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Karpat Y.,Bilkent University | Polat N.,Turkish Aerospace Industries TAI
CIRP Annals - Manufacturing Technology | Year: 2013

Carbon fiber reinforced polymers (CFRP) have emerged as the material of choice to satisfy increasing demand for lighter aircrafts. Machinability characteristics of CFRPs are quite different than those of metals; therefore, special tool designs have been developed for CFRP machining. The double helix end mill design compresses the upper and lower sides of the laminate using opposite helix angles that eliminate delamination. A mechanistic force model for double helix tools is developed based on milling force data obtained on flat end mills. The proposed model can be used to improve double helix tool designs and to optimize milling process parameters. © 2013 CIRP.

Karpat Y.,Bilkent University | Bahtiyar O.,Turkish Aerospace Industries TAI | Deger B.,Turkish Aerospace Industries TAI | Kaftanoglu B.,Atilim University
CIRP Annals - Manufacturing Technology | Year: 2014

Carbon fiber reinforced plastics (CFRPs) possess desirable material properties that satisfy the aerospace industry's high strength to weight ratio objective. Therefore, CFRPs are commonly used in structural parts, either alone or together with aluminum and titanium alloys. Drilling of CFRPs has been studied extensively in the literature in recent years, with special emphasis on process parameters and delamination. This study identifies mechanical properties of uni-directional CFRPs through drilling tests. Drilling of uni-directional CFRP plates with and without pilot holes has been performed, and cutting and edge force coefficients are identified. A polycrystalline diamond (PCD) drill was used in tests since this type of drill is commonly used in practice. Finally, validation tests on multi directional CFRP laminates have been performed and good results have been obtained. © 2014 CIRP.

Karpat Y.,Bilkent University | Bahtiyar O.,Turkish Aerospace Industries TAI | Deer B.,Turkish Aerospace Industries TAI
International Journal of Machine Tools and Manufacture | Year: 2012

Carbon fiber reinforced polymer (CFRP) usage in the aerospace industry has been steadily increasing due to its superior material properties such as high strength, low weight, high resistance to corrosion, and a low thermal expansion coefficient. In addition, CFRP parts are produced near-net-shape, a process that eliminates rough machining operations. However, machining operations such as drilling, side milling, and slotting are still necessary to give the CFRP parts their final shape. A majority of the studies on machining of CFRP laminates are on drilling. The number of studies on milling of CFRPs is quite limited. In this study, a mechanistic cutting force model for milling CFRPs is proposed based on experimentally collected cutting force data during slot milling of unidirectional CFRP laminates using two different polycrystalline diamond cutters. Cutting force coefficients in radial and tangential directions are calculated as a function of fiber cutting angle. The relationship is represented with simple sine functions. The mechanistic model is shown to be capable of predicting cutting forces during milling of multidirectional CFRP laminates. The experimental milling force measurements and predicted milling forces agree well with each other. Surface milling experiments were also conducted to investigate the relationship between milling forces and surface quality. Some suggestions on surface milling of CFRP laminates are given based on these observations. © 2012 Elsevier B.V. All rights reserved.

Karpat Y.,Bilkent University | Bahtiyar O.,Turkish Aerospace Industries TAI
Procedia CIRP | Year: 2016

Carbon fiber reinforced polymers (CFRPs) are widely used in the aerospace industry due to their light weight, high strength, and low thermal conductivity. Drilling is a critical process that affects the quality of CFRP parts. This work studies the influence of process parameters on delamination and tool wear. Polycrystalline diamond helical drills are used in the experiments. It has been shown that drilling energy calculations can be used to set appropriate feed and speed parameters and for increasing drilling performance of CFRPs. The results also indicate the importance of thermal modeling of CFRP laminate for better understanding of the drilling process. © 2016 The Authors.

Cakmakcioglu S.C.,TOBB University of Economics and Technology | Sert I.O.,TOBB University of Economics and Technology | Tugluk O.,Turkish Aerospace Industries TAI | Sezer-Uzol N.,TOBB University of Economics and Technology
Journal of Physics: Conference Series | Year: 2014

In this study CFD investigation of flow over the NREL S826 airfoil is performed. NREL S826 airfoil was designed for HAWTs of 10-15 meter diameters. However, it is used in the NTNU wind turbine rotor model and low Reynolds number flow characteristics become important in the validations with the test cases of this rotor model. The airfoil CFD simulations are carried out in 2-D and 3-D computational domains. The k-rn SST turbulence model with Langtry-Menter (γ-Reθ) transition prediction model for turbulence closure is used in the calculations. The Delayed DES is also performed in the stall region for comparisons. The results are compared with the available METUWIND experimental data, and are shown to be in fair agreement. It is observed that 3-D CFD analysis provides increased accuracy at increased computational cost. © Published under licence by IOP Publishing Ltd.

