For over 40 years, UAVs have been a part of NASA’s fleet and range from full-scale solar-powered versions to those using electric motors or propellers. Uses have included remote sensing for Earth sciences studies, hyperspectral imaging for agriculture monitoring, tracking of severe storms, and serving as telecommunications relay platforms.
Under the ERAST Joint Sponsored Research Agreement, NASA Dryden (now Armstrong) joined with AeroVironment, Inc., headquartered in Monrovia, CA, to design, develop, manufacture, and conduct developmental flight tests of the Centurion, the first aircraft believed capable of achieving sustained horizontal flight at altitudes of 90,000 to 100,000 feet.
A series of research flights at NASA’s Dryden (now Armstrong) Flight Research Center in the summer of 2005 validated the premise that using thermal lift could significantly extend the range and endurance of small unmanned air vehicles (UAVs) without a corresponding increase in fuel requirements.
Just as sailplanes use thermal lift and updrafts to soar for extended periods of time, the Autonomous Soaring Project flew a lightweight 15-pound motor-glider to demonstrate that the same concept could be applied to small, powered UAVs to increase their endurance and save energy.
UAVs may be classified like any other aircraft, according to design configuration such as weight or engine type, maximum flight altitude, degree of operational autonomy, operational role, etc.
Based on the weight
Based on their weight, drones can be classified into five categories — nano (weighing up to 250 g), Micro air vehicles (MAV) (250 g – 2 kg), Miniature UAV or small (SUAV) (2-25 kg), medium (25-150 kg), and large (over 150 kg).
Based on the degree of autonomy
Drones could also be classified based on the degree of autonomy in their flight operations. ICAO classifies uncrewed aircraft as either remotely piloted aircraft or fully autonomous. Some UAVs offer intermediate degrees of autonomy. For example, a vehicle that is remotely piloted in most contexts but has an autonomous return-to-base operation. Some aircraft types may optionally fly manned or as UAVs, which may include manned aircraft transformed into uncrewed or Optionally Piloted UAVs (OPVs).
Based on the altitude
Based on the altitude, the following UAV classifications have been used at industry events such as ParcAberporth Unmanned Systems forum
- Hand-held 2,000 ft (600 m) altitude, about 2 km range
- Close 5,000 ft (1,500 m) altitude, up to 10 km range
- NATO type 10,000 ft (3,000 m) altitude, up to 50 km range
- Tactical 18,000 ft (5,500 m) altitude, about 160 km range
- MALE (medium altitude, long endurance) up to 30,000 ft (9,000 m) and range over 200 km
- HALE (high altitude, long endurance) over 30,000 ft (9,100 m) and indefinite range
- Hypersonic high-speed, supersonic (Mach 1–5) or hypersonic (Mach 5+) 50,000 ft (15,200 m) or suborbital altitude, range over 200 km
- Orbital low earth orbit (Mach 25+)
- CIS Lunar Earth-Moon transfer
- Computer Assisted Carrier Guidance System (CACGS) for UAVs
Based on the composite criteria
An example of classification based on the composite criteria is U.S. Military’s unmanned aerial systems (UAS) classification of UAVs based on weight, maximum altitude and speed of the UAV component.
The primary difference from manned aeroplanes is the lack of need for a cockpit area and its windows. However some types are adapted from piloted examples, or are designed for optional piloted or unmanned operational modes. Air safety is also less of a critical requirement for unmanned aircraft, allowing the designer greater freedom to experiment. These two factors have led to a great variety of airframe and engine configurations in UAVs.
For conventional flight the flying wing and blended wing body offer light weight combined with low drag and stealth, and are popular configurations. Larger types which carry a variable payload are more likely to feature a distinct fuselage with a tail for stability, control and trim, although the wing configurations in use vary widely.
For vertical flight, the tailless quadcopter requires a relatively simple control system and is common for smaller UAVs. However the mechanism does not scale well to larger aircraft, which tend to use a conventional single rotor with collective and cyclic pitch control, along with a stabilising tail rotor.
Traditional internal combustion and jet engines remain in use for drones requiring long range. However for shorter-range missions electric power has almost entirely taken over. The distance record for a UAV (built from balsa wood and mylar skin) across the North Atlantic Ocean is held by a gasoline model airplane or UAV. Manard Hill “in 2003 when one of his creations flew 1,882 miles across the Atlantic Ocean on less than a gallon of fuel” holds this record
Besides the traditional piston engine, the Wankel rotary engine is used by some drones. This type offers high power output for lower weight, with quieter and more vibration-free running. Claims have also been made for improved reliability and greater range.
Small drones mostly use lithium-polymer batteries (Li-Po), while some larger vehicles have adopted the a hydrogen fuel cell. The energy density of modern Li-Po batteries is far less than gasoline or hydrogen. However electric motors are cheaper, lighter and quieter. Complex multi-engine, multi-propeller installations are under development with the goal of improving aerodynamic and propulsive efficiency. For such complex power installations, Battery elimination circuitry (BEC) may be used to centralize power distribution and minimize heating, under the control of a microcontroller unit (MCU).
Communication
UAVs use a radio for control and exchange of video and other data. Early UAVs had only narrowband uplink. Downlinks came later. These bi-directional narrowband radio links carried command and control (C&C) and telemetry data about the status of aircraft systems to the remote operator.
In most modern UAV applications, video transmission is required. So instead of having separate links for C&C, telemetry and video traffic, a broadband link is used to carry all types of data. These broadband links can leverage quality of service techniques and carry TCP/IP traffic that can be routed over the Internet.
The radio signal from the operator side can be issued from either:
- Ground control – a human operating a radio transmitter/receiver, a smartphone, a tablet, a computer, or the original meaning of a military ground control station (GCS).
- Remote network system, such as satellite duplex data links for some military powers. Downstream digital video over mobile networks has also entered consumer markets, while direct UAV control uplink over the cellular mesh and LTE have been demonstrated and are in trials.
- Another aircraft, serving as a relay or mobile control station – military manned-unmanned teaming (MUM-T).
Modern networking standards have explicitly considered drones and therefore include optimizations. The 5G standard has mandated reduced user plane latency to 1ms while using ultra-reliable and low-latency communications
