Tuesday, March 03, 2015

There's Legal Analysis, and then there's REAL Analysis

I usually enjoy Eugene Volokh's stuff. Though his move to the WP was disconcerting, I got over it.

But Volokh has a BIG swing and a miss in describing a 'growing' rationale for an argument that the Supreme Court should revist/reverse previous court opinions and enforce Interstate Sales Tax collection the way the greedy little state and local politicos/taxmen WANT it enforced.

The ' rationale' goes like this:
This argument has grown stronger, and the cause more urgent, with time. When the Court decided Quill, mail-order sales in the United States totaled $180 billion. 504 U.S., at 329 (White, J., concurring in part and dissenting in part). But in 1992, the Internet was in its infancy. By 2008, e-commerce sales alone totaled $3.16 trillion per year in the United States.
Sorry, those are RAW numbers. What does the trend look like factoring the overall economic environment.  I'm  assuming internet sales have supplanted most catalog sales:  a pretty safe assumption I believe.  And even if it's not, if someone can compare them directly across the timeline--well then! So can I.

Here's those two numbers Volokh used, lined up against GDP figures for private and government consumption and investment:

The big number for 2008 compared to the little number in 1992 overstates the growth in interstate sales. Complaints about 'missing revenues' from politicians reeks of unwillingness to compete, and worse a certain sense of entitlement to other people's dollars--no matter what the source--to feed the allmighty 'state'.

A rising tide lifts all boats

As the 'Barf Box' at the bottom of the chart indicates, NO ONE ever seems to ever ask the question as to what entities receiving all that internet revenue spend it on -and where? I would normally presume states with the better internet-friendly tax laws would benefit most and the local governments would be happy about it. But I realize we're dealing with people whose lives are often immune to the direct effects of the real economy, and I think more than a few resent the denial of an "opportunity for graft".

And I'm not buying any argument that the Local and State governments NEED those taxes on internet revenues: All hail the rise of the "Social Spending-Entitlement-Complex...
  State and local government is THE 'Growth Sector' of government. 


When the data is on your side, argue the data....

Sorry Eugene.

Tyler Rogoway Sets Off a Global Disinformation Cascade

He Owes the World an Apology but Don’t Hold Your Breath: It’s His 'Job'

Rogoway opens with the little Punk Journalistic lie in the title: F-35 Can't Carry Its Most Versatile Weapon Until At Least 2022Rogoway’s ‘article’ is about the F-35 and the Small Diameter Bomb Increment II (SDBII) , or GBU-53/B.
The BIGGEST problem with Rogoway’s hit-piece is that even within the article itself, it is recognized that all of the F-35 variants CAN carry the GBU-53/B, it is just at the moment the F-35B can’t carry as many internally as it will after 2020-22:
When it comes to the F-35B and this game-changing weapon, the problem is fairly simple. The F-35B, even with its truncated weapons bay compared to its A and C model cousins, was supposed to be able to carry eight SDB IIs internally.
The SECOND biggest problem with Rogoway’s hit-piece, is that it is written up as if it is the F-35 program’s and in particular the F-35B’s ‘fault’, when anyone who’s been following both programs for any length of time knows that the SDBII development contract was awarded to RTN in 2010, quite a few years after the ‘size’ of the F-35B’s weapon bays were set, but not before development was completed. The SDB II program went into that situation with eyes wide open.
Further, Rogoway's article (through his ignorance?) was/is written up without recognizing certain protocols of aircraft weapons design and integration. ESPECIALLY the one that requires new planes to be compatible with selected existing weapons for integration, AND that new weapons are required to be compatible with existing planes on which they are designated to be used. Normally, any conflict between the F-35B and the SDB II would be a ‘hit’ against Raytheon, but since Raytheon wasn’t being beat up for busting some Outer Mold Line envelope, it took me all of a few minutes to locate information WHY no one was pointing fingers (other than Punk Journalists that is).
The first clue is in the 2012 SAR
The SDB II program office has made considerable progress on the F-35 risk reduction effort to address the ongoing F-35 System Development and Demonstration program delays. The SDB II team successfully conducted F-35B and F-35C weapon’s bay fit checks utilizing production jets. The data collected during these fit checks will be used to finalize the modification of the F-35B weapon's bay. These efforts are on track and serve as a critical risk reduction event for both the SDB II and F-35 programs.
Talk about ‘concurrency’! This is multi-program concurrency! I'm surprised someone's not b*tchin' about that angle in this....yet. By 2012 both programs had experienced delays. More were to come. Those fit-checks referenced in the SAR 'happened' by the way.

So,,. is Rogoway spotlighting some F-35B issue not known before, not in work, or likely to ?
No.

Rogoway doesn’t like how the SDBII deployment is ‘happening’. Is that the F-35B’s “fault”?
No.

It’s a program decision made a while back (at least years ago: pg 117):
According to program officials, the biggest risk facing the program is integration with JSF. If JSF cannot meet its design specifications, then SDB II may not meet its requirements for weapon effectiveness or availability on that aircraft and may need design changes. The JSF is now integrating other weapons which will allow the program to determine the accuracy of its design documents. Many of these weapons have more stringent thermal and vibration requirements than SDB II. Additionally, SDB I will begin integration with JSF about 2 years prior to SDB II. By integrating with JSF after these weapons SDB II will be able to leverage data from these efforts.
Did Rogoway expect the entire F-35 program to re-jigger it’s schedule (no chance of delaying anything else is there?) JUST to put the FULL INTERNAL SDB II load on the F-35B earlier? I note here that if the program so desired, they could load 4 SDB II’s internally early and 8 later, and all the while put the full external pylon SDB II loadout on as well.

And it’s not as if the SDB II is any different than any other program. To hear Ol’ Ty tell it, the SDB is 'ready to go' and ONLY the F-35B weapons bay tweak is holding it up. Sorry, the SDBII story was still being written as of last year’s GAO rundown on selected systems (pg 119-120). I expect this year’s report will give the ‘all clear’, but it can cloud up again as well.
    
Writers don't always write the headlines to their stories. But I didn't hear anything about Rogoway being arrested for beating up his editor over it. Therefore either through commission or omission, Rogoway's culpable for this trash.

As an aside and IMHO, what this 'issue' looks like is a minor tweak of F-35B weapon bay content arrangements, including all the little ticky-tack pieces of suspension equipment, weapon interface busses, and stuff that is just passing through the weapons bay. I make this observation based upon the acute lack of hand-wringing in the responsible corners, the 2013 'fit check' that cleared the weapon-weapon and weapon-door clearance....and program spokesperson statements at Rogoway's source... that Rogoway didn't bother mention. Doesn't require much insight, just a lot less bias.

