J-20 5th Gen Fighter Thread IV (Closed to posting)

[latenlazy]

Oh NO ... certain members at both the Key- and the Secret-Projects Forum will instantly cry out that they were correct, that the J-20 is a lame duck or a huuuuuge long-range striker at best !!

Deino

Might have to clarify with the esteemed emile that our bet applies to the length of 2001 and 2002 XD. Then again, even with that added length it's still probably shorter than 21 meters

 

[Blitzo]

Not entirely correct. Everything else being equal the more surfaces or the larger surface area of an object overall the more RCS period! that is the fundemental law of physics that cannot be altered. As we all know things like canted or angle planes and RAM materials do minimize RCS but ultimately the surface area is still the fundemental problem.

Yeah, and that's what I'm saying -- that J-20's V tail + ventral fin total surface area may be equal to F-22's larger V tails alone.

I suppose a more interesting quesiton is whether a plane with more control surfaces versus a plane with less control surfaces are better for RCS reduction, if the total surface area of both plane's control surfaces are equal.

I presume this would depend on the orientation of the aircraft to the incoming radar, and of course, the orientation and shape of the respective control surfaces as well (which would be determined by the aerodynamic performance both aircraft are seeking to achieve).

Your example of V tail vs single is misleading. Yes a V shape tail do mimimize stealth due to it's angle of reflectivity however it doesn't necessary have less RCS than a single vertical tail. The reason it may have more RCS is because the vertical tail is well straight and vertical. If you angle a single vertical tail it will also minimize RCS as well although I don't know from an aerodynmics standpoint how well a plane with a single canted tail can fly but you get my point.

Well that's what I'm saying, that orientation matters.

 

[latenlazy]

Yeah, and that's what I'm saying -- that J-20's V tail + ventral fin total surface area may be equal to F-22's larger V tails alone.

I suppose a more interesting quesiton is whether a plane with more control surfaces versus a plane with less control surfaces are better for RCS reduction, if the total surface area of both plane's control surfaces are equal.

I would say the plane with fewer control surfaces would have a better RCS in that case, but I think the most important thing to consider isn't the number of control surfaces but how many planes there are. This isn't to say that we should conclude the J-20 definitively has a worse shape or performance than the F-22's, but it does suggest that the J-20's design might involve more complex computations and other considerations.

 

[Engineer]

looking at it strictly from geometry standpoint, canards might lead to a few more hotspots of rcs return compared to conventional layout, when looked at from the front. naturally, when in a chase, canard layout might offer less returns than conventional one, for the exact same reasons.

that's if canards/horizontal tail surfaces would be at the exactly the same level.

of course, having canards at the exactly same level as wings lowers their aerodynamic effectiveness. which is why j20 has them canted so their vortexes go over the wing.

so yes, j20 layout should lead to a few more rcs hotspots than j20 overall, both when looked from the front and the back. of course.

When people claim canards are disadvantage to stealth, these people tend to think radars see the same thing as naked eyes. If you think about it, the edges are designed to deflect radar signal away from the opponent, so would actually be difficult to detect by the radar. The more problematic areas should be the corners, where reflection is more unpredictable.

situation A) enemy radar is mostly in front of the j20/f22, both azimuth wise and elevation wise. leading surfaces of canards deflect to the side, no problem. leading surfaces of wings do the same. edges of canards, canard roots, slats on the wings and wing edges produce returns in all directions.

here j20 is potentially at a disadvantage because it has two canard edges, canard root edge, two edges from one of the slat/wing joints (other one is possibly covered by the canard) and the wing's edge. that's 6 sharp edges that might radiate in all directions. f22 in same situation would have two pairs of slat/wing edges plus a wing edge. its wings are obstructing line of sight to the tails so no edges are visible. that's 5 sharp edges per wing. (10 total for aicraft versus 12 for j20). BUT, since j20 canards are canted, it's possible the inner slat/wing joint isnt actually covered/obstructed by them. if so, that'd add 2 more edges per wing for a total of 10 versus 16.

situation B) enemy radar is front but is little below the level of the j20/f22. again, leading surfaces are not an issue. this time raptors tails' edges are exposed. so we have the same as above for raptor plus two horizontal tail edges plus tail root edge. thats 8 per side or 16 total. j20 this time has no chance of its canards obstructing any part of the wing so we're automatically talking about 8 edges per side or 16 total.

situation C) enemy radar is in front but is little above. this time j20 has theoretically a chance that its canted canards will obstruct one of the slat/wing joints so it may actually have 12 edges visible versus 16 on raptor.

but - one really designs all those shapes around the expected percentage of situations planes will find themselves in.

when one looks at a plane from the ground at 400 km away, one will have radar look at the plane at similar angle as situation A. as the plane moves closer, situation will change slowly to situation B. at which point does B become applicable? i don't know precisely. At 100 km away, a plane flying 15 km higher than the radar will be seen at an offset of some 6-7 degrees. that should be more than enough for a clear view of the horizontal tails. perhaps even half as large offset might be enough.

