Questions for Aerospace/Defense “Journalists”

A few questions for the Aerospace/Defense “Journalists” out there.

What are the odds that two different ‘news’ sources (one, two -- and two is worse) would come up with--shall we say, “twisted” articles concerning the standup of the first USMC F-35 squadron in Yuma within a day of each other without pre-coordination or as a reflex to information coming from a common pro-active source?

4:10 (F-35) to Yuma

What are the odds that the articles would also have the same basic twist (emphasis on the work ahead and not goals achieved, consistent repetition of old problems minimizing or ignoring progress made on known issues, and using the same sort of breathless reporting to make relatively minor technical problems appear as if they were of grave consequence) ? Again, without pre-coordination, or in reflex to information from a common, pro-active source?

What are the odds of these same articles being issued at essentially the same time after months of nothing but a long string of good news and successes coming out of the F-35 program?

What are the odds this ‘coincidence’ occurred after the election by design? It seems the closer we got to the election, the quieter the anti-JSF crowd became.

I don’t smell conspiracy coming from the usual circle of defense writers.

I smell sloth, smug ignorance, gullibility and possibly the spoor left behind by the usual suspects in the rabid Anti-Defense Establishment.

Fess up boys. Who’s writing your first draft these days?

Did your 'sources' get wind of this version of the story ahead of time and exhort you to perform damage control?   

Air Force Magazine on the Latest From the F-35 Mavens

Part 2 of How Ever Many It Takes
Part 1 Here 

Solomon over at SNAFU! posted a piece centering on an excerpt from an Air Force Magazine article “The F-35’s Race Against Time” (November 2012 issue). I had read it already, and didn’t see anything ‘earth-shattering’ at the time. But with Sol’s posting, it occurred to me that it would probably become more interesting to people the further you got away from those familiar with the current state of aeronautics, and it may draw secondary comments from the anti-JSFers to boot.

Dramatic Stealthiness (The Air Force Magazine Heading Not Mine)

The F-35’s radar cross section, or RCS, has a “maintenance margin,” O’Bryan explained, meaning it’s “always better than the spec.” Minor scratches and even dents won’t affect the F-35’s stealth qualities enough to degrade its combat performance, in the estimation of the company. Field equipment will be able to assess RCS right on the flight line, using far less cumbersome gear than has previously been needed to make such calculations.

 This will generate a major military culture change. If not, dollars to donuts either wing kings will start having strokes if they are told they shouldn’t touch “minor scratches and even dents” or someone with a paint can just won’t be able to resist making his jet ‘purtier’. The second alternative future already occurred when the Navy first fielded the F-18E/F.

Low Observability: In From the Start or Not At All

Stealth, said O’Bryan, has to be "designed in from the beginning" and can’t be added as an afterthought or upgrade. That means radar, electronic warfare, data links, communications, and electronic attack "need to be controlled" and must be fused from the start to work in concert with the special shapes and materials of the airframe itself.  

And that beauty has to be more than skin deep. You cannot underestimate the requirement to design the low observability into a plane as an integrated whole. Every academic/scientific text I’ve ever read on the subject stresses this point. Which makes Carlo Kopp’s latest as noted by David Axe over at Wired seem even more clownish. (I got a call from a friend and colleague who told me about it this weekend amid hearty laughter. I may slap it down with extreme predjudice in the very near future.)

The F-35A fighter has an active electronically scanned array radar and unique antennas spaced around the aircraft so that it can direct radar energy precisely, with minimal "bleed" in unintended directions. That puts more power where it’s wanted and reduces emissions that can give away the F-35’s position.  

In addition, it uses machine-to-machine communications with other F-35s. Emitters such as the radar and the electronic warfare system can flash on and off among all the F-35s in a flight.

AESA radars have most of this in common. What is intriguing is the question of what is meant by ‘unique antennas’? Could we guess what some are? Probably. Will we? No.
This also lists or hints at several advantages of modern radars with integrated EW suites. They’re not just 'spread-spectrum'. The 'low probability of intercept' idea is reinforced by beam control and short duty cycles, which tend to make the radars appear to be ‘noise’ more than anything else to most systems most of the time (nothing is certain).

'Only' 25% of “Too Much Signature” is… Still Way Too Much

O’Bryan took skeptical note of other fighter makers’ boastings that they have reduced by up to 75 percent the radar signatures of their fourth generation aircraft. He finds the claim perplexing; their original signatures are so massive, he says, that even a 75 percent reduction still leaves a huge radar return. These uprated fighters are visible within the maximum range of adversary air-to-air missiles, he said.  

"You basically haven’t really done anything, in terms of a practical tactical advantage against an enemy," said the Lockheed official. Worse, the RCS reductions evaporate once nonstealthy ordnance, fuel tanks, and other stores are hung on the "clean" aircraft.

"Until you have a first-shot, first-look, first-kill" capability, said O’Bryan, "you’re still at the same standoff [range], hoping that training and tactics are going to overcome a potential adversary." 

The most shocking thing is the fact that those points even needed to be said out loud.

Imitation is the Sincerest Form of Flattery

China and Russia have recognized the fallacy of trying to make a silk stealth purse out of a nonstealthy sow’s ear. That is why China is vigorously pursuing the J-20 and Russia the PAK-FA stealth fighter designs. If their programs pan out as expected, said O’Bryan, "fourth gen airplanes are really going to be at a serious disadvantage" against them.  

In a modern A2/AD environment, no fourth generation fighter can survive, O’Bryan insisted, no matter how much support it receives from jammers. In such an environment, however, the F-35 can fly in relative safety, with more range than the F-16 and with the same combat payload.  

When enemy defenses have been beaten down, and the need for stealthiness is not so strong, the F-35 will use both internal and external stations. That would boost its carrying capacity to a full 18,000 pounds of ordnance—more than triple the F-16’s max load of 5,200 pounds.

