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The F-35A's "wing loading" and why this is an archaic method

By: Dragon029 on Jun 17th, 2014  |  syntax: None  |  size: 5.67 KB  |  views: 752  |  expires: Never
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  1. The F-35A's "wing loading" and why this is an archaic method of deducing performance.
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  3. The quoted wing loading of the F-35A is 107.7lb/ft^2; according to Wikipedia. Pierre Sprey has said in one of his interviews that it's 108lb/ft^2 (rounded up, but that's fair).
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  5. Here's why it's not particularly relevant:
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  7. Wing loading is calculated by taking wing area and dividing by loaded weight (or another weight if specified).
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  9. Wing area however is calculated not by finding the area of the wings, or the area of the lifting surfaces of the aircraft, but rather by taking the leading and trailing edges of the wing, as well as the wingtips, and then making a hexagon out of them from where they meet in the middle of the aircraft. In the past, this was okay, because the fuselage's tubular shape didn't provide much lift; whatever deflection it provided could be approximated to be part of that "wing area" that extends into the fuselage.
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  11. On the F-35A, this is what it looks like: http://i.imgur.com/KluZpZc.png
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  13. Now, to confirm that this is true; we can scale the image so that the length and width of the image match the length and wingspan of the jet so that 1cm = 1 pixel, and then use a magic wand tool to find the area of that hexagon. This has already been done in that image, and as you can observe in the bottom, it comes out to be 432,808 pixels or cm^2. Converted to ft^2, that's roughly 465ft^2. If you divide the F-35A's 49,540lb loaded weight by that value, you get about a wing loading of approximately 106.5lb/ft^2; not exact (due to blurring in the image and the selection strength of the magic wand), but pretty close and definitely closer than what you'd get if you used only the wings, or the entire surface area of the jet.
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  15. But wait, seeing as aeronautics has advanced since we ditched slide rules and vacuum tube computers, why don't we at least see what the surface area of the jet is, seeing as it's a fairly wide aircraft?
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  17. If you do the same method, you get roughly 719,319 pixels (note that due to a larger perimeter and blurrier image, this value can swing a few thousand pixels): http://i.imgur.com/LlXQFSP.png
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  19. Convert to ft^2 and we get 774.27ft^2. Divide the loaded weight by that and we get a "surface loading" of 63.98ft^2 which is significantly lower. You can't call that figure the wing loading, because it's not equivalent, but I'll get to that soon.
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  21. But what about the F-15, F-16, F/A-18, etc?
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  23. Now allow me to point out that like the F-35, legacy aircraft utilise special techniques and features such as leading edge root extensions like on the F-16 and F/A-18. However, those aircraft were not designed with the computing power and additional ~30 years experience that the F-35 has. For example; one feature common on the F-35 and F-22 are chines; those sharp edges that come from the tops of the engine intakes and extend forward to the tip of the nose. Their primary purpose is to prevent radar from reflecting directly sideways, but they do also have another feature; when the jet has a positive angle of attack, they generate a pressure difference between the bottom and top of the edge, generating lift. Additionally, unlike the LERX of the F-16 and F/A-18, they go as far forward as is physically possible, meaning that when the jet needs to turn sharply, the additional lift at the nose generates more torque around the aircraft's center of mass.
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  25. Here's images of chines (and the other subtle advances of aerodynamic tweaking) in action:
  26. F-22 - http://i.imgur.com/6ejuu4e.jpg
  27. F-16 - http://i.imgur.com/WBoUxcU.jpg
  28. Note how further rearward the main body of the low-pressure zone starts; that difference, combined with the F-22's massive thrust, is what has made it the most maneuverable production fighter the US has ever produced.
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  30. Unfortunately the testing & weather conditions haven't allowed for any similar images of the F-35A to surface, but the simple fact that it shares the use of chines, as well as implements vortex-generating intake lips, etc means that the jet has a few tricks up it's sleeve.
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  32. The point is; while the F-35 is no F-22 or Su-35, the idea that it's a terrible fighter is simply unfounded. The use of wing loading in particular as a measure of performance also needs to change; a far more accurate measure would be to compare sustained and instantaneous turn rate as a function of airspeed and altitude. Obviously though that data either doesn't exist or is classified for many aircraft. In the mean time, one figure to consider (consider; not exclusively use; it shouldn't become 'the next wing-loading') is maximum take-off weight, as this provides a measure of the aircraft's ability to generate lift and change it's velocity vector, at at fairly low airspeeds.
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  34. Naturally a 747 has a much larger MTOW as an F-22, but when you're looking at aircraft with a similar size (eg F/A-18E and F-35A), the difference is more relevant (in case you're wondering; S.Hornet MTOW = 66,000lb, wingspan = 13.6m; F-35A MTOW = 70,000lb, wingspan = 10.7m).
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  36. Anyway, that's my rant / analysis - understand that while I think the F-35 is going to be a good aircraft, I most definitely do not deny that the development and procurement program was managed quite poorly (to use polite language). What I simply think is that it's issues have been severely blown out of proportion, simply by the scale & international involvement in the program; Sukhoi's PAK-FA / FGFA program for example has been facing quite a number of issues lately, yet the only complaints you'll hear are from India, because they're the only ones involved that have the will or ability to complain. When you involve the nations from which most internet users & mass media come from... well then you get what we hear today.
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  38. Thank you for reading.
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