Check-Six Forums

TOPOLO a écrit :Les bidons, c'est pour calibrer les radars de veille....

warbird2000 a écrit :Enfin il n'est pas exclu que la production continue encore un peu.

El Doctor a écrit :Ah ? La production va s'arrêter ?

El Doctor a écrit :Ah ? La production va s'arrêter ?

Ouais, ils ont produit à une cadence élevée * et vu le faible nombre d'appareils commandés, ils arrivent déjà à la fin.

* : relatif, comparé au Rafale dont on étale sans cesse la prod. Faut dire qu'ils ne doivent pas trop attendre de commandes export...

j'ai entendu dire que le f22 n'était pas si furtif que ça. la cellule est tellement invisible que les sorties réacteur et les perturbations de l'air dues au gaz chauds expulsés trahissent le f22. si le programme du radar cherche des retour de rréacteurs et des perturbations causées par le brassage d'air super chaud dans du froid le f22 est identifiable assez facilement.
vous en pensez quoi ?

Et puis c'est oublier que de l'air froid est injecté à la sortie des réacteurs pour diluer les gaz chauds (furtivité IR)

LighTning a écrit :Cependant de violents débats ont lieux en ce moment même entre l'USAF et l'USN.
Le F22 s'opposant au CVN (il est balèze le raptor quand même^^) C'est un remake du B36 contre porte avions de la guerre Froide... Ces 2 services se haïssent à un point c'est dingue. O_o

jojo a écrit :Oui enfin je n'y mettrai pas main non plus

Il y a un scoop là ou tu parles des réacteurs double-flux?

Tu penses que l'ambiance était meilleure entre l'armée de l'air et la marine au moment où il a fallu décider des spécifications du Rafale ?

There are two significant sources of infrared radiation from air breathing propulsion systems: hot parts and jet wakes. The fundamental variables available for reducing radiation are temperature and emissivity, and the basic tool available is line of sight masking.
Recently some interesting progress has been made in directed energy, particularly for multiple bounce situations, but that subject will not be discussed further here. Emissivity can be a double edged sword, particularly inside a duct.
While a low emissivity surface will reduce the emitted energy, it will also enhance reflected energy that may be coming from a hotter internal region. Thus, a careful optimization must be made to determine the preferred emissivity pattern inside a jet engine exhaust pipe.
This pattern must be played against the frequency range available to detectors, which typically covers a band from one to 12 microns.
The short wavelengths are particularly effective at high temperatures, while the long wavelengths are most effective at typical ambient atmospheric temperatures. The required emissivity pattern as a function of both frequency and spatial dispersion having been determined, the next issue is how to make materials that fit the bill.
The first inclination of the infrared coating designer is to throw some metal flakes into a transparent binder. Coming up with a transparent binder over the frequency range of interest is not easy, and the radar coating man probably won't like the effects of the metal particles on his favorite observable.
The next move is usually to come up with a multi layer material, where the same cancellation approach that was discussed earlier regarding radar suppressant coatings is used. The dimensions now are in angstroms rather than millimeters.
The big push at present is in moving from metal layers in the films to metal oxides for radar cross section compatibility. Getting the required performance as a function of frequency is not easy, and it is a significant feat to get down to an emissivity of 0.1, particularly over a sustained frequency range. Thus, the biggest practical ratio of emissivities is liable to be one order of magnitude.
Everyone can recognize that all of this discussion is meaningless if engines continue to deposit carbon (one of the highest emissivity materials known) on duct walls. For the infrared coating to be effective, it is not sufficient to have a very low particulate ratio in the engine exhaust, but to have one that is essentially zero.
Carbon buildup on hot engine parts is a cumulative situation, and there are very few bright, shiny parts inside exhaust nozzles after a number of hours of operation. For this reason alone, it is likely that emissivity control will predominantly be employed on surfaces other than those exposed to engine exhaust gases, i.e., inlets and aircraft external parts.
The other available variable is temperature. This, in principle, gives a great deal more opportunity for radiation reduction than emissivity, because of the large exponential dependence. The general equation for emitted radiation is that it varies with the product of emissivity and temperature to the fourth power.
However, this is a great simplification, because it does not account for the frequency shift of radiation with temperature. In the frequency range at which most simple detectors work (one to five microns), and at typical hot-metal temperatures, the exponential dependency will be typically near eight rather than four, and so at a particular frequency corresponding to a specific detector, the radiation will be proportional to the product of the emissivity and temperature to the eighth power. It is fairly clear that a small reduction in temperature can have a much greater effect than any reasonably anticipated reduction in emissivity.
The third approach is masking. This is clearly much easier to do when the majority of the power is taken off by the turbine, as in a propjet or helicopter application, than when the jet provides the basic propulsive force.
The former community has been using this approach to infrared suppression for many years, but it is only recently that the jet-propulsion crowd has tackled this problem. The Lockheed F 117A and the Northrop B 2 both use a similar approach of masking to prevent any hot parts being visible in the lower hemisphere.
In summary, infrared radiation should be tackled by a combination of temperature reduction and masking, although there is no point in doing these past the point where the hot parts are no longer the dominant terms in the radiation equation.
The main body of the airplane has its own radiation, heavily dependent on speed and altitude, and the jet plume can be a most significant factor, particularly in afterburning operation. Strong cooperation between engine and airframe manufacturers in the early stages of design is extremely important. The choice of engine bypass ratio, for example, should not be made solely on the basis of performance, but on a combination of that and survivability for maximum system effectiveness.
The jet-wake radiation follows the same laws as the engine hot parts, a very strong dependency on temperature and a multiplicative factor of emissivity. Air has a very low emissivity, carbon particles have a high broadband emissivity, and water vapor emits in very specific bands.
Infrared seekers have mixed feelings about water vapor wavelengths, because, while they help in locating jet plumes, they hinder in terms of the general attenuation due to moisture content in the atmosphere. There is no reason, however, why smart seekers shouldn't be able to make an instant decision about whether conditions are favorable for using water-vapor bands for detection.

On avait donné des credits pour juste maintenir la ligne de production et laisser ainsi le prochain président décider

faut lire le texte du premier lien... la production est prévue, et financée jusqu'à 2011. Ce que les senateurs demandent à Obama, c'est de prolonger ça pour continuer pendant un an de plus....

but critical DoD leaders approved spending just $143 million on parts to build only four

(Calendar Year)
Aircraft are to be delivered approximately two years after contract award.

par ailleurs, même si le délai initialementr prévu est de deux ans d'avance, qu'est-ce qui empêche de passer une nouvelle commande pendant que les chaines tournent encore?

Lockheed est sous cntrat pour délivrer 183 jusqu'à 2011. Le congrès a autorisé le département de défense à dépenser encore 500 millions supplémentaires pour construire des pièces de 20 chasseurs supplémentaires, mais les officiels du DoD ont approouvé la dépense de seulement 143 millions, pour la construction de seulement quatre appareils.

c'est un supplément sur la commande initiale

LighTning a écrit : Et puis c'est oublier que de l'air froid est injecté à la sortie des réacteurs pour diluer les gaz chauds (furtivité IR) alors à moins qu'il utilise la réchauffe ça risque d'être hard pour le trouver.