Vertical take-off and landing (VTOL) aircraft has good flight characteristics and system performance without runway. The multirotor system has been tried to expand into larger size for longer endurance or higher payload. But the motor power to endurance ratio has been limited. Due to the specific energy of gasoline being much higher than battery, introducing gasoline engine into multirotor system can be considered. This paper proposes a dual power multirotor system to combine a quadrotor using gasoline engines to provide major lift in shorter arm with another quadrotor using brushless DC motors to offer most controllable force with longer arm. System design, fabrication, and verification of the proposed dual power multirotor system development are presented. Preliminary flights have achieved 16 kg payload for long endurance flight. This is useful for various applications with advanced improvements.
The struggle to improve flight endurance and flight performance of multirotor has made efforts for decades. From the beginning of the past decade, the multirotor development has accomplished relatively small size for experimental use. It then becomes popular for its usefulness in aerial photography and other related applications that lead to requirement in payload and flight time [
Flight stabilization of multirotors has been currently achieved by fast and accurate response of BLDC motor with the advanced technology of miniature MEMS sensor in the past years. The MEMS sensors allow the mass production with lower cost and also reduce the size of the overall system [
The sensorless BLDC motor control enables the efficient and accurate brushless motor control system with smaller size [
Multirotor has advantage due to its simplicity, stability, flexibility, low cost, and being simple in manufacturing and maintenance. Mechanical simplicity is the major advantage over helicopter. The maneuverability and flexibility are the major advantage compared to fixed wing aircraft. However, its efficiency is far less than fixed wing aircraft and less than helicopter when compared at the same size. This is because the effective Reynolds Number at the lift generation plane of multirotor is much higher than helicopter and fixed wing aircraft. While helicopter also has rotating blades serving as primary lifting plane, it has less disc loading than multirotor and less influential flow from multiple rotors to result in higher efficiency [
The design concept of the proposed multirotor system preserves the simplicity of multirotor while increasing its payload and flight time performance. The causes of endurance problem of the traditional multirotors mainly result from the fact that the energy density of battery at present is not high enough to supply this type of helicopter for higher endurance. Several designs tried to extend quadrotor into hexarotor or octorotor for increasing weight capability. Increasing endurance requires heavier battery. This in turn leads to a heavier aircraft where after some point the effectiveness of this solution is too low [
Performance of battery capacity to flight time.
From Figure
There are many attempted methods to solve this inadequate endurance and payload problem. One of the popular solutions is to have many multirotor systems to operate as a swarm [
There are also hybrid designs in power propulsion system in order to change the gasoline engine power to electric or vice versa [
Quadcopter is a type of multirotor, as shown in Figure
Quadrotor axis and actuator definition.
Thus, the inertia is diagonal matrix
The rotor forces create thrust
From the above equations, to suppress the control power is to reduce the length between the rotor and center of gravity
The design concept builds a gasoline engine powered quadrotor on a BLDC motor powered quadrotor. It is termed as quadrotor in quadrotor, QiQ. The system design introduces the gasoline engines to supply the major lifting power while letting the electric motor handle the control stabilization, such that it is favorable for stabilization to increase the motor control effectiveness by reducing the effect of gasoline engine uncertainty to stability of aircraft. As discussed above, the control effectiveness of each motor or engine varies by distance of the unit to center of gravity of the aircraft. The reasonable configuration for this requirement is to move gasoline engine closer to center of gravity and extend the electric motor outward. The preliminary configuration of the proposed QiQ multirotor system is shown in Figure
The proposed QiQ multirotor configuration.
Two pairs of engine have each pair of rotation on opposite direction to generate counter torque as they spin the propellers. The pair of CW rotating engine counter the torque from CCW rotating engine and vice vesa. This idea is better than other alternative ideas such as tilting the engines or motors in order to compensate the force because the engines contribute lift force vector purely pushing upward vertically only. The power of engines can efficiently be used for lifting force alone. In addition, the byproduct torque generated from the propeller is cancelled by the opposite side propeller without any extra mechanism. This design is also optimized to utilize the lifting plane available to the maximum by placing the lift from the engine at the space between motor arms.
Considering (
Rotor force and torque distribution analysis.
