Before this devolves into a photoshop/global warming thread, can we please get back to topic? Thanks.
J-20 5th Gen Fighter Thread IV (Closed to posting)
- Thread starter siegecrossbow
- Start date
- Status
Hopefully shortly, it did seem to take a little time after the CNY before 2002 flew, and I am encouraged by your observations.
So I got bored today, and decided to print out some images of the J-20 and take out a ruler and protractor to figure out the wing sweep, wing span, and wing area of the J-20. I was motivated to do this because I'm kind of tired of trying to do measurements on my computer, and of seeing people making conjectures of the J-20's dimensions while referencing diagrams that I feel weren't accurately constructed.
I used the satellite picture from a while back that we used to confirm the J-20's length, two aerial shots of the J-20's bottom with minimal roll angles, and then some side and frontal shots, using height as a bridge scale for length and wingspan. For every picture I assumed the length was 20.5 meters. As I went along measuring and drawing lines, I noted that the errors were rather significant, and thus the measurements are rather rough. With the pictures I had on hand changing one or two millimeters in any dimension of scaling could generate some very significant differences. Also, each method, (satellite, bottom, front+side with height as bridge) seemed to suggest a different wing span.
The satellite photo suggested a wingspan of between 13.5-14.5 meters, but of course, given the poor resolution, it's hard to get something more precise. I'm a bit incredulous of the 14.5 meters estimate, and I believe it's simply a result of a more liberal wingspan to length ratio, from measurement error. Like I said earlier, one or two millimeters can generate huge differences.
I thought the front and side pictures would be more reliable and help corroborate the satellite photos, but they only indicated a wingspan of between 13-13.5 meters. The 13.5 meters I can buy, but the 13 meters seem unlikely, in part because while playing with the satellite photo I tried to shift around numbers to get the smallest wingspan estimate and only hit 13.5 meters. I suspect there might be problems of perspective with this particular method. It's possible that because the wing is set further back than the nose the wingspan will appear smaller, or it's possible that using height is an imperfect scaling bridge between length and wingspan due to height discrepancies from perspective at a vertical angle. Alternatively, it's simply a consequence of relying on four different measurements, which multiplies the error potential by that much.
The two bottom views were a bit of a surprise, yielding wingspan estimates of over 15 meters each. My guess is that a pitch angle shortened the ratio between the plane's length and wingspan, so I discarded those results.
After fudging around a bit with millimeter differences on a blow up of the satellite photo, my general gut feeling is that the J-20's wingspan is somewhere between 13.5-14 meters (probably around 13.8 meters). I lean towards the satellite picture's estimate, because, despite the poor quality and high chance of error it is the least likely to experience photographic distortions. I suppose I could try the method I used for the J-20 on other planes to see if using height to bridge front and side views is just a poor way to estimate wingspan. Either way, I've personally concluded that the J-20's wingspan is at least a good half to full meter wider than the most oft cited numbers.
After that, I went about measuring the sweep angle using the satellite photo. The satellite photo is a bit blurry, but seems to suggest a sweep angle of about 47-48 degrees, which seems consistent with what others have measured. The pictures from the bottom view both yielded an angle of 45 degrees, but again, perspective from a pitch angle probably has an effect.
Finally, with an estimate of the wingspan and the sweep angles (in addition to some rough estimates about other dimensions of the wing, like the length of the wing tip and sweep of the trailing edge in combination with some trigonometry), I tried to figure out the wing area, assuming that that is the area determined by line of the leading and trailing edges of the wing being extended until they intersect. Assuming that the wingspan is only about 13.5 meters, the J-20 should have a wing area of roughly 78-81 sq meters (lower and upper bounds given the range of numbers I used). Assuming a wingspan of 13.8 meters, the J-20 should have a wing area of around 81-84 sq meters. Assuming a wingspan of 14 meters, the J-20 should have a wing area of 83-86 sq meters, all assuming the J-20 is 20.5 meters long. This suggests that the J-20 isn't short on wing area compared to the F-22 at all. Of course, these are still just very rough estimates with a very high error range, so take it with a grain of salt.
