FPV frame materials have varied throughout the evolution of the hobby. Plastic, fibreglass and carbon fibre have all seen popularity throughout the years although carbon is the current standard. Aerospace and automotive industries employ many different materials into their vehicles, some of which have trickled down to the smaller multi rotor industry. In this article, I will discuss some basic material properties and FPV frame materials to answer the question: “will carbon fibre be replaced as the dominant frame material”.
Mechanical Properties (MP) & Physical Properties (PP)
Before diving into the FPV frame materials, it is important to know about physical and mechanical material properties. These are a set of characteristics, unique to each material. Understanding these properties allows for simple FPV frame materials comparison through testing and material datasheets. Mechanical properties measure the reaction of a material when specific forces are applied. Physical properties measure different aspects of a material which can be found without altering it. These include density, corrosion resistance and conductivity. Many properties exist however the ones listed below are generally considered when selecting FPV frame materials.
Tensile Strength (MP)
Tensile strength is a material’s resistance to pulling or a tensile loading. Its units are newtons per square meter (N/m^2), kilograms per square meter (kg/m^2) or pounds per square inch (psi). There are a few types of tensile strength for each material. Tensile yield strength dictates the maximum stress that can be applied to a material where it will not be permanently deformed when released. Ultimate tensile strength dictates the maximum stress which a material can withstand before it breaks. Materials with high strengths do not easily deform.
Compressive Strength (MP)
Compressive strength is a material’s resistance to compression. It also has the same units as tensile strength. Materials also have compressive yield and ultimate compressive strengths.
Shear Strength (MP)
Shear strength is the ability for an FPV frame material to withstand lateral loading or sideways stress. It also has the same units as tensile strength and respective yield and ultimate shear strengths.
Tensile Modulus/Modulus of Elasticity (MP)
Tensile modulus is a measure of a materials stiffness or elasticity in the tensile or compressive directions. It also has the same units as the previous three strengths. Materials with a high tensile modulus do not easily deform under loading but a material with a low tensile modulus will. For example, a diamond with a high tensile modulus will not easily compress however rubber with a low tensile modulus will compress.
Flexural Strength (MP)
Flexural strength or bending strength is the stress a material can endure before permanently deforming or fracturing. Its units are the same as the other strengths. A material with a low flexural strength such as chalk will easily snap if bent. A material with a high flexural strength will bend only under high loading before snapping.
Hardness is a material’s resistance to plastic deformation or indentation. Plastic deformation is a permanent material distortion.
Density is a material’s quantity of mass per volume of material. The units are kilograms per cubic meter (kg/m^3) or pounds per cubic inch (lb/in^3).
Why the Drone Industry Loves Carbon Fibre
Carbon fibre is one of the popular FPV frame materials commonly found in aerospace and automotive industries. Its popularity is mainly attributed to its low density yet ‘stronger then steel’ tensile strength. Although it is referred to as ‘carbon fibre’, its correct name is ‘carbon fibre reinforced plastic’ (CFRP). Woven carbon fibre cloth is made into hard sheets by combining them with a thermoset plastic known as a resin. The thermoset plastic has a lower tensile strength then the carbon fibres but a higher flexural modulus. The high flexural modulus of the resin is responsible for the stiffness and high flexural strength of the FPV frame material.
Disadvantages of Carbon Fibre
Due extensive production procedures, solid carbon fibre sheets are quite expensive to produce. Carbon fibre cloth isn’t overly pricey however it is quite difficult and time consuming to impregnate with resin to form glossy sheets. You can watch How It’s Made’s video on carbon fibre production here. Carbon fibre also cannot withstand a high quantity of flexing cycles under high loading. If carbon fibre is forced to continuously bend, the resin bonding the fibres can weaken. This can leave the carbon fibre permanently weakened and/or deformed. Another disadvantage of carbon fibre is that it cannot withstand nearly as high a degree of compression as it can tensile forces. Like string, carbon fibres can withstand tensile loading (pulling) but not compression (pushing). When compressed, most of the compression forces are transferred to the resin which fractures and deforms under high loading. The stiffness and compressibility of carbon fibre sheets varies between brands due to different resin compositions. Depending on the application, either softer or harder resins with various mechanical properties can be implemented.
Other FPV Frame Materials
Although carbon fibre is amazing, there are many other FPV frame materials. Listed below are some of the most noteworthy FPV frame materials and some general information.
Thermosets and Thermoplastics
There are two types of plastics, thermoplastics and thermosets. A thermoplastic can be moulded and continuously reshaped with heat. An example is 3D printer filament. Thermoplastics have varying degrees of elasticity and stiffness although they are characteristically softer than thermosets. Thermoset plastics vary from thermoplastics as they cannot be reshaped through heating once hardened. The primary advantage of thermosets is their stiffness (hence why it is used to create CFRP). Most plastics are relatively cheap FPV frame materials. Although not as strong as carbon fibre, plastics can be used to create cost effective frames and frame parts. I personally make use of thermoplastic polycarbonate canopies which act as sacrificial components, protecting expensive electronics and frame components. The ability to 3D print thermoplastics also allows them to be used for complex frame parts. Nylon and TPU are typical thermoplastic FPV frame materials. Both can withstand continuous flexing however nylon is significantly stiffer.