Under steady state,
Then the equation of motion can be constructed by
The equation of motion is transformed into
The attitude control in the body frame is represented by the following equations:
The size of gasoline engine has been developed smaller to increase its capability and reliability over past decade. Its capacitor discharge ignition (CDI) units allow accurate ignition timing and get easier to startup especially at cold start [
However, the gasoline still has its complexity. There are two to three needles for fuel-air mixture adjustment. For system setup, the O.S. GT-33 engine is selected [
Engine has one big disadvantage over electric motor for its serious vibration during operation. Vibration is dangerous to this system for structure and flight stabilization. In the QiQ design, there are four engines that run at the same time. The best way to handle this problem is to isolate the engine vibration from other parts of the structure. There are many types of the shock absorber available. They are molded elastomer mount, spring damper, air isolator, wire rope isolator, and so forth. In the QiQ developing phase, the wire rope isolator is found by experiments as the most suitable type of isolator. The wire rope isolator accepts high traction axial load, which is thrust force for the QiQ. Gasoline does not corrode the metal like it does to rubber or other kinds of hydrocarbon materials. It is also commercially available and customizable at reasonable cost. Experiment later shows more evidence about necessity of this shock absorber to multirotor stabilization.
To enforce the overall structure, two identical aluminum plates are designed and fabricated as the central structure, as shown in Figure
Aluminum central plate structure.
GT-33 engines are mounted on the end of the shorter angle aluminum beam which is fixed on the central aluminum plate. Wire type shock absorber is used to reduce the engine vibration, as shown in Figure
Wire type shock absorber and installation.
Plastic fuel tank is used in the prototype QiQ airframe for gasoline. The tube is set as pointing downward for refueling with a fuel filter at the end of the longer tube inside the fuel tank, as shown in Figure
The preliminary flight test of QiQ using plastic fuel tank.
The gasoline engine uses Master Airscrew 16 × 10 3-blade propeller. The electric stabilization part KDE5215XF-435 BLDC motor is selected from KDE Direct [
In order to achieve the maximum flight performance on each design scenario, the energy matching between gasoline and electric batteries has to be matched for both gas/electric system capability to use all the carried fuel and batteries in flight. The fuel consumption of engines at optimum condition is examined. These data are taken from the 4 averaged different engines which are adjusted to their optimum conditions. The tests were done in a 27°C calm day at 5 m elevation over sea level. The oil-fuel ratio was 3.75%. The Master Airscrew 16 × 10 3-blade propeller is used in this experiment [
Engine fuel consumption.
RPM | 5500 | 6300 | 7200 |
---|---|---|---|
Consumption (cc/hr/engine) | 540 | 720 | 1074 |
The engine performance and characteristics are studied by the test platform shown in Figure
Engine thrust and RPM test platform.
The test result shows that the thrust force output from GT-33 engine with Master Airscrew 16 × 10 3-blade produces considered linear thrust to engine speed changes, as shown in Figure
Engine thrust output versus engine speed.
In a similar manner, the electric motor power consumption is also examined by the test platform as shown in Figure
Electric motor thrust test platform.
Electric motor thrust output and current versus motor speed.
From Figure
Electric motor thrust output versus current.
The QiQ design has its target to the longest endurance at one hour. QiQ is also designed as adaptable for high payload application too. After fabrication of the first prototype, the total empty weight without fuel and main batteries is 19.6 kg. The battery unit are four of 6 cell lithium polymer, 5200 mAh capacity battery, each weight 820 grams. From the preliminary test, the maximum engine thrust can be 7.02 kg, and the BLDC motor thrust can vary from 2 kg to 6 kg at hovering condition. The maximum lifting force from both quadrotor systems can provide 66 kg maximum lift. While the maximum power is huge when compared to the empty weight of 19.6 kg, the force of motor comes associated with current consumption and affects flight time. There are several flight configurations of QiQ being proposed for reference as in Table
Various flight configurations.
Flight condition | Maximized endurance | Balanced | Maximized payload |
---|---|---|---|
Empty weight | 19.6 kg | 19.6 | 19.6 |
Fuel load | 3.2 kg | 1.6 | 1 |
Battery weight | 12.8 kg | 12.8 | 12.8 |
Payload | 0 kg | 12 | 20 |
Total weight | 36 kg | 46.4 | 53.8 |
Engine thrust | 24 kg | 28 | 29 |
Motor thrust | 12 kg | 18.4 | 24.8 |
Endurance | 60 min | 28 | 20 |
Simulation detail for each airframe configuration.
