Solar UAV Proposal

Table Of Contents


Autonomous Electric Aircraft using no Fuel (Unmanned Aerial Vehicle – UAV) Propeller powered electric UAV takes off on batteries and actively searches for updrafts. After encountering an updraft the UAV switches of the propulsion electric motor and soars. Air passing through the propeller during soaring revolves it and the movement is transmitted to the electric motor. Electro motor works as a generator in this mode. The produced energy recharges batteries and powers the electric equipment of the UAV. Energy gain is improved using solar power. The proposed UAV can stay aloft for long (indefinite) periods of time and can be used in reconnaissance and other applications. The control system of the UAV is responsible for autonomous behaviour (searching for updrafts, optimization of flight trajectory with regard to the mission objective and power management, solving critical situations, etc.) and for implementation of the human issued commands.

Outline of the Idea

We propose an electric unmanned aerial vehicle (UAV) capable to take-off and fly using electric motor and to land with fully charged batteries. Batteries of the UAV are recharged by regenerative soaring and solar power. The UAV is expected to stay aloft for a long time (hours and possibly days). During the time aloft the UAV searches autonomously for updrafts, optimizes flight trajectory with regard to the mission objectives and power management, solves critical situations and responds to human issued instructions. The instructions are expected to be defined as partial and general objectives instead of detailed commands. The advantages of the UAV are the following:

  • quiet and clean
  • fossil fuel independent
  • very low operational cost
  • can potentially remain aloft indefinitely
  • “parked” in air when not in use
  • has interesting application potential
  • study for manned regenerative soaring aircraft

The possible applications include:

  • Long time surveillance of road traffic, traffic jams avoidance
  • Bird eye view during rescue operations or catastrophic events
  • Search for missing persons at sea (flock of UAV’s to cover large areas)
  • Transponding radio signals in mountainous areas
  • Following migrant birds, marine mammals etc.
  • Weather forecast and study

Many of the ongoing projects of electric aircrafts are aimed on high altitude solar powered UAV’s. These try to avoid weather in order to get maximum exposure to sun and to protect their fragile lightweight construction. The UAV proposed here should operate in low altitudes in visual contact with the surface of Earth and take advantage of the vertical atmospheric motions.

Regenerative Soaring

Regenerative soaring, aircraft design.
Regenerative soaring, aircraft design.

Traditional sailplanes utilize updrafts to stay aloft and travel. In the typical scenario the sailplane searches for an updraft after initial climb (tow, self launch …), gains altitude by circling in an updraft and then glides in direction of the intended destination. Two important parameters the sailplane’s performance are minimum sink rate and glide ratio. Minimum sink rate determines how fast will the sailplane gain altitude and the glide ratio (usually expressed as X :1, meaning the sailplane will glide to the distance of X kilometers if starting 1km above ground) determines how good will the sailplane utilize the gained altitude. Most of the updrafts used by sailplanes are either thermal columns or upwind slope lifts.

Soaring in thermals
Soaring in thermals

Thermal columns (thermals) are basically bubbles of rising warm air which was warmed over sun irradiated surfaces. Upwind slope lift arises when air is forced to flow over an obstacle. It is also possible to apply dynamic soaring – technique used by many migrant birds and remote control pilots.

Upwind slope lift
Upwind slope lift

This technique uses the change of wind speed in the wind profile close to surface to gain energy. Albatrosses travel almost effortlessly thousands of kilometers in any direction using dynamic soaring. However, this technique requires to make sharp, high speed maneuvers close to the ground.

Regenerative soaring feature is easily added to most of the self launching sailplanes. In this case the propeller (or propellers) serves as a wind turbine while flying in the updraft. Because of the increased drag during regen the aircraft can not climb as fast as a clean sailplane but the energy generated by the turbine can be stored for future use. The most common means to store the energy are batteries, pinwheels, springs or twisted rubber bands. This energy can be used for free or emergency cruising, for new start and climb or to power devices on board of the aircraft. In the optimal case the aircraft will fly entirely without fuel or recharging on the ground.

