DIY Drone: Lesson 2. Frames.

Content

Introduction

So, before you start assembling the drone, the first step is to choose a frame. You can execute it yourself, or use ready-made solutions (UAV frame kit). As you have already noticed, various types of frames and configurations can be used to create multi-rotor UAVs. Therefore, in this section we will consider common or basic types of frames, materials of execution, as well as issues related to design.

UAV frame types

Tricopter

• Description: UAV that has three beams, each of which is connected to a motor. The front part of the tricopter is considered to be the side of the junction of two beams (Y3). The angle between the beams can vary, but is typically 120°. To counteract the gyroscopic effect of the uneven number of rotors, as well as to change the angle of rotation, the rear motor must be able to rotate (achieved by installing a conventional RC servo motor). To exclude the use of a servo from the assembly, the Y4 design is used, which implies the coaxial installation of an additional motor on the rear beam.
• Advantages: Unusual appearance of the drone. It achieves the best flight characteristics when flying in a straight direction. Price (requires fewer motors and ESCs to build).
• Flaws: Asymmetrical design. Requires a servo drive. The complexity of the rear beam (because the servo must be mounted along the axis). Not all flight controllers support this configuration.

Quadcopter

• Description: A “quadcopter” is a drone that has four arms, each of which is connected to a motor. For the “X configuration”, the front part of the quadrocopter is considered to be the side of the junction of two beams, for the “+ configuration”, the longitudinal beam can be considered the front.
• Advantages: The most common multi-rotor design. The simplest and most versatile design. In the standard configuration, the beams/motors are symmetrical about two axes. All flight controllers available on the market can work with this multi-rotor assembly.
• Flaws: Lack of redundancy (if the system fails, especially in the elements of the power plant, the drone falls).

hexacopter

• Description: “Hexakopter” has six arms, each of which is connected to the motor. The front part of the hexacopter is considered to be the side of the junction of two beams, but the longitudinal beam can also be considered the front.
• Advantages: If necessary, the design of the hexacopter allows you to easily add two additional beams and a motor, which will increase the total thrust, as a result of which the drone can lift more payload. In case of failure of one of the motors, it is possible that the drone will be able to make a soft landing, and not crash. Modular frame design. Almost all flight controllers support this configuration.
• Flaws: Bulky and expensive design. Additional motors and parts increase the weight of the copter, so in order to get the same flight duration as a quadcopter, you need to install more capacious batteries.

Y6

• Description: The Y6 design is a type of hexacopter based on not six beams, but three, each of which is connected to a pair of coaxially mounted motors (6 motors in total). At the same time, it is worth paying attention that the lower propellers project thrust downwards.
• Advantages: Fewer components compared to a hexacopter. Lifts more payload compared to a quadcopter. When using counter-rotating propellers, the gyroscopic effect, like the Y3, is eliminated. In case of failure of one of the motors, it is possible that the drone will be able to make a soft landing, and not crash.
• Flaws: More expensive compared to a quadcopter due to the use of additional parts that are equivalent in cost to hexacopter parts. Additional motors and parts increase the weight of the quadcopter, which means that in order to get the same flight time as a quadcopter, you will need to use a larger battery capacity. In practice, the thrust obtained on the Y6 is slightly lower than that of a conventional hexacopter, probably because the lower propeller affects the thrust of the upper propeller. Not all flight controllers support this configuration.

Octocopter

• Description: The octocopter has eight arms, each of which is connected to a motor. The front part of the hexacopter is considered to be the side of the junction of two beams.
• Advantages: More motors = more thrust, and therefore more redundancy, allowing the drone to fly with heavy and expensive DSLRs with confidence.
• Flaws: More motors = higher price and bigger battery. Due to its high cost, it is relevant only for the professional sphere.

X8

• Description: The X8 design is still an octocopter, only not with eight, but with four beams, each of which is connected to a pair of coaxially mounted motors (8 motors in total).
• Advantages: More engines = more thrust, and therefore more redundancy. More likely to land the drone softly in the event of a motor failure.
• Flaws: More motors = higher price and bigger battery. Due to its high cost, it is relevant only for the professional field of activity.

UAV size

Drones come in a variety of sizes, from Nanos that are smaller than the palm of your hand to oversized ones that can only be transported in the back of a truck. For most users who are just getting started with the drone hobby, the optimal range of sizes that offer the most versatility and value is between 350mm and 700mm. The frame size is the diameter of the largest circle that intersects each of the motors. UAV parts of this size have a wide range of prices and the largest selection of products available.

UAV execution materials/Design

Below are the most common materials used for the manufacture of frames for multirotor drones, so the list is not complete. Ideally, the frame should be rigid with as little vibration transmission as possible.

Foam rubber (Foam) – as the only material for the manufacture of UAV frames, it is rarely used, and, as a rule, in combination with a rigid frame or reinforced structure. It can also be used for strategic purposes; as a protection for the rotors (propellers), the chassis often acts as a damper. Foam rubber can be of different types from soft to relatively hard.

Wood – if the priority is the cheapness of the design, then the tree is a great option, which will significantly reduce the assembly time and manufacture of spare parts. Wood is quite hard and is a time-tested material. It is important that perfectly straight wood is used in the manufacture of the frame (without bends and deformations).