Simulation profile | Motor | Propeller profile | Propeller size | Frame weight (g) | ESC max current (A) | Battery C rating |
---|---|---|---|---|---|---|
Small | EMAX (MT1806 2280KV) | Master Airscrew Electric | 4.7 × 3 | 250 | 30 | 45C/60C |
Medium | EMAX (GT2215-10 1100KV) | Master Airscrew Electric | 10 × 4 | 600 | 30 | 35C/50C |
Large | EMAX (GT5325-09 325KV) | XOAR PJN Electric | 28 × 4.7 | 1500 | 100 | 35C/50C |
Small-size quadrotor’s configuration for flight time simulation and result.
Battery capacity (Wh) | Battery weight (g) | AUW (g) | Battery resistance (ohm) | ESC internal resistance (ohm) | Battery specific energy (MJ/Kg) | Flight time (min) |
---|---|---|---|---|---|---|
13.32 | 96 | 605 | 0.0108 | 0.008 | 0.4995 | 4.8 |
24.42 | 174 | 691 | 0.0059 | 0.008 | 0.5052 | 7.6 |
33.3 | 237 | 760 | 0.0043 | 0.008 | 0.5058 | 9.4 |
44.4 | 315 | 846 | 0.0033 | 0.008 | 0.5074 | 11.1 |
49.95 | 354 | 889 | 0.0029 | 0.008 | 0.5080 | 11.9 |
55.5 | 393 | 932 | 0.0026 | 0.008 | 0.5084 | 12.5 |
66.6 | 471 | 1017 | 0.0022 | 0.008 | 0.5090 | 13.5 |
Medium-size quadrotor’s configuration for flight time simulation and result.
Battery capacity (Wh) | Battery weight (g) | AUW (g) | Battery resistance (ohm) | ESC internal resistance (ohm) | Battery specific energy (MJ/Kg) | Flight time (minute) |
---|---|---|---|---|---|---|
24.42 | 174 | 1319 | 0.0066 | 0.008 | 0.5052 | 5.8 |
36.63 | 258 | 1412 | 0.0044 | 0.008 | 0.5111 | 7.8 |
46.62 | 330 | 1491 | 0.0035 | 0.008 | 0.5086 | 9.2 |
55.5 | 393 | 1560 | 0.0029 | 0.008 | 0.5084 | 10.2 |
88.8 | 627 | 1818 | 0.0018 | 0.008 | 0.5099 | 13 |
155.4 | 1095 | 2333 | 0.0012 | 0.008 | 0.5109 | 15.3 |
177.6 | 1251 | 2504 | 0.0011 | 0.008 | 0.5111 | 15.7 |
Larger-size quadrotor’s configuration for flight time simulation and result.
Battery capacity (Wh) | Battery weight (g) | AUW (g) | Battery resistance (ohm) | ESC internal resistance (ohm) | Battery specific energy (MJ/Kg) | Flight time (minute) |
---|---|---|---|---|---|---|
106.56 | 752 | 5407 | 0.0081 | 0.0025 | 0.5101 | 8.9 |
195.36 | 1376 | 6094 | 0.0044 | 0.0025 | 0.5111 | 13.8 |
296 | 2096 | 6886 | 0.0029 | 0.0025 | 0.5084 | 17.5 |
473.6 | 3344 | 8258 | 0.0018 | 0.0025 | 0.5099 | 21.5 |
592 | 4176 | 9174 | 0.0015 | 0.0025 | 0.5103 | 23 |
710.4 | 5008 | 10089 | 0.0012 | 0.0025 | 0.5107 | 24 |
947.2 | 6672 | 11919 | 0.0009 | 0.0025 | 0.5111 | 24.9 |
1302.4 | 9168 | 14665 | 0.0007 | 0.0025 | 0.5114 | 25 |
The stabilization of multirotor requires fast RPM response from the stabilization motor. Individual stability from all units providing lift to the system may affect the overall flight stability and quality of the QiQ multirotor system. Although BLDC motor provides fast response to the given input, the gasoline engine does not act in the similar response. The gasoline engine power output is usually controlled by air inlet value which controls the quantity of fuel and air which goes into the combustion chamber. Carburetor acts as a mechanical controller for the engine. The performance and characteristics of the engine are adjusted by fuel-mixture needles. Just like any other air-breathing engine, gasoline engine is affected by environment such as atmosphere pressure, air temperature, and humidity. Engine temperature varying during runtime plays an important role to affect engine performance and characteristics. These are major challenges that would not allow open-loop control for engine RPM. The uncertainty from environment which affects engine might induce unwanted characteristics of engine, lead to instability flight, and affect flight performance dramatically.
