Lithium Polymer batteries are currently considered the best electric power source available for use in electric rc helicopter kits. The reason for this is that lithium battery packs are significantly lighter than NiMH or NiCD packs, and have higher capacity for the same size. Over the last five years they have worked their way into the hobby market, and in addition to being used in electric rc helicopter kits they are also common in rc boats, cars, and airplanes. Their low weight and high capacity make them ideal for long flight times, while also providing more power.

Understanding Lithium Polymer Battery Specifications:

Lithium Batteries are also know as LiPo, Li-Po, LiPoly, and Li-Poly. What do all the lithium battery specifications mean? The values that are the most important are:

• Pack voltage (depends on cell count; each cell is 3.7 V)
• Capacity (mAh)
• “C” rating (describes the rate at which power can be drawn from the battery)

Lipo Battery Cell Voltage

The nominal voltage of each cell in the pack is 3.7V, but this can go down to 3.3V during discharge, and up to 4.2V when fully charged. A battery pack is composed of two or more cells put together in series for increased voltage, or in parallel for increased capacity. A 2 pack with 2 cells in series would be rated at 7.4V (2x 3.7V).

The battery pack configuration is denoted by the number of cells in series and the number of cells in parallel. A 3s2p pack would have three cells in series, and 2 cells in parallel, using a total of 6 (3×2) cells. A 4000mAh 3s2p pack would have a capacity of 4000mAh (2 x 2000mAh), and a voltage of 11.1V (3 x 3.7V). It would internally consist of six 3.7V 2000mAh lithium polymer cells. The cells would be doubled up (the 2p part of 3s2p) to get 4000mAh, and there would be three in series (the 3s part of 3s2p) to get 11.1V (3 x 3.7V).

The “C” Rating

The “C” rating describes how quickly a battery can be discharged. A 2000mAh LiPo battery with a “C” rating of 1C continuous would mean it should not be discharged any faster than 2000mA or 2A, which would take one hour. A 2000mAh pack rated for 12C continuous would be able to discharge at 12 times its capacity (12 x 2000mA = 24000mA or 24A) at which rate it would discharge in 1/12th of an hour. Using a pack with a higher C rating than you require will leave some room for safety, and extend the life of your battery.

Continuous Current:
If you know how much continuous current will be drawn and the capacity of the pack you want to use, you can easily determine what C rating you require. If you are drawing 5A from a 1320mAh pack, simply take the current and divide by the capacity: 5A = 5000mA, 5000mA / 1320mAh = 3.8C.

Burst Current:
LiPo batteries are also given a C rating in terms of burst, which is how quickly the battery is able to discharge for a short time. A burst rating of 20C would mean a 2000mAh battery could supply 20 x 2000mA = 40000mA or 40A for a few seconds.

Selecting the Best LiPo Battery for Your RC Helicopter

To select a battery for your rc helicopter, you first need to know what voltage you require and how much current you will be drawing continuously.

• If the motor in your rc helicopter is rated to work with 11.1V, you would need a 3 cell battery.
• If you need to draw 20A, and you would like your rc helicopter to have a 10 minute (1/6th of an hour = 6C) flight, you would need a battery with 20A / 6 = 3.3A = 3300mAh.
• This means you would need to put a 3s 3300mAh battery with a C rating of 6 or higher in your rc helicopter.
• Thunder Power RC has a LiPo battery for almost any electric RC helicopter.

Always Remember These LiPo Safety Instructions

• Remove the battery pack from your rc helicopter before charging.
• Only use chargers designed to work properly with lithium polymer batteries.
• Always place your LiPo on a fire-proof surface while charging or use a LipoSack.
• Never leave your battery unattended while it is charging.
• Never charge a lithium battery if it is below 3.0V per cell.
• Never charge a lithium battery that is puffed up, or damaged in any way.

To get the most out of the electric power system in your electric rc helicopter, you will need to know if any of the components are under-powered, or over-powered and just adding weight. This article discusses what you can measure, and how you can measure it. This article is fairly advanced. We recommend reading What Does An Electric Power System For An RC Helicopter Consist Of first.

