As mentioned in the introduction, the “drone” is frequently misunderstood as a quadcopter. In fact, UAVs originate from fixed-wing devices that mimic real planes far, before the first multirotor appeared in the sky.
In the following sections, we discuss features of the particular construction (airframes). Still, to understand it, one needs to understand elementary principles on, why actually something heavier than air can fly.

In the beginning, it was natural for humankind to assume, nothing heavier than air can fly. However, birds used to break this rule. First observations brought the concept of the bird's wing, as the main feature that enables them to fly. It was particularly clear while observing raptors (eagles, falcons, ospreys) able to soar without a meaningful waving of the wings.
The phenomena that brought people to the sky is called a “lift force” (shortly referenced further as “lift”). A special shape of the wing causes the flow to travel the long way through the upper part, comparing to the flow over a short way, under the wing. That generates perpendicular (well, close to perpendicular) force to the flow direction called lift (Figure 1). Obviously, the existence of the flow is essential to generate lift and keeping it simple; faster flow generates bigger lift force (that relation is not linear, however, but square, regarding velocity). Air can be considered as a sparse fluid, and fluid mechanics apply here. More about principals and physical model of the lift creation can be found on the Experimental Aircraft Info website ^[1].

To control an object, 3 axes cross within the centre of gravity of the drone, and each drone must be able to control it (Figure 2). Those are Roll, Pitch and Yaw. The composition of those three rotations can locate drone within 3D space in any direction and position. Controlling each axis requires the ability to apply a force, and it is implemented differently, depending on the airframe. By the aforementioned, we used to consider also thrust as the 4th force that enables full control.

There are 3 main categories of drone airframes:

• Fixed-wing: mimicking birds and planes;
• Helicopters: following large helicopters with underlying DaVinci's project;
• Multirotors: a pretty new concept, that actually is not directly related to nature or human-made flying object.

A long time before UAV and drone terms were introduced to the world, the hobbyists implemented RC planes that build fundaments for current technologies driving the drone market.
The main concept of the fixed-wing is to follow full-scale constructions like passenger aeroplanes, soarers, combat flying wings and military jets. The main source of the lift force are wings (usually two, located symmetrically). Those constructions also used to have a vertical stabiliser (tail, one in the centre or two by sides) to stabilise direction. This is so much different compared to the birds. Axes are controlled with control surfaces, and thrust is generated with propellers or jet engines. There are many variations of this airframe model but elementary one, mimicking plane, has three main control surfaces (Figure 3):

• ailerons - controlling roll,
• elevators - controlling pitch,
• rudder - controlling yaw.

Many drones (particularly bigger ones) require additional surfaces, helping, i.e. to slow down or to increase lift (Figure ##REF:planectrlsurfaceadv##):

• flaps - when deployed, increase lift and also slow down - flaps work partially as air brakes, but please note, many planes, including soarers/sailplanes, do have separate airbrakes located in the mid-wing, not related to the flaps, while some jets may have it implemented on the wing or even at the tail on their body,
• slats (singular is slot) - provide an additional surface to increase lift - not very common in small and medium UAVs but in the large scale ones, may be essential to increase Maximum Take-Off Mass (MTOM).

Flaps and slats are deployed usually during take-off and landing.

A flying wing is an evolution of the regular plane, where the whole body has been transferred into a single wing to maximise generated lift. Flying wings have a better volume/area to MTOM ratio than regular planes, as virtually almost the whole airframe generates lift (Figure 5). In the case of the flying wings, control surfaces are integrated and usually limited to just only two of them, integrating ailerons and elevators. It requires mixing of the control channels implemented in the RC controller or the flight controller (or both, depending on the current flight mode). Many industrial and military drones are implemented as a flying wing because of the heavy payload (professional cameras, weapons) they carry. Flying wings do not use rudder nor tail, and that is one of the reasons they're hard to be detected by Doppler radars, as side reflection is shallow. On the other hand, lack of rudder increases stability problems on straight-line flights with a crosswind, during takeoff and landing. In full-scale construction this problem is tackled with thrust vectoring, usually requiring jet engines. For this reason, many flying wing UAVs introduce side sharklets (small “tails” at the end of each wing), similarly to those introduced by the Airbus company in their A320 series, now present in most of the passenger planes constructed in the world.

There are many approaches to improve flight performance and stability. One of the variations that made it popular to the market is V-tail (Figure ##REF:F117##). It integrates rudder and elevator and in the case of UAVs requires technology discussed in the case of the flying wings, regarding control of the V-tail surfaces.