Electric Aircrafts

Helios prototype
Helios prototype

Electric motors are widely used in remote controlled (RC) airplanes and in several manned airplanes. The limiting factor in application of electric motors in aviation is the energy storage. Batteries or ultracapacitors provide low energy to weight ratio which means that electric airplanes have small range. Solar and atmospheric energy can be used to increase the range and the time in air. Helios prototype was a UAV of NASA, ultra- lightweight flying wing aircraft with a wingspan of 75.3m, powered by solar cells, batteries and hydrogen-air fuel cell. Sunseeker II was as of Dec, 2008 the only manned solar powered airplane in flying condition.

Sunseeker II
Sunseeker II

In 2009 it became the first solar powered aircraft to cross the Alps. Its solar array charges Li-Polymer battery powering a 6kW electro motor. Max speed on solar power is 64kph. Sunseeker II takes advantage of thermals if possible. Several self launching electric sailplanes are available on the market, e.g. Antares 20E of Lange Aviation. Antares 20E is equipped with 42kW electric motor. It climbs to 3000 meters in app. 13 minutes when the batteries are depleted. Electric aircrafts slowly start to appear also as commercial products.

Electraflyer trike, Electric Aircraft Corporation
Electraflyer trike, Electric Aircraft Corporation
Electric Aircraft Corporation produces two types of electric aircrafts: rigid wing Electraflyer-C and Electraflyer trike (motor hang glider with Stratus wing). Their lithium-polymer battery pack with capacity 5.6kWh lasts for 1-1.5 hours flying. The electric motor used is 13.5kW brushless motor with 90% efficiency.

Features, Equipment and Instruments of the Proposed UAV

Propulsion system components:

  • propeller – large (more efficient) with symmetrical blades cross section possibly mounted as a ducted fan
  • brushless electric motor (dual role as turbine)
  • battery pack – (Li-polymer, Li-Ion … )
  • battery heating system – to ensure optimal performance of the batteries
  • solar cell array
  • motor-generator controller
  • charger

Flight controls (fly-by-wire):

  • primary controls: 3 (roll, pitch, yaw)
  • secondary controls: elevator trim, wing flaps, airbrakes etc. (optional)

Other equipment:

  • retractable landing gear (optional)
  • rocket parachute – deployed in critical situations

Avionics and Instruments (digital):

  • GPS
  • altimeter
  • airspeed indicator
  • rate-of-climb indicator (variometer)
  • attitude indicator (gyro horizon)
  • turn coordinator
  • indicators (battery, g-force, fire, stall, landing aids etc.)

Special devices:

  • Force feedback (aerodynamic load on the steering surfaces)
  • Diagnostic system
  • Radio receiver: for data and commands
  • Cameras and optional sensing devices
  • Video and data transmission system
  • Control computer

Functions of the Control System

The proposed control system intends to use ground based and on-board AI. The ground based system will analyse available meteorological data and 3D model of the surface of Earth. This system will provide hints to the UAV regarding areas with high likelihood to produce updrafts. The ground based AI system would be vital for night operation when thermal updrafts and solar array do not provide energy and UAV depends mostly on upwind slopes lift. The onboard AI will be responsible for the following:

  • Automated take-off and landing
  • Reaction in critical situations: stall and spin recovery, parachute deployment etc.
  • Optimization of flight trajectory with respect to the mission objective and to the atmospheric conditions (power management)
  • Localization and utilization of updrafts (variometer, vision and ground AI data)
  • “Parking” in air – when not in use aircraft is commanded to stay within certain space and fly autonomously until new mission objective is uploaded.

Estimated Project Impact

Results of this project would contribute to several scientific fields: aeronautical engineering, electric engineering, robotics and artificial intelligence. The proposed UAV is intended to fly with or without a human pilot. The possibilities of this design are important because it enables straightforward step to manned flight without fuel. In this setup artificial neural network can be used to learn how to fly from a human pilot. Comparison of performance of manned and unmanned aircraft would be possible. Regenerative soaring has not been practically tested on an aircraft yet. The measurements taken could be helpful for future applied research. Important are also the results obtained in the field of artificial intelligence and robotics. These results could be important for development of completely autonomous UAV’s for extra-terrestrial research e.g. for Mars exploration.


This document is a redacted version of the following paper: