Browsing by Subject "Aircraft"

Now showing 1 - 12 of 12

Open Access
Aviation risk perception: a comparison between experts and novices
(Wiley-Blackwell Publishing, 2004) Thomson, M. E.; Önkal D.; Avcioǧlu, A.; Goodwin, P.
This article describes an exploratory investigation of the risk perceptions of experts and novices in relation to helicopter operations, under conditions where the participants are matched on various characteristics previously found to affect perceptions, such as demographic, gender, and background factors. The study reports considerable evidence of perceptual differences between the two participant groups (i.e., expert pilots and candidate pilots). We find that the experts' perceptions of relative risks are more veridical, in terms of their higher correlation with the true relative frequencies. A significant positive correlation between the flight hours and the contextual risk-taking tendency is also shown, leading the experienced pilots' choices toward risky alternatives in scenarios - a potential result of their overconfidence based on superior task performance. Possible explanations are offered for the findings and potential avenues for future research are identified.
Open Access
Efficient solution of the combined-field integral equation with the parallel multilevel fast multipole algorithm
(IEEE, 2007-08) Gürel, Levent; Ergül, Özgür
We present fast and accurate solutions of large-scale scattering problems formulated with the combined-field integral equation. Using the multilevel fast multipole algorithm (MLFMA) parallelized on a cluster of computers, we easily solve scattering problems that are discretized with tens of millions of unknowns. For the efficient parallelization of MLFMA, we propose a hierarchical partitioning scheme based on distributing the multilevel tree among the processors with an improved load-balancing. The accuracy of the solutions is demonstrated on scattering problems involving spheres of various radii from 80λ to 110λ. In addition to canonical problems, we also present the solution of real-life problems involving complicated targets with large dimensions. © 2007 IEEE.
Open Access
Fast and accurate solutions of scattering problems involving dielectric objects with moderate and low contrasts
(IEEE, 2007-08) Ergül, Özgür; Gürel, Levent
We consider the solution of electromagnetic scattering problems involving relatively large dielectric objects with moderate and low contrasts. Three-dimensional objects are discretized with Rao-Wilton-Glisson functions and the scattering problems are formulated with surface integral equations. The resulting dense matrix equations are solved iteratively by employing the multilevel fast multipole algorithm. We compare the accuracy and efficiency of the results obtained by employing various integral equations for the formulation of the problem. If the problem size is large, we show that a combined formulation, namely, electric-magnetic current combined-field integral equation, provides faster iterative convergence compared to other formulations, when it is accelerated with an efficient block preconditioner. For low-contrast problems, we introduce various stabilization procedures in order to avoid the numerical breakdown encountered in the conventional surface formulations. © 2007 IEEE.
Open Access
Flight network-based approach for integrated airline recovery with cruise speed control
(Institute for Operations Research and the Management Sciences (I N F O R M S), 2017) Arıkan, U.; Gürel, S.; Aktürk, M. S.
Airline schedules are generally tight and fragile to disruptions. Disruptions can have severe effects on existing aircraft routings, crew pairings, and passenger itineraries that lead to high delay and recovery costs. A recovery approach should integrate the recovery decisions for all entities (aircraft, crew, passengers) in the system as recovery decisions about an entity directly affect the others' schedules. Because of the size of airline flight networks and the requirement for quick recovery decisions, the integrated airline recovery problem is highly complex. In the past decade, an increasing effort has been made to integrate passenger and crew related recovery decisions with aircraft recovery decisions both in practice and in the literature. In this paper, we develop a new flight network based representation for the integrated airline recovery problem. Our approach is based on the flowof each aircraft, crewmember, and passenger through the flight network of the airline. The proposed network structure allows common recovery decisions such as departure delays, aircraft/crew rerouting, passenger reaccommodation, ticket cancellations, and flight cancellations. Furthermore, we can implement aircraft cruise speed (flight time) decisions on the flight network. For the integrated airline recovery problem defined over this network, we propose a conic quadratic mixed integer programming formulation that can be solved in reasonable CPU times for practical size instances. Moreover, we place a special emphasis on passenger recovery. In addition to aggregation and approximation methods, our model allows explicit modeling of passengers and evaluating a more realistic measure of passenger delay costs. Finally, we propose methods based on the proposed network representation to control the problem size and to deal with large airline networks. © 2017 INFORMS.