Each airframe has features and drawback. Here we discuss the most noticeable ones.
Pros:

• Simple flight control in RC mode: RC controllers were designed to handle plans control without an extra flight controller (early RC hobbyists);
• Passive generation of the lift force through wings;
• Ability to soar/sail without active propulsion or even with no throttle force generation at all (no engine);
• Possibility to recover on failure;
• Model of the dynamics of such construction is well documented and deeply studied;

Cons:

• Cannot hover in one position - drone must move forwards;
• Requires runway to take-off and land;
• Cannot fly upside down (usually, unless thrust is reasonably bigger than lift force, to replace it to some extent) as wing's section is not symmetrical;
• Minimal turning radius exists;
• Problems “following” slow objects - a necessity to circuit round followed object if its speed is below critical (minimum) flight speed for the drone;
• Fixed wings are fragile to the crosswind;

It was Leonardo Davinci's idea, to use a big, screw-like device (aerial screw, Figure 7) to “drill” air and generate an airflow downwards, thus creating lift force oriented upwards.

This idea has grown in the first half of the XX century into the full scale and models/UAVs, but as helicopter construction is a pretty complex one (both natural and scale), it is not very common to be used as a UAV. A helicopter's body mimics a dragonfly, but the nature of the generation of the lift and control is different than in the case of insects.
A regular helicopter has a large rotor with at least two blades. Each blade can be rotated parallel to its length, this way changing the lift force generated by each blade. Moreover, each blade can be virtually rotated independently; thus, the main rotor can “vector” the lift, enabling the helicopter to roll and pitch.

To operate a helicopter up and down (change total lift) one uses collective pitch, so changing all blades angle of attack (aforementioned rotation parallel to its length) increases or decreases the total lift generated by the main rotor (Figure 8).

To roll and pitch, there is a way to change the position where each blade generates a larger lift, using so-called “cyclic pitch”. Angles of the blades are altered while rotating, dynamically (Figure 9), so the same blade generates a different lift depending on its current position. That causes the lift to change its effective direction that no longer is perpendicular to the main rotor rotation surface.

The most notable part of the helicopter is a mechanism, that drives the main rotor and controls blades (rotor hub, Figure 10).

In many micro drone constructions, the collective pitch is implemented with the change of the rotation speed of the motor driving the main rotor. This construction, however, excludes cyclic pitch thus rotating in pitch and roll axes are implemented other way (see below, Figure 12) and usually limits the number of rotation axes the device can use while in the air.

Additionally, base helicopter construction has a tail rotor (anti-torque), and its main responsibility is to compensate for force generated by the main rotor. The tail rotor is perpendicular to the main rotor and pushes or pulls the tail, thus also enables the helicopter to yaw (Figure 11).

In full-scale helicopters and large UAVs, the main rotor and tail rotor are usually driven parallel, as the rotation of one impacts another. Hence, the tail rotor has rotatable blades that can change the force generated even at the constant rotation speed. In the case of the tail rotor, all its blades are controlled parallel, which is different from in case of the main rotor where each blade can be virtually controlled independently. In the case of smaller UAVs, the tail rotor motor is usually separate from the main rotor motor and controlled independently of the main rotor with an electronic controller. In large scale helicopters, the tail rotor can be exchanged with the air-jet outlet of the hot gases leaving turbine (or turbines) that drive the main rotor: this seems to be a more efficient solution, as it uses exhaust gases to implement anti-torque force, but also problematic to control thus not very common.

Torque compensation can be implemented using counterrotation. There are two known solutions:

• coaxial - counter-rotating, pretty common in small RC models (Figure 12), but is also present in full-scale ones: Kamov Ka-50;
• tandem - as in well known Boeing CH-47 helicopter - popular Chinook (Figure 13);

Small scale RC helicopters are usually simplified on their construction, not to include tail rotor classically, while rather equipped with two rotors mounted on the coaxial shaft, driven by separate electric motors and counter-rotating. This way torque of one rotor is compensated with the torque of another one. This construction is additionally equipped with a tail rotor that is parallel to the main one, driven by a separate, 3rd electric motor, that enables the helicopter to pitch. This construction can only pitch and yaw, cannot roll. Yaw is implemented with a change of the relative rotation speed of two main rotors (Figure 12).

In commercial helicopters, coaxial, counter-rotating main rotors are used in lightweight constructions.

Another solution is used in the heavy lifting helicopters with two main rotors mounted on their endings, so-called “tandem”, i.e. CH-47, where there is no tail rotor at all. Still, both main rotors provide collective and cyclic pitch (Figure 13).

