking at the following photo, I've labeled where we'll attach the balance leads.
Solder the balance leads onto their respective pads. Again, be very careful not to short anything. When complete, it should look like this:
Once the leads are attached, connect the pack to your balancer and test. At this point all that's left to do is enclose the pack with some fresh heat shrink covering. Make sure you reinstall any insulating material that you may have removed.
You have now successfully added a balance connector to the pack.
Conclusion
Adding a balance connector to a Li-Po battery pack is a relatively simple modification that will undoubtedly extend its service life and improve its performance. Your battery will be much happier for it.
Safe flying!
Wired For Balancing
Build A Receiver Pack
Your receiver battery is the heart of your radio-controlled helicopter. If that battery fails, a crash is virtually inevitable. There are good receiver batteries on the market, but learning how to build one will give you extra confidence that your battery is up to the task.
Picking the right cells
Selecting the right batteries is first. You want to pick a high-quality cell that has enough capacity and can be built in a configuration that will fit your model. The 1100-2500 mAh AA packs will work okay for 30- or 50-sized helis. I prefer to go with sub C cells for 50- to 90-sized models, although people are often successful with A-sized batteries for the larger models. Whatever cells you decide to go with, make sure they are from a reliable source and have adequate capacity. Smaller cells tend to have higher internal resistance and less capacity, as well as less ability to deliver current.
Building your pack is as easy as following these step-by-step instructions:
1. Rough up the surface of the cells on the ends to help the solder stick.
2. Pre-tin the ends of the cells.
3. Arrange the cells in the layout you want so that you can solder the cells together in a series configuration. (diagram of a simple series circuit)
4. Glue the cells together with Goop.
5. Begin at the positive terminal at the end and solder the red servo lead to it.
6. Begin with the first cell to the second with a battery tab or solder braid.
7. Continue connecting the cells in the pack one at a time.
8. Solder the black wire of the servo lead to the negative terminal at the end of the pack.
9. Measure the voltage of the pack to make sure you are getting your expected voltage.
10. If the pack is providing adequate voltage, proceed to shrink-wrap the pack.
11. Cycle the battery several times before use to make sure it is working properly.
Things to Watch For
Make sure your new battery is working properly before committing a model to flight with it. Use a charger that can display how many mAh the battery actually took, and cycle the battery several times to confirm that it's working. To cycle the battery, simply charge and discharge it several times to get an accurate idea of its useful capacity. When you solder the cells, use a high-quality iron and avoid getting the cells too hot. Your iron should be hot enough so that you can just touch it to the cell and almost immediately start melting solder onto the surface. If you have to hold the iron on the cell to get the surface hot enough to accept the solder, the whole cell may heat up and cause damage.
(closing picture)
Conclusion
Sometimes you just can't find the right pack to fit your needs. After a crash that's caused by a suspicious failure, a little peace of mind goes a long way. Making your own battery gives you some quality control over your model's power supply. If you want to get really precise, you can even charge and discharge the cells before construction. This will ensure that they're delivering similar capacity and are at the same voltage, which means they'll be balanced when you assemble the pack.
Understand FlyBar Mixing
The flybar has a very important job for our model helis. A term commonly used that is related to the flybar is its "mixing ratio," or "flybar ratio." Have you ever looked at a heli manual and noticed that there are installment location options for flybar mixing arms? Have you ever wondered what that adjustment is really doing? The flybar ratio is one of those adjustable parameters that can dramatically change the way a heli moves through the air. A better understanding of this topic will allow you to make such adjustments with confidence.
Background:
Most RC helicopters in use today have a flybar. The flybar provides stability without the expense of maneuverability to our helis. In order to better understand why flybar ratio adjustments do what they do, we need to understand how they work through the history of its design.
Bell and Hiller
There are two systems that are combined and mixed together to control our model heli rotor heads: the Bell and Hiller systems. Both the Bell and the Hiller systems get their names and designs from full-size helis. Let's explain them both briefly:
The Bell control system is based on the Bell Stabilizer bar. The Bell stabilizer bar was used in many early Bell helicopters and is most commonly seen on the Bell UH-1 Huey. The stabilizer bar is basically a weighted flybar without paddles that spins with the rotor head providing gyroscopic stability. A mixing arm on the stabilizer bar takes inputs directly from the swashplate and mixes them with the gyroscopically stabilized bar to the blade grips. Flight control systems using a stabilizer bar benefit from direct control input, but offer limited maneuverability due to the overriding stability of the system.
