Servomotors in model aircraft

Servomotors in model aircraft

Different technical points characterize the servos: format/dimensions, torque, rotational speed, mass, solidity… A brief overview!

A servo, more precisely servo-motor, is, as its name suggests, a servo motor.

It is used in model aircraft, and more widely in the field of drones and robotics.

Many brands share the market, some of which are very well known because they are very old and famous.

We can thus cite Futaba, Hitec, Protronik, Savox, etc. To these brands are added other more or less known brands, but also many "no name" brands, that is to say often Chinese manufacturers who manufacture on behalf of other brands or retailers. This has the effect of making this market more difficult to “read”.

Among the most recognized manufacturers, Futaba is essential. This Japanese manufacturer delivers to the industrial world, puts forward a quality management system that makes the performance of its servos systematic and not random like other basic brands. The internal components of servos can vary greatly from one manufacturer to another! This brand is widely used in competition, and often makes the difference in terms of power consumption which is optimized.

The protronik brand offers servos with excellent value for money, often directly in HV, i.e. they can be powered directly from a 2S lipo battery without a voltage step-down device, which is practical and saves weight on a model aircraft! (recommended li-ion battery for reception)

Before choosing a servo, you have to consider the place of the servo in the machine you want to fly. For example, a competition model will not need the same order execution speed as a leisure model. The scale of the model aircraft also matters a lot.

Let's see the features one by one.

The format / dimensions of the servo.

You might as well specify it right away: a standard servo will not be suitable for a miniature model such as for indoors, for example. Different formats exist: nano, micro, mini, standard, etc.

It is first important to know how much space you have to install your servo.

However, if you don't have the space to fit a standard format servo to control the camber flaps on a little big one, you'll still have to find it because a nano will never do the trick! Many tips and accessories especially exist to easily integrate a servo into an aircraft, such as this or that!

The couple

In physics, the torque of a motor (here our servomotor) is the force which acts with a lever arm on a pivot point. The calculation of the motor torque is done using the formula of the law of the lever arm: motor torque = force (N) x lever arm (m). For a servomotor, we adapt the units to its scale, and we speak of kg.cm. (10N corresponds to approximately 1kg).

It is imperative to know what mechanical constraints will be applied to the servo in order to size it correctly. The most sensitive servos in model aircraft are often those for the camber, elevator and aileron flaps. Which are also doubled or even tripled functions on large models (2 or 3 servos per flap or 2 or 3 flaps for one function!)

The golden rule is to always oversize! This is what will make your servo last over time and will not fail overnight, without warning.

For example, a 3kg servo (implying 3kg.cm) is just right for a standard beginner aircraft, with a small wingspan (1m20-1m40). It is not reasonable to put less powerful. Example of a modern but first price standard servo for beginner aircraft here: the S-U300

We recommend to follow the instructions given by the manufacturer of your model or to proceed by comparison by taking the servos recommended for a similar model!

rotational speed

The speed of rotation of the servo motor is important so that the model reacts quickly to the solicitation of the control in flight. Sometimes a few fractions of a second can be the difference between a crash and survival. But it is especially between 2 competitors that the difference can be felt, and it is often the competitors who attach greater importance to this parameter, because it influences the quality of a figure in F3A or F3C for example.

The type of sprockets

Several possibilities: plastic, reinforced plastic, metal including titanium… It is a compromise to be found between robustness and resistance to shocks and snags, and lightness. Metal gears quickly increase the mass balance on an indoor model...

But beware, inside a servo, there are several gears, and most remain plastic, even in the presence of 1 metal pinion! The best is to disassemble an old servo to understand!

Titanium is particularly recommended, because it is a metal that is both light and strong, like on this 9842 servo for example, where metal is omnipresent, at all stages of the gear system.

Note that there is a lubricant for the servo gears, which makes your equipment last!

The mass of the servo

We often only look at it closely at the level of the servos for indoors… On larger models, the mass becomes incidental. It is therefore in the world of nano and micro servos that this “battle” takes place!

Hitec positioned itself very early in its history on small format servos, for indoor use but also gliders, such as this HS85MG, small but strong! (and at a nice price…)

Ball bearings

Ball bearings affect servo performance and durability. With bearings, the servo is stiffer overall and more accurate.

A bearing on ball bearings (especially when there are 2), relieves the servo in its efforts to act on a command. It also reduces wear by preserving the servo motor and all gearing. Nevertheless, the presence of bearings unfortunately has an impact on the price of the servo, and on its mass. They are therefore less present on very small servomotors.

Example of a servo with 4 ball bearings (!!!) for competition: the BLS253 from Futaba!

The type of motor

Servomotors are equipped with very small electric motors. For a long time, these were only coal-fired motors, even when the first brushless motors arrived for the motorization of our models. But some manufacturers were then able to introduce brushless motors into servos, such as futaba with its “BLS” range. Brushless motors, as a reminder, are more efficient and more durable.

Supply voltage

For a long time, servos had to be supplied with 4.8V, or even 6V max, at the risk of damaging the components. But things have changed.

2 things to know here:

1) tension affects performance (higher tension brings more torque and higher speed)

2) there is clearly a practical aspect (along with a lower risk of destroying a servo by overvoltage). Having an HV servo allows you to directly plug in a 2S lipo battery to power the receiver and servos without a step-down or voltage regulator.

To finish :

-There are metal servo boxes, with fins. Since metal is a good conductor (unlike plastic), the case allows the heat to be evacuated and the servo motor to be cooled, in addition to increasing the resistance of the servo body. The assembly is also more rigid and therefore more precise!

-2 words on the SBUS: this system makes it possible to put in series all the servos which equip an aircraft with a complex configuration. We install hubs like this at Futaba, which allow each servo to know exactly when it should intervene. This logic saves wires, mass and simplifies the whole configuration!

- There are servo testers: power peak mutliplex or ST8

-And finally, there are continuous rotation servos for specific applications, like the HSR-2645-CRH and the linear servo, present at the beginning of aeromodelling, have made a comeback!

- to last, a servo must only withstand the constraints of the flight itself and not those due to poor installation: there should be no hard points in the controls!

Bonuses:

- A servo can also be used to drop in flight with this type of accessory or to tow a glider with this one!

- To control a servo motor from an arduino: see here