Many scale helicopters introduce the flybar: a coaxially mounted bar, usually with extra mass by its endings, mounted over the main rotor (sometimes parallel, sometimes perpendicular), to stabilise small models, where rotor blades present small inertia, due to their low mass thus causing instability in flight. Introducing a flybar increases helicopter stability but also lowers its manoeuvrability and response.

Each airframe has features and drawback. Here we discuss the most noticeable ones.
Pros:

• Helicopter can hover;
• Can even move backwards;
• There is no minimal turn ratio (as in fixed-wing) and it can pivot in place;
• Uses vertical take-off and landing (VTOL);

Cons:

• Active generation of the lift (far less efficient than a fixed-wing);
• Main rotor failure causes immediate fall, still, there is a rescue procedure called autorotation but so far hard to implement in scale models;
• Complex mechanics and servicing;

So far, a quadcopter (multirotor with 4 propellers) is a synonym for a drone or UAV. This construction comes partially from the helicopter idea and is simplified a lot as in most cases it uses fixed blades. There is no natural (animal) to mimics, and construction is purely artificial, human-invented. Multirotors can operate freely in 3D space and in most cases they use at least 4 motors (eventually less, with force vectoring, i.e. using servomotor). Most popular is the quadcopter construction, but hexacopters (with 6 rotors), octocopters (8 rotors) and even multirotor with 16 motors and propellers (hexadecacopters) are not rare (Figure 15). Lightweight constructions usually do not go beyond the “Hexa” configuration (Figure 14) as motors are the heaviest part of the drone (along with the battery pack) and there are limits on the maximum take-off mass (MTOM) inexperienced operator can control.

There are dozen of different structures of multirotor airframes, each with particular features and drawbacks. Propellers generate lift, and in most cases, propellers are fixed (there is no pitch variation like in helicopters) and the lift is controlled independently for each motor via changing rotation speed. Composition of all lifts generated drives drone operation. There do exist multirotor, that share the same idea as a helicopter's main rotor construction. There is a central motor and the change in the lift is controlled via a variable pitch of each propeller. Multirotor requires an advanced flight controller to stabilise in the air, using a gyroscope and accelerometer (at least). Opposite to the fixed-wing, human operators are unable to control multirotor directly as it requires at least 100 Hz position update (currently control loop is up to even 32kHz).

Because of the simplicity of building and ready components availability, that they are adaptable for virtually any variation of the multirotor construction, multirotor is the most frequently used construction for UAV even, if its flight dynamics is still hard to model on the theoretical level. Because of the lack of a detailed model, most of the flight controllers use the PID controller for each degree of freedom of the copter extensively. PID parameter tuning is related to the particular airframe and usually obtained experimentally. Of course, ready sets provide pre-tuned airframes, and the Internet is full of advice and parameter sets for popular frames, but changes to the payload and centre of gravity can infer stability.

Multirotors are universal and can hover in place as helicopters, yet far more stable. Movement is possible in any direction as most of the constructions are symmetrical. Lift is generated collectively via all motors and propellers.

In Figure 15 there are presented variants for quad-, hex- and octocopters, where their geometry differs. In general, X-shaped constructions (A, E, F) are more popular than plus-shaped ones(B, C, D), because of two major factors:

• pitch (tilt, roll) of the copter is done by more than one motor that speeds it up, of course, the cost is extra energy,
• from facing camera won't have an arm, motor and propeller in the centre of the field of view.

Aerial operations using a multirotor can be easily explained considering quadcopter as an example. Principals of the operations are extendable straight forwards to the Hexa-, octa-, and more propellers. The general rule is to variate the rotation speed of the motors, thus affecting generated lift and this way to pitch the multirotor in the desired direction (Figure 16). Yawing uses inertia to rotate; thus, this operation is the least efficient in the case of multirotor. In Figure 16 we present the motor's rotation speed variation to obtain the desired effect of the pitch, yaw and roll rotations.

As a multirotor drone can virtually fly in any direction, yaw rotations are not frequently used, yet in case of drone equipment is directional (i.e. camera mounted only in front of the drone and not rotatable) ability to yaw is still essential.

In Figure 15 one can observe that any multirotor has a + (plus, as in 15D) or X version (as in 15A). X version is popular in drone racing and video filming, as front-mounted camera view is not affected with front arm (as in case of the plus configuration) yet and this is the majority of the multirotor UAVs. Plus configurations are considered obsolete and niche.