• On a pure Hiller setup, the cyclic input from the swashplate is sent to the flybar; the flybar is then flown to the desired disk angle. The main rotor grips are attached to the flybar, and the tilting flybar will move the main grips to follow the same circular path of the flybar. The Hiller flybar is gyroscopically stabilized as the Bell Stabilizer Bar but now offers increased maneuverability as it can be flown to a new position. The downside to a pure Hiller setup is the lag in controls. The flybar needs to move itself into its new position before the main blades do, as the flybar in a Hiller system pulls the main disk behind it. This results in a small delay before the effect is realized at the main rotor disk.
Bell-Hiller System: The best of both worlds
How do you enjoy the pros of each system while diminishing the cons? Simple. Just combine them! Most RC helis use a combination of the above systems called a Bell-Hiller setup. By offering a direct input from the swash to the main grips via a mixing arm (Bell input) and controlling a maneuverable flybar (Hiller input) you get stability and maneuverability without any delay in control input.
The Bell-Hiller mixer arm is where all the magic happens. It takes the combined input from the swash and the flybar and transmits the resulting output to the blade grips. The use of a mixer allows the blade grips to get the desired instant input from the swashplate while also getting the needed control and stabilization input from the flybar. When the flybar tilts, it raises or lowers the mix arm, and will thus raise or lower the pitch on the main blades. By making adjustments to the mixing arm you can fine-tune how much influence you want the flybar to have over the main blades.
Bell input Adjustment:
The Bell component is usually set at factory to what the manufacturer has determined to be the best for the given heli. There are very few model helis on the market with adjustable Bell input; most let you adjust only the flybar (Hiller) side of the mixer arm. If you have a machine that allows Bell input changes, and you decide to experiment with these settings, be sure to check the total collective pitch range as you do so. Changes in the Bell input affect total pitch range much more than Hiller changes. This is because the washout removes all collective pitch influence from the flybar (Hiller side), leaving the Bell input to transmit all collective pitch changes to the grips. Making changes to the Bell mix is generally done by more experienced pilots looking for the perfect mix for their flying style. Making changes to the Bell side of the input is done using the same steps as making changes to the Hiller input, so the procedure below is more or less the same.
It can seem confusing at first, but if you read the manual and try the different settings out on the bench, it will make more sense. Try tilting the flybar with the heli on the bench while watching how much the blade grips rotate on the spindle. The amount of movement generated at the grips by the flybar input increases as the flybar ratio is changed to a higher setting—just as lowering the flybar ratio will reduce the amount of movement you see with this exercise.
How to Measure Flybar Ratio:
With all the prerequisite info behind us, we can move on to working with the flybar ratio. First, let's define what exactly we are measuring, here, so it's not confused with the Bell and Hiller ratios. The flybar ratio is simply the correlation between the flybar tilt and main blade pitch, measured in degrees. The standard form is to measure how much main blade pitch is given per each degree of flybar tilt. For example, a Raptor 50 Titan is equipped stock with a 1:1 flybar ratio. What this means is that for each degree of flybar tilt, the main blades actuate one degree as well. It's a 1 to 1 relationship in this case. 1:1 is considered a fairly high flybar ratio, while .5:1 would be a low flybar ratio, as the main blades would only move .5 of a degree for each degree of flybar input.
An interesting sidenote on the Raptor is that due to its 1:1 ratio, the flybar does not need to be level when setting the pitch up on this heli if you use the flybar as a guide to set pitch to. Any heli with a flybar ratio other than 1:1 needs to have the flybar leveled while setting pitch.
Calculating the flybar ratio on your heli is quite simple as long as you have a pitch gauge with a bubble level on it. If your pitch gauge does not have a level, you can simply attach a small one to the top or bottom rail. The other tool you'll need is a protractor, or anything else that measures angles.
1. The first thing you do is tilt the flybar in one direction (it doesn't matter how much tilt you put in). Measure the angle that the flybar is tilting by using your protractor (make sure that the protractor is level). Keep the flybar locked at that position and use your pitch gauge to find out what pitch the main blades are at, and again check for level. Write both of these numbers down.
2. Tilt the flybar in the other direction and re-measure in its new location. Also check the main blade pitch at this new position. Write these numbers down as well.
3. Use the two main blade pitch numbers to figure out the total travel between the two. For example, if one end was -3 and the other end was +4, you would have 7 degrees of travel between these two points. Now do the same with both flybar angle readings. Let's say, for our example, that we had 9 degrees total range on the flybar tilt.
4. Simply divide the main blade travel by the flybar tilt travel we just found to get the ratio. In this exercise it would be 7/9 or .78, meaning we have a .78:1 flybar ratio in this exercise. You want the main blade travel in the numerator because you are trying to get a ratio of how much main blade pitch each degree of flybar tilt creates. In our example, you get .78 degrees of pitch change for each degree of flybar tilt.