Engine response in open loop has unpredictable characteristics. A preliminary study shows the response characteristics of engine and motors, as shown in Figure
Step response comparison between gas engine and electrical motor.
Engine linearity is another important issue in the engine RPM stabilization and its controller. The O.S. engine GT-33 carburetor data from manufacturer stated that 70–80% of maximum engine power develops within half of air inlet value. This serves as throttle input for the engine. From the preliminary experiment, the engine response to the throttle input is shown to be highly nonlinear. Before implementing PD controller, it is necessary to linearize the throttle input to reduce the complexity of introducing a PD controller. The incremental PD controller is then implemented to the system. These two combinations allow the system to respond to instant movement of RPM command as well as accurately tracking for servo optimum point.
In a mission starting from take-off to landing, engine controller has to change its mode in several times. There are three major modes required for the QiQ. (
In addition, the engine controller has to receive other commands or parameter settings from the flight controller or the ground station. Those parameters include controller gain, servo limit, and idle position. It is necessary to have data transfer mechanism to send modes, parameters, and other commands from flight controller to the engine controller. Nevertheless, it is still necessary to keep a necessary bandwidth of RPM command stream from flight controller to engine controller. The RPM command is different from other kinds of data transfer because the command has to be synchronized with flight controller control loop. In addition, the RPM command data is streaming one-way from flight controller to engine controller.
One more thing to consider about communication between engine controller and flight controller is the reliability of the communication system. Engine itself generates a lot of EMI from its ignition system. Vibration from engine and other sources can affect the reliability of data transfer. Flight controller needs to ensure that the command it assigned is completely and correctly transferred to engine controller to keep machine state being tracked. The CAN Bus protocol meets all the requirements. CAN is a bus type of communication and two-way transmission with error-checking mechanism. Each message in CAN Bus required acknowledgment in transport layer [
If it appears that the message is not received by other nodes, the CAN Bus controller supports automatic retransmission. It is also proven in the automotive standard, where the scenario is similar to the QiQ design.
The CAN Bus serves as parameter and status report feedback from engine controller. This includes every message between flight controller and engine controller but not RPM command. The RPM command in CAN Bus is used instead of a dedicated wire for each engine controller. The RPM signal is Pulse Width Modulation (PWM) signal with frequency of 50 Hz.
Since control loop requires feedback signal from engines, it can theoretically and simply read from hall effect sensor installed on the shaft of the engine. The RPM sensor generates a pulse for each engine revolution. Engine controller then reads the pulse, calculates time difference between each pulse, and gets the RPM data. However, in practice, the signal is severely disturbed by a very high frequency and very high amplitude noise. If the RPM sensor is connected directly to the main electronic system, it affects all of the signal input/output readings of the microcontroller. Moreover, the signal level output from the hall sensor is 0~1.5V, which is too low to trigger the MCU readings. The EMI noise also propagates through servo motor wires because they are necessary to be placed close to the engine.
To prevent the disruption of electronic system from EMI noise of engines, the isolation of electric system between noisy system and main system needs to be considered. The isolator needs to have a signal amplifier, where its robustness and simplicity are required. Its frequency response is better to be in the RPM signal than the high frequency noise range. In order to achieve the isolation, a transistor is introduced to amplify the RPM signal from the engine. The amplified signal drive TLP181 optoisolator is adopted to separate two power systems by the light transmission and light reception, as shown in Figure
Engine controller diagram.
The servos also are found to be an EMI receptor to the power system after preliminary tests. Because it has to be installed close to the engine, the EMI from ignition can propagate through the air to the servo chassis. Although the EMI does not affect the servo itself, it induces a severe disturbance effect on the engine controller electronics. It is better to separate the power system of the servo as well. The optoisolator is used on the signal line from engine controller in a similar manner to RPM signal but without amplifier. It is shown in Figure
Engine sensor and shock absorber installation.