The transition to a non-LO configuration will always be situational: a judgment call, based upon a Commander’s weighing of the relative risks and benefits. Immediately after Desert Storm, the assumption was that once the IADS were taken care of, the switch to non-LO assets could be made in short order. After Operation Allied Force, where the Serbs made a decision to shepherd and conserve their air defense assets as much as possible, the thinking shifted away from simply assuming the transition from LO to Non-LO operations would occur.

O’Bryan said the F-35 is an all-aspect stealth aircraft—that is to say, stealthy from any and all directions. 

Now, this is not news. But watch the watchers parse the above as consistent with their current views of F-35 low observability. Expect exclamations akin to “He didn’t say it was all aspect VLO or other such drivel. I can call it drivel because if the F-35 low observability meets the design requirements, it is ‘stealthy enough’. If it were more ‘stealthy’ than it needed to be, the same people would b*tch about it driving cost.

No Vectored Thrust, No Woe Is Us

Cost and performance trade-offs were made when it came to designing the F-35’s exhaust system, O’Bryan said. Lockheed Martin chose not to employ a two-dimensional thrust-vectoring nozzle, as it had on the F-22 Raptor.  

For one thing, the decision reduced cost. For another, it eliminated one of the larger practical challenges to maintaining the stealth characteristics of the F-35.  

The classified "sawtooth" features that ring the nozzle help consolidate the exhaust into a so-called "spike" signature, while other secret techniques have been employed to combat and minimize the engine heat signature.  

"We had to deal with that, and we dealt with that," O’Bryan said, declining to offer details.

This is kind of funny, because by logical extension, the question of “why not F-22-like thrust vectoring?” could also be logically carried forward to “Why only two-dimensional thrust vectoring?” for the F-22. The answer is of course the same: "insufficient performance return for the investment".

O’Bryan certainly couldn’t go into the subject of the fighter’s EW/EA suite in any detail, or the way it might coordinate with specialized aircraft such as the E-3 Airborne Warning and Control System, RC-135 Rivet Joint, E-8 JSTARS, or EA-18G Growler jammer aircraft.  

He did say, however, that F-35 requirements call for it to go into battle with "no support whatever" from these systems.  

"I don’t know a pilot alive who wouldn’t want whatever support he can get," O’Bryan acknowledged. "But the requirements that we were given to build the airplane didn’t have any support functions built in. In other words, we had to find the target, ... penetrate the anti-access [defenses], ... ID the target, and ... destroy it by ourselves."  

O’Bryan said the power of the F-35’s EW/EA systems can be inferred from the fact that the Marine Corps "is going to replace its EA-6B [a dedicated jamming aircraft] with the baseline F-35B" with no additional pods or internal systems.  

Asked about the Air Force’s plans, O’Bryan answered with several rhetorical questions: "Are they investing in a big jammer fleet? Are they buying [EA-18G] Growlers?" Then he said, "There’s a capability here."  

O’Bryan went on to say that the electronic warfare capability on the F-35A "is as good as, or better than, [that of the] fourth generation airplanes specifically built for that purpose." The F-35’s "sensitivity" and processing power—a great deal of it automated—coupled with the sensor fusion of internal and offboard systems, give the pilot unprecedented situational awareness as well as the ability to detect, locate, and target specific systems that need to be disrupted.  

Translation: I’m not ‘saying’, but look at what you know already. Nudge, Nudge, Wink,Wink.

DEAD vs.SEAD

When it comes to electronic combat, the F-35A will make possible a new operational concept, O’Bryan said. The goal is not to simply suppress enemy air defenses. The goal will be to destroy them.  

"I don’t want to destroy a double-digit SAM for a few hours," he said. "What we’d like to do is put a 2,000-pound bomb on the whole complex and never have to deal with that ... SAM for the rest of the conflict."  

At present, that is difficult to do. Adversaries, O’Bryan pointed out, recognize that the basic American AGM-88 High-Speed Anti-Radiation Missile has a light warhead able to do little more than damage an air defense array. Thus, they have adapted to the threat by deploying spare arrays with their mobile systems.

I’d say 1. DEAD vs SEAD is not new and 2. O’Bryan is somewhat over-simplifying. One 2K bomb’s blast radius wouldn’t take down a double digit SAM complex, He’s obviously talking about going against the control van or other control node with a single weapon. I think more likely would be single pass with SDBs en mass against multiple aimpoints. Basically a scaled down version of what the B-2 did in Allied Force against targets such as the Krivovo Support Base:

Krivovo Support Base, Post Strike, Operation Allied Force

In the photo above are very large warehouses struck in one pass by 2K JDAMs in Operation Allied Force. Note how the bombs were placed at alternate ends of the inside two warehouses for each four building set. The overpressure/blast also damaged the adjoining buildings, staggering and spreading the weapons allowed for maximum destruction in minimum time and aimpoints. Note also how the ends of the buildings that were struck were selected to also impede access to the target by the adjacent road as much as possible.

Since 1999, weapon accuracy and flex target capabilities have only gone up, and while the subject SAM radar, control, and transporter-erector-launchers are smaller targets, they are also "soft" when you find them. And between the EOTS (video) and the AESA(video), they will be found. And we won’t even go into how easy they are to find post-launch when you can track a missile back to the launch point (video at http://www.youtube.com/watch?v=qF29GBSpRF4, link will not embed for me right now). Postt launch detect and destroy is not a trivial point because there are alot fewer TELS than missiles. Destroying TELs early pays off big as you go down the timeline of a conflict.   

End of Part 2
Part 3 Here

Air Force Magazine on the Latest From the F-35 Mavens

Part 1 of How Ever Many it Takes

Earlier this week, Solomon over at SNAFU! posted a piece centering on an excerpt from an Air Force Magazine article “The F-35’s Race Against Time” (November 2012 issue). I had read it already, and didn’t see anything ‘earth-shattering’ at the time. But with Sol’s posting, it occurred to me that it would probably become more interesting to people the further you got away from those familiar with the current state of aeronautics, and it may draw secondary comments from the anti-JSFers to boot.