At 400 km away, it would actually be situation B. Both aircraft are flying parallel with respect to the sea level, but there is the curvature of Earth which you need to take in account. The radar of both aircraft sees the opponent from slightly below.

on the other hand, having platforms in the air that will look at j20 from above is hard. unless we're talking about awacs and a ground hugging j20. those situations will of course be much less common than a variety of ground radars searching for a high flying j20.

situation D - radar is at just the angle offset from the longitudinal axis of the plane so it can catch the big return of wings' leading surfaces. j20's surfaces are caught the same way as in situation A, while raptor still obstructs view to tails. but all that is less important since big return of whole length of leading surfaces dwarfs the combined returns of those tiny edges in situations A/B/C.

all this is very simplified, and there's a bunch more potential sources of return on the planes which i havent mentioned.
and none of this mentions materials, which may have progressed to be point where its practical to rely on them negating those edge returns for select radar wavelengths. so, shape wise, canards may be slightly worse solution, but in the end, when materials and angles at which planes get looked at and number of rcs spots are calculated - overall increase in rcs may not be even close to order of magnitude higher, and for certain wavelengths it may be basically the same.

People should buy a J-20 model then bring it to a rural area on a completely dark night. There, they can shine a flashlight at the model to see where the reflections are coming from.

 

[shen]

As to Lockheed Martins early JAST/JSF Concept...here's how that looked.


As to the number of control/stabilizor surfaces between the J20 and the F-22 that can impact to one degreee or another the RCS, here's what I count:


Most people foget the ventral fins on the J20.

If we are going to count non-moving surfaces, let's not forget two more on F-22

Please, Log in or Register to view URLs content!

 

[paintgun]

Might have to clarify with the esteemed emile that our bet applies to the length of 2001 and 2002 XD. Then again, even with that added length it's still probably shorter than 21 meters

Regarding the length of J-20, it is rather long compared to the F-22
While the difference in actual length is not much, the fuselage is at least 10% longer than the F-22

If someone tries to extrapolate this as a handicap in maneuverability it might or might not hold merit, again as there too many factors to weigh in and simple eyeballing will not suffice, unless you've worked in various projects concerning similar aircraft i.e canard config, so much that past experience working with those data can give you a ballpark estimate of J-20's aerodynamics, then again no sane engineer will do that as well, no data no cigar

 

[paintgun]

If we are going to count non-moving surfaces, let's not forget two more on F-22

we are getting into A vs B now

 

[paintgun]

I suppose a more interesting quesiton is whether a plane with more control surfaces versus a plane with less control surfaces are better for RCS reduction, if the total surface area of both plane's control surfaces are equal.

I suppose the one with less control surface, a tailless delta for example, or a flying wing

 

[latenlazy]

Regarding the length of J-20, it is rather long compared to the F-22
While the difference in actual length is not much, the fuselage is at least 10% longer than the F-22

Oh I don't dispute that at all. This length debate is mostly about three things: 1) Whether the J-20 is some 23 meter long flying behemoth, 2) Who's preferred method of measurements are right, and 3) Ego.

 

[kwaigonegin]

Yeah, and that's what I'm saying -- that J-20's V tail + ventral fin total surface area may be equal to F-22's larger V tails alone.

I suppose a more interesting quesiton is whether a plane with more control surfaces versus a plane with less control surfaces are better for RCS reduction, if the total surface area of both plane's control surfaces are equal.

I presume this would depend on the orientation of the aircraft to the incoming radar, and of course, the orientation and shape of the respective control surfaces as well (which would be determined by the aerodynamic performance both aircraft are seeking to achieve).




Well that's what I'm saying, that orientation matters.

Assuming the surface area is identical and everything else being equal (ram, material etc) I would say a movable surface has the 'potential' to reflect more waves back to the transmitter and has less chance of scattering which plays a fundemental role in making something stealthy.. Not that the specific surface has a net more RCS but because it moves, it therefore increases the odds of reflecting more back due to the unpredictability of the angle the moment a radar hits it... unlike a fixed surface.

In a fixed plane the designers and computers obviously tries their hardest to make it at an optimum angle relative to the rest of the body of the plane but if the entire surface moves it gets exponentially more complicated to calculate and that's why I say it has the potential to reflect more back.

With that being said maybe computers are powerful enough these days to take into account every single possible angle of deflection not only of the movable plane itself but also relative to the actual orientation of the aircraft once it's flying. It also has to take into account every single possible angle the radar waves may hit on that particular angle of said surface relative to the entire aircraft and calculate every possible scattering pattern of reflective waves AND... then after that make sure whatever the calculation is the placement and the orientation of the control surfaces is still optimum for supersonic flight, high AoA and or enhanced maneuveriblity.
.. AND to make things more complicated we're not even talking a straight square or something of a rectangular shape either. The plane itself like the stabilisers, canards etc are triangular in shape and in the case of the new J20 even with the notch on the corners.

This is the reason why while making something stealthy is not difficult however to make it stealthier is exponentially more difficult. It is thereotically possible that if every possible calculation can be acquired of an object 's reflectivity it can be made to be practically invincible to radar. That's the Holy Grail of stealth I guess.