What's the point? 

Now, we’ll ignore all of Rogoway’s usual and lesser affronts. such as alternating between effusive awe and hellfire damnation over things that must seem to be like magic to him, use of ridiculous hyperbole (SDB II ‘A.I.’ -- ROFLMAO on that one), and all the self-referential linking to past articles where he didn’t do his research then either.

What’s important, is how Rogoway’s spew of disinformation flew around the world at the 'speed of heat' by the know-nothings. Example? First, Rogoway’s hit piece gets picked up by a New York free-lancer who got the UK Daily Mail to put out this verbose headline online at the top:
Pentagon's most expensive fighter jet which is set to be used by the Royal Navy on HMS Queen Elizabeth CAN'T carry most advanced weapons because of design flaw

Never mind, that the Brits aren’t planning on even buying the SDB II yet (even says so in the UK Mail piece), the title 'says it all'. Really?  REALLY.
90% of readers won't read the article, 90% of those who bother won't recognze the cognitive dissonance between the title and content.

But EVEN the free-lance writer, one ‘Zoe Szathmary (who may have even less consequential knowledge on just about anything 'defense' than anyone), provided more hard information than Rogoway:
Joe DellaVedova, a spokesman for the F-35 program, said in an Inside Defense interview last week that changes are being made to the jet so it can be released on schedule.
DellaVedova also said that Lockheed Martin will likely get the alterations contract down the line in 2015.
'This is not a new issue to us,' DellaVedova told Inside Defense. 'We've been working with the SDB II program office and their contractors since 2007.
'The fit issues have been known and documented and there were larger and more substantial modifications needed to support SDB II that have already been incorporated into production F-35 aircraft.
'These minor or remaining changes were put on hold until the aircraft reached a sufficient level of maturity to ensure that the needed changes would not adversely impact any ongoing SDB [II] developments.'
Inside Defense reported that the Joint Strike Fighter (JSF) program office so far has not publicly recognized problems - and that eight internal and 16 external SDB IIs are in fact meant to go on the F-35B.
Note the part above about the last "alterations" not yet being on contract? Is that important? I'm pretty sure it is. 

 Inside Defense had this out last week, and the DellaVedova quote above was in Rogoway’s source. But Rogoway made nary a mention of it. What’s the matter Ty, it didn’t support your meme? Or are you perhaps piqued because Joe won’t give you an interview too?? 

Moving on....

The story then bounced from the UK Mail to some Italian rag, who warps the UK Mail article into a message that the UK wants to but CAN’T use the SDB II.  Some Indian defense news website in turn, now limps in with another perversion of the story.

And I know a story has pretty much reached the sub-basement when this guy picks it up .

But this story is a smashing success by Gawker Media standards: A low-brow Dis-Information Cascade.

~Sigh~ 

We are being memed to death on the F-35 these days as the mass media races to  the bottom of the trough to make a headline. We’ll feel the interwebs hum for days about this crap, and then this 'non-factual factoid' will probably pop up for years--argued by the 90% who didn't read the article at the source but will remember Rogoway's lying headline.

Update: If I'd realized a thread at F-16.net had already started washing Rogoway's spew overboard while I was at work I would have just pointed people there. I got this ready for publication last night, decided to check in and relax and found Rogoway's piece pretty much in tatters

Sunday, March 01, 2015

The F-35 and the Infamous Transonic Acceleration Change

Part 3 : KPP Requirement or no, ‘What If’ the F-35 will someday really need to get rid of all or part of extra seconds in the new KPP enroute to Mach 1.2?

(Part 1 here: Part 2 here)
The F-35 Transonic Acceleration KPPs were ‘changed’. This change was almost immediately decried far and wide as a failure by the ill-informed and self-serving critics. I’m certain some have forgotten or missed the discussion in Part 1 where I referenced the DoD documentation describing the process by which KPPs are established, and changes are made, and that they are only changed if the changes are validated as being operationally acceptable. I’m certain others will miss the references in Part 2 where it was highlighted that they changes were always going to happen, if only because you cannot violate the laws of physics. (Those people may be sputtering at their computer screen soon if they aren't already). That the F-35 KPP changes were implemented at all proves the changes were acceptable by those responsible for the integrity of the requirements and supporting system engineering process.

This post and series is not for those people.

It has been for people who want to understand more about the changes and their ramifications. Agreement or approval by me or anyone else as to the acceptability of these KPP changes is irrelevant. The agencies to which they are relevant have already decided.(Right about here those ‘other people’ would mentally register a need to utter a Circumstantial Ad Hominem against those responsible that when you strip away all the unsupported assertions, logically translates into little more than “those people are the experts, what do they know?...oh and “they must be lying!” or “what are they hiding?”)

BUT!... probably, someday, someone flying one of the F-35 variants is going to REALLY need to accelerate M.8 to M1.2 as quick as the original KPP specified or even more quickly (just not often enough to pursue it as a KPP). Truth be told, there’s probably going to be a time or twelve where somebody is going to wish ‘instantaneous’ translation to Mach 10, but you have to draw the line someplace within the laws of physics.

IF there is some operational need to shave all or part of 8 seconds (or the longer times for the other variants) going from .8M to 1.2M, can it be accomplished? If so, HOW would it be done, and is the remedy needed so onerous that it will adversely impact the F-35’s viability or operational utility? With the F-35A needing only to overcome 8 seconds, the program office could have easily just specified a lower weight to accomplish the feat, and a checklist item added to the effect of “if you want to do ‘this’, then ensure fuel and weapons on board do not exceed X lbs”. But as fuel on board at a mid-mission point tends to be a valuable commodity, and weapons carriage is a fighter aircraft’s reason for being, I presume everyone can see why the users, JROC and program office didn’t take that route.

“One Weird Trick”

If, by chance, “8 seconds” is a big deal, even for an aircraft with a ‘greater initial acceleration than an F-16’ and it still means those 8 seconds crossed some breakpoint minimum needed operationally, then the good news is it is rather easy to accommodate. In fact, fighter pilots have been using ‘this one weird trick’ (Man, I hate those ads) for decades to squeeze extra acceleration out of the transonic region.

It’s called ‘Unloading’

Hat tip to Pat ‘Gums’ McAdoo, USAF Lt Col (Ret), at F-16.net for first reminding me of this ‘way back when’:
“RE: transonic accel....... I'll bet that the profile was st-and-level and then gofor it. We who have done it know that all ya gotta do is reduce AoA by lowering the nose a tiny amount and shazaam!”
You can also bet any future opponent that the F-35 runs into WILL also be doing the same thing. Sometimes ‘unloading’ will be the best thing for a pilot to employ, sometimes something else will be the best thing to do. Aeronautics has advanced to exploit the medium to its maximum. There is no magic airplane that does everything better than any other airplane within each generation or two: there's always a 'catch'.