500th Sortie of an F-35A Lightning II joint strike fighter from the 33rd Fighter Wing at Eglin Air Force Base, Fla., flies over the Emerald Coast Sept. 19, 2012 preparing to land. (From Original U.S. Air Force photo/Master Sgt. Jeremy T. Lock)

The magazine itself is the monthly publication of the Air Force Association, so one could almost view it as an ‘unofficial’ Air Force’s newsletter. It’s a glossy-photo publication designed mostly for consumption by AF/Defense insiders and Hoi Polloi, so you won’t find improperly leaked technical details or rants about programs, policy or planning. There are special topics the magazine takes on from time to time where they go into great depth on the topic, but that is not their normal fare. What you will find frequently are general info articles that sometimes yield nuggets of gold: first glimpses of information as it is officially released into the public domain. The excerpt Sol was excited about was the first public revelations of the F-35s capability to travel some distance over Mach 1 without afterburner. Not really ‘supercruise’, but surely a performance surprise for the F-35’s more ardent critics.

I thought I’d add to Sol’s posting with some more excerpts and observations on some things that may be of interest in the same article.

Lockheed Martin Vice President Stephen O’Bryan, the company’s point man for F-35 affairs, declared that the fighter meets requirements. A former Navy F/A-18 Hornet pilot, O’Bryan said the combat capability of even the earliest baseline model will greatly exceed that of the most heavily upgraded fourth generation fighters and strike aircraft, such as the F-15, F-16, and F-18.

The one thing I think we should take away from this point is the implied fact that the first full-rate production versions of F-15, F-16, and F-18 aircraft were NOT representative of the fully capable type, and many were bought in relatively large numbers compared to the total buys. ALL underwent significant, mostly preplanned design ‘upgrades’ before their definitive types were finally fielded. By ‘definitive’ I mean most or all the capabilities that were programmed to be part of the platform when production began were either installed or ruled out.

Example: The Lightweight Fighter Mafia and their fellow travelers consider the early F-16s to be ‘definitive’ from their POV, but until the Block 30/32 aircraft were brought to realization, there were considerable shortcomings seen in the type, and they were seen by the responsible players from the start:

Gen John J. Burns, a man with impeccable fighter-pilot credentials antedating any of those belonging to members of the “Fighter Mafia,” enthusiastically endorsed missiles— especially the BVR variety. This inclination largely accounted for his skepticism about the lightweight fighter. As it stood at the time, because the F-16 did not have a sufficient radar for semiactive AIM-7s, it could usually fire on an enemy only from the rear quadrant— whereas an enemy with a radar missile could shoot one in the face of the F-16 pilot. Since World War I, the plane taking the first shot has a rather pronounced tendency to win.  

As noted, not until the AMRAAM got its initial operational capability in the early 1990s did the Viper acquire a BVR weapon—itself a “high-tech” answer. General Burns’s attitude is neither new nor limited to senior officers. From the beginning, one could find in the Fighter Weapons Newsletter of the late 1950s great enthusiasm for the new missiles among junior fighter pilots. For example, Capt Robert Thor, writing in 1958 while Boyd was still assigned to Nellis, argued that in the near future a fighter pilot who came back claiming a gun kill would be confessing a failure to use his missiles properly. -------Dr David Metz “Boydmania” (some solid debunking of ‘Boyd’ myths at the link)

Capabilities

The Air Force Magazine article continued…

The fighter’s capabilities will make it a three- or four-for-one asset, said the Lockheed briefers, meaning that it will be able to simultaneously perform the roles of several different aircraft types—from strike to electronic attack, from command and control to battlefield surveillance.  

O’Bryan pointed out an important truth about air combat: Fourth generation strike aircraft assigned to hit targets guarded by modern anti-access, area-denial systems (A2/AD, in military parlance) require the support of "AWACS, electronic attack, sweep airplanes, SEAD" (suppression of enemy air defenses) aircraft and cruise missiles. Such a package could run to dozens of aircraft.  

The same mission, he claimed, can be achieved with just a quartet of F-35s. Each would be capable of operations that go well beyond air-to-ground missions. The four-ship would be a potent factor in any scenario calling for the employment of airpower, O’Bryan asserted.

The first paragraph is OK as long as we’re talking about missions versus numbers. Lanchester’s Square laws still apply, though the ratios may vary, and keep in mind one airplane can’t be in more than one place. The assertions made in the two paragraphs following the first indicate that this is O’Bryan’s intent, but I can see people confusing missions and end strength if they don’t know any better.

Next the discussion moved to ‘Maintainable Stealth’….

When it comes to maintainable stealth design, the F-35 represents the state of the art, O’Bryan said, superior even to the F-22 Raptor, USAF’s top-of-the-line air superiority aircraft.  

The F-22 requires heavy doses of regular and expensive low observable materials maintenance. F-35 stealth surfaces, by contrast, are extremely resilient in all conditions, according to the Lockheed team. 

"We’ve taken it to a different level," O’Bryan said. The stealth of the production F-35—verified in radar cross section tests performed on classified western test ranges—is better than that of any aircraft other than the F-22.  

This, he went on, is true in part because the conductive materials needed to absorb and disperse incoming radar energy are baked directly into the aircraft’s multilayer composite skin and structure.  

Moreover, the surface material smoothes out over time, slightly reducing the F-35’s original radar signature, according to the Lockheed Martin official. Only serious structural damage will disturb the F-35’s low observability, O’Bryan said, and Lockheed Martin has devised an array of field repairs that can restore full stealthiness in just a few hours.

This is a mixed bag to comment on. On the one hand, yes the F-35 LO design approach was heavy on incorporating lessons-learned from prior systems, and it appears the result is solid. But if you want to make commentary on any earlier LO designs, you have to also acknowledge the reason why the lesson ‘took’ was that designers figures out that the first peacetime priority for wing commanders is flying schedule and pilot proficiency.

I suspect the F-22’s peacetime LO maintenance burden is skewed by commanders opting to NOT fix LO discrepancies when they appear and let them fester and grow lest they threaten the flying schedule. Funny thing how metrics can drive the performance instead of measure it: if Commander performance reports are involved they usually cause people to care about what they measure MORE than measuring what they should care about.

