UAV airframes refer to the main physical structure of an Unmanned Aerial Vehicle or drone, on which all vital components such as avionics systems, payloads, engines are installed.
Quality of airframes is dependant on the nature of mission, the weight of payloads it has to carry, and the takeoff and landing approach of the drone. High strength with minimum possible weight, large payload carrying capacity, excellent maneuverability, and high hover efficiency are the essential requirement of all airframes. Military UAVs require high endurance to stay in the air for a long period.
All modern drones are equipped with an array of sensors, and other systems, inevitably increasing the overall weight and reducing the flight time.
Weight reduction, therefore, is critical, and to build the airframes, manufacturers today use non-conventional materials such as composites (generally made using fibers and resins), which reduce the weight of UAVs without compromising on their strength.
There are some of the common materials and composites used for UAV airframes:
Plastic is easy to mold into solid objects of different sizes and shapes. Lighter than metal alloys, plastic has high malleable properties and offers high resistance to corrosion and chemicals. It also has low electrical and thermal conductivity and excellent durability and high strength-to-weight ratio. Plastic is very cost-effective. Propellers and skids of a drone are usually made of plastic.
2. Aluminum Alloys
Aluminum is a common metal used to construct drone airframes. Known for its low density and high strength, aluminum alloys can resist corrosion through passivation, making it an ideal choice for the aerospace industry.
Compared to aluminum, composites reduce weight by 15-45%. Besides high strength, composites are resistant to corrosion from saltwater and electrolysis. In the case of bird strikes or accidents, they absorb the impact energy, instead of shifting it to the lower units. They produce lower noise or vibrations as opposed to aluminum or any other metal.
Some of the conventional composites used for airframes are Carbon Fiber Reinforced Polymers (CFRP), Glass Fiber Reinforced Polymers (GFRP), Boron Fiber Reinforced Polymers (BFRP), and Aramid Fiber Reinforced Polymer (AFRP).
a. Carbon Fiber Reinforced Polymers (CFRP)
Carbon fiber is a combination of carbon fibers and thermosetting resins, offering weight reduction, strength, enhanced durability, and low thermal shrinkage. For creating carbon fiber, carbon atoms are aligned parallel to the main axis of the filament. For commercial use, thousands of filaments are wound together. Carbon fibers are cost-effective, stronger than steel, lighter than aluminum, and stiffer than titanium. It can be easily mass-produced. Hexcel Corporation is one of the leading players developing carbon fibers. They make HexTow carbon fiber by combining all types of thermoset and thermoplastic resins.
b. Glass Fiber Reinforced Polymers (GFRP)
The second-most widely used material in airframes, Glass Fiber, offers low material elongation and high material strength. Moreover, it is easy to produce and requires little maintenance. Glass Fiber is suitable for a variety of applications due to its high strength, increased flexibility, long durability, excellent stability, and high resistance to heat, temperature, and moisture. It is lightweight and can be molded to design radomes and antenna substrates. Owens Corning (US) is one of the leading players developing glass fiber reinforced polymers. Other leading manufacturers are Jushi Group (China), Owens Corning (US), Taishan Fiberglass Inc. (China), CPIC (China), Saint-Gobain Vertex (France), Nippon Sheet Glass (Japan), and Johns Manville (US), among others.
c. Boron Fiber Reinforced Polymers (BFRP)
Boron Fiber is the strongest and expensive material commercially available for airframes. BFRP is used in F-15 fighters, B-1 Bombers, Black Hawk helicopters, space shuttles, and Predator, owing to its excellent compressive strength. This polymer is six times more modulus of elasticity as compared to GFRP. Boron fiber is used in the US military aircraft such as F-14 and F-15. The limited application of boron fiber is attributed to its toxic nature, high costs, and increased brittleness as compared to other fibers. It is not preferred for ground and underwater vehicles. Specialty Materials, Inc. is a leading manufacturer of boron fiber products.
Aramid Fiber Reinforced Polymer.
d. Aramid Fiber Reinforced Polymer (AFRP)
Aramid fiber is a synthetic fiber that offers high impact resistance and increased stiffness. Cutting AFRP requires high accuracy and precision, which makes them expensive and difficult to use. Aramid fiber is known in various trade names such as Nomex (a meta-aramid) or Kevlar (a para-aramid). It is widely utilized for military ballistics and body armors, owing to its susceptibility to light, compression, and hygroscopy. Kevlar and Twaron are the two most popular aramid fibers. They are used in aircraft components, helicopters, space vehicles, missiles, canoes, kayaks, powerboats, brakes, clutches, etc.
These are some of the main advantages of using composites for UAV airframes, instead of metals:
- lightweight, marking the drone energy efficient
- incredibly strong and very had to break
- resistance against corrosion and compression
- low machining errors
- design flexibility. Easy to fabricate intricate parts
- maximum stiffness and strength
- less number of assemblies and fasteners
- higher ‘stealth’ capabilities with low radar and microwave absorption
- low thermal expansion in high altitude flights
- low maintenance
Composites also have some disadvantages compared to metals. They are:
- expensive to build
- structural degradation at high temperature and wet conditions
- delamination and cracks
- low energy absorption, resulting in high impact during hard landing
- labor-intensive and complex fabrication process
- higher maintenance cost