Electric R/C Flight
By Heather Mardon
Part 8: To Gear or not to Gear
Yes gearboxes are the subject of this months column. Because of the low vibration and relatively smooth torque of an electric motor, gearboxís are quite easy to implement from a mechanical point of view when compared to there IC engine counterparts.
Why Use a Gearbox?
Most electric motors exhibit high rpmís but low torque. This means that in direct drive they would only be able to turn rather small diameter props. Yes there are now a few exceptions to this with the advent of new types of brushless motors but in general the majority of electric motors available have a high Kv rating.
Note: Kv is an abbreviation for RPMís per Volt. The higher this value the faster the motor will turn on a given voltage.
If you want to turn a small prop at high speed ie a speed 400 pylon racer, then there is no need for a gearbox. However most sport and scale models will benefit from driving a larger prop.
The larger the prop, the better the efficiancy. There is no way around this fact, because although we can scale a plane down, the air molecules still stay the same size!
A larger prop, as well as being more efficient will also produce a lot more static thrust, which is needed by draggy airframes or models that rise off ground.
What you do need to be sure of is that you still have enough pitch speed to fly your plane above itís stall speed.
Sample calculation - pitch x (rpm/1000) yields a pitch speed in mph. (It's not exact, but close enough.) A pitch speed of 2.5x stall speed is recommended as a safe starting point.
Types of Gearboxís
This is probably the simplest type of gearbox available. A pinion gear is fitted to the motor shaft by either grub screws or pressfit with Loctite. The pinion gear then meshes into the larger spur gear. The ratio of the gearbox will simply be the number of teeth on the spur gear divided by the number on the pinion.
Astroflight use spur gearboxís with a twist, they use helical cut gears which give better efficiency and quieter operation.
Can be made simply and relatively cheaply. A wide range of ratios are available.
Often has a large amount of offset to the motor.
Can be a bit noisy
If not sealed are susceptible to foreign matter damage
Can strip teeth on a prop strike if your unlucky
Motor rotation direction must be reversed which may require retiming.
Uneven force on the motor shaft, best if a Ballbearing motor is used.
In this type of gearbox there are two stages of reduction. The pinion turns the three idler gears which then drive an outer gear as shown in the picture.
Due to having three idler gears the force on the motor shaft is distributed evenly.
The gearbox is inline with the motor, great for mounting in slim glider noses.
The motor does not need reversing as the output shaft turns the same way as the motor.
More expensive to produce.
Limited ratios, mostly 3.3:1 and higher
These work much the same as a spur gearbox except that the spur gear is turned "inside out".
Usually have a small amount of offset to the motor.
Most are sealed from foreign matter (Except for some smaller parkfly models)
Does not require reversing the motor.
Still applies an uneven force on the front motor bearing.
Here the gears are replaced with toothed pulleys and a belt transfers the rotation force.
Quiet smooth operation
Are more tolerant to the odd prop strike or foreign matter ingestion
A few models offer swappable ratios
Does not need reversing of the motor
Relatively bulky with a large amount of offset
Limited to about 20,000rpm at the motor due to belt wear & heating factors
Whats on the building board?
Iíve finally finished my Lethargik aerobatic parkflyer. So far have only had two flights on it due to the atrocious weather. My second flight lasted 11 minutes and seemed to have plenty of power so I think it has a lot of promise for some fun evenings or early mornings down at the park once the days stretch out a bit. Oh and of course it uses a geared motor to get plenty of thrust at low speeds.
New on the board is an ARF AT-6 Texan by Global that I got back in April for my birthday. It was designed for a glow motor 0.15-0.25 size. Iíll be using a Model Motors 2810/20 Brushless motor on about 9xCP1700 cells. So far the only major deviation from the plan is my decision to install dual Aileron servos instead of one central servo and snakes. The curve of the preinstalled tubes was just too tight so I ended up using them as channels to run my servo wires!
Until next time
Keep your Watts Up