I also have a minor problem with the blanket F-35 RCS performance “is better than that of any aircraft other than the F-22”. If he would have said ‘fighters’, I’d MIGHT be fine with it. But since LO design is tailored to the mission and operating environment, non-fighter LO systems’ LO performance are not comparable. I won't even mention that he couldn't possibly be briefed on every program to make such an assessertion, nor would the people running the range likely tell him more than he needed to know, and he wouldn't need to know the performance of other systems.

End of Part 1
Part 2 Here

Extreme Angle of Attack: What is It Good for?

Not as much as you might think.

In looking at AoA papers related to the series I just concluded, I ran across a paper that was able to express what good Aeros everywhere already know. Higher maximum AoA does not translate into more "vector" change if it sucks down your "smash".

Perhaps this gives some indication as to why there is a 'specified' AoA for the F-35?

(and all the AoA over and above that point probably won't count for much.)

From: EVALUATION OF FUNCTIONAL AGILITY METRICS FOR FIGHTER CLASS AIRCRAFT, B.W. Cox and D.R. BrDowning University of Kansas, AIAA-92-4487-CP, 1992.[boldface emphasis mine]

 

The following conclusions are drawn from the evaluation of the metrics:

1) The Combat Cycle Time metric is dominated by the sustained turning and acceleration phases which are already assessed by traditional measures of merit. However, the metric provides insight into two features of aircraft capability not considered by traditional measures of merit. First, the metric provides a measure of the ability of the aircraft to transition between two sustained aircraft maneuver states. Second, the metric's time based value rewards quick flight path whose pointing maneuvering while simultaneously balancing this reward with a penalty for large energy losses.


2) The Dynamic Speed Turn plots do not measure any new aircraft capabilities and thus do not assess capabilities not already addressed by traditional measures of merit. Rather, the strength of these plots is derived from a) their ease in depicting the penalty paid for achieving certain performance levels at various airspeeds and b) their ability to clearly compare the maneuver capability of dissimilar aircraft. To obtain the same information from traditional energy maneuverability diagrams requires a more timely and detailed analysis and the resulting comparisons between dissimilar aircraft are not as concise.


3) The Relative Energy State metric has two features not addressed by traditional measures of merit. First, the metric stresses the importance of retaining maneuver potential since either defensive or offensive maneuvers may be required after a turn in a multi-bogey environment. Second, the deceleration to corner speed is clearly quantified.


4) The quantification of these metrics showed that the angle of attack load factor limiter in the F-16A provided that aircraft with a definite advantage over the F-18A, F-5A, and X-29A. The limiter, although reducing the maximum turn rate, prevented the aircraft from reaching high energy bleed rate conditions. This limiting essentially relieves the pilot of the responsibility of having to set up an optimum maneuver. However, the metrics quantified here do not provide an ability to assess the benefits of performing high angle of attack maneuvering - a capability which could potentially offset the associated energy losses.

THE DESTROYER MEN: A Veteran's Day Tribute

The evolution of a WW1 tribute to destroyer crews.

(Something different for Veteran’s Day )

Berton Braley is little heard of today, but early in the 20th Century he was quite a celebrity…and prolific poet. I wonder how much his obscurity today is due to him being a commercial success AND a ‘Philosopher of Freedom’? I discovered him quite by accident, trying to track down the origins of a poem titled “Destroyer Sailors” that my Grandfather had scribed in the back of his ‘Cast Iron College’, aka Machinists Mate School, notebook in 1926. He attributed the work to ‘Sunset Red the Poet’. Using that as a starting point, trying to find who wrote the original turned into an interesting journey: it appears the poem has been adapted to different eras, with slightly different titles, and attributed to various people as it has been handed down through time. The original poem was written during World War I, titled The Destroyer Men, and back when ‘destroyers’ themselves were relatively new:

Source: Navy.mil

 
THE DESTROYER MEN
There’s a roll and pitch and a heave and hitch
To the nautical gait they take,
For they’ re used to the cant of the decks aslant
As the white-toothed combers break
On the plates that thrum like a beaten drum
To the thrill of the turbines might,
As the knife bow leaps through the yeasty deeps
With the speed of a shell in flight!

Oh ! their scorn is quick for the crews who stick
To a battleship s steady floor,
For they love the lurch of their own frail perch
At thirty-five knots or more.
They don t get much of the drills and such
That the battleship jackies do,
But sail the seas in their dungarees,
A grimy destroyer s crew.

They needn’t climb at their sleeping time
To a hammock that sways and bumps,
They leap kerplunk ! in a cozy bunk
That quivers and bucks and jumps.
They hear the sound of the seas that pound
On the half-inch plates of steel
And close their eyes to the lullabies
Of the creaking frame and keel.

They scour the deep for the subs that creep
On their dirty assassin s work,
And their keenest fun is to hunt the Hun
Wherever his U-boats lurk.
They live in hope that a periscope
Will show in the deep sea swell,
Then a true shot hits and it s "Good-bye, Fritz"
His future address is Hell!

They’re a lusty crowd and they’re vastly proud
Of the slim, swift craft they drive ;
Of the roaring flues and the humming screws
Which make her a thing alive.
They love the lunge of her surging plunge
And the murk of her smoke screen, too,
As they sail the seas in their dungarees,
A grimy destroyer s crew!

The Poem Evolves

As you can tell from the original, it would have been quite dated post-WW1 if it hadn’t been allowed to evolve. Here is the version of the poem again as my Grandfather put it to paper less than a decade after WW1, titled ‘Destroyer Sailors. I submit this is may be the earliest record of the post-Braley versions, as it is the earliest I could find.

Read more Berton Braley online here, or more about the man and his works here.

Barry Graff's "Why the F-35" Added to My Blogroll

A Force Multiplier for Truth

I've been meaning to reciprocate a link to Barry Graff's Why the F-35? blog for some time now. He's doing yeoman's work making sure the good news about the F-35 doesn't get drowned out by the anti-JSF drumbeat.
If you haven't visited, please do so frequently from now on. There you will find all the news that should be getting out in the usual venues, but 'somehow' never seems to reach their 'front pages' (at least without a trip through the spin machine). Barry's keeping abreast of the news cycle let's me spend time on other topics and activities just as near to my heart. So... Thanks! 

