Choosing the right motor and battery are both essential for building a successful ornithopter. Here, I will explain some of the different options and how to choose the right power system. How much power is needed, exactly? For typical membrane-winged ornithopters, about 100 watts per kilogram coming out of the motor should be enough power to give your ornithopter a good rate of climb. As ornithopter wing designs approach the efficiency of real birds, it should be possible to get by on somewhat less. Estimates of the mechanical power required for bird flight vary over the range of about 10 to 50 watts per kilogram.

Rubber Band Power

The simplest ornithopters are powered by rubber band. The rubber band combines the functions of motor and battery in one super-convenient package. The rubber band can produce a large amount of torque, so you don't need any gear reduction to flap the wings of your ornithopter. You can simply use a thicker rubber band if you need more power. For all these reasons, rubber-powered ornithopters are by far the easiest to design and build, and definitely the place to start!

The energy is stored by winding the rubber band with a twisting motion. This causes the rubber to elongate in a helical path. The result is just like stretching the rubber band, except it takes up less space. The rubber band should be lubricated, because the twisting motion causes a lot of rubbing. The grade of rubber is very important. Contest rubber intended specifically for flying models will store several times more energy than standard, office-grade rubber bands. Impressive flight times can be achieved in lightweight models. Roy White holds the record for indoor, rubber-powered ornithopters at 21 minutes, 45 seconds!

Electric Motors

Most radio-controlled ornithopters are powered by an electric motor and battery. There are several types of electric motor that may be used in an ornithopter. The selection of motor type will depend on your specific project.

brushed				brushless
standard		coreless		inrunner		outrunner

The brushed motor is the oldest type and the most familiar to most people. These motors have some coils of wire attached to a central, rotating shaft. The coils of wire serve as electromagnets. The motor also has some permanent magnets in the motor casing. The force exerted between the electromagnets and the permanent magnets causes the motor shaft to rotate. Electrically conductive "brushes" transfer power to the rotating shaft or armature. The electrical contacts are designed so that, as the motor rotates, the various coils or windings are switched on and off in a specific sequence, which allows continuous rotation. Brushed motors are the least expensive type, and a properly selected brushed motor will supply enough power for most ornithopter needs.

• Tip: Tiny "pager" or coreless motors are great for indoor and micro-sized ornithopters. These inexpensive motors are what makes pagers and cell phones vibrate. They are a special type of brushed motor that has no iron core in the armature.

In a "brushless" type motor, the electromagnets are switched on and off electronically, so there are no mechanical contacts. This is more efficient, but it requires a special electronic device called a brushless motor controller. This makes them more expensive than brushed motors. Brushless motors come in two varieties: inrunners and outrunners.

The inrunners are the simplest type of brushless motor. These motors have some coils of wire around the outside, which serve as electromagnets. They also have a permanent magnet on the rotating shaft inside the motor. The electromagnets are switched on and off in sequence, causing the permanent magnet to rotate.

There is also an outrunner type of brushless motor. These motors work on the same principle as inrunners, but they are inside-out. The electromagnets are at the center of the motor, around the motor shaft but not attached to it. Instead, they are secured to the non-rotating face plate. The permanent magnets are on the outside, attached to the motor case. The case rotates and is attached to the output shaft, so that rotates too.

Outrunners tend to operate at a lower speed and higher torque compared with intrunners. For that reason, outrunners don't need as much gear reduction. This will usually make the outrunner a better choice for most ornithopters. Less gear reduction is likely to mean less friction and less weight. However, the inrunner might be advantageous if you have an existing ornithopter that's geared for a high-RPM brushed motor.

Whatever type of motor you use, there are several factors that influence the power output of the motor. For example, the gear ratio must be carefully selected in order to achieve a high enough power output, while not causing the motor to overheat. This design process is explained in The Ornithopter Design Manual.

Batteries

There are several types of battery on the market. None of them are very good, compared with the fat birds use to store energy. The amount of energy stored in fat is about 10 watt-hours per gram. Can you imagine a 1 gram battery putting out ten watts of power for a whole hour? Probably not, because the best batteries on the hobby market today supply something like 0.14 watt-hours per gram, or seventy times less than fat. These are the lithium-polymer batteries, abbreviated "Li-poly". There are some other drawbacks to lithium-polymer batteries, such as safety, and a relatively short life in terms of the number of charge cycles they can endure. However, they offer the best solution available at the present time. Nickel-metal-hydride batteries can be used, but they have an even lower energy density. Nickel-cadmium batteries are worse still, and they have the added "benefit" of containing a deadly toxin. Are the Li-poly batteries starting to look better now?

Many different lithium polymer batteries are available, varying in size, number of cells, capacity, and discharge rate. The Ornithopter Design Manual describes how to select the best battery for your particular ornithopter.

Storage Medium		Energy Density (Wh/g)		Li-poly batteries
Rubber		0.007
Battery, NiCd		0.04
Battery, NiMH		0.07
Battery, Li-poly		0.14
Nitromethane		3.3
Methanol		6.4
Fat		10.5
Gasoline		12.2

Other Power Systems

Internal combustion engines have been used in ornithopters. Normally, a pull-start engine equipped with a centrifugal clutch and a large heat-sink would be used, similar to what would be found in an RC helicopter or car. Despite the high energy density of the fuels, internal combustion engines are far less efficient than electric motors. Combined with the difficulties of starting, cooling, exhaust, and noise, this weighs against the use of internal combustion engines in most ornithopters. They should be considered for manned ornithopters, or in special applications where the required flight duration cannot be achieved using batteries.

The ideal ornithopter drive system would be some kind of artificial muscle. Compressed-air cylinders have been used for this purpose, but the reliance on a tank of compressed air is limiting. Other devices include hydraulics, "muscle wires" made of shape-memory alloys that contract when heated, and special polymers that contract in an electric field. These systems have held great appeal for ornithopterists, but none of them offers a high enough power-to-weight ratio for flapping-wing flight. Someday, there will be a really powerful and efficient artificial muscle, and its existence will greatly accelerate the development of ornithopters. That day has not yet arrived.