ROAR (Remotely Operated Auto Racers) is a national non-profit organization dedicated to promoting rc cars and trucks. ROAR also provides liability and bodily injury insurance for it’s members who race at ROAR sanctioned events and keeps a database of approved lithium polymer batteries. Tested at ROAR’s laboratory, approved lithium polymer batteries need to meet criteria including safety and performance. If you’re racing your electric car or truck at a ROAR sanctioned event, you should use approved batteries. We carry all of ROAR’s approved Thunder Power RC batteries, listed below:

You can browse through our huge selection of Thunder Power RC lipo batteries on our site. We pride ourselves on carrying every battery that Thunder Power RC manufactures, so if you see one that we’ve missed, feel free to contact us.

All RC airplanes and helicopters are controlled by servos - small, electromechanical devices that allow everything from controlled flight to payload releases. So what are servos?  How do they work? And how do you choose the ones that will work best in your model? We’ll answer all these questions, and take you through everything from the basics of servo operation to their technical details.

What is a Servo?

A servo is a device that can rotate to an arbitrary position, as set by the user. Servos usually consist of a small DC (direct current) electric motor, several gears, and a head where an arm or wheel can be attached. When the user tells the servo what angular position to move to, the servo rotates and holds that position until further input is specified. The servo holds position because external forces are always interacting with the aircraft, and would set control surfaces to undesired positions unless stopped. Servos exert a torque on external forces, that prevents them from changing the position of any control surface.

How Servos Work

A servos job is to convert the angular movement of a servo arm to the linear movement of a control surface. This is done by attaching linkages, called control rods to the servo arm and the associated control surface. When the servo head rotates, it pushes the control rod back and forth. The rod is linked to a control surface, and can move it up or down as the servo rotates.
Servos are controlled by three wires: two to provide the DC power that the motor needs, and one that sends the signal, controlling the servo. The signal wire works by sending the servo a series of pulses, which are interpreted by its internal circuitry. By varying the timing of each pulse, the servo knows exactly which position to move to.

RC Servo Motors

The motors that drive RC servos come in several different types. Here’s a list of the most common varieties, and some information on each to help you decide which ones to use:

• Coreless – Conventional motors use copper wires wrapped around metal cores to form electromagnets. In a coreless motor, there is a metal mesh that rotates around the permanent magnets. Coreless motors respond more quickly than conventional motors, because they don’t have to overcome the momentum associated with heavy metal cores.
• Brushless – Servos can be powered by brushless motors, giving them longer life, faster response time, and more torque.
• 3 Pole and 5 Pole – Electric motors have permanent magnets, called poles, that electromagnets are attracted to. Servo motors can have either 3 or 5 poles, with more poles providing better torque. If you’re new to RC or have a regular sport model, you probably won’t notice the difference between 3 pole and 5 pole servos.

Choosing the Right Servo

Servos have a number of defining properties that make them suitable for different applications:

• Torque – This is a measure of the servos strength, or how much “push” it has. More precisely, torque is the product of force and the radius at which it acts. This is shown graphically in the figure on the right. Bigger planes need high torque servos to move their large control surfaces. In general, servo size goes up with rated torque.
• Dimensions – Servos come in many different sizes, which you can choose from depending on your application.
• Weight – The weight of a servo depends on several variables. Most often recorded in grams, the weight of a servo is always reported on the package.
• Bearings – There are two ways to support the output shaft of a servo – bearings and brushes. Brushes are cheaper, but bearings last longer and operate more smoothly. Very small and very cheap servos tend to be brushed, while high end and very large servos generally have bearings. It’s possible to upgrade a brushed servo to bearings, with several upgrade kits being available on the internet.
• Gears – Most hobby grade servos use nylon gears, while higher end servos use metal gears. Metal gears add more weight, but their advantage is that they can’t “strip”, causing an RC helicopter or airplane to crash. Metal gears wear over time, which can cause “slop” in their rotation, but the gears can be replaced somewhat economically. In general, nylon servos are adequate for sport flying. If you’re particularly worried about losing a model in a crash, or are flying intense aerobatics, a metal geared servo is probably the right choice.
• Speed – Speed measures how fast the servo can move from one position to another. Different RC airplanes and helicopters will need servos with different speeds. For example: a trainer doesn’t need to change control surface positions rapidly, but a 3D helicopter or plane does. High speed servos are many times more expensive than standard ones.
• Digital / Standard – Servos can be of two types: digital, or standard. Both digital and standard servos can be used with a normal receiver, the real difference is performance. All servos use a series of short pulses as signals that determine what angular position they should maintain. This series of signals is usually very fast, somewhere around 50 pulses per second maximum. On a standard servo, this rate is so fast that small movements of the control sticks may not have an affect. This means that there’s a small deadband on the control sticks, in which no servo movement takes place. Although it’s not a problem on trainers and most sport class models, the deadband becomes a significant issue with 3D aircraft. Even a small delay with a 3D aircraft could cause a crash.
  Digital servos remove the deadband by speeding up the rate at which it receives      pulses. Usually, this is increased from around 50 to 300 pulses per second. This increase in resolution allows the servo to operate much more precisely.