Control loop of the engine controller is running at 50 Hz update rate, which matches the servo motor input frequency. The incremental PID controller is used in this design. The desired engine rotational speed (in RPM) is called setpoint. The difference between measured RPM and the setpoint is the error (
From the design of carburetor, the response of given input to the output RPM is not linear. PID controller is based on the linear system. Its parameter is not suitable for the whole range of operation. The engine response curve is steep at lower throttle and becomes less sensitive at higher throttle. The actuator linearization is adopted based on the captured data to normalize the response rate of the engine throughout the throttle range from 4000 RPM to 7500 RPM. Data is cut into two sessions for 4000~6000 RPM and 6000~7500 RPM to form two sections of linearization equation.
Attitude stabilization of multirotor is the key to make its fly. In fact, it is almost impossible to fly multirotor without the stabilization system. In the dual power QiQ case, more challenges lie on the higher inertia airframe. The smooth attitude response is required to make sure that engine and fuel will not be affected by the movements. The stabilization of the proposed design is based on those four outer electric motors. They act similar to a conventional quadrotor system. The difference of power in each motor generates moment that tilts the frame around its center of gravity. In the conventional electric multirotor, only motors and propellers are moving parts. However, in the dual power QiQ design, there is liquid gasoline with engines that are sensitive to quick movement. With too fast angular velocity of the body frame, the gasoline may splash to affect the center of gravity and form bubbles in the fuel. This will cause the engine to have unstable trembles due to the carburetor suction with air bubble from the fuel inlet. This may cause engine to get into intermittent instability or even shut down. Since these fast angular movements are caused by attitude stabilization controller in fast response, it is practical to set a limit over attitude response speed. Nevertheless, the accuracy and precision of the controller have to be preserved.
The flight control board uses STM32F427 as a main microcontroller in the proposed QiQ system [
The flight controller implements PD controller with PID inner loop to control roll and pitch angle, as shown in Figure
Roll and pitch PID stabilization controller diagram.
The outer loop PD controller takes input from transmitter and outputs the desired angular rate. It has angular rate limiter to control the rate of response. In some cases, the angular rate should be slower, such as application with the onboard gimbal stabilization for onboard camera. The device may have poor response if the airframe rotates too fast. The proportional part of the outer PD describes how fast the control should respond to the error. Large proportional gain gives crisp response for the proposed QiQ system while slightly increasing its accuracy in overall. The derivative part prevents overshoot and controls system in smoothness.
For heading controller, the simple PID controller is used as in Figure
Yaw stabilization controller diagram.
Shock vibration from reciprocating engine is not suitable for automatic stabilization system, especially when the accelerometer is used as a part of attitude estimation. Vibration also damages structure in long term by inducing fatigue, loosing fastener, disturbing other devices, and so forth. In the QiQ design, there are four engines running simultaneously such that the shock vibration of the engine should be minimized and prevent the vibration spreading.
Shock absorber performance is validated by comparison with the vibration of the test platform between two installation configurations. The configurations mount the engine with or without shock absorber. The MPU-6500 IMU is used to measure the acceleration of the frame. This IMU is similar to the one implemented on flight control. The acceleration limit of this accelerometer is
Measured acceleration data without shock absorber.
Measured acceleration data with wire type shock absorber.
The shock acceleration data are collected from the accelerometer in the axis of piston movement. It can be seen in Figure
After installing the engines on the shock absorber on the test platform, the vibration on the structure is dramatically reduced, as shown in Figure
In the QiQ engine controller development and experiment, the gasoline engines are equipped with 16′′
As discussed earlier, the linearization of the actuator has to be done before the actual PID is implemented. This is done by starting and warming up the engine until its temperature is saturated and stable. After that, the throttle should be slowly increased while monitoring the throttle and engine RPM relation. Since the stability of engine speed under 4000 RPM is poor, the engines may be subjected to cooling down or even shutdown when a sudden change signal is received at very low RPM. The controller design should be working at the region higher than this point.
From Figure
Relation between engine RPM and throttle input.
