F-35 vs J-20

The F-35 compared with other modern jets.
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inst

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Unread post23 Mar 2017, 03:58

@garry: you're right about requiring a fourth-power increase in transmission energies to get 2x increase in range, it's been a while and I've forgotten. About AESA vs PESA, scaling the diameter of the radar aperture by 2 gives you a 4x increase in total transmit power as well as a 4x increase in total signal received. PESA, since it's based off a magnetron, gives you only a 4x increase in total signal received, and the magnetron has to be independently scaled in power to match AESA scaling.

About E-2D, APS-138 on E-2C had about 224 km vs 0 dBsm. The APS-145 is claimed to have been a 40% improvement on the APS-138, giving you about 314 km vs 0 dBsm, which already outperforms the stated ranges of PESA PAC-3 and S-300. The APY-9 is supposed to be a two-generation jump from APS-145, using the APG-68 on the F-16 to the APG-80 as a baseline, you'd get a rough doubling of range, implying detection of 0 dBsm at over 628, and implying over 353 km detection vs -10 dBsm (the typical RCS of stealth aircraft vs anti-stealth radars).
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gta4

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Unread post23 Mar 2017, 04:17

Another reason why we need to double check the source before citing it:

inst claims that F-35's yaw rate is limited to 25deg/sec, but in fact it is way beyoud 90deg/sec and it is still under control:

https://www.youtube.com/watch?v=5hERYdmjZWA
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garrya

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Unread post23 Mar 2017, 04:30

inst wrote:@garry: you're right about requiring a fourth-power increase in transmission energies to get 2x increase in range, it's been a while and I've forgotten. About AESA vs PESA, scaling the diameter of the radar aperture by 2 gives you a 4x increase in total transmit power as well as a 4x increase in total signal received. PESA, since it's based off a magnetron, gives you only a 4x increase in total signal received, and the magnetron has to be independently scaled in power to match AESA scaling.

It doesn't work like that !!!
A bigger aperture result in higher gain, which mean more power concentration.But the relationship is not linear.

Detection range also depending on radiation factor whether it is Taylor , Blackmann ,Hamming or Cosine

inst wrote:About E-2D, APS-138 on E-2C had about 224 km vs 0 dBsm. The APS-145 is claimed to have been a 40% improvement on the APS-138, giving you about 314 km vs 0 dBsm, which already outperforms the stated ranges of PESA PAC-3 and S-300. The APY-9 is supposed to be a two-generation jump from APS-145, using the APG-68 on the F-16 to the APG-80 as a baseline, you'd get a rough doubling of range, implying detection of 0 dBsm at over 628, and implying over 353 km detection vs -10 dBsm (the typical RCS of stealth aircraft vs anti-stealth radars).

a) you should provide some source or photos for your claims

b) For those figures you provided, what was the radar mode ? ( look up or look down?, Cued search or volume search ?) what were the probability of detection (Pd) and probably of false alarm (Pfa) ? (since they can mean very different SNR)
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c) using generation jump to estimate radar range is ........, i can't even ........ :doh:
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TBH, before trying to estimate radar range, you should learn the basics first
https://basicsaboutaerodynamicsandavion ... ermeasure/
https://basicsaboutaerodynamicsandavion ... s-part-ii/
Last edited by garrya on 23 Mar 2017, 04:36, edited 2 times in total.
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inst

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Unread post23 Mar 2017, 04:32

@garrya: in the CNN (actually CCTV) video, the J-20 makes a 90 degree turn in about 3 seconds between 0:20 to 0:30. That's about 30 degree STR. There's also the initial 6.5 second 180, so if we assume 180 degrees implies STR, it would come out to about 28 degree STR.

But notice how the F-35, at sea level, produces vortices, and classically speaking, 180 degrees is not considered STR. So we either accept that the J-20 has 28 degree ST and the F-35 has 22.5 degree STR, or neither figure is STR and both are about 28-30 degree ITR craft.

There's one other interesting calculation I've located. The 5G requirement for the F-35 is at 15000 ft, at Mach 0.8. That implies a turn rate of only 7.57 degrees / second under that regime. This is about half of the F-16 running at the same conditions.

@gta4: First, what people think of me on the internet isn't such a big deal, it's not like you're going to dox me and harass me with e-mails from the F-35 fanclub; I assume the NSA knows who I am but the NSA isn't going to run these kinds of ops. Second, I don't care about Yaw rate, and if you'd check the extrapolation of figures, it seems reasonable that STR on the F-35 isn't that hot.
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inst

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Unread post23 Mar 2017, 04:33

@ GarryA: I'm measuring by diameter, so scaling by 2x gives you 4x the aperture area. Using diameter, range increases are roughly linear, using area, they're square root. Do you get my point?
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garrya

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Unread post23 Mar 2017, 04:48

inst wrote:@garrya: in the CNN (actually CCTV) video, the J-20 makes a 90 degree turn in about 3 seconds between 0:20 to 0:30. That's about 30 degree STR. There's also the initial 6.5 second 180, so if we assume 180 degrees implies STR, it would come out to about 28 degree STR.

