FlyRider - Half Scale Prototype

Why a 50% Prototype?

It would be completely nonsensical to assume that the development of an eVTOL would be possible without errors and crashes. If all flight tests were carried out with a 1:1 prototype, it would not only be extremely expensive but also very dangerous. The construction and build of a prototype is therefore unavoidable.

Why 50%? The prototype should not be too big, because it will also be very expensive, but it should also not be so small that the flight behavior is extremely different from the real aircraft. I think that 50% represents a pretty good compromise between cost and flight behavior. The most important goal of the pototype is to gain experience with with the flight controller, the Ardupilot software and all the required technology. The prototype also serves to test the planned safety measures.

Frame

Propulsion

Motors for the prototype are EMAX MT-2216, according to the supplier they shall provide 0,95 kg of thrust with a 10 x 4.5 prop at 14,8 V (4S) and 14.6 A. The real FlyRider aircraft is planned to have a 1,5 / 1 thrust to weight ratio. With a total peak thrust of 5,7 kg the weight limit for the prototype is 3,8 kg to achieve the same thrust to weight ratio.
As motor speed controller (or ESC = Electronic Speed Controller) I use a Hobbywing X-Rotor 20A ESC. Since the EMAX MT 2216 consumes 14.6A at full power, a 20 Amp ESC should be sufficient, especially since no long duration flights are planned with the prototype.

    
    Parts Required for Prototype 

    6 x EMAX MT 2216 motor (or similar)
    6 x Hobbywing X-Rotor 20A ESC (or similar)
    2 x 5000 mAh LIPO with min. 20 C  (or similar)
    1 x Pixhawk Flight Controller
    1 x LIDAR

Power

As batteries I use a set of 4S Lipos (14.8 V) which can at least provide 45 A to be able to use one battery for 3 motors. A 5000 mA battery is able to provide 45 A for more than 6 minutes, enough time for the planned test flights. You also have to make sure that the batteries have the C rating to be able to provide the current needed to get full thrust out of the engines. The C rating, quite simply, refers to the capacity of energy the battery can safely discharge, represented as a multiple of its overall capacity. Thus a 5000 mAh 20 C battery should be able to provide 100 Amp peak current (5A * 20C = 100A) - at least in theory. It is best to have a discharge rate overhead of 30%. For a 5000 mAh 20C battery a discharge with 45 Amps should be no problem.

Control = Key to Success

The most difficult thing about flying is to keep control of the aircraft at all times. For this you need either a capable pilot, or a powerful flight computer, which has the necessary sensors (IMU, GPS, lidar) to check and correct the flight attitude and position at any time. Ideally you have both.

The first flight tests with both the prototype and the full-scale FlyRider aircraft will be carried out remotely (for the prototype test flights there is no choice since its unmanned, for FlyRider ist just for safety reasons).

Flight Controller

Pixhawk is much more than a flight controller, pixhawk has set a standard for the entire industry, and today controllers from various vendors around the world are built to its specifications. Initially Pixhawk was a student project at ETH Zurich. Little did the team know that it would end up having such a large influence on the drone industry. Today it is one of the leading flight controllers for UAVs and drones.

GPS and Compass

I use the Holybro GPS and compass module as it is very easy to connect to Pixhawk and provides stable and accurate position data.

The whole setup for the FlyRider flight control system is pretty simple and that's a good thing! You should always keep in mind that in terms of safety, a component is at its best when it is not needed - what is not installed cannot fail!

Ardupilot

Pixhawk is a sophisticated flight controller, but only the hardware. Without suitable software it just can't work! Fortunately, there is a variety of software that can run on Pixhawk, my choice is "ArduPilot". ArduPilot is one of the most advanced and reliable open source autopilot software available, it has been under development since 2010. The software is capable of controlling almost any vehicle system imaginable, from conventional airplanes, multi-copters and helicopters to rovers, boats and even submarines.

Mission Planer

Mission Planner (MP) is a full-featured ground station application for ArduPilot. Mission Planner can be used as a configuration utility or as control supplement for ArduPilot controlled vehicles. With MP you can install the firmware into the flight-controller hardware, setup and configuration your vehicle, plan autonomous missions with simple point-and-click way-point entry on maps, download and evaluate mission logs created by autopilot and monitor the vehicle status while in operation.

Most importantly: All required settings for the flight controller can be made in Mission Planner.

First Flights And Tuning

Unfortunately, the first flights were extremely unstable. There was always a lot of movement around pitch and roll, yaw hardly worked at all. So I started to tune the Ardupilot. You can find all the relevant information on the Ardupilot website.You can use the graph below to estimate the correct MOT_THST_EXPO value for your aircraft.

PID Controller Initial Setup

The next settings are meant to get your PID controller acceleration and filter settings into the right approximate range for your vehicle. These parameters are critical to the tuning process. The PID controller default values for axis P/D/I values are usually safe for first test hovers of most vehicles.

INS_ACCEL_FILTER: 10Hz
INS_GYRO_FILTER: 40Hz for 10 inch props
ATC_ACCEL_P_MAX: 110000 for 10 inch props
ATC_ACCEL_R_MAX: 110000 for 10 inch props
ATC_ACCEL_Y_MAX: 27000 for 10 inch props


I have changed INS_ACCEL_FILTER to 10 and INS_GYRO_FILter to 40 Hz.

Motor Setup and Vibration Check

it is very important to ensure that the thrust curve of your VTOL motors is as linear as possible. A linear thrust curve means that changes in the actual thrust produced by a motor is directly proportional to the thrust being demanded by ArduPilot.

MOT_BAT_VOLT_MAX: 4.2v x 4 Cells = 16.8 V MOT_BAT_VOLT_MIN: 3.3v x 4 Cells = 13.2 V ATC_THR_MIX_MAN to 0.1 MOT_THST_HOVER to 0.25 (I had to set it to 0.35 to have enough thrust)

The vibrations are within the normal range (at least until the hard landing).