FOS UAV

The aim of this project is to develop an open source aerial mapping platform using standard hardware and to develop a workflow from data acquisition to data processing.

open uav

FOS UAV is part of the Drone Research Program by ICFOSS (International Center for Free and Open Source Software)

Overview

Most UAVs today are multirotor, they have the advantage of being able to hover and carry more payload like sensors and other auxiliaries. But with the current battery technology, multirotor have flight time in the range of 15- 20 mins, without compromising on the payload. They make good inspection drones in tight spots and hard to reach places. But because of the way they fly, they burn through a lot of power, hence limiting the flight time and maximum range.

Fixed winged aircraft fly on the principle of aerodynamic lift and are far more efficient at covering ground than multirotor. Even though they are considerably harder to design and build, they make up for with efficiency.

Our goal is to build a suitable fixed-wing platform that can carry the necessary payload and provide usable data for aerial mapping. The craft needs to carry a good GPS system, a downward-facing camera for taking images and be able to fly missions autonomously.

Our constraints being, it must be simple to make, and easy to use. All the hardware and software are low cost and open-source, without sacrificing safety and usability.

4-Axis Hotwire Foam Cutter
CNC Drag knife

Mapping Process

The mapping is done by a process called Photogrammetry. The UAV carries a camera that is angled down to take photos of the ground when the crafts fly overhead. The UAV then flies along with a defined GPS way-point mission which covers the area to be mapped with sufficient overlap. The software then reads all the images and then stitches them into a 3-dimensional orthomosaic.

Multiple data can be inferred from the orthomosaic like, ground elevation, water level, cubic volume of materials. With sufficient overlap and good camera altitudes, centimeter-level accuracy can be obtained. But in order for the process to work, the UAV must provide reasonably stable data over the entire course of the mission.

Software

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Ardupilot is one of the most advanced open source autopilot solution, it has been there the longest and has an active community developing it. The firmware supports a wide range of vehicles from airborne to terrestrial and even underwater. It is paired with suitable ground control software to bring out its full functionalities.

In the latest release of Ardupilot, they moved onto the Chibios platform and have now extended support for some of the common STM32 F4 flight controllers like the Omnibus F4 and the Matek F405 wing. We will be using this version and testing the features, the new firmware is still in beta and being actively developed.

INAV

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Inav is a flight control software, which was forked from Clean Flight. The software supports both multi-rotors and fixed wings and has good GPS modes like Return to launch (RTH) etc. The software has a mission planner, which allows for setting full autopilot GPS way-point navigation. It supports a variety of Flight controller boards and is actively being developed by the community.

Open Drone Maps

OpenDroneMap is an open-source toolkit for processing aerial drone imagery. Open drone maps turn the images captures by the drone into three-dimensional geographic data that can be used in combination with other geographic datasets.

it can process a collection of images into point clouds, digital surface models, Digital Elevation Models, etc.

Mission Planner

Mission Planner is a ground control station for Plane, Copter, and Rover. It can be used as a configuration utility or as a dynamic control supplement for your autonomous vehicle. It can be used to configure and tune your vehicle, plan, and load autonomous GPS way-point missions. With proper telemetry hardware, it can provide the live status of the vehicle, record telemetry data, and operate the vehicle in FPV (first-person view)

Hardware

Design Considerations

The flight characteristics need to be kept in mind when choosing an aircraft. We wanted to keep things simple and minimize the number of moving components. Hence no tail. The aircraft will be a flying wing, simple and monolithic in construction, good performance, and agility.

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WIng aerodynamics 101

The two most important parameters when designing an aircraft are the Center of Gravity (COG) and the Center of Pressure (COP). The COG is the point where the weight of the aircraft acts and the COP is the point where the aerodynamic lift forces act. For the plane to be stable, the COG must be in front of the COP. But this means the airplane has a tendency to nose down when flying. That’s where the tail comes in, the tail on a conventional airplane is set at a slightly downward angle compared to the wing, a term known as Decalage.

So the wing is lifting up and the tail is pushing down to keep the plane stable. If the COG is too far forward the wing has to push down even more and the wing has to make more lift to counter this effect. The inverse is also true, if the COG is too far back then the tail has to push up to keep the plane level, this is not a stable condition and it’s very difficult to fly a tail-heavy plane.