Unloading the wing during an acceleration run through the transonic region pays off in increased acceleration (or reduces the decline of acceleration if that is all that is desired) immediately. It is also the key part of employing the Rutowski Profile (Ref #10 in Part 2) in reducing climb times for the same reasons. When a pilot completely 'unloads' the wing, all of the wing wave drag coefficient contribuion due to lift goes to zero, and the aircraft ‘sinks’ (altitude decreases). This is a relatively slower rate of descent compared to an actual ‘dive’, by the way. But a dive is also an option for even greater acceleration, and as Shaw observes (Fighter Combat, p. 407) it is even more effective the heavier and ‘cleaner’ an aircraft is-- assuming the dive does not take the aircraft past its structural limits. When that much wave drag suddenly disappears, the acceleration rate increases dramatically (Shaw recommends a “sharp pull-down” to a dive attitude then an ‘unload’ as particularly effective). Higher acceleration rates are like compound interest: the more you have earlier, the more the distance you cover and the more speed you have at the end: If the wing is unloaded early in a run when acceleration rate is already high but late enough that the aircraft is near Critical Mach, the time the plane would need to be unloaded might be exceedingly small for an F-35A to achieve 1.2M those eight seconds faster. It might help people to be able to visualize how little difference there is between M.8 and M1.2 at 30K feet if we use a graphic (we're keeping the numbering of figures as beginning in Part 1):
Figure 20: Every tenth of a Mach is about 40KIAS at 30K ft Altitude
An eight second difference for the F-35A in all probability translated into achieving something like Mach 1,1-Mach 1.18 in the original 55 seconds. That's small potatoes as far as overall speed goes. 

Here’s the example of a straight wing drag profile shown earlier:


Figure 21: Typical Drag Curve for a 'Loaded' Straight Wing 
Now, here’s a representation of what the ‘straight wing’ drag curve looks like if unloading the wing reduces the wing wave contribution by a conservative 40%:

Figure 22: Typical Drag Curve for an 'Unloaded' Straight Wing 
The total peak drag coefficient in this instance is reduced by 25%. Observe in the next figure that the peak drag coefficient ‘unloaded’ at around M1.1 is about the same as just before Mach 1 ‘loaded’, and that even in the typical Critical Mach range for fighter aircraft between M.8 to M.9 (Shaw, “Fighter Combat”, Pg. 399) the drag coefficient is reduced. 


Figure 23: Straight Wing Drag, Loaded vs Unloaded
IMHO, the F-35 Critical Mach Speed, like most advanced fighters, should be well into the upper half of that range IF the performance measures in the Bowman paper are to be believed (We’ll get to that in a moment). 

The actual amount of drag reduction will vary depending upon the total aircraft design (and remember it will also vary by airspeed and altitude). Shaw’s “Fighter Combat” provides a convenient graph highlighting the increases in acceleration that can be expected at different altitudes over a wide speed range for legacy aircraft. It is modified here to illustrate the altitude and Mach range of interest:

Figure 24: Typical Benefits of Unloading on Acceleration (Original source as noted, modified by Elements of Power)
We see in Shaw’s chart that for legacy fighter designs flying from .8 Mach to Mach 1.2, unloading increases the acceleration rates between ~17% at M1.0 to as much as ~20% at Mach 1.2 and ~25% at .8 Mach at the 30K ft KPP altitude (Ref #9). This appears consistent with what we would expect from the differences in the drag curves shown in our examples. It should also be readily apparent that both the F-35B and F-35C could employ the ‘unload’ technique in the same manner as the F-35A. Obviously, the F-35B would only need a small ‘bump’ compared to the F-35C. The smaller the time/speed difference needing to be closed, the more flexibility on timing and duration is available for the pilot to choose when and how long to ‘unload’.
Figure 25: Unloading and/or Diving to Achieve Original KPP times.
The F-35C’s obviously much greater wave drag due to lift than the other two variants (expanded upon in Part 2) also means it would just as obviously benefit MOST OF ALL from the unloading technique. If a dive would be necessary for the F-35C (as the ‘worst case’) to meet the old transonic acceleration standard, it could be able to carry that acceleration beyond 1.2 to perhaps even M1.6  (F-35 Max Speed in level flight. We do not know what the 'never exceed' speed is, so we’ll stick with what we know the F-35C can do. If ending the acceleration run at the same altitude (30K feet in the KPP) is important, the F-35 pilot can accelerate beyond M1.2 and so after climbing at the end of the run, trading smash for altitude, the plane is still flying M1.2 or better.

This might be a good time to interject the observation that if there was ever evidence the original KPPs were established before the aircraft design was even undertaken, it is the original common KPP of 65 seconds for the F-35B and F-35C: two airplanes with vastly different weights and wing areas. This tells me the trade space for the F-35C design might not have been fully understood when the Navy laid down the requirements or the KPP really was a 'let's see what we can get' figure.
We should also note here that the KPPs were/are against a predicted weight standard and that all indicators are that the predicted weights are being beaten, with all variants coming in (for now) below the modeled weights used for the KPPs.  Those lower weights mean lower wave drag to begin with, and in turn better transonic acceleration.   
Shaw asserts that unloading for best transonic acceleration is done close to ‘Critical Mach’. Bowman (Part 2 Ref #9) identifies the F-35A KPP threshold for max speed at 30K ft altitude without afterburner at >.96 Mach. This speed cannot be too far above or below F-35A Critical Mach because “at speeds faster than the Critical Mach number the drag coefficient increases suddenly, causing dramatically increased drag”. Afterburners (and/or unloading) would soon be required to go any faster once Critical Mach is reached.
Bowman BTW, is now a Navy Captain, and as far as I know continues to NOT be an F-35 'fan'. I'm still hoping for some karma backlash that will tie him to the F-35 program as a developer or operator, just for the entertainment value alone.   

Conclusions?

Other than changing the KPPS evidently was not a 'big deal', there's not much to conclude: just a lot of open questions remain.
That the program was warning far ahead of the KPP change that the spec was unrealistic (Ref #3 and #4 in Part 2), makes the initial acceleration KPP looks more like it was a ‘show us what you can do and we’ll revisit the issue later’ placeholder. An entirely likely possibility is that as the program matured, different advantages were seen in areas of aircraft capabilities that shifted design emphasis to 'someplace else'. A KPP may be set based upon an assumption of X capability would require KPP X to be one thing, and then as the design matures, and operational concepts evolve or even threat perceptions shift, it becomes apparent that X capability isn't as important as Y capability, which if is addressed would require more or less out of the KPP X.  

