It is composed of. Short filaments of Synthetic man made mineral fibers with highly thermally stable Synthetic rubber (Styrene. Butadiene/ Acrylonitrile Butadiene ...
Proceedings of The nineteenth National Seminar on Aerospace Structures, Feb 23-25, 2017, VIT –Vellore.
Development of Mechanical Components for Autonomous Mini Helicopter Sanjay Kumar Viswasa, Muralidharan Shanmugama, Sandeep Kumar Prajapatia, C Venkatesanb a
Project Associate, Helicopter Lab, Department of Aerospace engineering, IIT Kanpur, UP- 208016, India b Professor, Department of Aerospace, IIT Kanpur, Kanpur UP-208016, India
Abstract
This paper presents the study performed on mechanical components such as main gear, landing gear, clutch liner, rotor head damper and fuselage so that these can be developed indigenously. Several tests like material testing, load testing and estimation of radial stiffness of rotor head damper are performed. Material test involves Differential Scanning Calorimetry (DSC), X-Ray power Diffraction (XRD), X-Ray Fluorescence Spectrometer (XRF), Energy Dispersive X-Ray (EDX), and Thermo Gravimetric Analyzer (TGA). Laser displacement sensor and National Instrument (NI) – PXI system are employed for vibration test of landing gear and estimation of radial stiffness of rotor head damper. Main rotor gear, tail transmission coupler, rotor head damper, clutch liner, and fuselage structural component have been manufactured indigenously. Finally, these components are assembled on mini helicopter and test flights are carried out to evaluate their performance.
Keywords: Mini helicopter; main gear; landing gear; clutch liner; rotor head damper; fuselage.
1. Introduction Nowadays, unmanned aerial vehicles (UAVs) technology has improved significantly. UAVs are increasingly used in many applications in which ground vehicles cannot access the desired locations due to the characteristics of the terrain and the presence of obstacles. [1] Also main advantage of unmanned aerial vehicle is the absence of the pilot on board the vehicle. This feature allows using them in dangerous and risky missions for the safety and the lives of pilots. The role of human being in the vehicle would be taken over by sensors, control algorithms and computation units to perform desired tasks [2]. Since rotary wing vehicles can take off and land in limited space and hover above targets [3], these vehicles have certain advantages over conventional fixed wing aircraft for surveillance and inspection tasks. UAVs have attracted a great deal of attention all over the world due to their use in military applications, environment exploration, structure inspection, mapping and remote surveillance [4]. When operating small unmanned aerial vehicles, robustness and fault-tolerance are important to ensure safe and trouble free flying. However the cheap materials and few redundancies used for UAVs are in contrast to this [5]. Department of Science and Technology, Govt. of India initiated a research activity at IIT Kanpur to set up a laboratory for design and development of autonomous mini helicopter. The major focus of this research activity is to develop system level architecture for a mini helicopter such as mechanical system, ground station unit, onboard systems, communication systems and control logic algorithm to provide the autonomous control [6]. The research related to mechanical systems focused on designing test rigs, gears, landing gear, engine performance test, rotor load measurements etc. The MTOW of procured mini helicopter is 6 kg. It is customized to MTOW of 10.80kg which can take payload of 2.80kg. Due to this customization, the critical parts which are likely to fail are identified and research activities are initiated. The critical parts such as main gear, clutch liner, landing gear, fuselage and rotor head damper are indigenously designed and manufactured.
Proceedings of The nineteenth National Seminar on Aerospace Structures, Feb 23-25, 2017, VIT –Vellore.
Nomenclature F K L1 L2 L3 MTOW MTTF X0 Xa Xb θ
1.
force applied radial stiffness of damper distance between centre of head and centre of damper distance between centre of rotor head and laser point distance between centre of rotor head and point of application of force Maximum Take-off weight Mean Time to Failure displacement at O displacement at A displacement at B rotation angle at O
Rotor head damper
The helicopter rotor system is the combination of rotor blades, rotor head, main shaft, linkages and swash plate. It generates aerodynamic lift force that supports the weight of the helicopter. Rotor head of helicopter connects the main shaft and blades. It consist of feathering shaft, hub, blade grip, c-clip, bearing, thrust bearing and rotor head damper. The rotor head damper is one of the important component of helicopter which damps the vibration produced at head due to rotation of blades.
Figure 1 Rotor head
1.1. Set-up The set-up for testing radial stiffness of damper consists of National Instrument’s PXI system, laser displacement sensor, weight holder, 24volts DC power supply and clamps. Laser displacement sensor reads the displacement as a function of loading. 1.2. Theory: Considering the moment of equilibrium at O in figure 2: 𝑀𝑜 = 𝐹𝐿3 − 𝐾𝑋𝐵 𝐿1 − 𝐾𝑋𝐴 𝐿1 = 0 𝐹𝐿3 = 𝐾𝐿1 (𝑋𝐵 + 𝑋𝐴 )
Proceedings of The nineteenth National Seminar on Aerospace Structures, Feb 23-25, 2017, VIT –Vellore.
