At KU Leuven we have been developing a new generation of drones to double the speed and range of conventional multi-rotor drones.
For quite some years already, multi-rotor drones are the preferred type of drone for typically applications such as filming or inspection, in which the drone flies at low speeds and usually stays rather close to its pilot. On the one hand, the design of these drones is not very well suited for covering long distances at high speeds, which are beneficial in multiple applications such as parcel deliveries or emergency services. On the other hand, fast and efficient fixed-wing drones require a person or catapult to take off and a runway, parachute or net to land which limits their possibilities in fully automated missions.

Our team has been focusing on improving the speed and range of multi-rotor drones. We therefore combined the benefits of fixed-wing drones with those of multi-rotor drones resulting in a hybrid solution which performs a transition from hover flight into an efficient cruise flight. The only moving parts of the drone are the four propellers which are used to provide lift, thrust and control. This results in a robust, lightweight and low-maintenance solution.  Back in 2014, our first prototypes took off.  These designs made a transition from hover to cruise flight of 90°, with a large wing producing 100% of its required lift in cruise flight. Flying in windy conditions, however, was challenging due to this large wing. Therefore, we decided to improve the wind resistance by reducing the wing size and compensate the loss in lift by improving the aerodynamics and only make a partial transition such that the propellers still produce part of the lift in cruise flight. The result is a very wind tolerant, efficient and fast new generation of drones… The CargoCopter.

Our designs have been evolving fast: a parametric selection algorithm optimizes every new CargoCopter design for its mission-specific requirements. The designs are specifically engineered to be suited for lightweight 3D printing. Thanks to this agile methodology, we were able to design, manufacture and test-fly dozens of prototypes with gradual improvements leading up to doubling speed and range compared to standard multi-rotor drones.

The CargoCopters designs are meant to transport payloads up to 5kg, achieve speeds up to 150km/h and cover distances up to 60km.

With the increasing interest in the use of multi-rotor UAVs for long range applications such as pipeline inspection, large-area mapping, transporting packages or even people, the question arises: “how far can they fly?”.  

In this post I’ll present a theoretical approach, an experiment and a conclusion to answer this question. Let’s start with some basic theory that can be used for all flying things. The distance d that a UAV can cover, flying at a constant mass and velocity v is presented by:

With t the flight time calculated as the available energy in the battery divided by the total required electrical power:

Assuming the power required for propulsion is some orders of magnitude larger then power for payload or avionics, the total required electrical power can be calculated as the power required for propulsion divided by the overall efficiency of the propulsion system (losses in cables, ESC, motors, propellers, air).

For the same battery chemistry and manufacturing process, we can assume that the energy available from the battery linearly depends on its mass:

For Li-Po batteries, a value of 180Wh/kg is possible or in SI units:

The mass of the battery is a fraction of the total mass of the UAV. For example a UAV with a total mass of 4kg, including 1kg of battery has a 25% battery fraction:

The theoretical power that is needed for propulsion is calculated as the flight speed multiplied by the drag:

For fixed-wing aircraft this formula makes sense but multi-rotors also consume propulsion power to create lift, right? The propellers create both lift AND propulsion. This is analog to an airplane which uses wings and a propulsion unit (propeller or jet) to create its lift and propulsion respectively. So just consider the propellers of a multi-rotor acting as a wing and a propulsion unit simultaneously.

We do not know the drag of the UAV and we cannot measure it. Why not? For a fixed wing aircraft you can put the aircraft in the wind tunnel and measure the drag with a force sensor with the propulsion unit producing zero thrust. Or we can measure the thrust of the propulsion unit in flight with a force cell. We can do this because the propulsion unit and the ‘lift unit’ (=wings) are separated systems. Because the propellers of a multi-rotor act both as wing and propulsion unit, we cannot simply remove them and measure the drag in a wind tunnel. This would be the same as measuring the drag of a fixed-wing aircraft without the wings. Because we cannot separate lift and thrust (= drag in regime flight), we continue with a generally accepted ratio in aviation: the maximum lift-to-drag ratio, also known as the aerodynamic efficiency or glide ratio. A glide ratio of 10 for example means, if you shut down the propulsion unit, that you can travel maximum 10 meters while descending 1 meter.