Note: I'm also keeping a couple of links in my blogroll that some may wonder why, as they don't get updated very often. It's just that when they do, I don't want to miss it!  

GO VOTE!



Election 'After Battle' Report: 

It 'Aint Over'-The job just got a whole lot tougher.

 

With the exception of the usual brain dead zones (including Moscow on the Brazos), and the usual NAACP illegalities in Houston, Texas pretty much did what it could against the Rise of the "Loser Nationtm" . Too bad about much of the rest of the country. Expect more polarization at the state level as the sentient beings who can cut their strings with the 'Blue' states move to where Makers outnumber the Takers. Welcome! -- just leave any silly ideas at the state line. Concealed Carry permit applications are first door on the right.

 



Image Courtesey of the Chicago Boyz

The F-35: What Will Happen While Exploring ‘High Angle-of-Attack’, Part 4

‘Old School’ F-15: Entry to the modern world of ‘High-Performance’

Part 4 (and last) in a series of posts where we document The Profound Truth of High Angle-of-Attack (AoA) flight testing of high performance aircraft. What is “The Profound Truth”?

Discovery and rectification of undesirable aircraft behaviors during High Angle-of-Attack testing of High Performance Aircraft is not only the ‘Norm’, but those behaviors needing rectification/mitigation are usually complex, sometimes bizarre, and often ‘spectacular’.

The F-15 and AoA: A Surprisingly Short Story

My whole family had just moved to Antelope Valley to be near my Dad while he worked another flight test program at Edwards AFB-- only the month before the F-15’s first flight at the same location. I joined the Air Force before the whole family moved back to Texas six months later (Dad would often refer to himself as a “migrant aerospace worker” or “Aero Bracero”).



F-15 First Flight, July 1972 With Square Wing Tips Shown to Good Effect (US AF Photo)

I joined the Air Force before Dad’s assignment ended, but still got to see the first F-15s in the skies over Eddy and the AV quite a bit before I left. In a way, the F-15’s ‘career’ began the same time mine (paying anyway) did, and I got to watch most of the F-15’s maturation, successes, and evolution in ‘real time’ before and after I retired in 1993.

From knowledge gained going way back to the early 70’s and the heady days of the F-15’s flight test and OT&E, I thought I’d just be able to search up some early scholarly papers using a select few keywords like ‘F-15 flutter’ and ‘F-15 buffet’ and then just quote the source data as to what actually ‘happened’. But my research surprised me. There was almost NOTHING written in peer-reviewed or official literature on F-15 High AoA exploration and behaviors while they were being discovered that I could find in the public domain.

Some of this void could be due to the time frame: we were going toe to toe with Russian fighter designs in Vietnam until at least six months into the F-15’s flight test program, and the Cold War was still ‘freezing’. Absence of hard data could also be due to the priorities given to the challenges, controversies, and manufactured scandal surrounding the F-15’s engine development history. That was always in the news at the time, and the news was (usually) wrong about what was really happening with the F100 engine and why.

From forensic examination of the larger body of available ‘retrospective’ literature, it seems for the most part the Air Force was just extremely happy to be able to do what the F-15 design was intended to do: fly much deeper into the High AoA regions before onset of buffeting and stall at higher speeds than its predecessors.

In the past 10 years, U.S. military aviation has progressed from the generation of F-4/F-8 air superiority fighter to that of the F-15/16/18 aircraft, which are demonstrating significant improvements in maneuver performance. These improvements result from more sophisticated aerodynamic design, lower wing loading, and higher thrust-to-weight ratio, and they permit the newer fighters to maneuver as well at 7 to 8 g’s as the earlier aircraft did at 4 to 5 g’s. The limited assessment to date of the newer fighters indicates that they also track as well at 7 to 8 g’s as their predecessors did at 4 to 5 g’s. This is attributed largely to their improved aerodynamics and more sophisticated control systems, which permit them to operate at higher load factors with lower levels of buffet intensity and wing rock than their predecessors. – Precision Controllability of the F-15 Airplane, T.R. Sisk and N.W. Matheny, NASA Technical Memorandum 72861, 1979.

Sisk and Methany’s 'Precision Controllability of the F-15 Airplane' is particularly valuable to our examination because it recounts NASA’s experience exploring F-15 handling qualities using a pre-production F-15 (Airframe number 8). Sisk and Methany offers some insight into what needed to be ‘fixed’ on the early F-15s and why, when after 6 of 10 Gunsight Tracking tests, key systems had to be upgraded to ‘production standard’. The ‘Gunsight Tracking’ test involved making a windup turn from 1 g (trimmed) to the “maximum allowable load factor or angle of attack”.
That “adverse pilot comments concerning the airplane’s handling qualities” in the early tests drove the testers to upgrade the control system to “meet production standards with regards to friction, hysteresis, and breakout forces”, ‘replace roll and trim actuators”, and replace the ARI (Aileron-to-Rudder Interconnect) with a production unit, among other changes is a pretty good indication that the first F-15s were hairy (or at least hairier) beasts at higher AoAs.

Findings of particular note are 1) that the F-15’s buffet as experienced in the cockpit is considerably higher than that experienced in the YF-16 and YF-17 prototypes and 2) the F-15 wing buffet is “severe” at higher AOAs, with mild to moderate wing-rock at airspeeds of interest when the AoA is above “Approximately” 10 degrees.[This raises an interesting question: If the comparison holds for F-16s and F-18s, and the F-35 exhibits buffeting somewhere in between the spread, may we expect F-15 pilots transitioning to the F-35 to think of the F-35’s buffeting as “meh” and pilots transitioning from the F-16 or F-18 react to the same as “OMGWTFO!”?].

A key point to remember here is that the F-15 has no lift augmentation devices, either trailing or leading edge, so there is no way to alter wing high AoA performance without redesigning the wing itself or tweaking your primary flight controls.