Examples? Program news over the years seemed to talk a lot about range and carrier approach handling as being 'big deals'. Did range become more important than acceleration? If so, fuel could have been added which increased weight and that reduced acceleration. For the C model, did the Navy decide a bigger, higher drag wing meant fewer carrier landing mishaps? It is also entirely possible one or more of the variant KPPs started simply as a wish-list item.

We've heard before about he F-35 KPPs being based upon aerodynamically 'clean'  legacy aircraft specs. Was that a 'miss' where the initial KPP-setters failed to recognize the physics involved, or they did recognize it but since there was no design to evaluate at the time, they let it go: leaving it as an admirable goal but also knowing the KPP could always be changed later? That last explanation makes most sense to me. I've dealt with the acquisition system bureaucracies for decades, and that kind of development  seems totally in character for how the system works. But whatever the reason, we can only 'guess'. We don’t really know 'why', and I wouldn't be surprised if no one is still around within the program who remembers the 'why'. It would make a terrific question for some politico to enquire about, especially if there was some dark secret behind the change as F-35 detractors seem to often insinuate.  

The Bottom Line, Again

For the F-35A model, the 8 second difference between new and old KPP appears to be trivial. The F-35B would probably have to unload a few seconds earlier or for a few seconds longer to meet the old KPP and the F-35C may have to unload a lot earlier and for a few more seconds to do the same.

But it is hard to say for certain, because the B and C early acceleration profiles may be just as good as pilots flying the F-35A assert about the CTOL version. The F-35C might have the toughest time making up the acceleration difference because it has a much larger non-lift contributor to wing wave drag to go with the larger lift contribution. But as it is also slightly longer overall and especially longer in the wing and tail surface areas, the F-35C shape in its entirety may (I don't think so but I can't rule it out) be lower drag than the others above Mach 1.1+. Or the F-35C may have other lower drag advantages due to something like its ‘cleaner’ wing attachment transition on the bottom surface. Who knows?

The newer 'changed' KPPs are as far as we know, still based upon modeled F-35 performance using “end-of-life, worst-case” scenario relative to the F135 engine’s power capacity” (Ref #7 Part 2) and “two per cent thrust” penalty (Ref #8 in Part 2). These ‘wedges’ against the F-35's performance could also be the difference between meeting the old KPP and needing the new KPP in the case of the F-35A, and part of the difference for the F-35B and C. 

The changes might also (almost certainly for the F-35C) have been due to encountering an ever-so-slightly higher than planned/predicted peak transonic drag coefficient, or some combination of the above. Contrary to what some might think, computational fluid dynamics and wind-tunnel testing do not prevent small surprises when the aircraft finally flies, they just lower the chances and severity. Sometimes those surprises go undiscovered for years (Bitburg Roll anyone?), and frankly I'm STILL surprised at how little difficulty the F-35 has had in some performance areas compared to legacy aircraft.  Beginning at about .8 Mach, even the tiniest drag [design] differences affect an aircraft’s [drag] performance out of proportion to the differences. The ability to cruise in military power at Mach 1.2 for some distance (Part 2, Ref#2) indicates that for the A and the B model at least, that ‘peak’ in the Drag Coefficient around Mach 1.1 is a fairly narrow one. 

The importance of the KPP change all comes back to what Tom Burbage was quoted as saying in Ref #4:
“...the biggest question is: are the acceleration characteristics of the airplane operationally suitable?”
The people buying and flying the airplane apparently say ‘yes’. And they've got the data and required knowledge and judgement to make the call. The voices outside the program do not.


Acceleration Sidebar: Is Acceleration More Important to Offensive or Defensive capabilities?

If I had to make a guess as to whether or not F-35 transonic acceleration was more critical in the A2A arena for closing on an enemy or escaping an enemy, I would say it is more important as an offensive attribute for closing on an enemy. Why? For two main reasons. First, I seriously doubt there is a fighter in existence or on the drawing board that can be expected to consistently attempt to overtake an F-35 without hesitation: or any aircraft that will competently shoot missiles over the shoulder at their pursuers. Second. The F-35 is being fielded as an optimal pack hunter: any aircraft pursuing one F-35 will always be a little more cautious if the pilot has reason to expect there to be another F-35 or two that he has no clue as to where it or they are.

Next Up: 

Stimulated by Commenter 'Tim A.' in Part 2, I'm going to do a quick parametric examination  of transonic acceleration differences between the 'ultimate' F-16 (Block 60) and the F-35A at A2G and A2A weights and loadouts. Nothing too extensive-- just something to ponder.

  

Sunday, February 22, 2015

The F-35 and the Infamous Transonic Acceleration ‘Spec’ Change:

Part 2:  Top-Level Analysis of F-35 Transonic Performance

(Part 1 Here)

Analytical Approach

To make the next part of our exploration as widely accessible and understood as possible, our goal will be to continue to minimize the amount of math and physics to the greatest practical extent: keeping things as simple as we can without doing violence to the phenomena. Think of it as a ‘studies’ approach. 

Therefore, I will try to base analyses upon relative differences in ‘sum totals’ and relative percentages or fractions to some baseline. We can’t get away from a minimal discussion of the nature of acceleration versus drag, but we can illustrate the things about drag and acceleration that we need to grasp without crunching too many of the numbers. Which is a good thing, because without the need to know, we don’t have the actual F-35 numbers (ex: drag values) available to us in the public domain for plugging into any calculations anyway.

First we’ll summarize the relevant things we now know-- or think we know-- about the F-35’s Transonic Acceleration KPP and overall performance. We will focus on the F-35A model in examining the relevant physics of accelerating through the transonic region and how they affect all aircraft and in particular the F-35A. We’ll then follow with a discussion examining just one likely ‘workaround’ that the F-35A might operationally employ (if it is even needed), and discuss the ‘tactical’ impacts and whether or not what may obviate or mitigate whatever the ‘popularly-perceived’ operational challenges the F-35 variants endure passing through the transonic region.

What we know or think we know

Here is what we know or has been reported as stated by the F-35 Program Participants including the user communities that will help guide the discussion.  Note: We’re numbering the references for easing later analysis and possible discussion.

1. The 2012 DOT&E report mentioned the following about the F-35 acceleration from .8M to 1.2M:
A.) A Model: Extended the time for acceleration from 0.8 Mach to 1.2 Mach by 8 seconds
B) B-Model: Extended the time for acceleration from 0.8 Mach to 1.2 Mach by 16 seconds
C) C Model: Extended the time for acceleration from 0.8 Mach to 1.2 Mach by “at least” 43 seconds

2. A Lockheed Martin representative was quoted in Air Force Magazine (cached) as stating the F-35 can maintain Mach 1.2 for a dash of 150 miles without using afterburners. Adding: "Mach 1.2 is a good speed for you, according to the pilots,".