The glide ratio of a fixed-wing aircraft is easily measured this way and are in the order of 10 to 40. For helicopters, it can also be defined by gliding down in auto-rotation. Therefore the blades are given a negative pitch and they act like a wing of a glider, but then rotating. Typical ratios are around 4 or lower. Because we cannot change the pitch of the blades of multi-rotor propellers, it is not possible to determine the lift-to-drag ratio by gliding down in auto-rotation.

We’ll have to find a way to determine exact the lift-to-drag ratio for multi-rotors later on, but let’s assume for now we know the ratio and it is less than a full-size helicopter in auto-rotation (4) and more than a wingsuit (2.5).

We also know the lift, which is equal to the weight:

Rearranging former equations allows us to calculate the maximum range:

Since the propellers act as both wings and propulsion unit, it is also not possible to determine solely the efficiency of the propulsion unit. Therefore, we have to consider the two factors marked in yellow as one unknown value. We can make an educated guess however: the efficiency will always be <1 and the lift-to-drag ratio will be probably <4. Therefore, we can calculate the maximum range of a multi-rotor with 180Wh/kg batteries and 25% battery as:

The energy density of the battery and the fraction of the battery are easily measured and should be maximized to get the highest range.

We assumed that  

but we need a way to know this number for a multi-rotor. This number is a combination of aerodynamic and electric/mechanical efficiency of a multi-rotor and would be a good benchmark to validate the capability of a multi-rotor to cover large distances.

Doing some more math with former equations allows us to determine the highest value of this number by performing some test flights at different flight speeds (yay! No wind tunnel test or force cells required, only a speed and a power measurement!)

If we want to consider only the aerodynamic performance of the multi-rotor, including the propellers, we can define a ‘theoretical glide ratio’, which we cannot achieve because auto-rotation is not possible:

With P_mech the total mechanical power delivered to the propeller shafts during cruise flight. This can be measured or calculated if the efficiency of wires, ESC and motor is known for all possible operating points.

Now the big question: what is the theoretical glide ratio of a multi-rotor? During my PhD, I performed many test flights with different multi-rotors and here are the results for a popular setup:

The best total flight efficiency of this setup was:

occurring at a total mass of 1.6 kg and a speed of 16.3 m/s. With a 25% battery fraction and 180Wh/kg batteries, this results in a range of about 20km.  Because I measured the efficiency of the motor and ESC in all possible operating regimes on a test bench, I was able to also compute the theoretical glide ratio:

So that is somewhere in between a flying squirrel and a wingsuit (not too good!).

Conclusion: The range of multi-rotor can be easily computed if you know the ‘flight efficiency’, which is a combination of the efficiency and aerodynamics of the ESCs, motors, propellers and the body/arms/wings. The best flight efficiency of a conventional quadcopter was calculated from flight data as 1.2 and the accompanying ‘theoretical glide ratio’ was 2.13. People working on VTOL hybrid solutions should be able to increase this number towards glide ratios of fixed-wings (> 6). However… battery fraction is of equal importance if you want to maximize your range.

Question: what is your flight efficiency and battery fraction? I’d love to see some comparison between multi-rotors, helicopters, fixed wings and hybrids!

If you like to learn more about the performance of multi-rotors for high speeds, long range, endurance, payload capacity, you want access to all the data on different vehicles or for more specific questions contact me on bart.theys@kuleuven.be

At the University of Leuven (KU Leuven) we are working hard on improving performance of multi-rotor UAVs in terms of flight speed, endurance and possible payload, with the application of automated aerial transportation in mind. Our approach is to add wings to a multi-rotor and make a transition between hover and cruise flight in order to decrease the required power for flying at high speeds. In order to reduce the number of moving parts, and therefore extra weight and points of failure, we only use differential thrust for controlling the UAV throughout all flight phases

In the summer of 2014, our first prototype was presented, the “ VertiKUL "

This UAV made a transition of 90°, producing 100% of its required lift from the wing in cruise flight. Also see this post. We noticed that flying in windy conditions, however, was challenging. Especially the automated landing was hard. Therefore we decided to improve the wind resistance of the UAV by reducing the wing size. Therefore, part of the required lift in cruise flight is still produced by the propellers that now operate at 45° with respect to the direction of flight. The result is a very wind tolerant, efficient and fast multi-rotor: VertiKUL2

Design specifications:

-          5kg total mass

-          60 to 70km/h cruise flight

-          + 25km of range (we still have to validate this)

-          Pixhawk flight controller with adapted ArduCopter 3.2.1 firmware

-          1kg of payload possible (20x15x10cm)

Because of the transitioning, it was hard to include a landing gear to land on a flat surface. Therefore we decided to land on an inclined surface, or a box as in the video.