Altay O.,Turkish Aerospace Industries TAI | Kalenderli O.,Technical University of Istanbul
IET Science, Measurement and Technology | Year: 2015

Wavelet-based de-noising is used to separate partial discharge (PD) signals from the noises resulting from measurement circuits or the surrounding environment. PD de-noising by using the wavelet shrinkage method is capable of separating the noise component to some extent, but the selection of the wavelet base has a remarkable effect on the denoising results. The wavelet base is directly related to the distortion of the PD waveform and quality of the de-noising process. Although there are applications on PD noise separation in the literature, the selection of the wavelet base, which affects the evaluation of the PD characteristics, is still challenging. Instead of using correlation-based wavelet base selection for de-noising PD data, in this study a novel wavelet base selection method based on the most informative sub-band energy and entropy for separating noise from PD pulses is introduced and successfully applied to raw data obtained from the PD measurement set-up. The advantage of the proposed method is that the wavelet base selection solution is automatic and independent of the original noise-free pulse waveform. This study shows that the proposed method is useful for the extraction of noisy PD pulses by describing the basic discharge parameters such as discharge amplitude and the duration and time of occurrence more clearly. © The Institution of Engineering and Technology 2015.

Cayan F.N.,Turkish Aerospace Industries TAI | Sezer H.,West Virginia University | Celik I.,West Virginia University
Fuel Cells | Year: 2016

An engineering analysis based on calibrated numerical predictions was performed to estimate the minimum allowable impurity concentrations in coal syngas intended to be used in Solid Oxide Fuel Cells (SOFCs) operating for over 10,000 h. Arsine and phosphine, impurities that are known to have the most deleterious effects on the cell performance due to their affinity to have strong relations with the anode catalyst by formation of secondary phases, were investigated. Time to failure was taken as the operation time when 60% performance loss is incurred, estimated by the previously developed one-dimensional degradation model. Limiting concentrations were determined for arsine and phosphine fuel contaminants for electrolyte and anode supported SOFCs. Predicted lifetimes for single cells can provide a basis for estimation of SOFC stack lifetimes operating on coal syngas. Extrapolation of results from the numerical simulations based on accelerated laboratory tests at relatively higher concentrations can provide guidance into predicting the cell failure at low impurity concentrations. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Manavoglu B.,Turkish Aerospace Industries TAI | Yazgan E.,Hacettepe University
Proceedings International Radar Symposium | Year: 2014

Nowadays, Multistatic passive radars using commercial signals (e.g., FM broadcast, digital TV) as illuminators of opportunity have increasing significance in air defence systems because of their promising technical performance and low projected costs. They have significant advantages against stealth targets and provide resistance to jamming. On the other hand, various complex application aspects present challenging issues. Hence a number of research and development studies are currently active. In this paper, obtained target motions using real turboprop and jet aircrafts flight data are degraded into three dimensions in Cartesian coordinate then using designed multistatic passive radar simulation model tracking of target motions process is done. In target tracking, Extended Kalman Filter (EKF) and Unscented Kalman Filter (UKF) are used for estimation. For the estimation of target state vector Doppler frequency, azimuth, elevation and range measurement data are used. Measurement noise is added to measurements and Monte Carlo simulation is used to asses target tracking performance. © 2014 Warsaw University of Technology (WUT).

Dikmen E.,Turkish Aerospace Industries TAI | Isci H.,Turkish Aerospace Industries TAI
Proceedings of ISMA 2014 - International Conference on Noise and Vibration Engineering and USD 2014 - International Conference on Uncertainty in Structural Dynamics | Year: 2014

In this study, the effect of an elastomeric damper on the coupled dynamics of the tail rotor shaft and the tail cone of an unmanned prototype helicopter has been examined by generating equivalent stiffness for the O-ring used as a simple elastomeric damper. The O-rings used in three locations on the tail rotor shaft are made from nitrile rubber (NBR) with nonlinear hyperelastic material behavior. Therefore the deformation history is significant and should be included in modeling. Nonlinear quasistatic analysis sequence of the O-ring has been performed to obtain equivalent spring stiffness. The quasistatic analysis sequence includes mounting of the O-ring on the bearing mount; the bearing mount to tail cone and the movement of the shaft during operating. Finally the equivalent stiffness for the O-ring has been used in the dynamic analysis of the tail rotor shaft-tail cone assembly.

Kayayurt B.,Turkish Aerospace Industries TAI | Yayla I.,Turkish Aerospace Industries TAI
AIAA/IEEE Digital Avionics Systems Conference - Proceedings | Year: 2013

STANAG 4586 [1] is the ground control station architecture and messaging standard for the interoperability of UAV (Unmanned Aerial Vehicle) systems between NATO nations. The standard aims to provide a common architecture for UAV Ground Control Systems and provides a set of standard messages for communication. It has the concepts of Core Unmanned Control System (CUCS), Vehicle Specific Module (VSM) and Data Link Interface (DLI). © 2013 IEEE.

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