Disclaimer.

The Free-Drones Company, owner of this site, is not liable for any damages arising from the use of the technology presented, or any material contained in it, or from any action or decision taken as a result of using this site. The information, technology and software contained in this site is provided "as is" for the readers convenience, without warranty of any kind, and may be superseded by updates. It is the readers responsibility to make sure that his activities in relation to the technology presented is in compliance with applicable laws.

Note: With ongoing progress, the Site will be updated with new results.

Project Objectives and Discussion.

Different type of drones have been developed over the years. They can be disdiguished in two main categories: drones equipped with wings, and drones without wings. Winged drones are comparable to airplanes, and characterized by long reach capabilities. Multirotors without wings are comparable to helicopters and characterized by great mobility, however have limited reach. Winged drones may or may not be equipped with VTOL i.e. V(ertical) T(ake)-O(ff) and L(anding) capabilities. VTOL-Drones combine the cruising flight efficiency of fixed-wing aircraft with the convenient vertical take-off and landing of multirotors. However this option has negative impact on the reach of the drone. This site will focus on fixwinged drones and investigate the feasibility of developing an autonomous low cost E(xtended) R(each) D(rone). As mentioned already, the extra battery weight for VTOL, resulting in reduced reach, this technology will not be considered in this site. Lets discuss what, in the scope of this site is meant with an autonomous ERD. For a drone to qualify as an autonomous ERD, it has to have the capability to deliver a 10Kg payload at a 500km distant target, this with no external intervention:

a: Once the drone has been loaded with the max.10Kg payload and the electronics "woken-up" by the user, the user has to specify the GPS values of the target. From now on, the following steps are controled by the flight-control electronics.
b: All sensors are checked by the system if OK.
c: At start the GPS position and barometric (air) pressure are recorded by the system.
d: Flight distance to user specified target(s) is checked if in reach i.e.500km.
e: Roll, pitch and yaw controlers are activated.
f: As part of the take off procedure motor RPM is increased . The drone takes-off and climbs straight to 200m altitude under roll- and pitch control only.
g: The drone continues climbing to cruise altitude under pitch, roll and yaw control in the direction of the target. The cruise altitude is "hard coded" in the flight-control software. h: Cruise to target.
i: Arrived at target:
- in onw-way mode (max.reach is 500km): spiral down and execute landing procedure at target.
- in return mode (max.reach is 250km): spiral down to user set altitude and drop payload. Climb to pre-set cruise altitude, orientate "nose" in direction home, and cruise homeward. Spiral down and execute landing procedure at home base.

In Section 4 a full set of functions is presented by which the flow in the "main loop" can be constructed such that a flightprogram as listed above will automatic be executed.
Rather than developing a fixedwing drone, there are many models on the market. Up to what level these commercial drones qualify as ERD needs to be investigated and tested.

The Drone Body.

Rather than developing and building an own winged drone, the management of the Free-Drones Company has decided to scan the market on available comercial hybrid fixedwing drone that after modification could satisfy the 500km reach and 10Kg pay-load capacity requierments as mentioned above.

Intro-Fig.1: A potential 3.6m wingspan Drone.

This example of a commercial fixedwing drone, is built of Wood and composit material, and has a wingspan of 3,6m. The drone is hybrid, i.e.equipped with a gasoline Tail-motor. Max.take-off weight is ~30Kg. Max.payload ~10Kg, Payload volume 30 Liter. Flight altitude 3000m. Max.cruising speed ~100km/hr.

As mentioned already to develop this drone into an autonomous ERD, some modifications will be required. For example an optimal gasoline motor as shown below needs to be selected.

Intro-Fig.2: DLE 120cc 2-cilinder ERD Drone motor. Price ~eu900 a copy.

Assuming the specs of the hybrid Foxtech Great Shark max. are more or less valid for 3.60m wingspan drones, it is reasonable to expect the following performance: with 10Kg payload on board, an estimated max.reach of 500km at 100km/hr seams feasible with estimated 12L fuel. However, in case of a 500km flight to the target, no fuel will be left for the return flight. The drone have to execute a landing procedure at target.

The Flight Controler.

To modfy the drone into an autonomous ERD, a dedicated flight-controler is required. To better understand the problems encountered with the development of an autonomous flight-controler, and find solutions to these problems, it was decided to develop one from scratch. As back-bone the powerfull 32 bit, 600mHz Teensy 4.0 micro-processor is selected. The strorage capacity of the Teensy 4.0 is more than sufficient to store the data and code. The microprocessor is highly Arduino compatible, and can be programmed with the Arduino IDE.

The Sensors.

An autonomous drone has to rely on its sensors. A brief critical review of the sensors will follow.
To control the attitude, the gravity vector is measured with a 3D accelerometer. Unfortunately this 3 axis meaurement may be inhibited by accelerations originating from other sources like vibrations etc.
A 3D gyro can be used to overcome this problem, however on its turn, a gyro is plagued by drift and this needs to be compensated.
Orientation is derived from the earth magnetic field using 3 axis magnetometers. However, for reliable orientation data, magnetometers require a heavy calibration procedure.
The specified altitude for cruising to the target can be controled with a barometric pressure sensor. Unfortunately air-pressure tends to fluctuate. A tiny pressure fluctuation of 1 mbar equals a height difference of +/-8m.
For guiding the drone from home to the planned target and back, GPS will be used. Some of the GPS scentences also provide the info of altitude above sea-level. Unfortunaely this value is highly inaccurate, Also the position derived from the GPS Latitude and Longitude readings fluctuate, resulting in position errors in the order of +/-7 meters.

Test Bench.

In the following sections it will become clear that quite some mathematics and data filtering is required to reduce the errors in the different sensor readings.

Before building a full scale ERD, a Test Bench comprising all the essential electronics, including all sensors and controls is constructed. The test-bench is presented in Sec-1_Util-1 and intended to help with a better understanding of the above listed difficulties. A considerable number of support utilities and sensor calibration applications are developed like:
- A tilt-compensated compass is developed, including calibration software. The compass is based on Euler angle description. Use of Quaternions will be considered in a later stage.
- Ultrasone and barometric sensors are investigated, and software developed for altitude and landing control.
- GPS software for target finding is developed.


During the development phase of the prototype, and for safety reasons, a radio transmitter and receiver for motor control is added to the prototype. In case the drone runs out of control, the tester can use the transmitter, reduce the motor rpm, and force the drone to an emergency landing.
This all, including the developed software code will be presented and discussed in the following sections of this Web.site.