(3, 4,and 5 are all from the same source)

3. “Based on the original spec, all three of the airplanes are challenged by that [acceleration] spec,” said Tom Burbage, Lockheed’s program manager for the F-35. “The cross-sectional area of the airplane with the internal weapons bays is quite a bit bigger than the airplanes we’re replacing.”…

4.“We’re dealing with the laws of physics. You have an airplane that’s a certain size, you have a wing that’s a certain size, you have an engine that’s a certain size, and that basically determines your acceleration characteristics,” Burbage said. “I think the biggest question is: are the acceleration characteristics of the airplane operationally suitable?”…

5. …U.S. Air Force Lt. Col. Eric Smith, director of operations at the 58th Fighter Squadron at Eglin Air Force Base, Fla., and F-35 test pilot, said that flying the aircraft is a thrilling experience.
“I can’t even explain the adrenaline rush you get when you light the afterburner on that thing,” Smith said. “The acceleration is much better than an F-16.”

6. The F-35′s acceleration is “very comparable” to a Block 50/52 F-16. “Again, if you cleaned off an F-16 and wanted to turn and maintain Gs and [turn] rates, then I think a clean F-16 would certainly outperform a loaded F-35,”Kloos says. “But if you compared them at combat loadings, the F-35 I think would probably outperform it.”   The F-16, Kloos says, is a very capable aircraft in a within visual range engagement–especially in the lightly loaded air-to-air configuration used during training sorties at home station. “It’s really good at performing in that kind of configuration,” Kloos says. “But that’s not a configuration that I’ve ever–I’ve been in a lot of different deployments–and those are the configurations I’ve never been in with weapons onboard.” – 

7. The weight purge of a few years ago was so “intensive,” Crowley said, that “there’s not thousands of pounds” of weight left to be saved on the F-35. However, even with a three percent annual weight growth, the key performance parameters, or KPPs, won’t be affected.
“All of our predictions for performance are based on an end-of-life, worst-case” scenario relative to the F135 engine’s power capacity, “so the true performance of the jet, throughout its life, will be much better.”

8. PARLIAMENTARY JOINT COMMITTEE ON FOREIGN AFFAIRS, DEFENCE AND TRADE 20 MARCH 2012 (Tom Burbage) Transcript
For any performance-related requirements, we artificially penalise [sic] the engine by five per cent fuel flow and two per cent thrust. Those margins are given back as we mature the design and get more and more solid on exactly what it is going to do. They are there for conservative estimation up front. We have not taken back any of those margins yet so, when those margins are taken back, the airplane will continue to be well in excess of its basic requirement.

Information from ‘other’ sources.

9. The baseline transonic acceleration specifications for the three variants prior to the spec change are believed to be:
A) A Model: The original “threshold” Key Performance Parameter (KPP) specification time for transonic acceleration (.8 to 1.2 Mach) was ≤ 55 seconds at 30Kft Altitude.
B) B Model: The original “threshold” Key Performance Parameter (KPP) specification time for transonic acceleration (.8 to 1.2 Mach) was ≤ 65 seconds at 30Kft Altitude.
C) C Model: The original “threshold” Key Performance Parameter (KPP) specification time for transonic acceleration (.8 to 1.2 Mach) was ≤ 65 seconds at 30Kft Altitude.

These times were asserted in an oft-referenced Air Command and Staff College paper authored in 2008 by a then-Lt Commander Geoffrey P. Bowman (USN). These numbers have been repeated in the popular aviation press (example) and Australian Air Force Air Vice Marshall Osley came close to ‘anecdotally’ confirming the F-35A KPP as asserted in the ‘Bowman Paper’ in testimony before an Australian Parliamentary Committee in 2012. The aircraft gross weights and fuel loads for the KPPs have not been revealed, though the Bowman Paper observes they are for internal-only weapons load configurations.

10. …This next reference is included because I believe the article that it is from was at the root of the comment that provided me with the impetus for these posts. It also hints at a way forward within 
An interesting factoid, one of the USMC test pilots mentioned this little tidbit—they have to use a modified Rutowski profile in order to get the F-35B and C up to Mach 1.6. Basically, you do one push over, unload the jet and accelerate, get up to 1.2, turn and repeat until you hit 1.4 Mach, turn and repeat till you hit Mach 1.6. It just barely gets there and barely has any gas left over afterwards. The kinematics are basically F/A-18C-like, though that was apparently exactly what was expected….

FYI and FWIW, here’s one of the more decent explanations of a “Rutowski maneuver” that I’ve found online: 
If the final speed is near the aircraft’s maximum speed, the large speed increase necessary renders the conventional method of using the peaks of the Ps curves useless. However, the energy method works well. Note in this example the optimum climb path includes an acceleration in a dive. This optimum energy climb path is also known as the Rutowski climb path, after its developer. The path (Figure 7.14) [at the link] consists of four segments to reach energy state E in minimum time. Segment AB represents a constant altitude acceleration from V = 0 to climb speed at state B. The subsonic climb segment follows a path similar to the one illustrated to the tropopause at state C. This subsonic climb is usually a nearly constant Mach number schedule. An ideal pushover or dive is carried out at constant Eh from C to D. The acceleration in the dive is actually part of the optimum climb path.
 (It's better at the source with the graphic)

11. There’s a Block 50/52 (F110-GE-129 or F100-PW-229 powered) Dash-1 supplement (Hellenic Air Force) out on the web that provides significant insight as to the F-16 performance baseline used to set the original F-35 transonic KPP.  

Deduced Transonic Acceleration KPP Times Analysis of the Variants

The most important performance ‘data’ in our hands are the believed-to-be-actual acceleration times
From References #1 and #9 above, we find evidence that the F-35 variants are currently expected to perform the transonic dash from .8 to 1.2 Mach in the following times:

Model Original KPP Time (secs) Exceeded by (secs) New KPP Time (secs)
F-35A   55                                                  8 63
F-35B 65                                                 16 81
F-35C 65                                                 43 (minimum) 118

Why the differences, and why by ‘that’ much?

IMHO these differences would make an excellent case study for an introductory course on aircraft design if only to just illustrate how much even small differences in design can affect aspects of performance in large ways. I can’t think of any other case where there are three ‘operational’ combat designs that are so close to each other but still different enough to tease out likely drivers of performance differences. (I am excluding aircraft that have had all sorts of various bulges and blisters scabbed on post-manufacture). The first question that springs from observing these differences is how much of the variation is due to variation in aerodynamic shape, propulsion efficiency, and weight?  

