An unmanned aerial vehicle is one which can fly without an on-board human operator. Similarly an unmanned helicopter or a rotor unmanned aerial vehicle(R-UAV) is one which provides lift through aerodynamic forces like any other helicopter which is operated by an onboard pilot except for the fact that it is controlled by an operator with a ground station or they fly autonomously through pre-programmed flight paths. The R-UAV has an advantage of vertical take-off and landing, and hovering. These vehicles perform a wide range of functions such as remote sensing, rescue operations, military operations and transport.
In the Netherlands, R-UAVs have been used by the Government and public institutions such as the Police, Fire Department and the Military/Defence. These organisations have indicated a need for a more compact and robust system that can be used in case of calamities. Therefore, the use of R-UAVs reduces the risk of humans to be present at such dangerous places. To establish a better flight performance due to the violent gust conditions in The Netherlands, this project aims to design a robust rotor unmanned aerial vehicle which can withstand these extreme conditions and perform their respective tasks.
In the current situation different types of R-UAVs are being used such as single rotor UAV, coaxial rotor UAV, quad-rotors, ducted fan and tandem rotors. Hence one of the main objectives of this project is to determine the type of R-UAV which is best suitable for the turbulent wind conditions. To accomplish this, an exhaustive literature survey followed by a statistical analysis on the current trends in R-UAVs was carried out. Thereafter, keeping the gust control as the main factor among all these R-UAVs, a weightening was performed to determine the appropriate R-UAV. From these results it was found that the coaxial rotor UAV was best suited for the desired project. Further, a detail design will be carried out with the coaxial rotor UAV. While the scope of the Master Orientation Project is limited to the detailed design of the rotor alone(including the shape of the airfoil), carrying out a detailed design and analysis of the entire R-UAV can only be done as a master thesis project. The design of the rotor will include numerical calculations based on the preliminary design from the Principles of Helicopter Aerodynamics by J. Gordon Leishman and other related literature papers.
A rotor unmanned aerial vehicle (R-UAV) is one which provides lift through aerodynamic forces like any other ordinary pilot-operated helicopter except that it is controlled by an operator on ground with a ground station or they fly autonomously through pre-programmed flight paths. These R-UAVs are equipped with multifarious sets of sensors and other integrated circuitry required for accomplishing its goal which are already pre-programmed for the duration and plan of the respective mission . Currently, different types of R-UAVs have been developed for emergency and defence across the globe such as the single rotor unmanned aerial vehicle (UAV), coaxial rotor UAV, quad-rotors, ducted fan and tandem rotors. The field has shown expeditious growth during the last decade . The early small UAV systems that were developed were based on fixed wing aircrafts with radio controlled device. These were taken from the local electronic stores and augmented with features such as the Global Positioning System and the video download capability . Prior et al. have stated that in recent times the shift has been towards smaller, compact, lighter and more autonomous systems which have the capability to hover and perch with Vertical Take-Off and Landing (VTOL). This feature has now become essential than a desirable one .
Generally, most of the R-UAVs use either reciprocating internal combustion engines or the fully electric engines for their functioning. The main advantage of such R-UAVs is that they can be used for long range operations, more endurance and for high altitude surveillance. The internal combustion engines are restricted to steady state conditions and are operated near the constant torque output, whereas the electric engines reduce the acoustic, smoke and thermal signatures as compared to gasoline-powered UAVs . While the UAVs operating with electric engines are free from producing exhausts which might interact with the chemical detecting sensors. The electric system provides redundancy to gasoline engines and reduces the risk of losing expensive payloads while the UAV is operating in hazardous conditions .
The aim of the proposed project is to design R-UAV which in my case is a coaxial rotor, should have the following characteristics: light ranging from 10-20 kg and which uses an electric engine where the emissions are low, be operational with less time of about 5 minutes from its arrival at the place of calamity, capable of flying at 500 meters altitude and most of all it should be capable of operating 95% of the time during the Dutch weather in particular the violent gust conditions.