The long-term stability is very important in the proposed QiQ multirotor flight controller system. Even in situation of ambient environmental change, the controller characteristic and accuracy should remain the same. Temperature is known to be one major factor that affects the engine characteristics. The controller should be able to compensate engine throttle to give the expected power, while the engine has not reached the stable temperature. It also has to reduce compensation fast enough. The open-loop response experiment of the engines is studied to observe engine characteristics and performance without controllers. As engine is started from cool state, it is subjected to immediate throttle input. The reduction of throttle input observes the cooling down effect to the engine power. The response is shown in Figure
(a) Engine RPM response to step input. (b) Corresponding exhaust pipe temperature.
At the ignition moment (
At
The RPM control mode of engine controller using the incremental PID has been implemented. The engine starts from cool state at ambient temperature. The RPM setpoint is increased from 4750 RPM to 6700 RPM by pilot controller from the transmitter. After 75 seconds, the setpoint was changed to 4960 RPM and dropped to 4350 RPM after 60 seconds. The controller setpoint is once again raised to 6700 RPM for 60 seconds and then returned to 4350 RPM. The result is as shown in Figure
(a) Close-loop engine RPM response. (b) Controller throttle output and exhaust temperature.
From Figure
Another experiment is carried out before flight test. This experiment tests the controller characteristics before flight. It is important to ensure the similarity of the thrust actuator for the proposed QiQ multirotor system. Unbalance thrust may generate excess unwanted moment to destabilize the QiQ. This experiment is completed to ignite all engines and enables RPM controller. After that, step inputs are given to see their responses. The four engines performance is shown in Figure
Four engines controlled output response.
A smaller octorotor system as shown in Figure
Modified octorotor to simulate QiQ stabilization algorithm.
Experiment flight data of simulated octorotor.
From Figure
Figure
Flight test on this dual powered multirotor system with the proposed QiQ configuration has been successfully completed. The flight test was conducted in the morning of clear day. Ambient temperature was 27°C, with no sign of rain, and the wind was calm. The control strategy gives all engines to run at the same RPM. Since the engines are started at 10% throttle, the outer electric DC brushless motors are set at 60% throttle to maximize their possible control range. The engine RPM is continuously increased until take-off. The flight was stable. The aircraft was fully controlled by pilot. Its characteristic is similar to the conventional multirotor systems. However, the control in yaw direction has been slightly delayed due to high inertia in the axis. The test flight data are shown in Figure
Experiment flight data of QiQ prototype.
The control range was reduced because of flying area limitation and for safety of the experiment. The flight controller has ability to stabilize the QiQ airframe in every movement. Even though the movement is small, the precision in control has been demonstrated in Figure
Engine controller is able to hold the engine at desired throttle throughout the flight. The variation in engine RPM is about
The proposed QiQ prototype in 16 kg flight test.
After preliminary flight verifications, the proposed QiQ is added with a composite fuel tank for better performance. The fuel tank is made of glass fiber and is fabricated as shown in Figure
Composite fuel tank sectional view.
The purpose of the heavy lift multirotor is usually related to high inertia airframe configuration. There are some specific applications that require intensive heading control, for example, agricultural drone, where the spray array needs to be perpendicular to the flight path. The heading control for conventional multirotor relies on difference of propeller generated torque between clockwise and counterclockwise rotating direction of the motor as can be seen from Figure
In the top of Figure
Attitude control fin.
QiQ with composite fuel tank and attitude control fins.
This modification adds the direct control to the directional control of multirotor. The control authority also increases with size and reduces the required motor rotation speed change to control the attitude which expands flight envelope. The idea is suitable not only for QiQ but also for other large multirotors.
The idea of gasoline-electric dual power multirotor vertical take-off/landing vehicle is designed, fabricated, implemented, and proven to be feasible and valuable for more practical developments. Gasoline engines are able to provide major lift, while BLDC motor has better performance in system stabilization. The engine RPM controller can stabilize and normalize the engine characteristics into safe margin. Flight stabilization controller is also able to provide real-time response even in extreme case as in simulated airframe or precise control in real prototype flight test. The proposed QiQ design can be optimized and improved into further applications.
The authors declare that they have no conflicts of interest.
This work is supported by Ministry of Science and Technology (MOST) under Research Project MOST-105-2119-M-006-012.