Multirotor drones, and mainly quadcopters are the most popular and versatile type of aerial drones, which found use in many fields, such as aerial photography, research, competitions and even military. With their X configuration, they are stable, efficient, easy to control and maneuver. In this post, we will list and describe high level design goals for multirotors / quadcopters. We will focus on components of the drone itself (e.g. frame, motors, flight control system etc…) in future posts, however we will still mention these components here as applicable, while talking about each main design criteria.

When designing a multirotor, the following main criteria are taken into account, all of which affect the others, therefore they are interdependent. In other words, if we want to increase stability, we may need to sacrifice from maneuverability, or if we want to increase flight duration we must accept to have lower payload and so on. In all cases though, the safety and reliability of the drone should not be compromised. The main criteria when designing a drone can be listed as:

Now let’s take a closer look at each…

Stability:

Stability design, as the word suggests, aims for stable, controlled flight and dynamic response of the drone. In other words, stability refers to the drone’s ability to maintain steady flight, and return to its desired position or orientation after being disturbed by external force such as wind, or the internal effects such as sudden push from its propellers. So bottom line is that stability is how well can the drone autonomously hold itself steady, without the pilot constantly having to correct things manually, which is not easy.

The following factors affect stability of a multirotor drone:

Stiffness of the frame:

Stiffer materials have higher natural frequency. For those who are not familiar with the term, let’s explain natural frequency in plain English: Natural frequency is the rate at which something naturally (freely) “prefers” to vibrate when disturbed. If a material is stiffer, it resists bending or stretching more strongly, which means, it snaps back into place faster when it is disturbed. That quicker response makes its natural frequency of vibration higher (in other words it tends to vibrate faster).

Having stiff frame, therefore higher natural frequency is good, because this means, the rotation of propellers (which generally have lower frequency) will not cause resonance with the frame and create uncontrolled disturbing movements. In other words, if a frame is not stiff enough, it will just bend back and forth and experience uncontrolled movements, unbalanced forces during propeller rotation and may resonate with the propellers. This uncontrolled movements will also create sensor confusion.

Center of Gravity:

Weight of drone must be distributed evenly around its center of mass, otherwise there will be unbalanced forces. This means a symmetrical configuration for the frame and components. Not only that but the more outward mass is distributed, the more stable the drone is (which reduces agility though).

Flight Controller and Sensors:

One of the primary duties of flight controller and sensor is to ensure stability of the drone. These are done by stabilization algorithms, which greatly simplifies manual work of the drone operator. Sensors (such as gyroscopes and accelerometers) detect the drone’s movements and orientation, and the flight controller processes this data and quickly adjusts the motors to keep the multirotor balanced and stable.

Propeller Configuration:

Propellers are arranged so that their thrust and rotation balance each other. Opposite propellers spin in opposite directions, canceling out unwanted spins, and their placement ensures equal lift, which keeps the drone stable.

External Conditions:

External effects such as wind, turbulence, payload changes affect stability.

Flight Duration:

Longer flight duration is one of the major goals during design. The longer the duration, the more we can do with our drone. Flight duration is affected by battery capacity and energy density, weight of the drone itself, motor KV rating (how many RPM – revolutions per minute a motor will spin per volt with no load), propeller size and pitch, motor propeller compatibility, aerodynamics, flight style and control, power system efficiency, software and tuning.

Payload Capacity:

To be useful, a multirotor must be able to not only carry itself but also a certain amount of payload such as cameras, specialty and scientific sensors, cargo packages, other types of payloads which may be for the purposes military, agriculture, entertainment, industrial, law enforcement.

The main factor that affect the payload capacity is the thrust created by propellers. Usually a minimum of 2 to 1 ratio is required for thrust to weight, but for more intense uses this ratio can be more. Thrust in turn, is a result of propeller geometry, battery power and motor KV rating, torque and efficiency, quality of ESCs which can handle high currents smoothly, drone weight, frame design and aerodynamics.

Maneuverability:

Maneuverability is how quickly and precisely the drone can change its attitude (pitch, roll, yaw) and position. It is especially an important characteristic of racing drones and certain types of military drones.

High thrust to weight factor, as high as 5:1 is preferred, if we want high maneuverability (this ratio can be as low as 2:1 for camera drones for example). The higher this ratio, the more aggressive and quicker moves the drone can make. By aggressive moves, we mean the ability to make sharper climbs, tighter turns and flips, quicker acceleration and deceleration.

Propellers is another factor affecting maneuverability. Smaller propellers give quicker response, while larger props give more stability. Higher pitch propellers provide more speed and higher response, but at low throttle, they cause more difficult fine control. Higher number of blades give smoother control but they can reduce efficiency by adding drag.

As far as motors, high KV rating and high torque motors are preferred.

Size and compactness of frame greatly affect maneuverability. Smaller frames are are much more agile. Larger frames are preferred for stable flights, such as for filming. Stiffness of the frame is also important, where, stiffer frames can handle quick movements better with less unwanted shaking. Similarly to overall size of frame, mass (and in turn inertia) affects maneuverability, where lighter drones are more agile than heavier ones.

The drone’s electronics and control systems are crucial, since faster ESC refresh rates, quicker flight controller processors, and well-tuned control algorithms all directly influence maneuverability.

Safety and Reliability:

The last major design criteria we ill list when designing multirotor drones is safety and reliability.

The following factors are considered:

The health of power system components, such as battery condition, voltage to current matching by not undersizing ESCs or motors, designing connectors and wiring so that they are protected and will not easily be damaged.

Installing propeller guards, selecting high quality propellers which will not crack and cause imbalance easily.

Installing vibration isolators between the drone’s frame and flight controls to prevent or greatly reduce error readings of sensors.

Using quality radio links, and failsafe system upon connection loss between the drone and the operator.

As far as structural design, durable frames should be preferred. Note that stiff does not necessarily mean durable in this case, as stiffer materials can be brittle but as we mentioned above several times in this article, using stiff frame has many other benefits. The ideal design balances stiffness for performance with material choices that can absorb impact for durability.

The frame should also be weatherproof, protecting the drone and its components from dust, moisture and temperature extremes.

After building the drone, safety considerations do not end. Batteries, propellers, connections should regularly be inspected. Regular maintenance should be performed such as cleaning, tightening and parts replacement as required.

Post By: Ahmet Tuter

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