𝐾=
𝐹𝐿3 𝐿1 (𝑋𝐵 + 𝑋𝐴 )
From diagram, 𝑋𝐴 = 𝑋0 + 𝐿1 𝜃 𝑋𝐵 = 𝑋0 − 𝐿1 𝜃 Then, 𝑋𝐴 + 𝑋𝐵 = 2𝑋0 𝑅𝑎𝑑𝑖𝑎𝑙 𝑆𝑡𝑖𝑓𝑓𝑛𝑒𝑠𝑠 𝑜𝑓 𝑅𝑜𝑡𝑜𝑟 ℎ𝑒𝑎𝑑 𝑑𝑎𝑚𝑝𝑒𝑟 (𝐾) =
𝐹𝐿3 2𝐿1 𝑋0
Figure 2 Free body diagram of damper
1.3. Result First, stiffness estimation experiment is carried on the procured damper to determine its average radial stiffness. With this radial stiffness value as a reference, a new damper is indigenously developed and a similar stiffness estimation experiment is performed to determine its average radial stiffness. Three different dampers are shown in figure 3 and their properties are shown in table 1. The indigenously developed damper is now mounted on the mini helicopter.
Figure 3 Three different dampers
Proceedings of The nineteenth National Seminar on Aerospace Structures, Feb 23-25, 2017, VIT –Vellore.
Table 1 Properties of different rotor head dampers Properties
Damper (indigenously developed)
Damper (cylinder type)
Damper (ring type)
Density, gm /cm3
1.28
0.86
1.39
Hardness, Shore ‘A’
67-68
70-73
80-83
Estimated average radial stiffness, N/m
493594
554560
831580
Outer Diameter, mm
13
13.5
12.75
Inner Diameter, mm
8
8
8
Height, mm
6
7
NA
Base Material
Chloroprene rubber
Thermoplastic elastomer
Nitrile rubber
2. Main gear The main gear of mini helicopter is used to transmit torque from engine to main rotor and tail rotor system via centrifugal clutch system. As the MTOW of mini helicopter is increased by 5 kg, the original main gear fails to take this load. Hence, the main gear is designed and manufactured to suit the requirements. The original main gear is composed of Delrin material. Three main gears of different material are manufactured and subjected to load test. In load test, PEEK main gear is found be better than nylon and delrin main gear. Finally, peek main gear is chosen to serve the purpose which overcame the high noise and weight disadvantage of aluminum main gear.
Table 2 Properties of different main gear subjected to load test Gear material Properties
Main gear (PEEK)
Weight, gm. MTTF Outer Diameter (spur ), mm No. of Teeth ( spur ) No. of Teeth ( bevel ) Outer Diameter (bevel ), mm Pitch cone angle (degree) Tensile Strength, MPa
78.2 4.5 hr. 90 90 70 70 76.24 90-100
Main gear (6/6 nylon 30% carbon) 70.79 Less life 90 90 70 70 76.24 70
Figure 4 Peek Main gear
Main gear (Delrin) 48.3 Few runs of 10min duration 90 90 70 70 76.24 48-55
Proceedings of The nineteenth National Seminar on Aerospace Structures, Feb 23-25, 2017, VIT –Vellore.
3. Tail transmission Coupler Tail transmission coupler is used to transmit torque from bevel pinion to tail drive shaft and provides for angular misalignment condition. It can accommodate varying degrees of misalignment and some parallel misalignment. This coupler protects the driving gear and driven shaft members against harmful effects produced due to misalignment, sudden shock loads and vibrations.
Figure 5 Tail transmission coupler
4. Fuselage structure
The fuselage of a mini helicopter can be made of either metal, wood, or composite materials, or some combination of the two. Typically, a composite component consists of many layers of fibre impregnated resins, bonded to form a smooth panel. Tubular substructures like tail boom are usually made of aluminium alloy. The fuselage houses the engine, the transmission, avionics, flight controls, and battery. Mechanical tests like tensile test, density test, hardness test, Izode impact test are performed on fuselage structure and the properties are given in table 3: Table 3 Properties of our mini helicopter fuselage composite structural plate Name of properties
Values
Plate thickness
2.35±0.5 mm
Density
1.88
Hardness
89 - 91 Shore ‘D’
Flexural strength
466 MPa
Flexural modulus
23 GPa
Tensile strength
140 MPa
Izode impact strength
76 Kg.cm/cm
Several tests like XRF, XRD and EDX are done for elemental composition analysis; DSC and TGA for thermal properties of material. From these different tests, it is concluded that the fuselage material’s properties are similar to the commercial available Cotton Phenolic sheet HGW 2082. Table 4 Chemical composition of fuselage structure Chemical formula
Mass %
C
62.09
O
33.44
Al
3.47
Si
0.78
Ca
0.22
Total
100
Proceedings of The nineteenth National Seminar on Aerospace Structures, Feb 23-25, 2017, VIT –Vellore.