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A wing emulates a stable flying condition by having a forward COG and using its Elevons (Elevator+ Aeilerons) to pitch the wing up, A term known as Reflex in which the elevons are kept at a slight up to push the tail down and pitch up the wing. This is not without consequences, the more forward the COG the more the elevons have to pitch up, thereby creating more induced drag. The next parameter which influences the COG and hence the stability is the Sweep Angle.

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Sweep is important because it provides leverage for the COG to act. The COG needs to be at about 25-30% of the chord, in a rectangular wing, this is very close to the leading edge and will not provide enough leverage to balance the COG, so we need a long fuselage in front of the wing to carry the weight.

In a wing, the CG needs to be at about 20-30% of the Mean aerodynamic chord, which takes into account the root and tip chord and the sweep angle. In simple terms, the more the sweep, the farther back the COG can be, and the more leverage you can get. But the more the sweep the less the lift it produces per wingspan. Sweep also has another effect, it acts like a Dihedral

Dihedral is a slight angle between the wings to provide stability. When one wing dips, it produces more lift than the other wing and the aircraft comes back to level. A side effect being, it will cause the wing to wobble, as the side facing the wind will have more lift than the trailing wing.

Flying wings are a lot harder to control as they are inherently unstable aircraft. They are very sensitive to the changes in the Center of Gravity (CG) and flying them manually is a skill.

Since our objective is to get stable data, flying them manually is not an option. This is where a flight controller comes in, it will keep the craft stable and provide additional features.

Choosing an Airfoil

In our version #2 build, we used the Clark y airfoil for the wing but it did not perform as well as we expected, after a bit of research on tip stalling and how to avoid it in a wing, we stumbled upon the KFm airfoils. On paper they seemed very good, so we decided to try them out.

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The KF stands for Kline and Foggleman who were the designers of the KF airfoils. Unlike a conventional smooth airfoil. The KFm airfoil uses a series of layers to create stepped airfoils.

There are a number of KF variations, That use a combination of layers and positioning to produce a wing that can be made to emulate what a normal wing airfoil profiles.

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Drawn up by Dick Kline these give some basic guide numbers for the various combinations that have been tried. The guide numbers however are open to changes, going thinner will always work, going a bit thicker will often work, go too thick and it won’t work.

The percentage thickness refers to each section’s total thickness vs the total chord of the wing, and not the height of the step. The height of the step is only determined by the thickness of your foam sheet. Steps can be raised to give more height and the step effect increases proportionally with both height and airspeed.

The basic stepped airfoil idea is that there will be a vortex produced behind each step that fills in the gap and lets the airflow over the wing as though the gap was solid and profiled.

For more details, check the RCgroups form on KFm theory and science.

Kline-Fogleman-(KFm) Airfoils Advanced Theory Science

Some positive characteristics of KFm airfoils

  1. It can handle a wide range of speeds from very slow to fast.
  2. It has a much greater range for its center of gravity, it could be moved as much as 40% back since the entire rear section of the wing is producing lift.
  3. It has very good stall characteristics and it retards tip stalling in a flying wing.

Power plant selection

The power plant in an electric aircraft is not just the Motor alone, but a combination of Motor, Propeller, and the battery. The battery can be thought of as both the energy source and the supercharger. When a motor is under load, it will pull a lot of amps, and the battery must be capable of supplying it.

Batteries are rated for their capacity and discharge rating. The capacity is the total amount of charge it can hold, bigger mAh gives longer flight times, but are also heavy. The discharge rating, denoted by ‘C rating’ is the max amount of current it can deliver at a time.

Max current from battery = mAh x C rating.

Select good batteries with a high discharge rating, so that it can keep up with the motor.

The motors are rated in watts for the amount of power they deliver. Now how they deliver that power depends on the motor specifications namely,

  1. Higher Kv motor spin faster, use smaller propellers, consumes a lot of currents and are generally used for high-speed aircraft.
  2. Lower Kv motor spin slower, use a large propeller, produce lots of thrusts, consumes less current, Generally used for larger planes with big props.
  3. The numbers on the motors generally indicate the size of the stator eg – 2212 1000kv motor, which means stator dia is 22 mm and height is 12 mm.
  4. Larger diameter motors produce more thrust. Taller motors are stable at higher speeds.