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Göklerdeki istikbal: Sivil Havacılık Genel Müdürlüğü
(Bilkent University, 2020) Sülün, Ayşe Aybüke; Bümen, Ebubekir; Çelik, Fatma Selen; Atlı, Gülnur; Yılmaz, Nida Nur
1912 yılında başlayan ilk havacılık çalışmaları, Mustafa Kemal Atatürk'ün "İstikbal Göklerdedir!" sözünü temel alarak 1925 yılında ilk olarak Türk Tayyare Cemiyeti adı altında başlamıştır ve daha sonra Türk Hava Kurumu olarak devam eden kurumsallaşmış yapısını kazanmıştır. 1933 yılı itibari ile önceleri "Türk Hava Postaları" filosu ile ilk Sivil Hava Taşımacılığı gerçekleştirilirken, daha sonraki yıllarda Dünyadaki değişim hızına ayak uydurabilme, Türk Sivil Havacılık faaliyetlerinin uluslararası kimliği, denetlenmesi gibi amaçlar doğrultusunda 1954 yılında Ulaştırma Bakanlığına bağlı olarak Sivil Havacılık Genel Müdürlüğü kurulmuştur. Bu süreçte, çeşitli Sivil Havacılık kazaları yaşanmış ve bu kazaların ardından hava yollarının güvenli hâle getirilmesi için bir dizi önlem alınmıştır. Faaliyetlerini daha hızlı gerçekleştirebilmesi adına 2005 yılı itibariyle Ulaştırma Bakanlığı bünyesinden finansal anlamda özerkliğini kazanmıştır ve Sivil Havacılık Genel Müdürlüğü teşkilatlanmasında değişikliğe gidilmiştir. Sivil Havacılık Genel Müdürlüğü şu an hâlâ çalışmalarını sürdürmektedir.
Open Access
An integrated approach for airline scheduling, aircraft fleeting and routing with cruise speed control
(Elsevier, 2016) Gürkan, H.; Gürel, S.; Aktürk, M. S.
To place an emphasis on profound relations among airline schedule planning problems and to mitigate the effect of unexpected delays, we integrate schedule design, fleet assignment and aircraft routing problems within a daily planning horizon while passengers' connection service levels are ensured via chance constraints. We propose a nonlinear mixed integer programming model due to the nonlinear fuel consumption and CO2 emission cost terms in the objective function, which is handled by second order conic reformulation. The key contribution of this study is to take into account the cruise time control for the first time in an integrated model of these three stages of airline operations. Changing cruise times of flights in an integrated model enables to construct a schedule to increase utilization of fuel efficient aircraft and even to decrease total number of aircraft needed while satisfying the same service level and maintenance requirements for aircraft fleeting and routing. There is a critical tradeoff between the number of aircraft needed to fulfill the required flights and overall operational expenses. We also propose two heuristic methods to solve larger size problems. Finally, computational results using real data obtained from a major U.S. carrier are presented to demonstrate potential profitability in applying the proposed solution methods.
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İstanbul - İskenderiye uçuşu ve Mehmet Fethi Bey
(Bilkent University, 2018) Okşaş, Muhammed Musab; Yılmazer, Aybüke; Kıraç, Hasan Furkan; Canatar, Berker; Çakır, Ahmet Batuhan
Bu projede 1914 yılında İstanbul-İskenderiye (Mısır) arasında düzenlenen hava seyahati, hava seyahatinin öne çıkan ismi Yüzbaşı Mehmet Fethi’nin uçuşu üzerinden işlenecektir. Bu uçuş günümüzde dahi birçok devletin halen çıkar çatışması yaşadığı Ortadoğu topraklarında, 20. yüzyılın başlarında Osmanlı Devleti’ni parçalamak için iç karışıklıkların çıkarılmaya çalışıldığı bir dönemde düzenlenen ilk kıtalararası hava seyahatidir. Hem Balkan Savaşlarının acısını unutturmak hem de Ortadoğu halkına Osmanlı Devleti’nin gücünün halen yerinde olduğunu göstermek adına 1914 yılının başında düzenlenen bu seyahat dönemin iç ve dış politikasına da ışık tutmaktadır.
