UAV platforms

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

Aerodynamics principals

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 in 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 applies here. More about principals and physical model of the lift creation can be found on the Experimental Aircraft Info website [1].

Lift force
Figure 1: Lift force

The angle between the incoming airflow and the wing surface affects lift generated (when angle grows, lift increases, as air has to travel on even longer distance on the top surface) and it is named Angle of Attack (AOA). But increasing the angle cannot be done infinitely: each wing (construction) has some rated maximum (AOA). When exceeded, lift force suddenly drops, as airflow becomes turbulent causing the wing to stall. Non-laminar airflow is a common reason for low lift force and has been a reason for many serious accidents. Polished wing surface is essential to ensure non-turbulent (laminar) flow, thus i.e. icing decreases it significantly.

To control an object there are 3 axes that 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. 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 a different way, depending on the airframe. By the aforementioned, we used to consider also thrust as 4th force that enables full control.

Rotation axes
Figure 2: Rotation axes: RPY (Roll, Pitch, Yaw)

Airframes

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.

Fixed-wing

A long time before UAV and drone terms were introduced to the world, 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.
Figure 3: Plane control surfaces (basic)

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.

Figure 4: Plane control surfaces (additionals)
Flying wing

A flying wing is an evolution of the regular plane, where the whole body has been transferred into the single wing to maximise generated lift. Flying wings have better volume/area to MTOM ratio than regular planes, as virtually almost whole airframe generates lift (Figure 5). In 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 in 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 very low. On the other hand, lack of rudder increases stability problems on the 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.

Figure 5: Flying wing (here X8 frame)
Tail and rudder variations

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 case of UAVs requires technology discussed in case of the flying wings, regarding control of the V-tail surfaces.

Figure 6: Lockheed F-117 Nighthawk with V-tail
Pros and Cons

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 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 od 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;

Helicopter

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.

Figure 7: Leonardo's “aerial screw”

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 UAV. Helicopter's body mimics a dragonfly, but the nature of the generation of the lift and control is different than in 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 can change. Moreover, each blade can be virtually rotated independently thus the main rotor can “vector” the lift, enabling the helicopter to roll and pitch. Additionally, base helicopter construction has a tail rotor (anti-torque) and its main responsibility is to compensate 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 ##REF:tailrotorheli##.

Anti torque tail rotor
Figure 8: Anti torque tail rotor

In the full-scale helicopters and large UAVs, main rotor and tail rotor are driven usually parallel, as the rotation of one impacts another so tail rotor has rotatable blades that can change the force generated even at the constant rotation speed. All blades are controlled parallel, different than 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 separate (second) and controlled independently of the main rotor with an electronic controller.

The most notable part of the helicopter is a mechanism, that drives the main rotor and controls blades (rotor hub, Figure ##REF:rotorhubheli##.

Rotor hub
Figure 9: Rotor hub
Dual main rotor helicopter

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

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

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.

Pros and Cons

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;

Multirotor

So far, a quadcopter (multirotor with 4 propellers) is a synonym for a drone or UAV. This construction comes partially from helicopter idea and is simplified a lot. There is no natural (animal) to mimics and construction is purely artificial, human-invented. Multirotors able to operate freely in 3D space use at least 4 motors (eventually less, with force vectoring, i.e. using servomotor). Most popular is the quadcopter, but hexacopters (with 6 rotors), octocopters (8 rotors) and even multirotor with 16 motors and propellers (hexadecacopters) are not rare (Figure 11). Lightweight constructions usually do not go beyond the “Hexa” (Figure 10).

Figure 10: caption

Constructions with more than 6 motors have a great feature: the ability to fly controlled way even if one of the motors is down (or even more).

Figure 11: Multirotor variations (selected, but not limited to)

There are dozen of different structures of multirotor airframes, each with particular features and drawbacks. Lift is generated by propellers and in most cases, propellers are fixed and the lift is controlled independently for each motor via changing rotation speed. There do exist multirotor, that share the same idea as helicopter's main rotor construction: there is a central motor and change in the lift is controlled via variable pitch of each propeller. Multirotor requires advanced flight controller to stabilise in the air, using 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 lack of detailed model, most of the flight controllers use extensively PID controller for each degree fo freedom of the copter. 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.

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.

On Figure 11 there are presented variants for quad-, hexa- and octocopters, where their geometry differs. In general, X-shaped constructions (A, E, F) are more popular over 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 12). Yawing uses inertia to rotate, thus this operation is the least efficient in case of multirotor.

Figure 12: Multirotor aerial rotations and moves and their physical principals

UAV building components

Sensors (specific for UAV)

Actuators (specific for UAV)

Power sources (specific for UAV)

en/drones/platforms.txt · Last modified: 2020/07/08 13:00 by pczekalski
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