The propeller converts the mechanical energy from the motor into thrust. They are specified by their diameter and pitch. A “5X4 prop” has a 5-inch diameter and 4-inch pitch.

Generally,

  1. Larger props produce more thrust, smaller props are more responsive.
  2. Always match the motor to propeller. Lower Kv pairs with larger props and vice versa.
  3. For efficiency, go for the larges prop you can fit on the aircraft.
  4. Larger props produce more thrust, higher pitch gives faster speed.

Flight controller

A Flight controller is basically an inertial measurement unit (IMU) plus a couple of sensors and a processor to read the sensors and act on the motors. It is constantly monitoring the orientation of the craft and trying to keep it level against outside disturbances. Most flight controllers can run software that provides a host of abilities like GPS navigation, Auto-land/take-off etc.

There are many open-source flight controllers designed for fixed-wing like the APM and Pixhawk. But we can use the mini race quad boom to get cheap and efficient flight controllers.

Most mini quads today use a flight controller which has an STM32 chip on it, depending on the performance required, they range from F1 to F7 series. we settled on an STM32F4 flight controller which is a happy medium between performance and the number of devices that can be connected to it.

The flight controller needs software to run. There are a couple of opensource software to choose from like Ardupilot, Betaflight, Cleanflight, INAV etc. Most software is geared towards multirotor but there are a few which support fixed wing crafts.

We chose Ardupilot as the flight control software as it met all the requirements and has good mission planning capabilities.

Hardware

The hardware is low cost and reliable, and can be obtained off-the-shellf

Bill of Materials

SL NO ITEM DESCRIPTION QTY
1. Matek F405 wing STM32F405 Flight Controller 1
2. Beitian BN-880 GPS module GPS Receiver 1
3. DYS D3536 1400KV 1000Kv 700W motor 1
4. SkyWalker 60A Esc 60A ESC 1
5. Towerpro MG90S Servo 9g Metal Gear Servo 2
6. Pushrod connectors Servo pushrod connectors 2
7. Gemfan 9X6 Prop 9X6 prop 1
8. Tattu 5000mAh 30C Lipo Battery 1
9. Frsky S8R Receiver Frsky Telemetry receiver 1
10. Sik Telemetry Radio 433Mhx Dual-way Telemetry Radio 1
11. 5mm Depron Sheet 5mm XPS Sheet 10

All previous design iterations can be found on the Build Log

Version 4.0 (Stingray, KFM6)

Wingspan: 1250mmm
Sweep: 30 deg
Root chord: 380mm
Tip chord: 220mm
AUW: 1300g
Power plant: 3536 1400kv motor (9X6 prop)

The design of the wing was influenced by our previous iteration (Tomcat), depron is a good material for construction. The current model is designed to carry a lot more weight than its predecessors, hence the sweep angle is reduced to 30 deg to account for the change in center of gravity.

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The wing is made by stacking the depron sheets on top of each other and glued together. This allows for rapid prototyping and repair in case of a crash. The design can be cut by hand with a ruler and knife, there are very few curves to cut.

For ease of fabrication, we exported the design as 2D profiles and cut them out on a Shopbot 3-axis mill.

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The parts are easy to fit together and designed in a way to prevent the concentration of stress points (Glue joints) in a single place. The two base pieces provide a foundation on which the rest of the pieces are stuck. The joint is reinforced by top plates and the KFm airfoils are stuck on top.

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The arrangement provide a easy and rugged frame for the wing.

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All the pieces are stuck together with synthetic resin adhesive, a polyurethane glue like Uhu Por is recommended. Sticking the parts together is simple and provides a strong bond, which prevents bending.

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The Elevons sanded to give an airfoil shape and attached to the body using Nylon hinges. The Nylon hinges provide smooth travel for the elevons.

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The airframe is finished by spray painting and covering it with a layer of tape. The tape provides tensile strength to the body and acts a protective outer shell. Stripes added for visual effect!

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The wing flies well, it needs a 5000mah battery and a GoPro camera in the front to balance out the CG but once it’s balanced, it flies like on rails, good glide slope, and stall characteristics. The KFm6 seems to work well with this airframe.

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Flight Test

Version 5.0 VTOL Tailsitter

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A 1m wingspan 2.5Kg AUW Vertical take-off and landing Tail-sitter aircraft.

Link to repo – https://github.com/rahulsarchive/FOS_UAV