Delete comment from: Elements Of Power
Hello SM, thanks for bringing this up.
As I didn't take the time to really drive certain points home that I've made elsewhere before and since when I wrote the piece, I appreciate the opportunity to do so here.
First, KPPs are not requirements, they are literally "Parameters" for evaluating the weapon system against the Requirements within a trade space. Those parameters can be adjusted as needed to ANY value so long as the overarching requirement involving multiple KPPs is not violated or requires change. It is not unusual to change KPPs for a reason or several reasons: from discovering they're not representative of what the requirement's intent is to the requirement itself being unachievable (not unheard of in new weapons system development where the reason the new weapon system program even gets going is because existing tech will not suffice for the future). The latter is ONLY a problem if a reduction/change would subvert the reason for the program existence. If there was a set specific time that the F-35 had to meet without external stores that could not be violated (for some unknown reason) to ensure its classified Lethality and Survivability requirements would be met, the transonic acceleration time KPP would not be raised above that time and a redesign would be undertaken. If a redesign could not solve the problem, a program could be cancelled if the problem made the value of the program unworthy of pursuit. You can’t “cheat” on a KPP if the KPP values still support the requirements.
Second, and as it happens, it was publicly acknowledged years before the KPP change that the KPP was established based upon CLEAN F-16 and F-18 performance and that adjustments would have to be made to recognized that the F-35 acceleration ability with internal stores was not degraded as it was for the F-16 and F-18 (http://www.f-16.net/forum/viewtopic.php?t=16745). I would re-emphasize here that NOBODY goes through the transonic regime easily, which means whoever accelerates better early getting to the transonic region has the edge, and pilot reports to date point to the F-35 as superior in that regard to an armed F-16 and F-18. I suspect further that the loaded F-35A may get to M1.1 much faster than a clean F-16, but that the last little ‘nth’ of V costs a few or more seconds due to higher drag rise than predicted, or perhaps incurred due to design features that support other KPPs.
Third, the ‘unloading’ advantage would not benefit an armed F-16 or F-18 to the extent that it benefits an F-35 because of their lighter weight (less drag from lift) and greater frontal area (non-lift) drag increases compared to the frontal area drag in their ‘slick’ state. I should have also emphasized more about ‘unloading’ not necessarily involving a ‘dive’ as well. Simply unloading the stick allows acceleration to increase speed and lift, which allows more unloading that gives more speed and lift, up to the point where minimum AoA is reached to generate the lift needed to fly level at the weight and speeds flown. Theoretically, you could unload carefully without losing any altitude at any time. Diving on the other hand involves a tradeoff: converting more potential energy to kinetic, that might have to be recovered later, but possibly on the other side of the transonic drag region peak (at a lower cost in V). Both are techniques already well known and in use. They also certainly allow greater latitude for designers and programs to meet hard requirements.
Sep 9, 2017, 12:48:05 PM
Posted to The F-35 and the Infamous Transonic Acceleration Change