The Geometry That Makes Fighters Invisible to Radar

[00:00:00]This is a view from above of the F-35.

[00:00:03]This [clears throat] other one is the F-22. This is the B-2 Spirit. And this one is the Sukhoi 57.

[00:00:12]>> [music] >> Did you notice anything strange?

[00:00:18]The four planes that we have seen at the beginning [music] are all planes that incorporate a stealth design in their airframes.

[00:00:27]And in this video, we cover a specific aspect of stealth [music] design that is not really well known. Did you ever notice how many parallel lines are there in a stealth aircraft? [music] Aerodynamic surfaces leading and trailing edges are generally aligned, but also the edges of [music] the openings of the airframes are aligned in the same way.

[00:00:56]So, look how many parallel lines.

[00:00:58]Uh, for example, here. Or here.

[00:01:02]Or even here.

[00:01:04]An area where this type of design is really evident [music] is on the doors of the weapon base, which seem more saw blade >> [music] >> than an aeronautic structure.

[00:01:16]Well, we are all used to the mix of jagged and round design of stealth planes, [music] but why these parallel lines? And to understand that, we need [music] to step back and explain a few concepts.

[00:01:31]Radar stealth is obtained by two different classes of [music] technical solutions.

[00:01:37]Radar absorbing materials and [music] structural geometry.

[00:01:42]In this video, we are not covering the radar absorbing materials, but we just focus >> [music] >> on the geometry. The basic principle is rather simple.

[00:01:52]Do not reflect the radar waves back toward the emitter.

[00:01:57]Often to explain [music] stealth, texts refer to an optic analogy. For example, the reason why the forward fuselage of [music] a stealth plane is shaped like a speedboat is because in this way the reflection angles are such that the radar waves [music] are not reflected toward the emitter.

[00:02:21]This is a correct analogy and this is the reason why you will never see a vertical fins or vertical [music] side walls in a stealth plane.

[00:02:32]And it is also the reason why there are no exactly 90° [music] corners on a stealth plane. A 90° corner has the property to always reflect the radiation back to the direction from [music] where it came.

[00:02:46]Underwing pylons for weapons or engines, since they are at a 90° with the wing itself, are among the most powerful reflectors.

[00:02:58]And for the same reason, the air intakes [music] of the F-22 and the Sukhoi Su-57 are not square but rather irregular lozenges.

[00:03:08]Can you imagine the radar echo of a plane like the MiG-31? [music] One consequence of the optic analogy is that stealthiness [music] depends from the angle between the plane and the impinging radiation. Planes do not fly straight and flat all the time. Radars are not always exactly [music] the same altitude and so on. So, as the plane maneuvers, it might cause detectable radar reflection. And [music] for example, a look-down radar might receive a much stronger echo than a radar at the same level even [music] if the plane is flying horizontally.

[00:03:45]This is the reason why pilots are trained on how to manage the radar signature of the plane.

[00:03:52]And they fly the plane trying to minimize the reflection toward the radars that could potentially [music] identify them.

[00:03:58]This is also the reason why stealth is very effective against a single isolated plane or a group of non-integrated radars, [music] but it definitely becomes less effective when various radars connected with a data link and integrated with a proper information fusion capabilities, they work together in the same [music] airspace.

[00:04:20]If I'm not picking up an echo, my friend might, >> [snorts] >> and it will tell me where the track [music] is.

[00:04:26]Actually, China and Russia are working feverishly to achieve this type of [music] integration in their air defense systems.

[00:04:35]There are actually other counter measure that are progressively eroding the [music] advantage of stealth, but this will be the subject of another video.

[00:04:44][music] In this video, there is more. We got to the point where the optical analogy that brought us so far >> [music] >> now is breaking down.

[00:04:53]Actually, if it was so easy, we would have had stealth many decades ago. So, the first consideration is that reflections depend from [music] the frequency of the impinging radiation. In general, the longer the frequency, the less accurate is the no optical analogy.

[00:05:09]But, uh the radar frequency has another implication.

[00:05:13]>> [music] >> Radiation is not only reflected, but it also travel through the metallic [music] structure of the plane that in this case acts like an [music] antenna. If the size of the metal parts is right and some characteristic dimensions [music] are in a specific relationship with the radar wavelength, then the structure may [music] radiate energy itself, and guess what? The tip of the aerodynamic surfaces is one of those points where the radiation happens.

[00:05:49]The wing leading edge, for example, [music] reflects some energy optically, but also emits a diffuse lobe of energy from the wing [music] tip parallel to the leading edge itself. This effect in practice always happens with the most [music] commonly used wavelengths, and while an accurate design can minimize it, obviously, uh you can never in [music] practice get rid of it.

[00:06:15]To be honest, this effect is actually beneficial in some cases, for example, [music] when it comes to confuse automated target recognition systems. [music] But, for a stealth plane, it is just a giveaway.

[00:06:30]So, designers choose to have these lobes all aligned together in two directions.

[00:06:36][music] True, the echo will be stronger, but it will be just [music] in those two directions.

[00:06:45]If the plane doesn't keep the wing tip pointed at [music] the receiver, the radar may catch it just a glimpse, uh which would be quickly gone. And this is the reason why on stealth [music] planes, you have so many parallel lines.