5. Clutch The function of clutch is to engage and disengage the power transmission from driving shaft to driven shaft. It is classified into three main categories: Methods of transmitting torque, engaging force and method of control. Under Methods of transmitting torque, the clutch is further divided into Positive clutch, friction clutch and hydraulic clutch. Based on method of transmitting torque, there are spring type clutch, centrifugal clutch, semi-centrifugal clutch and electro-magnetic clutch. Finally, upon method of control, manual clutch and automatic clutch are available. Clutches used in mini-helicopter are centrifugal clutch and freewheel clutch. These clutches belong to the category of friction type in transmitting torque, uses centrifugal force to engage the driving and driven shaft automatically.
Figure 6 Clutches at Mini-Helicopter and its parts
5.1. Centrifugal Clutch The centrifugal clutch engages or disengages the engine to transmission system. It consists of clutch base, clutch liner, bell clutch, pinion gear, grub screws and bearings. The MTTF of original clutch liner is more than 7 hours. So, in order to enhance the operational life time of clutch liner, a detail study is initiated. As a first step, EDX test is done to find its material composition. With this material composition as a reference, suitable clutch liner materials are selected. They are garlock blue-gard 3000 and COMPO HC AF 11. Garlock blue-gard 3000 is used for making gaskets. The material composition of garlock blue-gard 3000 is equivalent to Original clutch liner’s material composition. Most of RC helicopters use this as a clutch liner. The MTTF is estimated to be 4.53 hours by flight tests. Next, the automobile clutch liner material COMPO HC AF 11 also tested for this purpose. It is composed of Short filaments of Synthetic man made mineral fibers with highly thermally stable Synthetic rubber (Styrene Butadiene/ Acrylonitrile Butadiene rubber) as a binder and fused in a matrix. It has an operational lifetime of 29 minutes only.
Proceedings of The nineteenth National Seminar on Aerospace Structures, Feb 23-25, 2017, VIT –Vellore.
Original 40.7
Blue-gard
40.27 37.07 27.45 20.29
14.12 11.3 5.24 0.36 0 C
O
Mg
2.13 1.07
0 Al
Si
Ca
0 Br
Figure 7 Mass percentage of clutch liner material composition
Conclusion The research activities to develop the mechanical components for mini helicopter led to detailed understanding of every individual component’s properties and function. Based on the knowledges obtained via this work, the components are designed and tailored to meet the requirements. Future work will be focused on further performance enhancement of these components and to apply the knowledge to develop components for higher weight class of mini helicopters. Acknowledgements We would like to acknowledge the financial support of Department of Science and Technology, Govt. of India for carrying out this research work and DMSRDE Kanpur for providing material test facilities and manufacturing of components with new materials. References [1] G. Heredia, A. Ollero, M. Bejar and R. Mahtani, Sensor and actuator fault detection in small autonomous helicopters, Mechatronics – Elsevier, 2008 [2] G. Buskey, J. Roberts, P. Corke, M. Dunbabin, G,. Wyeth, The CSIRO Autonomous Helicopter Project, in Experimental robotics VIII, edited by B. Siciliano and P.Dario, vol. 5 of STAR, Springer-Verlag, New York, 2003 [3] Johnson, Andrew., Montgomery, James., and Matthies, Larry., Vision guided landing of an autonomous helicopter in hazardous terrain, Proceedings of the IEEE International Conference on Robotics and Automation, Spain, 2005. [4] Acevedo, J., Arrue, B., Maza, I., and Ollero, A., Cooperative large area surveillance with a team of aerial mobile robots for long endurance missions, Journal of Intelligent and Robotic Systems, vol.70, no. 1-4, 2013, pp. 329-345 [5] Søren Hansen, Mogens Blanke, In-flight Fault Diagnosis for Autonomous Aircraft via Low-rate Telemetry Channel, 8th IFAC Symposium on Fault Detection, 2012 [6] Haritha, Pathuri., Venkatesan, C., Development and Evaluation of System Level Architecture for Autonomous mini Helicopter Journal of Unmanned System Technology, vol.3, No 1, 2015, pp 24-32