Composites are FPV frame materials composed of two or more materials. Apart from carbon fibre, common aerospace composite materials include fibreglass and aramid fibre (Kevlar is a well-known variant of this). In general, the advantage of composites is the combination of each material’s mechanical properties. Kevlar and fibreglass, like carbon fibre are also quite strong however carbon fibre is significantly stiffer. Concrete is another composite with high stiffness, but its high density makes it unattractive for drones.
A composite sandwich is two panels sandwiching an inner core material. Composite sandwiches can be made of two or more materials such as carbon fibre sheets and an impact absorbent foam core. Common core materials include impact absorbent foams and aluminium honeycomb. Honeycomb structures have high compressive strengths however the material cost is quite high. The main advantage of a composite sandwich is the high panel strength and stiffness for the weight. In the drone industry, carbon fibre & foam sandwiches are informally referred to as ‘Oreo carbon’. An Oreo carbon panel of equal dimensions (excluding thickness) and weight to a carbon fibre panel, in most cases, will be significantly stiffer. The main disadvantage of composite sandwich materials is their low ability to withstand direct impacts. Aluminium honeycomb loses its compressive strength once deformed and impact absorbent foam crumbles when compressed. On a racing drone, Oreo carbon parts have been experimented with however they are known to break easily in crashes.
Metals are used in thousands of applications. Through heat treating, quenching and annealing, metals can be hardened or softened to better suit their application. Aluminium, steel and titanium are the most common metals in the drone industry. Aluminium is quite malleable and ductile meaning it bends or deforms easily. It has a relatively high tensile strength although steel’s is quite higher. Steel, however, is also a lot denser which allows for aluminium structures to use a lot more volume whilst maintaining an equivalent weight. If designed correctly, aluminium structures can be extremely rigid. Titanium possesses the best properties of steel and aluminium. It is quite a stiff material yet is almost half as dense as steel. The main disadvantage is its cost which is significantly more than steel and aluminium.
Various woods were quite common in the early days of the hobby as they are relatively cheap, semi-rigid and moderately strong (well, strong enough back when drones couldn’t easily exceed 60mph). Wood is also extremely easy to manufacture. Although uncommonly used in modern frames, woods such as plywood are excellent materials to prototype carbon fibre frames with. Plywood frames can be laser cut in seconds. Because plywood fractures similarly to carbon fibre, plywood frames can also assist in determining any weak points in a frame design. My frame, the OSprey Racing Fletch 210 went through several plywood design iterations before manufacturing the final carbon prototype.
FPV Frame Materials Application
The ideal FPV frame material for a specific drone frame is defined by its application. Where cost isn’t the predominant factor, carbon fibre is usually the ideal material. For applications that are cost sensitive, plastics are usually the ideal material with nylon being a common selection. Larger commercial drones will often use aluminium tubing where carbon fibre would be an expensive alternative and plastics would lack the required rigidity. For racing/freestyle applications, carbon fibre is often the primary selection as it suits the ‘low weight and high durability’ mini-quad material criteria.
Frame design is equally as important that the implemented FPV frame materials. When designing a frame, the designer must consider and design around the limitations, manufacturing methods, advantages and disadvantages of each material. Current frame designs have been optimised for production in flat carbon fibre sheets. To further optimise carbon frames, many designers have implemented thermoplastic parts such as impact absorbent canopies to increase durability or vibration damping motor pads. This evolution in frame design is cost effective whilst improving frame durability and weight. These thermoplastic parts which can be manufactured in almost any shape also make up for the design restrictions associated with flat carbon fibre sheets. Innovators such as FU-RC however have developed more organic frame designs featuring moulded carbon fibre parts, but the cost is significantly higher than a standard sheet carbon frame. For slower micro drones, plastic frames lead over carbon fibre due to their moderate durability and ability to be cheaply injection moulded/3D printed. Unless cost is a greater consideration then durability, plastic mini drone frames are unideal as carbon fibre sheets have roughly the same density as most plastics but significantly higher strength. For more on FPV frame design, you can read Callum’s article here.
Will Carbon Fibre Remain Dominant?
Currently, carbon fibre looks to stay as the dominant FPV frame material. My prediction is that racing & freestyle frames will continue to be produced in sheet carbon fibre however plastic pieces will become more and more integrated into the design for mounting and impact absorbent purposes. Observing the cost, availability and mechanical properties of available FPV frame materials, carbon fibre still ranks at the top of the list in all three categories. New materials are continually being developed for aerospace applications however the sheer cost usually prevents it trickling down to smaller industries.