To help us investigate, we now need to take a look at the possible relevant differences. Those would be the differences between aerodynamic shapes, the engine installations, and relative weights: elements affecting the drag equation (Figure 3 from Part 1) which we now modify to remind us that the drag coefficient is but one element in the drag equation:
Figure 8 (Numbering Continued From Part 1
   
As the range of airspeed from .8 Mach to Mach 1.1 is the same, and the same air density (same altitude) are common to all variants, this reduces the number of variables on the drag side of the thrust-drag equation to two: the Drag Coefficient and the Cross-Sectional area. On the thrust-side of the equation, we will have less to work with but not so much less as to prevent us from making several observations and ‘educated’ estimations. Since this little exercise is about ‘understanding’ and not ‘quantifying’, I am comfortable working within these limitations.        

Relative Weights

We’ll model some likely weights (based upon the ‘Bowman Paper’ and acknowledged fuel and weapons carriage plans). Why ‘weight’? Because it is an excellent proxy for examining relative lift differences between the variants which “weight”, in level flight, must equal “lift”—the major constituent driver of ‘drag’.
If we take the empty weights, add half-internal fuel weight and the basic ‘day-one’ combat weapons load weight (with a wedge ‘rounding-up’ to account for the weight of weapon interfaces of each of the three variants), we arrive at a ‘likely’ set of aircraft weights for the Transonic Acceleration KPP:

            Empty Wt     50% Fuel Wt Weapons Load (2 JDAMS, 2 AIM-120s)     Total
F-35A 29300 9240 5000                                       43540
F-35B 32300 6750 3000                                       42050
F-35C 34800 9875 5000                                       49675

These configurations, whether exactly those used for the Transonic Acceleration KPP or not, are at least representative of the aircraft weights before release of air-to-ground stores and at the mid-mission point. They are therefore ‘realistic’ for use as a baseline for our explorations. They also cannot be too far off relative to each other to make too much of a difference in our search for understanding, as the only variable is fuel weight and the lower F-35B model JDAM carriage capability.
Using the F-35A Model weight as the basis, we find that over a range of mid-mission fuel loads, the F-35C weight, and therefore lift needed to be generated in level flight to be around 13-14% higher than the F-35A. 
Figure 9. Relative f-35 Variant Weights
We also see that largely due to a smaller total fuel load and lower weapons payload, the F-35B needs to generate approximately 3-4.5% less lift to maintain level flight. (This also again illustrates a point made in the Sustained G discussion about the lighter an aircraft, the more sensitive its performance is to weight changes.)

We can therefore conclude that the wing wave drag coefficient  percentage contribution to the total drag coefficient that is due to lift for the F-35B is less than the F-35A (under the same flight conditions in the speed region we are looking at of course), and that the F-35C’s wing wave drag coefficient contribution due to lift is significantly higher than for the F-35A. Further, we can conclude there is little, if any, difference between the wing wave drag contribution due to volume for the F-35A and B because the F-35A and F-35B wings are identical in area: they have the same fixed length (chord), and span. We  also know the area of the A and B wings are identical and their cross-sectional volumes very nearly so (more on this in the next section).  

We can also conclude that there is a large difference between the F-35A and F-35C in wave drag contribution as a percentage of the total drag due to both lift and volume. This is because the F-35C has a much bigger wing and control surfaces (more on this too in the next section). The overall coefficient of drag for the F-35C might theoretically be smaller than the A or B at some points along the curve, but that would have to come from reduction in the total drag via other means. For all we know, the total volume distribution of the F-35C comes closer to an ideal Sears-Haack distribution than the other two variants. We cannot definitively rule this out without more data, but given the differences between the F-35A and C in transonic acceleration, I believe we can assume with some relative confidence the F-35C has an overall drag coefficient much, much, higher than the A model, and even if the F-35C drag non-wing coefficient was lower than the A model, it certainly does not overcome the F-35C’s larger cross-sectional area. If the differences in drag coefficient due to wing lift were the only effective differences between F-35 models, we would expect the F-35B to have better transonic acceleration more on a par with the F-35A due to it weighing less when loaded, and we would expect the F-35C to be much slower in accelerating in level flight than either the A or B model. Since the A model has the best transonic acceleration, we should assume there is some other factor (or factors) affecting the F-35B performance beyond wing wave drag. Also, while it appears the C is the worst performer in transonic acceleration, it is almost certainly NOT due solely to wing wave drag contribution to total drag coefficient. This leads us to the next discussion, Aerodynamic shaping.

Aerodynamic Shaping 

Even this kind of comparison without hard data would be impossible if we were comparing completely different aircraft, or if we were seeking to quantify the differences instead of just understanding them. The good news is our limitations will make this section mercifully short.  

Cross-sectional area comparison 

First, we find that the cross-sectional differences between the F-35A and C are different in nature and scale than the differences between the F-35A and F-35B.
  
Figure 10. Cross-sectional Are Differences

The differences between the F-35A and B are more in the 'non-direct' lift regions of the cross-sectional area and are obviously due to the B model lift fan installation (#7). The wing profile is slightly increased at the lower wing root (#6) due at least in part to the STOVL roll-control system installation. 
The shape differences between the F-35A and F-35C are far greater. First, wing (#3) and horizontal stabilizer (#2) extensions as well as the taller vertical stabilizers (#1) add significant cross sectional area to the C model. Further, it appears the C model wing cross-section is indeed thicker (#5) than the F-35A’s to support the greater wingspan, but this could be an illusion from a longer curved under-surface and/or wing twist (either way, this presents an increased cross-section perpendicular to the airstream versus the A or B model). The F-35C wings also present an effectively increased cross-section to the airstream at the wing-folds (#4).  The F-35A and C do have a slightly increased cross-section versus the B model in the area of the tail hook enclosure (#8) but unlike the other cross-sectional areas, it is unclear how much of this is area that is presented perpendicular to the airstream in level flight.  Not known is if there are unseen relative differences in inlet design that affect not only installed thrust but also effective cross-sectional area, bur I believe we've captured the dominant difference drivers.

The Impact of Lift Surface Area on Wave Drag 

As already noted, Wave Drag due to ‘volume’ as a contributor is characterized in terms of a cross-sectional area for some fixed length. The F-35C is overall slightly longer than the F-35A and B, but its lift surfaces also have a significantly longer “fixed length” than the near identical F-35A and B surfaces. The F-35C’s wing and tail surface wave drag contribution due to ‘volume’ is therefore significantly greater than that for the F-35A and B. As a result we would expect the total drag of the F-35C in level flight to be much greater than the F-35A or F-35B due to having both a higher wing wave drag coefficient and greater cross-sectional area. 
     