Speaking of wing area, I stumbled upon this post by Kryptid from secretprojects (thanks siegecrossbow for posting the original) when looking for pictures to use, so you guys can compare notes with someone else who bothered to do some wing area estimates for the J-20. I should note that I disagree with this guy's calculations for wing loading, primarily in the use of the square cubed law, because I don't believe that the J-20 is proportionally bigger than the F-22 in every dimension, just length. Given that, a direct percentage increase in weight that correlates with increase in length should be sufficient to estimate a weight figure for the J-20, assuming that they have the same average density (which they may not). Even then though, his estimate probably overshoots a proper weight figure, because munitions and arms are being scaled in his weight figure too. It might be more precise to compare fuel and empty weight, and leave other loads out of the calculation.
I used the satellite picture from a while back that we used to confirm the J-20's length, two aerial shots of the J-20's bottom with minimal roll angles, and then some side and frontal shots, using height as a bridge scale for length and wingspan. For every picture I assumed the length was 20.5 meters. As I went along measuring and drawing lines, I noted that the errors were rather significant, and thus the measurements are rather rough. With the pictures I had on hand changing one or two millimeters in any dimension of scaling could generate some very significant differences. Also, each method, (satellite, bottom, front+side with height as bridge) seemed to suggest a different wing span.
The satellite photo suggested a wingspan of between 13.5-14.5 meters, but of course, given the poor resolution, it's hard to get something more precise. I'm a bit incredulous of the 14.5 meters estimate, and I believe it's simply a result of a more liberal wingspan to length ratio, from measurement error. Like I said earlier, one or two millimeters can generate huge differences.
I thought the front and side pictures would be more reliable and help corroborate the satellite photos, but they only indicated a wingspan of between 13-13.5 meters. The 13.5 meters I can buy, but the 13 meters seem unlikely, in part because while playing with the satellite photo I tried to shift around numbers to get the smallest wingspan estimate and only hit 13.5 meters. I suspect there might be problems of perspective with this particular method. It's possible that because the wing is set further back than the nose the wingspan will appear smaller, or it's possible that using height is an imperfect scaling bridge between length and wingspan due to height discrepancies from perspective at a vertical angle. Alternatively, it's simply a consequence of relying on four different measurements, which multiplies the error potential by that much.
The two bottom views were a bit of a surprise, yielding wingspan estimates of over 15 meters each. My guess is that a pitch angle shortened the ratio between the plane's length and wingspan, so I discarded those results.
After fudging around a bit with millimeter differences on a blow up of the satellite photo, my general gut feeling is that the J-20's wingspan is somewhere between 13.5-14 meters (probably around 13.8 meters). I lean towards the satellite picture's estimate, because, despite the poor quality and high chance of error it is the least likely to experience photographic distortions. I suppose I could try the method I used for the J-20 on other planes to see if using height to bridge front and side views is just a poor way to estimate wingspan. Either way, I've personally concluded that the J-20's wingspan is at least a good half to full meter wider than the most oft cited numbers.
After that, I went about measuring the sweep angle using the satellite photo. The satellite photo is a bit blurry, but seems to suggest a sweep angle of about 47-48 degrees, which seems consistent with what others have measured. The pictures from the bottom view both yielded an angle of 45 degrees, but again, perspective from a pitch angle probably has an effect.
Finally, with an estimate of the wingspan and the sweep angles (in addition to some rough estimates about other dimensions of the wing, like the length of the wing tip and sweep of the trailing edge in combination with some trigonometry), I tried to figure out the wing area, assuming that that is the area determined by line of the leading and trailing edges of the wing being extended until they intersect. Assuming that the wingspan is only about 13.5 meters, the J-20 should have a wing area of roughly 78-81 sq meters (lower and upper bounds given the range of numbers I used). Assuming a wingspan of 13.8 meters, the J-20 should have a wing area of around 81-84 sq meters. Assuming a wingspan of 14 meters, the J-20 should have a wing area of 83-86 sq meters, all assuming the J-20 is 20.5 meters long. This suggests that the J-20 isn't short on wing area compared to the F-22 at all. Of course, these are still just very rough estimates with a very high error range, so take it with a grain of salt.
