ORIGINAL: ckfinite
I'm interested - how does adding a few cubic meters of volume to the front of the fuselage drive the entire aerodynamic design of the aircraft? The aircraft was always going to be about the same width and length (see following diagram), due to the size and volume of the internal stores and the need for serpentine intakes. How, exactly, did the lift fan's volume compromise performance?
Two issues:
1. Area ruling is messed up by that extra volume. Poor area ruling causes a large drag rise in the transonic region of flight, which happens to be where most air combat takes place.
2. The wing is too small. This came from keeping the weight down for the STOVL version. If you look at the equations that define many of the important aircraft performance parameters (rate of climb, cruise range, sustained turn rate, etc.) they all contain wing loading (weight/wing area) as a parameter. In general, high wing loading = poor fighter performance. (There are some exceptions.)
It's the poor aerodynamics that result in the EM deficiency of the aircraft highlighted by the recent "F-35 versus F-16D" report recently leaked
Have you actually read the report that Axe selectively quoted from? The pilot was addressing the effects of some absolutely brand new control law changes, not trying to dogfight. Specifically, he found that in high AoA positions, positions only made possible by control law changes made weeks before, a lot of energy was bled off. Furthermore, another complaint Axe highlighted was insufficient yaw control - control that, if you read the actual report, was available but inaccessible thanks to the control laws. These changes culminated in other BFM testing, where the F-35 performed much better. The F-35's flight control software needed evolution, and tests like that inform the software development.
This was essentially telling the pilot "go out and test the software," in a plane that isn't really representative of the final product (AF-2 is limited to just 5G, for just one example). It didn't have much bearing on the F-35's overall performance at all.
I sure did read those reports. To me, the most important section is the "Energy Management" section.
Overall, the most noticeable characteristic of the F-35A in a visual engagement was its lack of energy maneuverability.
There are several other places where the issue is the control laws, such as the high-AOA regime. Great. That can be fixed with software. But you can't fix EM with software.
As for the aircraft not being representative of the final product, that cuts both ways. It was lacking much of the low-observable materials, which reduced its weight, which improves its EM performance. So there were advantages and disadvantages to using that pre-production model. Personally, I think those effects all wash out and are lost in the noise.
Someone else mentioned Dr. Kopp as a source for more information. You have to be careful with his writing because some of it is good and some is garbage. However, he does understand basic aerodynamics, and his early writings accurately predicted the poor EM performance of the aircraft. Of course, so was everyone else who was capable of basic math and knew the right equations.
By the way, I tried looking for Wikipedia entries for the equations so you can run the numbers yourself, but for once Wikipedia let me down. I was using Raymer's aircraft design book, but it's a bit on the expensive side.