Little boxes with levers control the control surfaces (rudder, elevator etc) - plug into receiver. Various sizes and strengths.
Servos are those little boxes with the arms sticking out - or a disk attached to the top - which transfer the electrical impulses from the receiver into mechanical motion.
The moving arm (or disk) pushes/ pulls a rod or wire attached to the elevator or other control surface.
They normally come with the radio set (though you can by just transmitter from some suppliers) - you get 2 in a 2 channel set and 3 or more in a multi channel set.
If you are after smaller servos because of your particular need, you may be able to negotiate with your supplier to swap a couple of the larger servos (and a small amount of cash) for a smaller servo.
SIZE / TORQUE / COST
Servos come in different sizes and strengths. A standard servo weighs about 45 grams and has a torque of about 2.5 to 3 kg. This means that if a string were attached to the servo arm 10mm (1 cm) out, it could lift that weight. Cost is about A$26 each
The smaller servos that I use are 9 grams and have a torque of 1.4 kg and cost around A$60 each.
For a light electric plane, the savings in weight (72 grams) is significant.
The smallest are around 5 or 6 grams with a torque of about 0.6kg.
Some specialty servos have a higher torque capacity / metal gears / ball bearings - but these are matters which would not normally be required for beginners' models.
COMPATIBILITY / COMPARISONS
In Australia the servos are all compatible, with the exception of the difficulties of old style Sanwa gear (which not only a differnernt size plugs, the polarity was reversed).
Futaba plugs will fit JR and Hitec gear IF the edge of the plug is sanded off.
The arm or disk is attached to a spline. A range of arms and a disk or two, is generally provided with the servo. Different brands can have different spline sizes. JR kindly maintain the same size spline throughtout a range of their sizes.
Naro servo arms on the smaller models (that I use) are not as strong - but perhaps it is better to break a servo arm in a crash than to strip a gear.
You can buy replacement arms for some brands of servos.
It is possible to get replacement gears for some brands.
These are readily available in differing lengths. But be sure to ensure the plugs are compatible.
These are extension leads with three ends, and act like a "double adapter", allowing two servos to run off the same channel, as might be required for running separate ailerons in a large wing. When one goes up, the the other goes down.
It is NOT possible to run elevons (a combination of aileron and elevator) on flying wings off a Y lead.
CAN I PUT LARGE SERVOS IN AN ELECTRIC PLANE?
You can put them in a largish electric plane (2 metres), but doing so, particularly in a smaller plane. may produce disappointing performance.
New on the market - for specialised use. Not something you need to worry about as a beginner.
NEXT - LEARN ABOUT...........BATTERIES and CHARGERS.
Servos come in various sizes and strengths.
Basic servos weigh in at about 45 grams and cost aroundA$25.
The smaller you go, the higher the price.
Stronger servos are also available at higher prices.
A Y lead for joining two servos onto the one channel, such as might be used for ailerons. Cannot be used where the servos must be operated independently, such as for Elevons on flying wings or delta aircraft.