Fighter Weapons Center F-15C with Conformal Fuel Tanks and Speed Brake Deployed. When I was at Nellis in the late 70s, F-15 Crew Chiefs were kept 'moist' with cases of beer that pilots would have to buy them after pushing their aerodynamic braking 'show' too far and dragging their tail feathers.

Obvious Changes as a Result of Initial Flight Test

The two most obvious changes made to the F-15 as a result of flight testing were the wingtip design and the speed brake design and operation. I can find no detailed scholarly or otherwise authoritative references to the hows and whys behind the changes were made, but the reasons seem to just be presented now in the ‘everybody knows’ matter-of-fact manner. It is common to find references to the raking of the wingtips due to transonic flutter after the third F-15 was built, but only some sources mention the flutter was occurring at higher g’s and AoAs.

The speed brake design changed at some time during the F-15’s fielding, but I haven’t found an authoritative source I can corroborate as to exactly ‘when’. It is now apparently common knowledge that the dual changes of increasing the size of the speed brake and greatly reducing the angle it can be raised on a different schedule allows the same effectiveness at higher AoAs without adding to the F-15s total buffeting.

Other Issues for Perhaps Another Time

There’s a lot more ‘back story’ on the F-15 development and later discovery of peculiar flight characteristics that have nothing directly to do with ‘just’ high angle of attack. There is the twin vertical stabilizer buffeting issue [1], that seems to affect ALL twin-tail fighter designs to one degree or another, but caused severe cracking and redesign of the F-15 vertical surfaces. There is the “Bitburg Roll” [2] phenomenon, whereby the effects of aerodynamic asymmetry caused by having the 20mm Gatling gun only on one wing root “first surfaced in 1990 as an uncommanded yawing and rolling motion on a F-15C at Bitburg” Air Base in Germany. The Bitburg Roll manifests itself as either an uncommanded roll “up to 60 degrees per second” to the right or yawing motions at higher altitudes between 250 and 350 KIAS. There is also the horizontal stabilator flutter problem [3] that was ‘solved’ by putting a ‘snag’ in the leading edge of the stabilators. The solution was found by trial and error in a wind tunnel, and only recently has the art of computational fluid dynamics reached a point where, 40 years later, it is believed the ‘why’ of the solution can be understood.

There's more back story, but the point is made that:

Operation and test has shown that aerodynamic performance can and will remain unpredictable to any exactness when aircraft are operated in regions where non-linear (the definition of ‘turbulent’) airflow occurs.

__________________

Note: Apologies for the posting drought. I've been way 'out of pocket' part of the time on business travel and been working killer hours at work and home catching up on priorities that could wait no longer.

[1] F-15 Tail Buffet Alleviation: A Smart Structure Approach; Georgia Institute of Technology; WPAFB Contract Number F33615-96-C-3204; 1998.

[2] An Investigation Into The Effects Of Lateral Aerodynamic Asymmetries, Lateral Weight Asymmetries, And Differential Stabilator Bias On The F-15 Directional Flight Characteristics At High Angles Of Attack; D.R. Evans; AFIT/GAE/ENY/96M- 1; 1996.

[3] Flutter Mechanisms of a Stabilator with a Snag Leading Edge; R. Yurkovich; 19th Structures, Structural Dynamics and Materials Conference, 4 - 7 April 2011, Denver, Colorado; AIAA 2011-1847

F-35 Program Deputy Memo

Via Elements of Power’s Global Intel Network I’ve received the following memo. While we cannot completely ascertain its authenticity, the contents are hardly surprising or controversial. EoP offers the following as just another data point in the historical record of the journalism’s F-35 meme machine.

Internal F35 Program Memorandum

From: MG Christopher Bogdan
To: F-35 Teammates
Subject: AFA Statements 

Hi There!

Well by now I’m sure most of you have seen the reporting on my comments concerning the state of the F-35 program. I’m also pretty certain most of you have also figured out my motives for saying what I said, in the manner that I said it as well. But I want to be certain there are no misunderstandings between ALL of us on the F-35 team, so I’m going to describe what I said, and why I said it the way I said it, and do by covering my main points in as plain and clear terms as possible. Ready?

JSFPO-Contractor Relationship

When I said the relationship was the "worst I've ever seen" I meant it. But heck, even a wife-beater in his heart of hearts knows it takes ‘two to tango’ so while the press jumped on the statements as my telling the contractors to ‘shape up’, and just as I expected, think about it for a second: I’ve only been on board the program approximately two months. Two. Months. Two months on ‘the largest acquisition program ever’ is just about long enough to figure out there IS a problem and maybe get a feel for where my own side of the house is having problems. It’s certainly not long enough to determine root causes and failures. Even if I DID believe, as some claim, I single-handedly saved the KC-X  program, I STILL couldn’t possibly have enough chutzpa to think I had everything about the F-35 program ALL figured out in only two months. Rummage around in a few of those articles away from the headlines, and you’ll see I don’t lay the ‘relationship’ solely at the feet of any one group. After all it’s not like I can pretend there’s a surplus of experience and knowledge on the Government’s side of the relationship can I?

No More Money

No one in this business should be surprised the press has glommed on to the money angle: citing in their articles my point of having “no intention of asking Congress for any more money for the F-35 beyond what’s already in the pipeline”. While among us rational folks, that had to have come off as me just being “Major General Obvious” (Duh!) there IS method to my ‘madness’ as it were. This is the one comment that will buy me—our program—a little time out of the Petri dish to get our job done with minimal carping from the ignorant and uninformed. We will of course find the money we need that is already within the pipeline, and you can expect me to be open to suggestions in where to find it. Right now I just have a lot of questions and suspicions about things, such as perhaps those rather crudely constructed and outrageously high and unsupportable life cycle cost estimates. But in any case, WE will find a way. Even Major Generals can’t do it alone.

