34th Danubia-Adria Symposium on Advances in Experimental Mechanics University of Trieste, Italy, 2017

LASER WELDED INCONEL ROTOR BLADES FOR TIP-JET

Nebojša Kosanović1, Nenad Kolarević1, Marko Miloš1

1 University of Belgrade, Faculty of Mechanical Engineering, Kraljice Marije 16, 11120 Belgrade, Serbia. e-mail: [email protected]

1. Introduction On a tip-jet helicopter [1], hot cycle system transmits power pneumatically by lightweight ducting and a nozzle that directs high-energy gas Fig. 1. Blade cross-section from a turbine to the rotor blade tips to drive the rotor for helicopter flight. Aside from the difference in the cross-section, the blade also must have a different tip and root This requires a unique solution for the rotor (Fig. 2.). At the tip, there is a nozzle assembly (fig. blade of the tip-jet helicopter that will permit 3), and at the root there is a blade mount with distribution and acceleration of hot gasses from jet ducting and inlet for hot gasses. engine through its construction. Blades are developed to withstand additional internal overpressure loads at very high temperatures, up to 700°C, with strict aerodynamic demands. The idea was to use Inconel sheets, laser welded as basic material, and outside flow of cold air for cooling and reducing pressure loads. Numerical simulations are used to calculate stress-strain state of rotor blade structure. After that, blades are produced and experimentally verified in helicopter hovering tests. Fig. 2. Blade 3D model 2. Blade design 2.1 Nozzle design Design of rotor blades for tip-jet helicopter is unlike any other conventional helicopter blade The nozzle at the end of the blade could be design. A conventional blade would, in the most considered as an engine nozzle since its area cases, use composite materials and some sort of directly influences the performance of the engine. cross-section optimization [2]. In our case, the The area of nozzle exit is too small for easy engine blade cross-section is very different because it start, when a bigger nozzle is needed, so we used a needs to ensure efficient flow of hot gasses inside variable spring loaded nozzle mechanism actuated the blade. by centrifugal force. When the engine starts, helicopter’s rotor is stationary and the nozzle area To accommodate this difference in design, is kept maximal. As the power increases, the rotor instead of using one spar at 25% of airfoil chord, starts to move and rpm rise and centrifugal force we used two beams located as far as possible from increases, which reduces the nozzle area and each other (Fig. 1.). This allowed for a large hole increases power. Equilibrium is obtained at in the middle of the blade needed for ducting of hot operational rotor speed when the nozzle area is gasses. Additional stiffeners at the thickest part of minimal and the thrust is maximal. This design the blade cross-section provide required structural allows for easier and more reliable engine starts strength. Also, during rotation a centrifugal force and smoother operation of propulsion system. forces cold air in the small area between the stiffener and the blade skin and reduces thermal load of the blade.

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34th Danubia-Adria Symposium on Advances in Experimental Mechanics University of Trieste, Italy, 2017

Fig. 3. Blade tip with nozzle Fig. 6. Tower test results 3. Manufacturing processes The blade is completely made from thin Inconel 5. Remarks sheets that were laser welded. The welding was All Blades proved good for test purposes, done using special tools to hold the sheet metal in because of their toughness and reliability, and place and to ensure adequate quality and aerodynamic characteristics are better than we repeatability, especially because there need to be expected. However, the production process is very two same blades for one rotor. Fig. 4. shows the long and complicated with a possibility of middle section of the blade at the initial stage of repeatability difficulties, which could be solved manufacturing process. only by using different production methods and using Inconel sheet duct only as an insert.

Acknowledgements Design and tests were performed at EDePro d.o.o. in Belgrade as a part of UAV helicopter development program.

References Fig. 4. Laser welding of blade in special tool [1] J. Watkinson, the Art of the Helicopter, Elsevier Butterworth-Heinemann, 2004. 4. Design validation [2] Jae Hoon Lim, Sang Joon Shin, Young Jung After series of tower tests (Fig. 5.) [3], all data Kee, Optimization of Rotor Structural Design relevant for analysis of rotor lift characteristic was in Compound with Lift Offset, extracted and compared with theoretical Journal Of The American Helicopter Society calculations. Some parameters were recalculated 61, 012005 (2016). by using experimental data [4]. [3] N. Kosanović, M. Miloš, B. Jojić, Testing of tip-jet lift force, Proceedings of the 33th Danubia-Adria Symposium on advances in experimental mechanics, Portorož, Slovenia, 2016 [4] N. Kolarević, N. Davidović, P. Miloš, B. Jojić, M. Miloš, Experimental determination of light helicopter rotor lift characteristics with tip-jet propulsion system, Proceedings of the 30th Danubia-Adria Symposioum on Fig. 5. Tower test advances in experimental mechanics, The tests show that the drag on 0 angle of Primošten, Croatia, 2013, pp. 268-269 attack is smaller than theoretical value of [5] I. Abbott, A. Doenhoff, L. Stivens, Summarry ⁰ NACA0018 airfoil [5] which means that our airfoil of Airfoil Data, NACA REPORT No. 824, is thinner than the theoretical (Fig. 6.). 1945.

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