We developed a new approach for controlling these transitioning VTOL UAVs with promising results and applicable on a wide range of VTOL UAV designs that are controlled by differential thrust as multi-copters. More info will follow…

Last academic year at KU Leuven, we designed, built and test flown a VTOL UAV: the VertiKul. During this project we gratefully made use of the info and support of the DIY Drones community and therefore we would like to share our results on this project.

The VertiKul is designed for automated aerial transport of small packages and is optimized for maximum range and payload capability. The innovative design makes use of the benefits of both multi-rotors and fixed-wing airplanes. For take-off and landing, the VertiKul hovers like a quadrotor and for forward flight, the VertiKul pitches 90° and flies like an airplane.In airplane mode, the attitude is also controlled by differential thrust of the motors. Therefore, no additional control surfaces are required, reducing the number of moving parts, risk of failure and maintenance cost. The structure is made out of three carbon fiber tubes in a ‘H-configuration’ allowing an easy accessible space for a 10x15x20 cm package of 1kg. The tubes are connected using laser cut multiplex wood and wings are constructed using a polystyrene-balsa sandwich structure, covered with Oracover. For a good directional stability, the wings are slightly swept-back and winglets, that also help reducing the induced drag, are added. Since the wings introduce a high moment of inertia and strong moments because of wind around the yaw-axis, the propellers are tilted 10° to improve the yaw control.

Because of the two different flight modes and the transition in between, a new control strategy is needed. This strategy contains three levels. The first level, or low level, is the angular rate control as in “Acro mode”. Because of the -90° pitch in forward flight, it becomes hard for a human pilot to control the VertiKul since a roll command results in a yawing motion and a yaw command makes the vehicle roll (in counter-intuitive direction, yaw to the left results in roll to the right!). To make the control more intuitive, a mid-level controller is designed around the angular rate controller. This controller acts as “Stabilize mode” when the VertiKul is in hover and makes an automatic transition to forward flight when a switch is turned on the transmitter. The transition to forward flight takes around 5 seconds and gradually decreases the pitch angle to build up the speed required for enough lift of the wing in forward flight. Any input from the pilot is ignored during this phase.  A quaternion representation was required in order to avoid the ‘Gimbal lock’. In forward flight, the pilot inputs are only the desired altitude and heading, making it easy to fly by inexperienced pilots. Finally, the high-level controller generates a trajectory between two base stations and commands flight mode, altitude and heading to the mid-level controller.

In order to have a fully autonomous system, we also developed a docking system. The system includes an optical precision lading system, based on a PX4FLOW unit and a docking station at which a package or battery can be swapped. The VertiKul starts from one docking station with a fully charged battery and a package of 1 kg and then flies to its destination, 30km further, based on GPS. Once arrived at location, the VertiKul makes a precision landing on the docking station at that location. The battery is replaced with a full one and a new package is loaded so that the VertiKul can continue to its next destination.

The PX4FLOW camera we use for this autonomous precision landing is re-programmed in order to detect the center of the marker on the docking station and sends these coordinates to the autopilot on the VertiKul. Based on the altitude, roll and pitch angle of the VertiKul, the position of the marker is calculated and a position controller navigates the VertiKul to the landing spot. In order to be able to land at night, the marker is illuminated by leds under the surface of the translucent marker.

Check out the video here: http://youtu.be/omaxgFVDUWg

We haven’t yet been able to test the full performance of the VertiKul because of the limited test area where we can fly. During test flights we experienced a lot of influence of the wind on the big wings, making automatic landings very hard. Also the battery and package swap is not yet automated, leaving us with enough work to continue this project.