Open Access
Memory-efficient multilevel physical optics algorithm for fast computation of scattering from three-dimensional complex targets
(IEEE, 2007) Manyas, Alp; Gürel, Levent
Multilevel physical optics (MLPO) algorithm provides a speed-up for computing the physical-optics integral over complex bodies for a range of aspect angles and frequencies. On the other hand, when computation of the RCS pattern as a function of θ, φ, and frequency is desired, the O N3 memory complexity of the algorithm may prevent the solution of electrically large problems. In this paper, we propose an improved version of the MLPO algorithm, for which the memory complexity is reduced to O N2 log N . The algorithm is based on the aggregation of only some portion of the scattering patterns at each aggregation step. This way, memory growth in each step is prevented, and a significant amount of saving is achieved.
Open Access
MLFMA solutions of transmission problems Involving realistic metamaterial walls
(IEEE, 2007-08) Ergül, Özgür; Ünal, Alper; Gürel, Levent
We present the solution of multilayer metamaterial (MM) structures containing large numbers of unit cells, such as split-ring resonators. Integral-equation formulations of scattering problems are solved iteratively by employing a parallel implementation of the multilevel fast multipole algorithm. Due to ill-conditioned nature of the problems, advanced preconditioning techniques are used to obtain rapid convergence in the iterative solutions. By constructing a sophisticated simulation environment, we accurately and efficiently investigate large and complicated MM structures. © 2007 IEEE.
Open Access
Piloted evaluation of a control allocation technique to recover from pilot-induced oscillations
(American Institute of Aeronautics and Astronautics, 2015) Acosta, D. M.; Yıldız, Yıldıray; Craun, R. W.; Beard, S. D.; Leonard, M. W.; Hardy, G. H.; Weinstein, M.
This paper describes the maturation of a control allocation technique designed to assist pilots in recovery from pilot-induced oscillations. The control allocation technique to recover from pilot-induced oscillations is designed to enable next-generation high-efficiency aircraft designs. Energy-efficient next-generation aircraft require feedback control strategies that will enable lowering the actuator rate limit requirements for optimal airframe design. A common issue on aircraft with actuator rate limitations is they are susceptible to pilot-induced oscillations caused by the phase lag between the pilot inputs and control surface response. The control allocation technique to recover from pilot-induced oscillations uses real-time optimization for control allocation to eliminate phase lag in the system caused by control surface rate limiting. System impacts of the control allocator were assessed through a piloted simulation evaluation of a nonlinear aircraft model in the NASA Ames Research Center's Vertical Motion Simulator. Results indicate that the control allocation technique to recover from pilot-induced oscillations helps reduce oscillatory behavior introduced by control surface rate limiting, including the pilot-induced oscillation tendencies reported by pilots.
Open Access
RCS study of cylindrical cavity-backed apertures with outer or inner material coating: the case of e-polarization
(IEEE, 1993) Çolak, Dilek; Altıntaş, Ayhan; Nosich, A. I.
Cavity-backed apertures (CBA) are encountered as parts of any airborne or spaceborne radar targets. Most familiar of them are, probably, air inlets and engine tubes, known to contribute a great deal to radar cross section (RCS) of jet aircraft. In the present study, the dual-series-based solution is obtained for the scattering of an E-polarized plane wave from the cavity-backed aperture which is formed by a slitted infinite circular cylinder coated with absorptive material. The material coating can be done on the inner or outer surface of the cylinder. For both cases, numerical results are presented for the RCS and comparisons of the suppression of RCS are given for two different realistic absorptive materials.
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TAI’nin 1984 yılında Türk ve Amerikan ortaklar tarafından kuruluşu ve tarihi
(Bilkent University, 2023) Temizkalb, Ece; Akçıl, Zeynep; Çelik, Emircan; Düzenli, Tarık; Özcan, Yunus
Savunma sanayiinin öneminin anlaşılmasının ardından atılan adımlardan biri olan yarı Amerikan şirket TAI’nin kuruluşu, kuruluştan günümüze uzanan dönemde oldukça büyük etkilere sahiptir. Bu makalede, Amerikan ortaklar ve Türk Uçak Sanayi A.Ş. (TUSAŞ) tarafından kurulan şirketin kuruluş aşamaları ve geçmişi incelenmiş olup 2000’li yıllarda şirketin tamamen yerlileştirilmesi ve TUSAŞ çatısı altında birleştirilmesi üzerinde durulmuştur. Şirketin modern dönemi olarak tanımlanabilecek bu süreçte yaşanan gelişmelerden günümüz şirket yapısına ve projelerinden bahsedilerek şirketin tarihi hakkında detaylı bir çalışma yapılmıştır.