Figure 11. Extended Vertical surfaces on C model not shown

Engine Installations

Whereas the first two topics of weight and aerodynamic shape dealt with the drag side of the acceleration equation, the engine installation aspect is relevant more on the thrust side of the equation. This is actually a pretty simple thing to compare on the F-35, as all three engines are essentially the same, with the major differences (performance wise) being between the F-35A/C engines and installations being highly common and the F-35B engine installation being very different from the other two. The physical differences of the F-35B installation-- the integrated lift-fan at the front of the system, the articulated exhaust duct with a different afterburner and nozzle installation at the other end, as well as a roll control nozzle system tapping flow off of the core engine in the middle cannot ‘help’ the conventional-mode of operations. Any significant performance differences between the F-35A and C engines would have to be due to any installation differences that are not apparent/acknowledged. 
For all practical purposes, we should then expect the F-35A and F-35C engines to have “the same” installed thrust and efficiencies. On the other hand, with the F-35B we would expect installed thrust penalties due to the added mass of the lift fan drive shaft and open roll-control ducting, the shaft passing through the bifurcated air inlets where they meet, as well as the different exhaust nozzle. We don’t know for certain, but it makes sense to believe the F-35B engine installation may be somewhat less efficient than the A or the C model in the conventional operation mode.

A Summary of Thrust vs. Drag

To summarize the relative differences between the variants that would affect transonic acceleration in a table:

Figure 12.  Factor Differences  between variants, F-35A Baseline 
In these same factors and relationships we find the seeds of understanding the operational implications of the F-35’s performance relative to each other in regards to the Transonic Acceleration KPPs. 

Transonic Acceleration: What is it good for?

The first thing to understand about the transonic acceleration KPPs is that they are proxies for the reasons WHY a fighter aircraft would want to accelerate quickly through the transonic region in the first place. The (obvious) answer is to gain a position advantage against an enemy in the air or on the ground by reaching that position to achieve an advantage before an enemy can position themselves to counter it. In the Air-to-Ground scenario this might mean simply getting to a high speed sooner to reach a time-critical target before it disappears or for some similar reason. We will ignore the air to ground aspect in thinking about the F-35s transonic acceleration KPPs and assume that because they are set at a 0K ft altitude, there wasn't a lot of Air-to-Mud consideration behind them. In the Air-to-Air combat it is about either closing some distance to gain a superior position on an opponent or opening some distance to gain position or keep the opponent from gaining a position on you. So the key thing to remember, and I hope the reader finds this an irritatingly obvious point, is that an acceleration KPP is about the ability to cover some distance over a set time when starting at a relatively lower speed. A transonic acceleration KPP is about being able to cover some distance as quickly as possible while moving ever faster through the particularly challenging transonic speed region.  This seems to beg the question: how significant it is really that the F-35A is projected to exceed its KPP by 8 seconds? (Note: We’ll get to the B and C KPPs in passing as we proceed.)         

F-35A Transonic Acceleration Performance

It has been determined by the F-35A program’s analyses that the original Transonic Acceleration KPP for the modeled F-35A will be exceeded by 8 seconds, taking 63 instead of 55 seconds. But considering we have multiple sources (#5, #6 above) citing excellent acceleration characteristics compared to an F-16 that imply superior initial acceleration, and the analyses are based upon degraded engine performance (#7, #8 above) how likely is it that the 8 second difference is even operationally relevant? Take a look at a second hand on your watch or clock. Watch it for 8 seconds.  If you were going M1.2 at the end of eight seconds, how fast were you going at the start of the eight seconds? M1.19? M1.18? 
Just from the drag equation and the drag coefficient charts we know that acceleration will be much greater at the start of the acceleration run than at the end. Is it possible the F-35A exceeds the initial predicted acceleration but just barely misses the full expectation?  Answers to these questions could have bearing as to whether or not extending the KPP by 8 seconds was even operationally relevant.  As it stands, it should still be viewed as superior to a Block 50/52 F-16. As we will see, Kloos (Ref #6) understates the F-35A’s abilities compared to the Block 50/52, at least at the 30K ft KPP altitude (Ref #10). 

How the F-35A new KPP standard stacks up against the F-16 Block 50/2 

If we examine the F-16’s ‘Combat Max AB’ transonic acceleration data (Ref #11, Table A8-12), and compare it to the F-35A’s newest transonic KPP time (63 seconds) we find the F-35A loaded with two AMRAAMs and two 2KLb JDAMs has better acceleration than the F-16C/D in 20 of the 30 possible weight/drag index combinations shown in the tables (weights from 20K lbs to 41K lbs, and Drag Indexes from 0 to 250).

Figure 13. F-35A KPP Superior to most F-16C Weight/Drag Combinations

Figure 14: External Stores start adding drag quickly
The F-35A KPP is on par with the F-16 in one of the possible F-16C load-outs. At least three of the remaining nine F-16 load-out combinations I would characterize as ‘highly impractical/improbable’ (for those you can have a few hundred pounds of usable fuel, or you can have two AMRAAMs but you cannot have both).






This is about as much as a Block 50/52 F-16C can weigh/carry without increasing the drag count:

Figure 15. Max 'Clean' Block 50/52 weight.( Source Ref 11)
Figure 16: Empty Fuel Tanks and Store Stations Still Add Drag 
Three more F-16 combinations involve simply carrying two more 600 gallon fuel tanks with various degrees of ‘fullness’ along with just two AMRAAMs and no other weapons other than the ever present 20mm cannon.

On the other hand, for 6 of the 20 load-outs where the F-35 KPP performance meets or beats the F-16C, the F-16 can’t even GET to Mach 1.2 (for three combinations the F-16 performance is ONLY subsonic).

In short, even if the F-16 is running on fuel fumes carrying wingtip AMRAAMs and
LANTIRN pods, it can only carry just a little more internal fuel before the F-35A's latest transonic acceleration KPP standard can be said to be ‘better’ than an F-16C Blk50/52 in transonic acceleration.

Figure 17: F-35A KPP performance vs. F-16 w/ DI=100
  
Figure 18: Only 2 AIM-9s greater load than F-35 Internal Load Out. It'll need those AIM-9s long before te F-35 will. 


Even the F-35B and F-35C transonic KPP times meet or beat a similarly-loaded but much lighter weight Block 50/52 F-16s:

Figure 19: With comparable war loads: F-35A and F-35 B beats all & F-35C beats most F-16 configurations.   

 The ‘meaning’ we can derive from the revised F-35A transonic dash KPP is this: It still represents the stated ‘F-16 Like’ performance goals and overall, it exceeds them. Other than that, everything else is guesswork. 