Each component of your power system is good at something. Are you taking advantage of each of them? A 25A speed controller in a small coaxial helicopter with a motor drawing 5A is not being used properly. By measuring your electric power system you can discover mismatched equipment and correct it. In this example, a 10 amp or 7 amp ESC would save weight.

Measuring the Electric Current in Your RC Helicopter

The measurements you are interested in describe the flow of electric current (using unit of Amps, A), the electric potential (using unit of Volts, V), and power (using units of Watts, W). These can be measured with a wattmeter. A wattmeter can be inserted between your battery and speed controller and it will tell you the voltage, current, and power drawn from your battery. Measurements should be taken when your helicopter is at full throttle. For RC hobby use, there are small wattmeters that can be installed in you aircraft, and can log measured data over the entire flight. These are called flight data loggers, and in addition to current and voltage, they can measure rpm, temperature, and more with the addition of the appropriate sensors.

Measuring Current And Voltage in Your RC Helicopter

The current reading should be within the limits of the speed controller, battery, and motor. The voltage indicates how your batteries are performing. Usually you can confirm your battery is adequate by noting that current is below the battery’s rated max current (For Thunder Power batteries, this would be the continuous rating, not the burst rating). This rating generally says ‘this battery model is approved for this use,’ whereas voltage says ‘this battery is performing as expected’ or ‘this battery can’t handle this.’ As a rough guide, your lithium polymer battery voltage at full throttle should be above 3.3V per cell (i.e. a 3s lipo should be above 9.9V).

Measuring Temperature in Your RC Helicopter

The temperature gauge is used to tell you the temperature of your motor and batteries. For both, the key temperature is 60°C (140°F). Both should be under that temperature. Above this temperature, lithium polymer batteries are permanently damaged, while motors lose efficiency and become dangerously close to demagnetizing and burning the windings. However, being too cold isn’t ideal, either. Cold is a sign that the motor isn’t being properly used (i.e. it’s too big), and cold batteries don’t put out as much power as 50-60°C batteries.

Tips

• Put together a power system that is slightly conservative
• Test and measure that system, and then adjust based on those measurements
• Currents will be near every component’s limit, and temperatures after a flight will be ideal
• At that point, every component will be played to its strength.
• Have fun flying!

Your electric RC helicopter power system is a combination of battery, speed controller, and the motor that drives your gearbox and rotor blades.

The battery supplies the power. RC helicopters have used Nickel Cadmium (NiCd) packs, Nickel Metal Hydride (NiMH) packs, and the newer, lighter, and more powerful Lithium Polymer (LiPo or LiPoly) battery packs. All of these are rechargeable and can be used over and over.

The electronic speed controller (ESC) is the component that interprets the control signal coming out of you radio receiver (RX) and passes battery power to the motor to make it run at the speed you want. Speed controls are available for brushed motors with two motor wires, or for newer brushless motors with three motor wires.

The motor is the final component that converts the electric power to mechanical power. Brushed motors are less expensive, but less powerful. High performance helicopters tend to use newer brushless motors.

An example of a compatible RC helicopter electric power system is:

• LiPoly Battery: Thunder Power Pro Lite 3s1320
• Brushless ESC: Castle Creations Phoenix 45
  
• Brushless Motor: Hacker A20-6XL RC Helicopter Motor

What do you do if you want to learn to fly an RC helicopter, but you don’t want to take the chance of crashing it on your first try? If your RC transmitter has a standard trainer port or buddy port connector, you can use it as a controller for a computer based flight simulator. All that is required is a USB FMS interface cable with the correct end to plug into your transmitter, and the simulator software to run on your PC. Using your actual transmitter you use when flying your real rc helicopter on the flight simulator software allows you to familiarize yourself with the controls in a safe environment.

Screenshot of an RC helicopter model in the FMS Software

Free FMS Flight Simulator Software For Your PC

The most well known free software that can be used with these cables is called the Flying Model Simulator, or FMS. You can download this from the FMS Homepage. The FMS flight simulator is extremely good, it allows you to load many different models of aircraft and landscape that can be found on the internet. Even without an RC TX, you can install this program and fly aircraft with you keyboard. This software is compatible with Windows 95/98/ME/2000/XP. It will not work with Vista or Mac operating systems.

