Design Methodology for Quadcopter with Co-Axial Propulsion units to Achieve Thrust and Stability Demand During Flight

The usual Quadcopter has only four rotors and it achieves its Stability by nullifying the Rotational Moments produced by its corresponding rotors placed in the Frame.The addition of Co-Axial propulsion units , will leads to increase the Thrust, better Maneuverability and supports to achieve Stability in Emergency situations.A Quadcopter mounted CoAxial propulsion units will have less redundancy when compared to the Quadcopter having only four rotors, because the current requirement of 8 Brushless Direct Current Motor (BLDC) is comparatively larger than the current requirement of four motors so that the Flight time reduces for the Quadcopter having Co-Axial propulsion units.The usage of all Co-axial rotors produces lesser amount of Thrust as compared to the Thrust produced by only four rotors ,because in CoAxial Propulsion units the lower propeller loses thrust working in prop wash of upper propeller but this offers ability to provide significantly higher thrust for the platform in smaller volume.During Emergency situations where the Failure of motor occurs, in this case the presence of Co-Axial propulsion units will help to produce corresponding Rotational moment that was produced by the motor which failed to work.In some case where all the four motors gets failed, the co-Axial propulsion units can be used as a Back-up and the mission can be completed.Though the Co-Axial propulsion units has some drawbacks, it can be used with additional battery as power source in the Quadcopter, not for continuous usage but can be used during demands. Keywords—Quadcopter; Rotational Moment; Co-Axial Propulsion unit; Brushless Direct Current motor; propeller.


INTRODUCTION
The idea of coaxial rotors originates with Mikhail Lomonosov. He had developed a small helicopter model with coaxial rotors in July 1754 and demonstrated it to the Russian Academy of Sciences.In 1859, the British Patent Office awarded the first helicopter patent to Henry Bright for his coaxial design. From this point, coaxial helicopters developed into fully operational machines as we know them today.Two pioneering helicopters, the Corradino D'Ascanio-built "D'AT3" of 1930, and the generally more successful French mid-1930s Gyroplane Laboratoire, both used coaxial rotor systems for flight. In the recent years the technological and mechanical areas in the field of Unmanned Aerial Vehicles were drastically developed.The Drone Technology is used in most of the civilian operations related to Agriculture, border Surveillance operations, inspection of critical areas,rescue operations and in fire control.The Propulsion systems of the Drones is the most important component since it has higher power consumption.So that the propulsion systems must be efficient and appropriate to the mass of the Drone to have long Flight time.Additionally the propulsion units should produce higher Thrust so that the Thrust to weight ratio becomes significant to Lift the payload.
Propulsion units of the Drone are responsible for its survivability, endurance and other parameters crucial for its mission operation . Reliability is the main requirement of the Drone since the propulsion system failure inevitably leads to the Drone loss.The propulsion system should allow also a convenient drone climbing to high flight altitudes without power loss . A necessity to meet these requirements, as well as the special needs of specific applications, result in numerous technological and scientific challenges. This imposes a need for intensive research on UAV propulsion, which is constantly progressing and expanding.  [3] proposed a practical method to help designers quickly select the optimal products of the propulsion system to maximize the multicopter efficiency under the desired flight condition. First, the modeling methods for the components of the propulsion system are studied respectively to describe the optimization problem mathematically. Secondly, methods are proposed to find optimal motor and propeller combination with the maximum thrust efficiency according to the given design requirements. Finally, factors that may affect the hovering time of multicopters are analyzed, and the optimal battery parameters are obtained for maximizing the multicopter endurance. Experiments and simulations are performed to demonstrate the effectiveness and practicability of the proposed method.

II
Wayne Ong,Spot Srigrarom,Henrik Hesse.(2019) [4] design methodology for multirotor Unmanned Aerial Vehicles (UAVs). To specifically address the design of vehicles with heavy lift capabilities, and have extended existing design methodologies to include coaxial rotor systems which have exhibited the best thrust-to-volume ratio for operation of UAVs in urban environments. Such coaxial systems, however, come with decreased aerodynamic efficiency and the design approach developed in this work can account for this. The proposed design methodology and included market studies have been demonstrated for the development of a multi-parcel delivery drone that can deliver up to four packages using a novel morphing concept. Flight test results in this paper serve to validate the predictions of thrust and battery life of the coaxial propulsion system suggesting errors in predicted flight time of less than 5 percent. Marcin Biczyski,Rabia Sehab,James Whidborne,Patrick Chi Kwong Luk.(2020) [5] described the need for sizing of rotors for multirotor vehicle applications such as personal air transport, delivery, surveillance, and photography. A methodology for the propeller and motor selection is developed and augmented with flight time estimation capabilities. Being multirotor-specific it makes use of the platform's simplicity to rapidly provide a set of off-the-shelf components ready to be used in the vehicle. Use of operating points makes the comparison process fast, precise, and tailored to specific applications. The method is easily implemented in software to provide an automated tool. Furthermore, clearly defined input and output parameters make it also usable as a module in other multi criteria optimisation algorithms. The new methodology is validated through comparison with a consumer-grade drone and the calculated results are compliant with manufacturer's specification in terms of maximum hover time.