Technical Challenges

Make no mistake, we all know anything worth doing is never easy. If the United States and her International Partners didn’t NEED the capabilities the F-35 will bring to the Warfighter, we could have just squeezed a little more capability out of the basic legacy aircraft designs. By my acknowledging where we ALL already know where our challenges are and are actively working to conquer, when those successes come in the natural progression of time and engineering, WE will get credit for the accomplishments and blame for any shortcomings. It is an awful good thing we’re already well along in meeting those challenges, isn’t it?

Helmet Mounted Display (HMD)

The media still talks about the problems with the Helmet Mounted Display as if it were just found yesterday. That is to our advantage.  One would think that my mentioning the upcoming tests ‘in the next 60-90 days’ SHOULD tip them off that we believe our difficulties are substantially behind us:  just from our stating we are about ready to begin testing.
 

 

Software

Since when is software not a challenge? It’s the ‘death and taxes’ of systems development: software is always HARD. Of course I mentioned it.

Complex Logistics

That I’m citing logistics as a ‘hurdle’ we have to overcome should have been another ‘No Duh’ moment. Think about it. NINE partner nations around the globe with Foreign Military Sales customers like Japan also coming on board. Eventually, the F-35 will have the global footprint of the F-16 or greater. That’s a lot of airplanes to support in a lot of different places. But the planes they’re replacing are in some cases more numerous and in all cases have more maintenance overhead. This means the entrenched government bureaucracy will be fighting tooth and nail to make the replacement F-35 workload grow to protect their so-called ‘core capabilities’.  We’re not only going to have to stand up a global support system, we’re going to have to do it while parts of the US Government are trying to squirrel some of it away and out of our control.

F-35C Tailhook

The tailoring of the F-35C tailhook redesign isn’t yet complete, but we’re going to complete the task pretty quick. While the usual Palestinian Apologists at Reuters will frame my statements as we ONLY caught the wire “five of the last eight times”, real journalists will pick up on the trend and recognize the trend is three of the last five, and the two that weren’t caught was because the pilot didn’t hit his mark.  Maybe the smarter ones among their readers will eventually figure out that unlike our test configuration, that’s the reason there’s more than one wire on the big deck carriers. 



That’s all for now, but I’m sure we’ll be communicating again real soon as part of improving our team communications which is in turn a critical part of rebuilding our team relationships.

Chris

Achtung Lightning Und Spitfire!

This is the first of a couple of more light-hearted, but no-less relevant, posts I’m putting up before we close out the “F-35 High AoA” series. I just didn’t think these should wait.

Craig Hoyle at DEWLine Blog has a pic/’brief post’ up about the “UK Joint Strike Fighter Test and Evaluation Squadron” hosting a Battle of Britain commemorative event at Edwards AFB.

Hoyle poses a question:

An academic question, as there's no two-seater version of the F-35, but which aircraft would you rather fly in (not into combat, clearly)? As a Brit, it would have to be the R J Mitchell experience for me.

Hmmm. I wonder if he is aware that the ONLY two-seat Spitfires EVER built were conversions of existing airframes and not ‘production’ per se. (Do not doubt me on Spitfires). 

As of this time, Hoyle’s posted only one pic:



  (Credit All Photos: Matthew Short, Lockheed Martin)

Here are the other three shots of the past and future meeting:

The very large originals can be downloaded at the original article here.

The F-35: What Will Happen While Exploring ‘High Angle-of-Attack’, Part 3

F-18 E/F: Heritage F-18 All Fixed, But….Surprise!

Part 3 in a series of posts where we document The ‘Profound Truth’ of High Angle-of-Attack (AoA) flight testing of high performance aircraft.

 The Profound Truth?

Discovery and rectification of undesirable aircraft behaviors during High Angle-of-Attack testing of High Performance Aircraft is not only the ‘Norm’, but those behaviors needing rectification/mitigation are usually complex, sometimes bizarre, and often ‘spectacular’.   

As in the F-18A-D post, we will avoid mentioning all other problems that the F-18E/F program dealt with that did not have to do with the AoA performance, behaviors and testing. They still would be helpful highlighting in yet another way: illustrating how the F-35 program isn’t as ‘concurrent’ as some would lead us to believe. But I’ll (again) resist the temptation to beat that ‘dead horse’.

The F-18E/F is ANOTHER excellent exhibit of the Profound Truth.

In closing Part 2, I observed that “many of the design objectives behind the F-18E/F were focused on eliminating the extant problems and limitations of the Heritage Hornets”. In Part 3, we will observe how an entirely new set of challenges emerged for the F-18E/F. I would call the challenges “problems” if we were all thinking like Engineers: “A problem is normal, something to be expected, and something to be solved”. But too many people seem to think a problem is a reason to NOT do something (as if there is such a thing as a ‘problem-free’ path in any worthwhile endeavor). On the topic of solving problems, engineers in any field know that it is almost always the case that a feasible solution will involve making ‘tradeoffs’. The F-18E/F, like the Heritage Hornet, was no exception to the rule. 

First, the specific design goals of the F-18E/F were:

1. Enhanced departure resistance and post departure (should it still occur) elimination of “falling leaf” or unrecoverable spin modes.
2. Requiring the aircraft to meet all flying quality requirements with a centerline fuel tank since this is a common operational configuration in all services, foreign and domestic.
3. Elimination of high AOA hang up and the accompanying AOA/cg restrictions.
4. The aircraft must be able to land on an aircraft carrier following most flight control failures.
5. Improved roll performance at elevated AOAs in the gear up/flaps Auto configuration.
6. Expanded tactical utility with large lateral store weight asymmetries (since high value stores are frequently deployed one at a time and can result in significant lateral weight asymmetries and aircraft maneuvering limitations after release of one store).
7. Reduction of likelihood of encountering pilot induced oscillation/aircraft-pilot coupling tendencies.
8. Adequate control following a dynamic and/or static loss of one engine (which sized the F/A-18E/F vertical tail).
9. No reduction in flight envelope for the two seat F model over the single seat E-model, since both aircraft would be mission capable aircraft. (Source: Hanley, Et Al, p. 32-6)

The F-18E/F program objectives Hanley describes were rather admirably met in the end, but Hanley also cogently summarizes the REALLY big surprise that the F-18E/F designers had to overcome.

