Part 1: The Basics
Flashback...About two years ago, F-35 critics were agog over the news that the F-35 was reducing its “Sustained G” and “Transonic Acceleration” Key Performance Parameters. As (the once-but-no-longer-promising-and-now -‘Punk’) ‘Journalist’ Dave Majumdar reported on FlightGlobal.com:
Turn performance for the US Air Force's F-35A was reduced from 5.3 sustained g's to 4.6 sustained g's. The F-35B had its sustained g's cut from five to 4.5 g's, while the US Navy variant had its turn performance truncated from 5.1 to five sustained g's. Acceleration times from Mach 0.8 to Mach 1.2 were extended by eight seconds, 16 seconds and 43 seconds for the A, B and C-models respectively…Soon thereafter, I posted a short series where in the first part it was highlighted that the only truth one could conclusively draw from the Sustained G Spec change was that the F-35s would have slightly reduced sustained turn bank angles than planned. Anything else, including the relevance/significance of the change, would be speculation without additional knowledge.
Flash Forward: TodayI had left the transonic acceleration spec changes alone at the time it was ‘all the news’ because when I finished the ‘Sustained G’ posts, all the F-35 haters, anti-defense weak sisters, faint-of-heart, and the Joe Public mouth-breathers had pretty much moved on to complaining about something else. Also by the time I finished the Sustained G discussion, I didn’t really have the free time to quickly distill an explanation about transonic acceleration—or at least do so such that most people could understand the phenomenon if they put a little effort into understanding. After all, you can’t really simplify transonic acceleration with the same ease that you can with ‘sustained G’ because the former is about dynamic ‘change’ while the latter is about representing different states of equilibrium: nice and easy ‘steady state’ conditions.
A while ago though, I was reading a comment thread ‘someplace’ where there was ‘someone’ mixing claims about acceleration performance with top speed performance for the F-35C and complaining about the F-35 having to ‘dive’ to get to its top speed. I’m pretty sure he was referring to a comment made by a test pilot at PAX River (Naval Air Station Patuxent River)--also a while back--who talked about having to “accelerate, turn, unload, and accelerate” repeatedly within PAX’s range space to get the F-35C up to its top sustained speed of M1.6 using a ‘modified Rutowski’ procedure. I believe the commenter was incorrectly translating the ‘unload’ into a need to dive, versus the need to preserve speed during turns, just to make going through the exercise worth the effort within the limited range airspace allotted. This poor person’s mental flailing-about on something he clearly did not understand (alternatively, I suppose he could have been disingenuously misleading others--whatever) got me thinking again as to how we could best give some perspective as to what the announced changes to the transonic acceleration performance of the different F-35 variants might actually ‘mean’ without having someone pulling a synapse and then mentally limp right past the ‘Eureka!’ moment. Having thought about the subject for a while now, I now don’t think it’s too ‘hard’ of a write-up to produce – It’s just a tedious one.
Terminology HousekeepingBecause the media and others tend to use a shorthand to describe Key Performance Parameters (KPPs) as ‘specs’ or ‘Specifications’, we shall reluctantly do the same. KPPs are selected based upon their relevance to top level program requirements such as survivability, lethality, supportability, etc. The KPPs are the basis, as former F-35 PM Tom Burbage noted in 2012, “from which lower level detailed engineering specification are derived and Lockheed's job is to meet as many of those specifications as possible within the laws of physics”. In other words, KPPs are a vehicle used for deriving detailed engineering requirements from top-level operational requirements. They are initially established before the first design iteration comes out and it is not uncommon for them to be adjusted as more information about operational requirements and/or understandings of technical feasibility are refined. Though we will treat KPPs as requirements for the sake of simplicity, we need to understand that they are not immovable goals (or thresholds) that must be individually or collectively met, but instead are guideposts that show the way toward defining and then meeting engineering requirements that will support overall top-level program requirements. I’ve touched on this subject before, and this link still leads to the DoD Manual for the Operation of the Joint Capabilities Integration and Development System (JCIDS) which, with the references listed within the document, describe how requirements are determined and used, including the role of KPPs in the requirements process and the required steps/approvals to change KPPs.
Drag, Thrust and Acceleration
|Figure 1: Drag Contributors, Subsonic vs. Supersonic|
|Figure 2; 'Straight Wing' Drag Coefficient increases through transonic region|
The graphic above is scaled to reflect all values as a percentage of the maximum value. From this example we see that at Mach 1, the drag coefficient is but 75% of the maximum value reached around Mach 1.1, and that by Mach 1.6 the total Cd is less than 50% of the peak value. If we wished to calculate the total drag
force at any given speed, we could plug in the Cd value into the ‘Drag Equation’:
|Figure 3: Drag Equation|
While we do not know what the Drag Coefficient is for any of the F-35 variants at any of speed, we may have a general idea of what the ‘shape’ of the probable curve for each looks like. Here is a reconstruction of a typical swept wing aircraft drag profile, also expressed in terms of drag coefficient:
|Figure 4: Swept Wing 'Drag Rise' Curve|
|Figure 5. F-35 'Straight Wing', Extracted from a photo at www.JSF.mil|
Why Straight Wing?In case someone is asking the question, a simple NASA graphic drives home the point that a [swept] wing is ‘the way to go’ if a primary design concern is to reduce drag coefficient below about Mach 1.8. But there are other concerns, when it comes to fighter aircraft (such as 'maneuverability' and 'g-loading') that a straight wing provides certain advantages--such that a 'compromise' is often sought by sweeping the leading edges on an otherwise straight wing.
|Figure 6: Straight vs Swept Wing Decision|