III
CONSTRUCTION The prefix Quad-copter implies ("Quad" =four), is a drone configuration where there are four arms.The main frame is made of Carbon fiber due to its High Strength to Weight ratio, with each arm length of 230 mm.At each free end of the arm, a pair of Co-Axial motors will be fixed and propeller will be mechanically coupled to the motor.For all eight motors the output side of an ESC will be connected and the input side of the ESC will be connected to the flight controller. The other input of the ESC will be connected to the power distribution board where the power supply is provided by the Li-Po battery. In a similar fashion all the other ESC's, motors and propellers are connected. A receiver will be connected to the Flight controller to receive signals from the transmitter. .The landing frame of height 140 mm is connected to the main frame such that there is enough space for payload attachment and the clearance for the propeller rotating down side.

IV WORKING
The signals will be transmitted from Transmitter and it will be received by the Receiver in the drone. From the receiver the signal goes to the Flight controller where the signal will be processed with accelerometer and gyroscope sensors. The processed signal will be sent to the ESC, which allows the specific amount to the motor based on the signal it receives. The propellers are mechanically coupled to the motors so that they rotate and produce thrust.The additional battery is mounted on the drone to provide power supply to the Co-Axial Propulsion units.

C. ESC
It stands for Electronic Speed Controller and it is used to vary the RPM of the motor. 40A rated ESC is used asper the motor and battery specifications and this can handle upto 60A.

D. BATTERY
A 3s Li-Po battery of 10000mAh 25C current capacity and 11.1V is used to supply power to the motors at the topside of the Frame and another 3s Li-Po battery of 8000mAh 30c current capacity and 11.1V is used to supply power to the motors at downside (co-Axial propulsion units).The current capacity of battery to supply power to the motors at topside is higher than the current capacity of the battery to power the motors at the down side, because the motors at downside will not be continuously operated and it will be operated in an Emergency situation, where the motors should respond quickly, so that the battery used has higher discharge rate of 30c .  The weight of the Flight controller , Receiver and other fastening components were taken into Approximation, since any kind of Flight controller and Receiver can be used in the Drone.The overall weight of the Drone is approximated as 2251.4 grams.
G. THRUST CALCULATION Thrust to Weight ratio of the drone can be varied by engaging and disengaging the Co-Axial propulsion units.The additional rotors at the down side of the frame cannot be used continuously ,because the lower propeller loses thrust working in prop wash of the upper propeller but this offers ability to provide significantly higher thrust for the platform in smaller volume.
Thrust to Weight ratio of the Drone when only the rotors at the top side is used,

VII. STABILITY BACK-UP
The presence of Co-Axial propulsion units helps with Stability Back-up. When there is a Failure of rotor during Flight, the quadcopter loses its Stability and then goes out of control.In this case where the Quadcopter mounted Coaxial propulsion units, the rotors can act as a Back-up to Produce the corresponding Rotational moment as produced by the rotor which was Failed.Hence the moments were restored and the Quadcopter will attain its Stability.

Figure 7 Quadcopter configuration labelled Rotors
The configuration of the Quadcopters with Co-Axial propulsion units appears as in Fig 7. ,if the rotor 2 labelled in the Fig 7. gets failed during Flight, then the corresponding clockwise moment will be produced by the rotor labelled 6 in the Fig 7 will start rotate and bring the Rotational moment of the Quadcopter to Equilibrium.

VIII. STRUCTURAL DESCRIPTION
The Main Frame of the Quadcopter is designed by determining the factors like Propeller dimensions, Clearance between two rotating propellers and the distance between Co-Axial propulsion units. Carbon Fiber can be used to Fabricate the Frame, because of its High strength to Weight Ratio.The Frame is designed in such a way that it can be assembled and disassembled easily. The CAD model of the Quadcopter shown in Fig8.is made using CATIA V5 software.