Before building a micro helicopter some science is needed...
The following information should help defining what engine to use, how to calculate the gear and so on.

 

Engine gear calculation:
engine gear calculation is done by calculating the engine rounds per volt (check your engines manual) times the amount of cells divided by the number of rotorhead turnarounds. This equals the ratio. Now divide the number of the main gaer by the ratio.

Example: 4200 RPM x 3 cells/1200=10,5. 80/10,5=~7 teath


Existing energie per weight calculation:
Cellcapacity times Cellpower times amount of cells divided by flight weight. Sounds easy.

And how is Cellcapacity/Cellpower measured? - Ampere or amp:



Ampere or amp

The standard unit for measuring the strength of an electric current. The key word here is strength. The more amps the stronger, or more powerful. Often shortened to amp.

An ampere hour (abbreviated Ah, or sometimes amp hour ) is the amount of energy charge in a battery that will allow one ampere of current to flow for one hour.

A milliampere hour (mAh) is 1,000th of an Ah, and is commonly used as a measure of charge  batteries. The mAh provides an indication of how long the engine etc. will operate on its battery without having to recharge it.


amp minutes
A way to rate a cell's capacity in time. A 1 Ah cell has 60 amp minutes of power. A 0.5 Ah or
500 mAh (see mAh) cell has 30 amp minutes, while a 1.7 Ah cell has 102 amp minutes. This is useful in estimating motor run times. If a motor is pulling 20 amps then the 30 amp minute cell will last:

30/20 = 1.5 minutes, while the 102 amp minute cell will last 102/20 = 5.1 minutes.


mAh or mah

milliamp hour is (1/1000 of an amp) is the most common way ni-cad cells are rated for their
capacity. 1 Ah (see Ah) = 1000 mAh, .5 Ah = 500 mAh The popular Sanyo 1700SCRC cell has a capacity of 1700 mAh, 1.7 Ah or 102 amp minutes (see amp minutes)


Example: 3V x 430mAh x 3/120 g > 3V x 0,430 Ah x 3/120 g=0,03225 Wh/g
The higher the Wh/g value the longer is the flight time!


A nice freeware program for calculations for model helicopters, can be found on the FlyHELI webpage . It allows to calculate the rotor surface area,  rotor blade load etc.

This program runs under Win95, Win98 and Win-NT. It calculates important parameters of model helicopters and make comparison of different helis easy. Simply download the ZIP archive . Then unzip and start it. An installation is not required.