Nature has a fascinating review on drones – and especially microdrones!
For those who don’t have access, here are some highlights (somewhat technical):
Propulsive efficiencies for rotorcraft degrade as the vehicle size is reduced; an indicator of the energetic challenges for flight at small scales. Smaller size typically implies lower Reynolds numbers, which in turn suggests an increased dominance of viscous forces, causing greater drag coefficients and reduced lift coefficients compared with larger aircraft. To put this into perspective, this means that a scaled-down fixed-wing aircraft would be subject to a lower lift-to-drag ratio and thereby require greater relative forward velocity to maintain flight, with the associated drag and power penalty reducing the overall energetic efficiency. The impacts of scaling challenges (Fig. 3) are that smaller drones have less endurance, and that the overall flight times range from tens of seconds to tens of minutes — unfavourable compared with human-scale vehicles.
There are, however, manoeuvrability benefits that arise from decreased vehicle size. For example, the moment of inertia is a strong function of the vehicle’s characteristic dimension — a measure of a critical length of the vehicle, such as the chord length of a wing or length of a propeller in a similar manner as used in Reynolds number scaling. Because the moment of inertia of the vehicle scales with the characteristic dimension, L, raised to the fifth power, a decrease in size from a 11 m wingspan, four-seat aircraft such as the Cessna 172 to a 0.05 m rotor-to-rotor separation Blade Pico QX quadcopter implies that the Cessna has about 5 × 1011 the inertia of the quadcopter (with respect to roll)…This enhanced agility, often achieved at the expense of open-loop stability, requires increased emphasis on control — a challenge also exacerbated by the size, weight and power constraints of these small vehicles.
Improvements in microdrones will come from becoming more insect-like and adapting knowledge from biological models:
In many situations, such as search and rescue, parcel delivery in confined spaces and environmental monitoring, it may be advantageous to combine aerial and terrestrial capabilities (multimodal drones). Perching mechanisms could allow drones to land on walls and power lines in order to monitor the environment from a high vantage point while saving energy. Agile drones could move on the ground by using legs in conjunction with retractable or flapping wings. In an effort to minimize the total cost of transport, which will be increased by the additional locomotion mode, these future drones may benefit from using the same actuation system for flight control and ground locomotion…
Many vision-based insect capabilities have been replicated with small drones. For example, it has been shown that small fixed-wing drones and helicopters can regulate their distance from the ground using ventral optic flow while a GPS was used to maintain constant speed and an IMU was used to regulate roll angle. The addition of lateral optic flow sensors also allowed a fixed-wing drone to detect near-ground obstacles. Optic flow has also been used to perform both collision-free navigation and altitude control of indoor and outdoor fixed-wing drones without a GPS. In these drones, the roll angle was regulated by optic flow in the horizontal direction and the pitch angle was regulated by optic flow in the vertical direction, while the ground speed was measured and maintained by wind-speed sensors. In this case, the rotational optic flow was minimized by flying along straight lines interrupted by short turns or was estimated with on-board gyroscopes and subtracted from the total optic flow, as suggested by biological models