Connecting Your Transmitter to Your Computer

GWS sells a great FMS transmitter to USB interface cable in three types: Futaba, Hitec, GWS and JR. These GWS interface cables take the signal from the trainer/buddy port on standard RC transmitters and convert it into data that your computer interprets as coming from a USB joystick or game controller.

Getting Help with the FMS Software

The FMS Forum is a great community resource for learning and troubleshooting the FMS Flight Simulator Software.

It is possible for an RC helicopter to use more that two main rotor blades. With four evenly spaced rotors (quad-rotor) the thrust from all the RC helicopter’s rotors is combined to generate lift, and the thrust produced by each individual rotor can be varied to allow the RC heli to maneuver.

Decreasing thrust on rotors at or near the front will cause the RC helicopter’s nose to drop. This results in a forward tilt and forward motion. The same method can be applied to the rotors on the back or side, allowing the RC helicopter to move backward and side to side. Draganflyer RC Helicopters use quad-rotor design.

More information can be found in Wikipedia’s quad-rotor helicopter article.

The BladeRunner Recon is a soon to be released indoor electric RC helicopter with built in micro wireless spy camera. This radio control BladeRunner Recon spy helicopter has a video camera built right into the nose.

The video signal is sent by wireless signal back to a 2.2″ LCD screen built right into the transmitter. This displays a real time picture from the Recon rc helicopter’s point of view.

The BladeRunner Recon’s receiver can also be connected to your tv, for a larger view. It can even record still images and video clips of lengths up to one minute on its internal memory, which you can download to your computer through a standard USB port. A USB memory stick can be used for longer clips.

The BladeRunner Recon is a 3 channel rc helicopter. This means it can be maneuvered up & down, turn right & left, and move forward & backward. Two sets of blades are included with the BladeRunner Recon, one “beginner” set for practice, and one “pro” set for serious use. More Bladerunner parts can be purchased at RCToys.com.

The BladeRunner Recon comes ready to fly with no assembly required.

Manufactured by Interactive Toy Concepts, the BladeRunner Recon is expected to be available this fall, for a price of aproximately $200 US. View more Bladerunner Series RC Helicopters.

Most radio control helicopters use a common layout, with one large main rotor on top generating lift, and a small tail rotor. This is called the Sikorsky layout.

When the rc heli’s horizontal main rotor turns generating upward thrust, it also generates torque which causes the helicopter to rotate in the direction opposite to the rotation of the main rotor. This is countered by sideways thrust generated by a vertically mounted tail rotor. An explanation about the way this works can be found in the Antitorque section of the helicopter article on Wikipedia.

Also, by varying the thrust produced by the tail rotor the RC heli can remain in one position, or intentionally rotate clockwise or counter-clockwise.

An example of a conventional RC helicopter is the eXtreme 450 Brushless 3D electric helicopter made by Thunder Power RC. Smaller examples include indoor mini helis and mini bell IR helicopters.

An RC helicopter using a contra-rotating propulsion system will have two rotors spinning in opposite directions. This allows the torque generated by the rotor blades to be canceled out, and removes the need for a tail rotor to keep the RC helicopter from spinning. Without a tail rotor, the rc helicopter’s tail boom can be eliminated, saving weight.

One possible contra-rotating layout has one rotor blade mounted at each end of a long fuselage, as it is for the full scale Boeing CH-47 Chinook, and is known as a tandem rotor. This concept is explained well by the Tandem Rotor article on Wikipedia. Alternatively, both contra-rotating rotors can be placed on the same shaft, resulting in a coaxial design (sharing the same axis). Another good Wikipedia article on Coaxial Rotors explains the theory behind this.

Coaxial rc helicopters are very stable and easy to fly. The Bladerunner series of indoor rc helicopters made by Interactive Toy Concepts, and the ESky Lama V3 and Lama II V4 electric RC helicopters are examples of designs using co-axial rotor blades.