It's wise to run this calculation a couple of times using different flybar positions, in order to verify your results. While you do not need to know the flybar ratio of your machine in order to fly, it's interesting data to have and can be useful for fine-tuning a machine.
The flybar ratio is only one part of the equation regarding head adjustments. The length of the flybar and the dampers used will affect how a heli flies, as does the size, weight, and shape of the paddles. A flybar paddle that has a large surface area placed farther out from the main shaft (longer flybar) will have more influence than a small paddle placed closer in. For example, on a machine with a high flybar ratio you can get more powerful cyclic by using light paddles that have a large surface area, or use a longer flybar. To experiment, start with the stock heli setup and change one item at a time to get a feeling for how each option changes the heli's flight style.
Conclusion
The flybar is one of those seemingly simple, yet important parts to a model helicopter. Knowing what your flybar ratio is and what options you have on your heli makes you a more knowledgeable pilot in the long run. Next time you have a hankering to tinker, try adjusting the flybar a bit. The results achieved by small changes are amazing. In fact, a simple change on the flybar can make a heli feel like a completely new machine in flight. Whatever you do, enjoy!
Inverted Hover
Cool People Do It Inverted.
There is nothing like carbon fiber blades spinning at 2000 RPMs inches off the deck or the occasional blade scuff to get the adrenaline pumping during a flight. Now you can get your heart pumping a little faster by mastering the basics of inverted flight. Initially the awkward inputs will make the move a little tough, but with enough practice and effort you will be cutting the grass in no time.
SETUP
The proper setup for inverted flight is pretty straightforward. The first thing to make sure you have correct is your pitch setup. The conventional setup for 3D flight is to have at least +10 at the top of the collective and -10 at the bottom. You can also add more pitch for greater pop in 3D flight (see sidebar). When it comes to the throttle curve, if you are not running a governor you want to run an aggressive V-curve on your Idle-up condition(s) for 3D flight. The goal of a V-curve or a governor is to hold the headspeed as consistently as possible, allowing negative pitch to be used without losing headspeed. Make sure you adjust your cyclic travel enough until right before it binds.
Getting there
There are two simple ways to get your helicopter inverted, but first you want to make sure you're comfortable with flipping your helicopter and all orientations of upright flight. You also want to make sure that you're comfortable with the inputs that will be required for inverted flight, so practice on the simulator.
Rolling to inverted
One way to get your helicopter inverted is to simply do a sideways roll. Begin with a tail-in or a nose-in hover (whichever one feels more comfortable to you). Let me explain a tail-in hover. To start off, you want to fly your helicopter three mistakes high, giving you time to recover and save your heli in case you get in trouble. Begin adding a quick cyclic input either to the left or right, whichever one you prefer, but make sure it's fairly quick so that you have enough speed and momentum going into the inverted hover. You can also slow it down and apply more positive pitch as you enter, putting it in a climbing state; just make sure you stop the momentum as easily as possible. Once inverted, apply negative pitch and make sure you're level by compensating with the correct cyclic inputs to keep the helicopter steady. Be easy on the collective because if you apply too much your helicopter will climb more than necessary.
Flipping to inverted
You can also invert by doing either a back flip or a forward flip. When it comes to a back flip, just pull back on the cyclic (remembering, of course, to give yourself the necessary room so that you don't land the heli on its head); once you're inverted, apply negative pitch. If you started from a tail-in-upright hover, you will be nose-in inverted, so apply the nose-in orientation corrections stated below. With a forward flip, just apply forward cyclic, add negative input once again when inverted, and make the needed corrections.
Orientations
All the controls respond the same, but your orientation to the model makes it difficult at first to naturally know which input to give. With tail-in orientation, left and right cyclic input are the same, while forward and back input are backwards. Remember also that your pitch is reversed, so keep your stick in the negative range. You'll move the collective stick down to initiate an inverted climb-out. Tail rotor input can be thought of in one of two ways; you can either think of it as flying the nose backwards, or you can think of flying the tail. When nose-in inverted, forward and backward cyclic input remain the same, while left and right are backwards. Eventually you won't have to play mental games to remember which way to react, but when you are working your way into these uncomfortable orientations, knowing the above tips can help.
Bailing out
This is when knowing how to flip your helicopter comes in handy. For example, if there is a hiccup in any part of the maneuver, just apply the proper inputs to continue the flipping motion that you started. Just make sure to keep the heli away from you at all times. Once you pick a direction, stick with it and follow through; don't change your mind partway through and try to go in a different direction.
Conclusion
Inverted hovering is a little tricky at first, but with enough practice you will be able to impress everyone at the field with your grass-cutting skills. When mastering all orientations of inverted flight, it will help you expand your 3D flying by being able to bail out of maneuvers at any given time. Whether you're a few inches off the deck or high in the sky, inverted hovering is a rewarding maneuver that will never go out of style.