Plastic – for most users is available only in the form of plastic sheets. It tends to bend and as such is not ideal. Great for making a protective frame or chassis. If you are considering 3D printing, you should consider the time frame for manufacturing (maybe it’s easier to buy a UAV frame kit retrofit). 3D printing of parts has proven itself in the creation of small quadrocopters.

Aluminum reaches the consumer in various shapes and sizes. You can use sheet aluminum for the body, or extruded aluminum for the arms of the drone. Aluminum is not as light as carbon fiber or G10, but price and durability are the main advantages of the material. Instead of breaking or cracking, aluminum tends to bend. To work with the material, only a saw and a drill are required.

G10 (glass fiber type) – despite the fact that the appearance and basic properties are almost identical to carbon (carbon fiber), it is a less expensive material. Mainly available in sheet format and used to realize the upper and lower frame plates. Also unlike carbon fiber, G10 does not block RF waves.

PCB (Printed Circuit Board – Dielectric Plate) – essentially an analogue of fiberglass, but unlike the latter, they are always flat. Sometimes used as the top and bottom plates of a frame to reduce the number of parts used (for example, the power distribution board is often built into the bottom panel). Frames of nano/mini quadcopters can be made from a single printed circuit board that includes all the electronic stuffing.

The carbon fiber – the most popular material due to its light weight and high strength. The manufacturing process is still exclusively manual. As a rule, simple shapes are mass-produced, such as flat sheets, tubular components; execution of complex three-dimensional forms is carried out to order.

Additional Considerations

• suspension – most often used for camera stabilization (FPV / Aerial photography). As a rule, it is installed under the frame in accordance with the center of gravity of the UAV. It can be attached directly to the frame, or by means of guides. To stabilize the image, it is recommended to use two or three axial gimbals. Requires an increase in the length of the landing legs.

• Payload (shipping) – in the amateur field, it is something of a luxury, since any additional weight not only reduces flight time, but also leads to the rejection of additional elements that could add key functions to the drone. When designing, it should be understood that the transport case should be as light as possible and at the same time strong, and the cargo itself should be rigidly fastened, excluding any movement in flight.

• Landing legs – despite the fact that some UAVs land directly on the frame (usually excluded to reduce weight), the use of landing supports in the design will provide a gap between the bottom of the UAV and an uneven surface, and in the event of a hard landing, they take a hit on themselves, increasing the chances of rescue of such important elements of the drone as the camera, gimbal, battery and frame.

• Mounting – Despite the fact that it is much easier to design and manufacture a drone than a conventional helicopter, the location of each element should be considered at the very beginning of the design process.

General recommendations for installation:

1. When creating a frame from scratch, it is important to ensure the exact location of the four mounting holes through which the motors are attached to the frame.
2. Most motors for frame sizes from 400 – 600mm have the same mounting hole pattern, which allows you to use a frame from one manufacturer, and motors from another.
3. The location of all additional components should ideally be symmetrical about one axis, which later will help facilitate the search and adjustment of the center of mass of the drone.
4. Ideally, the flight controller should be located in the center of the circle (and as such the center of mass) connecting all the motors.
5. The flight controller is usually attached to the frame with struts, rubber dampers, or double-sided tape.
6. Many manufacturers use the same mounting hole pattern for the flight controller (eg 35mm or 45mm square), but there is no “industry standard” as a current “industry standard”.
7. The battery is quite heavy, and if the center of gravity of your build has shifted a bit, you can adjust it by moving the battery slightly.
8. Make sure that the battery mount is a little “playy”, but at the same time provides a secure fixation of the battery.
9. Velcro straps are often used to secure the battery, but it’s a good idea to add double-sided tape between the battery and the frame.

Guidelines

Step 1: See what materials and processing tools you have at your disposal.

• If your arsenal is not enough to implement a custom frame, or you just want to get a professional frame, consider purchasing a UAV frame kit.
• Even if a frame is made with the right tooling and basic materials, it can still have structural weaknesses that cause excessive vibration or misalignment. The manufacturing process requires keen eyesight and experience.
• When making the frame yourself, consider fastening all the necessary elements of the drone; motors, electronics, etc.

Step 2: List all additional (auxiliary) parts that you plan to include in the build.

• It can be a one-, two-, or three-axis gimbal for a camera, a parachute, an on-board mini computer, a payload, long-range electronics (as a rule, it makes it heavier and increases the assembly), floating equipment, etc.
• The resulting list of additional / auxiliary parts will allow you to get an idea of ​​​​the size of the drone and calculate the total mass.

Step 3: Consider your intended frame dimensions.

• A large frame is not necessarily a great potential drone, and it is not certain that a smaller frame will make it cheaper to build.
• A drone built on a frame size of 400 – 600mm is recommended for beginners.

Step 4: Design, build and test the frame.

• If you have purchased a UAV frame kit, then you have nothing to worry about in terms of strength, rigidity and assembly.
• If you decide to design and build a frame from scratch, it will be important to check its strength, weight, and see if the structure can withstand vibration (minimum flex).
• Consider using specialized modeling software (many are free, like Google Sketchup) to design the frame and make sure the dimensions are correct.

Now you have a frame and you can move on to the next lesson.