Researchers at the Swiss Federal Institute of Technology in Lausanne presented a paper earlier this month describing a drone that can boost its payload of food from 30 percent to 50 percent of its mass.

The IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) conference in Kyoto was told how drones have been useful in unmanned transport missions such as food and medical supply delivery to deliver life-saving nutrition and medicine to people in emergency situations.

However, commercial fixed wing drones can generally only carry 10−30 percent of their own mass as payload so some structures of a drone, such as the wings, could be made of edible materials, increasing its food-carrying mass ratio to 50 percent.

Should the edible drone be left behind in the environment after performing its task in an emergency situation, it will be more biodegradable than its non-edible counterpart and a flight-capable prototype can provide 300 kcal and carry a payload of 80 grams of water.

Some companies have already launched drone delivery services to reduce the cost of delivering small items on the last mile with multirotor-type drones most commonly adopted owing to their reliability when hovering and manoeuvring.

Drones can also be used to deliver life-saving nutrition for people in emergency situations, where approaches for ground vehicles are difficult. Consequently a fixed-wing drone is advantageous over a multirotor-type drone.

In general, the volume of the wing occupies the largest part of a fixed-wing drone, so the edible-wing has a wingspan of 678 mm. The remaining structures like fuselage, actuators and electronics use conventional materials.

The wing should be strong enough to avoid bending or material failure during flight, which favours foam such as expanded polypropylene (EPP) as a primary structural material for conventional fixed wing drones.

Rigorous Scientific Methods
Using rigorous scientific methods including Young’s Modulus tests, one of the most promising candidates was a puffed rice cookie, which is easily machinable by laser cutting and produced by applying high pressure to rice grains at high temperature.

A rice cookie has 3870 kcal per kg, lower than the number of calories of some sweets over 5000 kcal/kg for chocolate and candy, but the densities of those sweets are five to eight times higher than that of the rice cookie. Rice cookies offer a very similar nutritional value to other common foods such as oats, barley and pasta but are less dense and hence more suitable.

The typical size of a rice cookie is around 70 mm (in length) while a wing has a much larger area, which means that multiple rice cookies need to be laser cut and connected by using an edible adhesive.

Three types of edible adhesives were tested: corn starch, chocolate, and gelatin to also provide a small amount of nutrients to the edible wing, and gelatin maintained a strong bond until material failure of the rice cookie itself. The team concluded that it was stronger than both corn starch and chocolate, so gelatin was used as edible adhesive throughout the study.

Hexagonal Pattern
Wing loading is a critical design parameter in aircraft design, which impacts the lift coefficient, stall speed, and in the case of an edible drone, nutrition that it is able to carry. A hexagonal pattern was chosen to minimise the additional mass added by the edible adhesive and glued with gelatin to create a planar structure, as illustrated below.

The entire wing surface was wrapped in plastic film and tape to prevent humidity damage and the mass of one full wingspan edible wing was 100 grams including the protection film.

The fuselage was made of a 0.5 m long hollow carbon rod, with the electronics located at the front for balance during flight. A 300 kV brushless motor was used to generate thrust, while two micro servo motors served as actuators for the elevator and rudder of the tail wing.

Motors were remotely controlled by a standard 2.4 GHz radio control transmitter and receiver set. The edible-winged drone was powered by an 18.2 g Li-Po battery (7.4 V, 260 mAh) for at least 10 minutes of flight. The entire mass without payload was 200 grams.

Future development will focus on a novel way to store payloads, such as water, on an edible drone, without significantly increasing the surface area exposed to air. Owing to its simple fabrication, multiple edible drones could be easily deployed to deliver higher amounts of nutrition.