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irkutsk ::
Motori druge etape su ispočetka bili namjenjeni za PAK-FA. ovi sada su privremeni kako je 100 puta i rečeno do sad. Stoga sve za šta je pak-fa sposoban će bit nakon šta dobije svoje motore ... koji su sudeći po dosad rečeno brutalni. Iz tog razloga možemo očekivat bitno povećanje sposobnosti.
Ajde pokusacu jos jednom da objasnim.
Motori druge etape mogu da budu sposobni da poteraju avion i do 3500 km/h ali to ne znaci da ce avion biti u stanju da leti tom brzinom iz milion razloga. Posto ni ti ni ja ni niko na ovom forumu i sire ne zna koji je limit za max brzinu postavljen pred konstruktore, tvrdnja da avion sa motorima prve etape ne moze da postigne zadatu max brzinu je vrlo diskutabilna.
Zasto je to vazno da se zna?
Da bi se avion kretao velikom brzinom konstruktori moraju da naprave odredjene ustupke koji su daleko od zanemarljivih. Prost primer je Mig-31. U stanju je da se krece brzinom od M 2,83 ali nije u stanju da se suprotstavi ni jednom lovcu iz iste generacije u bliskoj vazdusnoj borbi.
Dakle, sve zavisi od toga sta se zeli i u principu svako resenje koje zahteva multifunkcionalnost (kao sto je to slucaj kod PAK FA) zahteva kompromise.
Da li ce konstruktori zrtvovati dosta toga (ojacanje konstrukcije, izbor drugacijih materijala, povecanje tezine itd.) zarad par stotina km/h ili ce ograniciti konstrukciju na odredjenu max brzinu (bez obzira na snagu motora) je stvar izbora i zahteva koji su postavljeni pred njih.
Ono sto ce se definitivno dobiti sa drugom etapom motora jeste bolje ubrzanje, skracenje duzine zaleta za poletanje, bolje manevarske karakteristike, smanjenje potrosnje i jos mnogo, mnogo toga.
Ali ako avion sa motorima prve etape ispunjava zahtev za max brzinu (koji je limitiran samom konstrukcijom aviona) onda necemo videti dalje povecanje max brzine prilikom ugradnje motora druge etape jer bi u tom slucaju doslo do strukturalnih ostecenja.
O kakvim se sve tu kompromisima radi moze da posluzi i primer uvodnika vazduha:
Na primer, S uvodnici vazduha koji su recimo primenjeni na F-22, F-35, Rafale itd. nisu promenjive geometrije.
To znaci da nemaju mogucnost generisanje veceg broja udarnih talasa i optimiziranja njihovog polozaja.
Na grafikonu jasno moze da se vidi zavisnost max brzine od tipa usisnika.
Da bi shvatili zasto se konstruktori opredeljuju za neka resenja na ustrb drugih i zasto u tom slucaju najradije zrtvuju max brzinu dacu ovaj pripmer:
Citat:Fixed-geometry normal shock intake was also the choice of the General Dynamics engineers when designing the lightweight fighter concept that was later to become the Mach 2.05 F-16 Fighting Falcon (see Fig. 6, right). The reader may suspect here that cost considerations have swayed them in this direction, but, in an article written in 1976, Hawkins [11] argued that this would have been the optimum configuration even if cost had not been an objective, as a variable geometry intake, whereas permitting a top Mach number of 2.2 would have incurred a 4% acceleration time penalty (from Mach 0.9 to Mach 1.5) and a 7% decrease in turn rate at Mach 1.2 as a result of the 250 lb of additional dry weight. A Mach 2.0 fixed geometry intake with an additional compression ramp intake (additional shocks) was also considered, but discarded for similar reasons (12% acceleration time penalty, 7% decrease in turn rate at Mach 1.2 and 250 lb of additional dry weight).
Incidentally, such an intake was later designed for the F-16/79, powered by the GE J79 engine, but, although it had a 20% higher total pressure recovery and 68% lower spillage drag at Mach2.0[12],
the resulting weight penalty turned out to be one of the main reasons why the F-16/79 never went into service.
I jos nesto, s`aspekta cistih performansi S uvodnici vazduha nisu najoptimalnije resenje.
Citat:On many installations, particularly on military aircraft, an element of the air induction system as important as the inlet itself is, of course, the duct. For high, uniform pressure recovery this should be as close to straight as possible, as any curvature may lead to separation and
thus loss of total pressure. Bends, usually S-bends, are, however, inevitable on many designs. Special care must be taken in extreme cases, where highly offset intakes are demanded by layout and center of gravity constraints (such as on the Harrier; see Fig.  or radar susceptibility requirements (line-of-sight shielding of the engine can be observed, for example, on many modern unmanned air vehicle designs). Computational studies have allowed careful shaping of S-ducts in recent years. The great challenge here lies in balancing the high computational expense of analysis codes that are accurate enough to predict such pressure losses against the need for analyzing a large number of candidate designs for a meaningful optimization study.
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