What is stealth and how does it need to be designed to get the status of being a stealth jet?
I'm gonna try to keep the whole thing as brief as possible...
The term 'stealth aircraft' is used to denote a plane that's designed to be very difficult to detect or track with radar. Otherwise known as Very Low Observable (VLO) planes. Examples are the F-117, B-2, F-22, F-35, PAK-FA, J-20 and J-31.
Note that stealth aircraft are not completely invisible - they just reduce the DISTANCE at which they can be detected by radar. For example, if a non-stealth aircraft like Su-30MKI can be detected by an X-band radar at a distance of 100km, then that same radar would be able to see an F-35 at around 10-20 km only. (Numbers are only for example).
How to make an aircraft stealthy? To understand this one first needs to understand how a radar works: a radar has a transmitter & a receiver. The transmitter sends out a waveform of radiowave energy which bounces off any surface they touch and scatter in different directions (depending on the nature of the surface) and some of the bounced waves return back to the receiver on the radar. By measuring the time between transmission & reception, and the direction they came from, a radar can register that there is an object at X distance from itself, at a set bearing. Radars of higher resolutions (such as those carried by jet fighters) can accurately scan the size & shape of the object they are looking at, helping the radar computer to identify the object.
Note that every object has a quantifiable amount of surface area which is capable of reflecting a wave back to the radar - this is called that object's
Radar Cross-Section (RCS) and is usually measured in square meters.
So how can an aircraft prevent getting seen by a radar in this way? The PRIMARY method for aircraft stealth today is AIRFRAME-SHAPING. The surfaces of the aircraft have to be designed in such a way that they reflect as less radar energy as possible. Note that a radar will see the aircraft only when a radiowave coming from the radar touches the aircraft and GOES BACK TO THE RADAR.
The surfaces on a stealth aircraft are designed to deflect the radiowave away from the direction it came from. If enough radiowave energy fails to travel back to the receiver, the radar registers nothing (note that a little bit of energy still goes back, but it's too little to matter. We'll get to that later).
If you look at the surfaces on stealth aircraft, you'll see that they have very specific angles designed to ensure that their surfaces manage to face a radar from an angled position as much as possible, avoiding perpendicular contact wherever possible, reducing it's RCS.
This attention to aspect angles removes a majority of the radar return which a plane would otherwise have if it were conventionally designed (with only aerodynamics in mind). But this is not enough to make a stealth aircraft. A stealth jet also needs to have internal weapon bays (as missiles & bombs are not designed to deflect radar and are therefore easy to detect) in order to remain combat-effective while still maintaining stealth.
However, a good stealth shape isn't always a good aerodynamic shape. So when designing an aircraft that's supposed to have great aerodynamic performance (like a fighter), it's imperative to make a few compromises with regard to stealth-shaping. Like sharp edges/corners etc. This is where the many SECONDARY stealth methods come into play, such as:
- Radar-Absorbent Materials (RAM). This malleable, paint-like material is applied on the parts of an aircraft where stealth-shaping is impossible as doing so would destroy it's aerodynamic properties. Such as the corners & wing leading-edges of the F-22 Raptor. Note that early stealth planes like F-117 had much more extensive use of RAM, but this is reduced today given the increased usage of less conductive construction materials and superior, computer-assisted shaping designs.
How does RAM work? Once a radiowave hits a RAM-coated surface, the material absorbs the waveform, dissipating it's energy within the molecular structure of the RAM, so that very little energy, if any, goes back to wherever it came from. As a side-effect, when a RAM-coated surface is exposed to radar waves from extended periods, it heats up a lot (making your aircraft more easily visible to Infra-Red optical systems).
- Radar-Absorbent Structures (RAS). These are entire structures/airframe components made out of materials which are inherently less reflective to radar. Like Carbon-Fiber Composites (CFC), Fiber mats, etc. which play a huge role in modern aircraft construction.
- Serrated edges & other deflective structures. These are machined components designed to scatter and/or deflect a radar wave as it follows a surface, so that the wave doesn't make it to a part of the airframe which could reflect it back. You can find such serrations & saw-toothed edges on the surfaces of all stealth aircraft (and even some non-VLO aircraft like Rafale, as a means of reducing RCS).
Coming back to radar basics, it's important to remember that no matter how well your aircraft is shaped & coated, it still reflects a tiny amount of energy back to the radar. However all radars have a programmable
Clutter-Rejection Threshold (CRT). This is a defining mark which stops the radar computer from reading every raindrop, bird & honeybee as a target - technically, even if a radar CAN see these extremely tiny signatures, it rejects them as clutter so as to reduce false alarm rates. In the following diagram you can see how a large signature stands out to a radar computer among many small signatures (the image is rendered to represent this and make it easy to present/understand, this isn't how a radar actually sees a target).
So as long as your aircraft's radar return is very small, the enemy radar won't register your presence and your signature will be lost among the various tiny signatures below the CRT -