The helicopters with coaxial rotors are a pair of helicopter rotors which are mounted one above the other on concentric shafts with the same axis of rotation but these rotors rotate in opposite directions also known as contra-rotating rotors. This type of contra-rotating configuration can be seen in a series of Russian Kamkov helicopters (Ka-50, Ka-32, Ka 26), Westlandâ€™s WG-25 Mote-Wisp-Wideye and Sharpeye prototypes, the Gyrodyne DASH developed by Peter James Papadakos . An exhaustive historical overview of the advancement of coaxial rotors has been given by Vanderover and Visser . Henry bright in 1859 was first awarded by the British Patent office for his design of the coaxial rotors, after which the coaxial R-UAV evolved 
The foremost advantage of using a coaxial rotor is the absence of a tail rotor which is used for anti-torque control, as the failure of the tail rotor causes many accidents . Hence this problem is eliminated in case of a coaxial rotor configuration. The disadvantages of a coaxial rotor include mechanical complexity, increase in parasite drag which might deter the overall flight performance. However, the coaxial rotor configuration has the capability of producing greater lift in the high speed flight where the lift is shifted to the dual advancing sides and away from the less efficient retreating sides . A tail rotor configured helicopter is estimated to consume about 5-10% of the total power which is supplied by the engines and about 20% during extreme cases [11, 12].
Some of the major advantages and disadvantages of a coaxial rotor configuration are listed below [13, 14, 15].
No drive train losses due to tail rotor absence.
No possibility of tail rotor failure which is a major cause of helicopter accidents.
Shorter fuselage and hence the overall size of the R-UAV is reduced..
Directional stability through cancellation of main rotor gear torque moment (Yaw torque reaction).
Compact size through use of concentric shafts.
Increased pressure differential over rotor system; increased thrust, higher efficiency for increase in thrust, which translates into a reduction in rotor diameter for a given thrust.
Complex linkages are required to operate pitching control. This disadvantage is predominantly id linked to full-scale aircraft.
Inter-rotor wash interference. Reduced efficiency of the lower rotor due to the upper rotor swirling the air in the opposite direction of the lower rotor which requires the lower rotor to run at higher speed to produce the same lift as the upper rotor.
Importance of flow interaction, requirement for rotor spacing. To ensure sufficiently clean flow for the lower disc, the spacing must be wide enough to allow as little interaction of the swirl of the upper rotor to impinge on the retreating component of the lower disc.
From the above observations it is very clear that though there are many favourable features for the coaxial rotor configuration, design and mechanical complexity are the major drawbacks. The in-flight advantages of the coaxial rotor configuration over the single rotor configuration, this technology are being developed by many research organisations and companies .
The development of small R-UAVs with coaxial rotors is focused on smaller, lighter and with more autonomous systems enabling it to hover and perch in areas where the humans and other larger UAVs find it difficult to reach. Hence this has become an essential feature rather than a desirable feature. To overcome the design complexities, simplified structural elements such as the fixed pitch rotors and low cost components, such as brushless outrunner motors, electronic speed controllers and standard sized rotors are used.
Thus with above mentioned advantages it is clear that a coaxial rotor configuration would be best suited for the proposed project.
Results and Outcome
All the design parameters will be found out with the aforementioned methodology. Now, this coaxial R-UAV would satisfy the given design requirements based on only statistical and numerical analysis. The results obtained from this have to be further analysed for dynamic stability when being operated during the violent gust conditions. Hence a gust model or a response to gusts by this coaxial R-UAV has to be evaluated.
Discussion and Conclusion
The aim of the proposed project is to design an R-UAV which would be fully operative during the extreme condition in The Netherlands, especially in violent gust conditions. The coaxial R-UAV with the absence of the tail rotor is beneficial because the stabilizing yaw moment is not required when the tail is not functioning, thus avoiding accidents. The coaxial R-UAV is favoured with greater lift, higher output power, compact size, Directional stability through cancellation of main rotor gear torque moment, and no possibility of tail rotor failure. Though the coaxial R-UAV has design and mechanical complexities, it can be overcome by the aforesaid methods.