12 Laws To Building The Perfect Clutch
The quest continues to build our helicopters as perfectly as we can. The last time we looked at "Building the Perfect Head." This time, we will show you the laws of building a perfect clutch and clutch assembly. Many times, we find our helicopters shaking and shivering before leaving the ground. Usually the first thing we look at is the head, making sure everything is balanced. Numerous parts on our helicopters, from the tip of the canopy to the end of the tail, can generate a vibration. A misaligned clutch can transfer its vibration throughout the heli and cause excessive wear on the clutch liner or bell.
1.There are many styles of clutch assemblies on the market. Identify what style of clutch you are using and what tools are required for each type. The most common type of fan and clutch are the thread-on style. This style allows for easy installation and does not require specialty tools. Collet-style fans require the use of a dial indicator, to ensure that the fan is spinning concentrically.
2.Start by removing the back plate of your nitro engine. This will give you access to the piston connecting rod. We use this rod to lock the engine's crankshaft so we can tighten the fan.
3. If using a collet style clutch assembly, install the first collet over the crankshaft and slide it all the way down until it bottoms out on the bearing. Some kits require using a washer that is included with the engine, before installing the collet. This will space the whole clutch unit correctly.
4. Install the fan onto the crankshaft. If using a screw-on type fan, apply a generous amount of thread locking compound to the crankshaft and carefully screw it in until it bottoms out on the bearing or washer. Use a crankshaft-locking tool and tighten the fan by using a rag over the fan. You can also use a fan hub wrench, which will tighten better than a rag and your hands. Using a collet-style assembly, slide the fan over the crankshaft until it sits on the collet.
5. Install the second collet, followed by a nut, if using that style. Give the fan a light tap on top of the hub to pre-set the fan. Tighten down the nut until you can feel the fan seat in the collet, but do not crank it down. The fan must be dial indicated as you tighten the fan. It is best to tighten down a little, dial indicate, then tighten down a little more, and repeat the process until the fan is tight. When using a threaded fan, install the nut and apply a small amount of thread lock on the nut and tighten down the fan.
6. Dial indicating the fan can be somewhat tricky the first time you install your clutch. Using a vise to hold the motor is one of the easiest methods to use to check the alignment of your clutch. Use a dial indicator with a magnetic base and mount it to the vise. Tighten down your engine using a rag around the motor mounts. Grip the mounts in the vise, but do not over tighten the vise or the crankcase may crack. Place the probe on top of the hub and zero out the indicator. Next, rotate your motor without a glow plug installed and find the high spot on the hub. Give the hub a tap on the high spot with the plastic end of a screwdriver. Repeat the dial indicating procedure as you tighten down the fan in small increments, until you have a perfectly level fan and hub.
7. After dial indicating the fan and hub, tighten down the nut and recheck the alignment. The clutch should not be more than .002 off.
8. Next, we move on to the clutch bell. The clutch bell uses a liner to grip the clutch when the shoes are accelerated and thrown outwards. This liner is easily installed using the Quick Tip tool in Issue 21. Test fit the clutch liner in the bell and trim off any excess liner. Apply a small amount of epoxy to the inside edge of the clutch bell, then place the liner inside. Use the clutch liner tool by compressing the PVC together and releasing after it has bottomed out inside the clutch bell. Let the epoxy dry.
9. This is a perfect time to lube the bearings in the clutch bell and the one-way bearing, if your clutch is equipped with one. Use high quality grease, like Tri-Flow, to lubricate the one-way bearing in the clutch and the ball bearings in the clutch bell.
10. After the clutch liner has dried, remove the tool and trial fit the clutch inside the clutch bell. You should be able to fit a piece of paper all the way around the clutch, in between the clutch and liner. If you cannot fit the clutch inside, then a little sanding is required. Sand the liner down evenly, using medium grit sandpaper, until the clutch will fit. Take your time here, because if you sand down the liner too much it will have to be replaced.
11. Next, install the clutch to the fan hub. Use thread-locking compound on the bolts and tighten them down slowly, alternating from bolt to bolt. This will ensure your clutch is aligned on the fan hub.
12. Test fit your engine and clutch assembly on the mechanics to make sure everything lines up. The clutch should set inside the clutch bell nicely without touching any of the sidewalls. You can check this by rotating the bell after the engine is installed. If you feel any restrictions then you must adjust the alignment. If everything lines up, install your engine and tighten down the screws.
Conclusion
Taking your time here will allow your machine to operate smoothly without any vibrations caused by the engine. Alignment is the key when using the collet-style or "colleted" clutch. Drag-free clutches will allow your heli to retain more energy in an autorotation. This will also improve the idling performance of your helicopter.