All RC kits and ARFs will specify the type and brand of servo required. Generally, you should adhere to these recommendations.

Now that you know what servos to get for your model, you can browse the large number of servos available on our website.

You can use any model aircraft equipped with a camera to measure distances on the ground. This is great for hobbyists interested in making their own maps, or if you’re just curious what the distance between two far apart objects is. With a little trigonometry (a gasp is heard throughout the room), all you need is to measure one angle and one distance. I’ll walk you through the math, it’s not actually that hard, and the end result is more than worth it.

What You Need

To do this project, you’re going to need a few materials:

• An RC helicopter, or RC airplane with 3 channel control or better – You’re going to need a stable platform to take pictures from. Helicopters are useful because you can hover in one position, but airplanes are cheaper. You might already have an airplane or helicopter around, but if you don’t, the Multiplex Easystar works well for this project. Note that to make a remote shutter function, more than 3 channels are needed.
• A lightweight camera, with remote shutter operation – You need to be able to remotely trigger the camera. This can be done a number of ways, but the simplest and most economical is to attach a servo to the top of your camera with rubber bands. Make it so that when you throw a spare channel switch on your transmitter, the servo arm will move and press down the shutter switch. As with any mod, feel free to create your own solution.
• A large protractor
• Some string and a weight – Clay works well as a weight, I’ll explain why you need it shortly.
• A drinking straw – You’re going to need this to build a sight for the protractor.
• Measuring Tape – You need to know the altitude of your aircraft, and to do this, you need to measure it’s distance from you. Landmarks with a known distance can also be used – consult any street map with a scale for these.
• A notebook / paper / pencil
• A friend to help – You can’t fly your aircraft and make measurements at the same time. Take a friend along to help you, and ask them to record the measurements.

The Math Involved

Now we come to the hard part – a little bit of math. You don’t really need to understand all these derivations, feel free to simply use the formula that I’ll give you. The problem is this: given an aerial picture, how can we figure out the scale?

We need the altitude of the airplane to figure out the image scale. This can’t be done directly, so we measure the angle (a), the distance (d), and use them to compute the height (h). This is a right triangle, and there are some handy trig functions that apply. In this case, we use the tangent function, which gives the ratio of the opposite and adjacent sides.  This is expressed as follows:

By taking the tangent of the angle a, and multiplying by the distance (d), we get the height (h). Here’s the formula that you would use:

So how do you get the angle? It’s simple: take the protractor and tape a piece of drinking straw to the flat bottom. Then attach a piece of sting to the bottom center of the protractor, so that it dangles straight down the 90 degree mark. Use the ball of clay to make a weight at the bottom of the string. Now, when you tilt the  protractor, the string will measure the angle. Be careful though: the angle with respect to the ground is not what’s read directly off the protractor scale. Reading the difference between the indicated angle and 90 degrees will give you the angle you need.

So why do we care about the altitude? Well, it turns out that the ratio of the altitude and the focal length of the camera is the image scale! You can find out the focal length of your camera by reading it’s manual. This is usually expressed in millimetres, so convert it to whatever  unit you have used to measured the distance, and thus the altitude in. Let’s put that all in a convenient formula:

And that’s the only formula you need. Just measure the distance, and the angle, know the focal length, and you’re done.

How to Do the Measurements

All that math was fun, but how do you actually measure distances using this method? I’ll illustrate with an example:

Suppose that you’ve just gone out to a field, and want to measure the distance between a tree and a building. The first step is to find a landmark you can fly over with a known distance. Using a measuring tape or map, you find that a nearby hill is 50 feet from where you’re standing. With a friend ready to measure and write down the angle, you launch your airplane and fly over the nearby hill, taking several pictures. Your friend sights the model aircraft through the protractor – straw device built earlier, and finds the angle to be 75 degrees.

Using a calculator, you find the altitude to be:

After landing, you download the pictures and print them full size, with no scaling. Your camera’s user manual reports that the focal length is 152 mm (millimeters). Converting this to feet is easy, just type it in Google or multiply by the number of feet per millimetre. 152 mm is 0.48 feet, so you plug that into the formula we derived earlier and obtain the following:

This means that distance measured on the picture is 0.000257 times as big as the real distance. You’re almost done: using a ruler, you measure the distance between the tree and building on the image to be 1 inch. Converting this to feet (because we want the distance between the building and tree in feet), gives a distance of 0.0833 feet. Now, multiplying this by 1 divided by the picture scale gives a final answer of 324 feet.

And that’s it – you’ve just measured the distance between two objects using nothing more than a RC aircraft, camera, and a little trigonometry. Just keep in mind that you have to know the distance between the airplane and you accurately for this to work – always take pictures right on top of the marker with a known distance.

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