Baigiamajame magistro darbe nagrinėjamas koncepcinio ultralengvojo lėktuvo „Kiras“ skrydžio stabilumas. Išilginio stabilumo charakteringi parametrai skaičiuoti rankiniu būdu bei Athenos sūkurių tinklelio principu veikiančia AVL stabilumo analizės programa. Šoninio judesio spiralės režimo tikrosios reikšmės, Fugoido, trumpojo periodo bei „olandiško žingsnio“ svyravimų tikrosios reikšmės įvertintos AVL. Darbas taip pat apima svyravimų dekremento, periodo T 1/2, laiko t 1/2, svyravimų ciklinio dažnio wn, ciklų skaičiaus N 1/2 ir kt. analizę esant skirtingam lėktuvo judesiui. Ankstyvoje projektavimo stadijoje buvo rastas ultralengvojo lėktuvo spiralinis nestabilumas. Ši klaida ištaisyta priartėjimo metodu keičiant uodegos geometriją ir sparno skersinį V kampą. Skaičiavimais bei kompiuteriniu skrydžio judesio modeliavimu įvertinus rezultatus buvo sukurta nauja kilio geometrija. Galiausiai pateikiamos baigiamojo darbo išvados ir siūlymai. Darbo apimtis – 54 p. teksto be priedų (67 p. su pried.), 9 iliustr., 10 lent., 12 bibliografinių šaltinių. Atskirai pridedami darbo priedai. / Stability analysis of conceptual design ultralight aircraft “Kiras” is presented in this master’s thesis. Longitudinal stability is evaluated by manual counting and Athena vortex – lattice based AVL program. Lateral derivatives, such as eigenvalues of spiral mode, “dutch roll“, Phugoid and short period oscillations were estimated by AVL . Thesis involves analysis of damping ratio, period T 1/2, time t 1/2, ocsillation circular frequency wn, number of cycles N 1/2 of distinct aircraft motions. The range of eigenvalues for specified run cases were automatically generated by AVL eigenmode meniu. At early phase of ultralight aircraft design, spiral instability was found. This problem fixed by approach method by correction of tail geometry and wing dihedral. According to the counting results and computerized motion analysis of an airplane, the corrected geometry of vertical tail is offered. Finally, the conclusions were made. Thesis consist of: 54 p. text without appendixes (67 p. with), 9 pictures, 10 tables, 12 bibliographical notes. Appendixes included.

General aircraft dynamic stability theory was used to predict the natural frequencies, damping ratios and time constants of the dynamic modes for three specific small UAVs with wingspans on the scale from 0.6 meters to 1.2 meters. Using USAF DatCom methods, a spreadsheet program for predicting the dynamic stability and handling qualities of small UAVs was created for use in the design stage of new small UAV concept development. This program was verified by inputting data for a Cessna-182, and by then comparing the program output with that of a similar program developed by DAR Corporation. Predictions with acceptable errors were made for all of the dynamic modes except for the spiral mode. The design tool was also used to verify and develop dynamic stability and handling qualities design guidelines for small UAV designers. Using this design tool, it was observed that small UAVs tend to exhibit higher natural frequencies of oscillation for all of the dynamic modes. Comparing the program outputs with military handling qualities specifications, the small UAVs at standard configurations fell outside the range of acceptable handling qualities for short-period mode natural frequency, even though multiple test pilots rated the flying qualities as acceptable. Using dynamic scaling methods to adjust the current military standards for the short period mode, a new scale was proposed specifically for small UAVs. This scale was verified by conducting flight tests of three small UAVs at various configurations until poor handling qualities were observed. These transitions were observed to occur at approximately the boundary predicted by the new, adjusted scale.