Speaking of wing area, I stumbled upon this post by Kryptid from secretprojects (thanks siegecrossbow for posting the original) when looking for pictures to use, so you guys can compare notes with someone else who bothered to do some wing area estimates for the J-20. I should note that I disagree with this guy's calculations for wing loading, primarily in the use of the square cubed law, because I don't believe that the J-20 is proportionally bigger than the F-22 in every dimension, just length. Given that, a direct percentage increase in weight that correlates with increase in length should be sufficient to estimate a weight figure for the J-20, assuming that they have the same average density (which they may not). Even then though, his estimate probably overshoots a proper weight figure, because munitions and arms are being scaled in his weight figure too. It might be more precise to compare fuel and empty weight, and leave other loads out of the calculation.
Last edited:
@latenlazy
I'm suspicious of printed evidence for shape. About 15 years ago I wanted to print an accurate shape so I tried my printer with a circle and, indeed the result was an ellipse. That was a matrix printer and likely your printer is better. But did you check for accuracy in this respect?
I'm suspicious of printed evidence for shape. About 15 years ago I wanted to print an accurate shape so I tried my printer with a circle and, indeed the result was an ellipse. That was a matrix printer and likely your printer is better. But did you check for accuracy in this respect?
How much did the shape deviate? If it's a minor error it shouldn't be a huge problem since I was working with pretty large images. Besides, as you said, that was 15 years ago
. I haven't had scaling problems before (that I noticed anyways)...so I don't suspect any scale issues. Furthermore, I've corroborated some of the angle measurements I took on paper and the computer. I'll take another look later though, but I don't think it would change my results very much. We're still dealing with a lot of potential errors given that we're ultimately using pictures which we can't identify discrepancies for.
Last edited:
A matrix printer is not designed for what I wanted to do and the distortion was obvious and needed to be corrected for. For more modern printer it should not matter but I think you should look at it, as you said you will.How much did the shape deviate? If it's a minor error it shouldn't be a huge problem since I was working with pretty large images. Besides, as you said, that was 15 years ago
Thanks for going to the trouble to do this!
This morning I tinkered around with the contrast, brightness, etc to see if I could tease out more details of the satellite pic and get a better estimate. I'm revising the sweep angle for the leading edge to 48 degrees, and am increasingly confident the wingspan is between 13.5 and 13.8 meters. I measured the sweep of the trailing edge to be between 10-12 degrees, and the length of the wingtip to be about 1.75 meters. All of these measurements are scaled using the assumption that the J-20 is 20.5 meters long. Using these estimates, the J-20's wing area should be between 82-87 square meters. The estimate seems larger than my first set of calculations because I assumed a sweep of 47 degrees in the earlier one, which would have shaved off a few square meters to the estimate. My estimates also seem larger than the aforementioned Kryptid's, which could be a result of miniscule differences in measurements. Either way, I believe that both sets of estimates provide a rather good ball park figure.
P.S. I double checked my printer. Everything checks out fine.
Last edited:
I am rather certain you are bang-on here with not only your calculations, but your conclusions as well, I think the J-20 is going to be a very capable aircraft even in her present configuration engine wise, and will likely go into LRIP configured as 2011 is presently. I doubt there are any big surprises, and when the WS-15 comes on line that will likely be incorporated as well, as some of your are aware there were some fairly substantial changes between the YF-22 and the F-22, and the flight control systems also took a little "tweaking" as the flight envelope was expanded.
So as we await the first flight of 2011, I am assuming that 2002 is at the Chengdu factory field and 2001 is still "out in the country", any word from the wall climbers of the flying status of 2001???
I had to do the same checking when printing my own smith charts. Luckily I don't think modern printers that accept vectorized format have such problems.
- Status