I learned in my intro to engineering class at my college that 5th gen fighters (which are stealth aircraft) are designed with similar angles to evade radar.

Why is this true?

Also, does the angle matter? Many of these modern fighters looks to have very similar angles (no pun intended).

Sukhoi PAK FA


4 Answers 4


Short answer: Yes.

Long answer: There are two ways to prevent the radar energy from being reflected back to its source in order to avoid detection:

  1. Absorption, and
  2. Directed reflection.

Absorbers work well only over a small wavelength band, so some reflection is unavoidable. Now the idea is to collect all reflected energy along specific directions, so the aircraft will reflect very little energy in all other directions.

Radars can adjust their sensitivity, and if they operate at their most sensitive setting, they will detect many false positives, but also a stealth aircraft. If the stealth aircraft maneuvers, it will momentarily shine one lobe of the reflected radar energy back at the radar receiver, causing it to reduce sensitivity such that the weak returns in all other directions will not be registered. Compare this to a diamond which will reflect light back at specific angles, causing it to sparkle.

There are two-lobe designs (Northrop YF-23 and B-2) and four-lobe designs (Lockheed F-117 and F-22), but the idea is the same in all cases. Reflected radar energy is focused in narrow beams, so very little is given off in all other directions.

This is not to say that such aircraft can evade detection. When the radar stations are networked, they can pretty well trace those sparkles and stitch together the aircraft's location. Also, NATO aircrews which had participated in international maneuvers were less than impressed with the stealth capabilities of their colleagues, but were under a gagging order to be more specific.

And then there is infrared. What does it help to be invisible on radar if the aerodynamic heating of the leading edge can be picked up by an IR receiver from 300 km away? When B-2s were flown over to the Farnborough airshow, British Eurofighters could detect them far out over the Atlantic already.

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    $\begingroup$ Does "their colleagues" mean Russia? The US? Someone else? $\endgroup$ Feb 15, 2015 at 18:35
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    $\begingroup$ @raptortech97: The Russians are much less sanguine about stealth, and those maneuvers were in the western states of the US. So the NATO aircrews were western European, and the colleagues were US pilots. $\endgroup$ Feb 16, 2015 at 8:12

The angle matters because radar works kind of like a mirror. Imagine holding a mirror out in front of you. If you start to twist it in any direction, you reach a certain point where the light coming off of you is no longer bounced back into your eyes, but is reflected off in another direction. Radar however cannot ever be completely reflected away, but this same principle allows a significant portion of the radar's energy to be deflected off in a useless direction, instead of back toward the dish.

The angles are similar because that angle was determined to have the smallest radar signature. So it makes sense that every other facet on the same (geometric) plane would be at the same angle. This means that the radar's energy is directed away in a very specific direction (away from the source), rather than a bunch of different angles (that would be more likely to return the energy to the source).

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    $\begingroup$ Good answer. I would add the following into analogy: imagining shinig a powerful flashligt on a disco ball and a flat mirror 100 meters away. You would not see the reflection on the mirror unless it's almost exactly pointed at you; disco ball, on the other hand, would give a clear "back signal" from any angle. That's why F-117 was all flat surfaces. $\endgroup$ Mar 27, 2014 at 12:36
  • $\begingroup$ @DarkWanderer Thank you, now I can't put the idea of F-117 shaped disco, ehm, ball out of my head. $\endgroup$
    – Pavel
    Jan 18, 2018 at 12:19

I assume you're using the term 'similar angles' in a geometric sense, to mean that the angle from a reference like the aircraft centerline to the lines made by two structural features are the same. Your photo shows this well, with the wing leading edge angle the same as the horizontal stabilizer leading edge angle and the same as the inlet (?) angle.

I believe the reason for this is that the overall likelihood of radar detection is lower because each of these features will reflect radar signal in the same direction as one another. This means that the reflection will be very much stronger in the direction they reflect, but that reflection has a very narrow spread. In contrast, the features will not reflect radar strongly at all other angles. Thus the radar dish has to be "lucky" to be at exactly the right angle from the aircraft to receive the strong signature, and so the overall probability of detection is reduced since at most angles the reflection signature is reduced. This is also the reason that flat surfaces are used in many stealth designs.

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    $\begingroup$ I think the OP is talking about the similar orientation of the leading edges of body, wings and horizontal stabs. What i am missing in all of the answers is why the leading edges would be important at all. I can understand that the large area parts acting as reflectors (wing undersides etc) would be oriented in particular ways. But the edges seem only having a tiny cross-section in comparison. $\endgroup$
    – Scrontch
    Jul 13, 2017 at 8:33

Define "with similar angles". In itself the phrase means nothing.
The basic shape of the airframe is of course a function primarilly of its required function to provide lift and enough inner volume for the aircraft's components.
Stealth depends on several factors:

  • absorption
  • scatter
  • deflection

These are functions of the shape and materials used. Creating surfaces that cause incoming EM waves to scatter in many directions rather than being reflected directly back causes the return signal received by a RADAR system to be much weaker, reducing the detection range.
But a big part is also played by placing grids in air intakes, shielding sharp edges with sawtooth overlays (see the canopy and bay doors, for example), and covering different parts of the aircraft in different materials.

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    $\begingroup$ The phrasing is fine. He's saying similar angles as in all of the new stealth aircraft look very similar to each other. Similar planform, similar fuselages. Not only that, but on each individual aircraft, the angle of the wing sweep is similar to the angle of the elevator sweep is similar to the vertical stabilizer sweep, and so on. $\endgroup$
    – StallSpin
    Feb 19, 2014 at 18:43

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