Iam pretty sure , just last page gta4 proved that your video was sped up.Moreover, assuming both turn was ITR that would mean aircraft lose energy over time so the first few second always result in faster turn rate than the total time

inst wrote:There's one other interesting calculation I've located. The 5G requirement for the F-35 is at 15000 ft, at Mach 0.8. That implies a turn rate of only 7.57 degrees / second under that regime. This is about half of the F-16 running at the same conditions.

How do you come up with 7.57 degrees /second ????
Moreover, the chart gta4 put up is for Mach 0.9 not Mach 0.8. Look at the F-15 chart i put up and you will see the different in sustained G between the 2 speed
Image
inst wrote:@ GarryA: I'm measuring by diameter, so scaling by 2x gives you 4x the aperture area. Using diameter, range increases are roughly linear, using area, they're square root. Do you get my point?

Radar do not work like that. I already explained why.
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eloise

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Unread post23 Mar 2017, 05:04

inst wrote:@garrya: in the CNN (actually CCTV) video, the J-20 makes a 90 degree turn in about 3 seconds between 0:20 to 0:30. That's about 30 degree STR. There's also the initial 6.5 second 180, so if we assume 180 degrees implies STR, it would come out to about 28 degree STR.

http://edition.cnn.com/2016/10/31/asia/ ... show-j-20/
J-20 made a 90 degrees turn in the time from 0:10 to 0:15 or total of 4 seconds => 22.5 degrees/seconds turn rate
J-20 made a 70 degrees turn then roll to other direction in the time from 0:20 to 0:24 or total of 3 seconds=> 23 degrees/seconds turn rate
Both are not full 360 degrees turn so it is questionable whether they are STR or not

Want to play the numbers game?
In video below F-35 made a 70 degrees turn and climb in the time from 1:29 to 1:31 or total of 2 seconds => 35 degrees/second turn rate LOL
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Unread post23 Mar 2017, 05:23

@garrya: Alternately, you could claim the second video was slowed down. In either case, I'm pointing you to a CNN source with the aircraft doing 30deg/sec turns both to the 90 degree and 180 degree point. The CNN source does show that the video was taken by CCTV, but CCTV is less likely to speed up the video compared to hobbyists taking videos with DSLR.


About F-35's turn rate, we have Mach 0.8 = 1160*.8 km/h at 15000 ft. Convert to m/s, and we get 2320/9 m/s

Using standard acceleration formula, with 5g inputted, we get r * 49 m/s^2 = (2320/9)^2, giving us a turn radius of 1356.1098.

The circle's circumference is ~8520 m, the airspeed in a sustained turn is 2320/9 m/s, meaning that the F-35 travels about 3% of the total circle per second. Multiplied by 360, we get about 11 deg / second, when the F-16, by comparison, is doing 14 deg / second in this regime.
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Unread post23 Mar 2017, 05:25

@Eloise: Actually, it's not a 70 degree turn, I was originally going to argue it was 2 seconds, but if you go up to a full 3 seconds, you get a 90 degree turn. For the F-35, likewise, if you apply the 3 second trick, you see 90 degrees worth of turn, so it's roughly a 30degree/sec turn.
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Unread post23 Mar 2017, 05:30

inst wrote: Alternately, you could claim the second video was slowed down. In either case, I'm pointing you to a CNN source with the aircraft doing 30deg/sec turns both to the 90 degree and 180 degree point. The CNN source does show that the video was taken by CCTV, but CCTV is less likely to speed up the video compared to hobbyists taking videos with DSLR.

CCTV has many reasons to speed up the video such as for propaganda purpose. It still measure to only 22.-23 degrees/seconds according to my MK1 eyeball estimation
Btw, what do you say about the video where F-35 done 35 degrees/second while climbing at the same time? :wink:
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Unread post23 Mar 2017, 05:31

inst wrote:@Eloise: Actually, it's not a 70 degree turn, I was originally going to argue it was 2 seconds, but if you go up to a full 3 seconds, you get a 90 degree turn

Pilot of the J-20 turned 70 degrees then he stopped, same for F-35 as it was climbing
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Unread post23 Mar 2017, 05:35

At :20, you see the bottom of the aircraft. At :23, you see the engines of the aircraft, which is a 90 degree turn.
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eloise

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Unread post23 Mar 2017, 05:42

inst wrote:At :20, you see the bottom of the aircraft. At :23, you see the engines of the aircraft, which is a 90 degree turn.

I don't think so If it was a 90 degrees turn you will not see the engine of the aircraft, because the viewer do not move with the aircraft and only stay in one point, so if it was a 90 degrees turn his view will be nearly perpendicular to the top of the aircraft
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Unread post23 Mar 2017, 05:45

Actually, I'm going to ignore eloise, you're free to ignore me as well; I find he's particularly mendacious. Perpendicular to the top of the aircraft is parallel to the edges of the XY-plane of the aircraft, or you're seeing the aircraft by its edges. You're seeing the engines, so it's 90 degrees.
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Unread post23 Mar 2017, 05:58

inst wrote:Actually, I'm going to ignore eloise, I find he's particularly mendacious.You're seeing the engines, so it's 90 degrees.

Ignore me because i make too much sense?
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