BUT!  The ubiquitous 'some' might ask.... 


‘What If’ Operational needs require the F-35A/B/C to get rid of all or part of those extra darned seconds enroute to Mach 1.2?  

THAT topic we will address in closing the series in Part 3.

Thursday, February 19, 2015

The F-35 and the Infamous Transonic Acceleration ‘Spec’ Change:

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.
We then explored what such a bank angle reduction MIGHT mean by performing parametric ‘what if’ exercises based upon certain assumptions. What we discovered was, is that the most important unknown appears to be aircraft total loaded weight and that the “baseline standard used [in developing the F-35 Spec] for the comparison was a clean Lockheed F-16 Block 50/52 with two wingtip Raytheon AIM-120 AMRAAMs”
In other words, the Sustained G spec was based upon an F-16 in a lightly loaded, and operationally-limited and therefore very non-typical ‘lightweight’ configuration. When an F-16A was loaded in a manner similar to the F-35’s ‘stealth’ weapons load, we found the F-35 may very well be significantly better than the F-16 in the ‘sustained turn’ category: It all depends on how heavily loaded each aircraft is relative to the other. 
For fun, we also showed that for the ‘spec’ F-35A to ‘turn like an F-4’ the F-4: 
1) had to be a late-model F-4E ‘slat bird’, 
2) had to be given every benefit of the doubt wherever there was any performance ambiguity, AND
3) it had to be loaded so lightly that it could only have a little over 8 minutes of fuel on board to achieve its maximum Sustained G turn performance. 

I also note here, once again, that ‘Sustained Turn’ is not now seen as important of a maneuverability parameter in the post-‘All Aspect’ missile era as it was before the all-aspect attack guided missile: Sustained Turn was more important when it was essential to get right on your enemy’s tail for a ‘kill’ while keeping him off yours.

Flash Forward: Today

I 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 Housekeeping

Because 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

If you know all this already, go ahead and skip the discussion about aerodynamics. If you know his stuff and read this part anyway, you will see there will be a lot I’m leaving out or perhaps oversimplifying. But I do so on purpose. Once again we’re conveying enough information to get an idea across, and not enough to go design an aircraft. These are just the ‘basics’, on the ‘basic concepts’ needed to understand what we’re going to discuss.


Drag: Subsonic and Beyond!
The most important things to remember about ‘drag’ for this discussion are that not only does drag increase due to friction the faster an object moves through the air, but that the ‘nature’ of drag changes as the object moves through the air at ever increasing speed. The airstream over the object begins transitioning from a ‘compressible flow’ to an ‘incompressible flow’. This occurs even before the object reaches Mach 1, as it enters the ‘transonic speed region’, with some surface areas of the object going ‘Mach’ (Critical Speed) before other areas on the object (aircraft). The transition from subsonic to the supersonic region is typically described as .8 Mach to Mach 1.2. [NoteAbove Mach 1.2 is considered ‘supersonic’ until much higher Mach Speeds where the nature of the flow changes again and is then viewed as ‘Hypersonic’ with the object moving through a ‘Plasma’ flow beginning at around Mach 5.]

This change in the interaction between the aircraft and airflow results in a change to the relative contributions of various contributors to total aircraft drag:
Figure 1: Drag Contributors, Subsonic vs. Supersonic

























Remember, not only do the relative contributions of the drag components change going from subsonic to supersonic, but the resultant drag for nearly all the ‘contributors’ increase as well. For example, here is a reconstruction of a typical straight wing drag profile, expressed in terms of drag coefficient (Cd): a dimensionless value used in calculating total drag (force). 


Figure 2; 'Straight Wing' Drag Coefficient increases through transonic region





























From Figure 2, it is obvious that the wave drag contributors, particularly the contribution due to the effects of the wing wave drag, are the overwhelmingly dominant factors. I’m going to risk oversimplifying wave drag somewhat here and just note that wave drag is composed of two contributors: Wave drag due to volume (cross-sectional area for some fixed length), 

and wave drag due to lift . One of the earliest advances in overcoming wave drag was the recognition that there were two contributors, which allowed designers to methodically attack the wave drag problem. (Though the occasional spark of genius didn’t hurt)
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
The only mysterious-to-some element in this equation should be the Cross-Sectional Area. This is the cross-sectional ‘slice’ of aircraft area presented to the air stream, and is perpendicular to the airflow passing over the aircraft. As the equation indicates, given the drag coefficient and cross-sectional area of any aircraft, drag increases as airspeed increases and when air density increases (density altitude decreases). Keep this equation in mind as we go forward: we will be relying on and referring to it from this point forward.
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
Note the effect of using the swept wing configuration. It delays the rapid onset of drag rise and also pushes the peak drag coefficient to higher Mach numbers. Remember also that this graph is scaled against peak drag coefficient at around Mach 1.55, with horizontal gridlines spaced at ‘25% of peak’ increments.

Figure 5. F-35 'Straight Wing', Extracted from a photo at www.JSF.mil 
The F-35 wing is a straight wing with swept leading edges. I suspect the F-35’s drag curve may be shaped something like a hybrid of the sample straight and swept wing curves shown, with a bias towards the straight wing drag rise curve shape. The acknowledgement that the F-35 can go some distance above Mach 1.2 without afterburner (to be shown in ‘What we know or think we know’, Ref #2 in Part 2 coming up) is a good indicator of the F-35A's drop in drag coefficient after Mach 1.1 as shown in the straight-wing graph vs a peak at Mach 1.55 as shown in the swept wing example.

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 

















Thrust and Acceleration

To accelerate at any speed, the thrust must be greater than the drag opposing the thrust. Jet engine thrust also decreases as the aircraft speed increases, because the difference between the aircraft velocity and the velocity of the engine exhaust becomes smaller as the aircraft accelerates. This isn’t a complete explanation, but it is the conceptual ‘bottom line’ and I don’t want to get wrapped around variability of air mass and internal engine drag among other things. For a more detailed discussion on the topic, NASA’s K-12 site has a pretty good overview (some high school math and physics employed).

In closing Part 1...

The most important point to remember going forward is the obvious one: At a given altitude, when airspeed is lower, the thrust is higher and drag is lower. Therefore, acceleration is greater. 


Figure 7.


As the aircraft moves faster through the airstream, thrust decreases and drag increases (Figure 7). Therefore the rate of acceleration will becomes less and less as the aircraft approaches the end of an acceleration run through the transonic region. If we can reduce drag and/or increase thrust between the beginning and the end of an acceleration run, then the rate at which acceleration (increasing speed over time) decreases will be lower (still accelerating, just doing so at an ever-slowing rate).

In Part 2, we will have a go at a top-level analysis of F-35 transonic performance.