Every flight test program encounters some “unknown unknown, “things that were not planned, thought of, or considered possible to occur in flight-testing. The Super Hornet was no exception. Early in the flight test EMD program, the aircraft experienced uncommanded “wing drop” during wind up turns and straight and level accelerations. As the program matured and the envelope expanded, it became clear that this was a serious problem that would impact aircraft performance if not corrected.
Over an eight-month period from August 1997 through March 1998, maximum resources were brought to bear to solve this problem. In all, over 10,000 wind up turns were executed on over 100 wing configurations before a solution was found. This effort required use of up to 4 of the 7 flight test aircraft to solve, causing significant rework to an already tight EMD schedule. The “wing drop” phenomenon was a rapid, uncommanded bank angle change of up to 180 degrees (if left unchecked by the pilot) that would cause a pilot to lose a guns-tracking solution on a threat aircraft. Wing drop occurred at all altitudes and from about 0.55 Mach to approximately 0.95 Mach. Extensive wind tunnel, simulation and CFD testing and analysis was conducted coincident with the flight-testing. (Hanley Et Al, p. 32-8)

I’m told that the total time to ‘fix’ the problem was close to 3 years. The Hanley paper doesn’t quite get into the root cause, and the authors’ explanation of the “fix” is really a list of corrective actions progressively tried and applied:

Some of the flight test “fixes” assessed included modified snag locations, vortex generators, grit, stall strips, modified flap scheduling, control surface biasing, fences and porous wing fold fairing covers. Eventually, the porous wing fold cover proved to be the most effective solution to the “wing drop” phenomenon, by dissipating adverse pressure gradients fore and aft of the shock forming on the wing and reducing the effect of the asymmetric stall. 

There was an interim step to arrive at the ‘porous cover’ solution, whereby enterprising Navy types tried removing the wing-fold cover entirely, finding it pretty much fixed the ‘wing drop’ but created too much drag to meet other requirements. That’s when NASA stepped in with the half-way between 'none and one' cover, to make it a 'porous' cover. What the Hanley paper doesn’t really highlight at all is that the two factors that made this behavior SO alarming were 1) the abruptness of the phenomenon and 2) the apparent randomness of the direction the plane would roll.

What was it that happened between the Heritage F-18 design and the F-18E/F that brought about the entirely new undesirable behavior? Checking my notes from the aerodynamicist’s lecture I mentioned in Part 2, I find a pretty plausible explanation: ‘Better Idea Creep.’

Some ‘Better Ideas’, in the End… Aren’t

Initially, the F-18E/F was to be a stopgap (for the A-12 and/or later, the JSF). The initial concept was to simply scale up the wing and control surfaces about 25%, put fuselage ‘plugs’ in to increase fuel capacity and put bigger engines in the stretched fuselage to power it all.

The first ‘better idea’ came from the structure guys. They’d felt the Heritage F-18 wings weren’t quite rigid enough and wanted a less flexible wing. They sold the idea of increasing the wing thickness to improve stiffness and allow a lighter structural weight ratio as well as adding more fuel volume. The next ‘better idea’ came from the “systems guys” who sold the idea of reducing the proportions of the leading edge flap chord, which meant lower hinge moments, which allowed reuse of the Heritage Hornet actuator designs and save money. That move also meant even more fuel could be carried in the wings.

“We ‘Aeros’ put twist and camber in a wing for very good reasons. We don’t do it to irritate Structural and/or Manufacturing Engineers. It’s a ‘bonus’, but it’s not why we do it.” (To paraphrase a certain Aerodynamicist)

Early F-18E/F Modeling showed that simply scaling up the F-18C/D wing, with the original twist and camber, would increase supersonic drag and threaten the ability to beat the maximum time requirement for dashing from subsonic to supersonic. This brought about the next ‘better idea’: ‘straighten’ the wing to reduce supersonic drag. Straightening the wing was seen as beneficial in lowering manufacturing costs as well.



The next ‘better idea’, theoretically to improve resultant high lift performance, was to put a leading edge ‘snag’ at the outer third of the wing that would generate a vortex. This move was puzzling. As one of my ‘Aero’ lecturers noted: it didn’t work on the F-18A, so it was taken out of the C model. What made the designers think it was a good idea to bring it back? My lecturer observed that it was probably due as much to the design culture at Boeing née McDonnell Douglas St Louis, which has seemed to favor using leading edge ‘snags’ for decades (most prominently the F-4 Phantom).

The F-18E/F designers then also modified the LEX, but that was believed to more likely have been done to aid in eliminating the ‘alpha hang-up’ mode and increase ability to point the nose down in such an event. I should also add here that the E/F’s wing drop ‘fix’ of making the wing fold cover ‘porous’ is believed to have a supersonic drag penalty greater than there would have been if the designers had left the twist and camber in place.

All the successive changes didn’t trigger alarms in (the right) people’s minds that maybe the F-18E/F would no longer be quite as simple a derivative as it was first proposed. The F-18E/F did not have to go through nearly as many ‘wickets’ as a new-start program (GAO,1994). The wind-tunnel and computer simulation effort was not as thorough as a clean-sheet design (such as the F-35) would have executed prior to design freeze and manufacture. Only AFTER the F-18E/F was actually flying was the scope of the problem revealed. Not that a full wind tunnel and simulation would have helped. The Abrupt Wing Stall (AWS) program instituted after the F-18E/F wing drop made its self known required increased computing power and more detailed wind tunnel data to adequately examine the phenomenon.

What the wing design changes DID produce was an airfoil system that had a very steep pressure gradient with a propensity to abruptly (there’s that word again) move forward with massive airflow separation at only a moderate angle-of attack in the high subsonic speed range. Observe the flow patterns resulting from taking a baseline F-18C wing, and adding F-18E/F design features:

Source: “Introduction to the Abrupt Wing Stall Program”; Hall, R.M., Woodson, S.H.; JOURNAL OF AIRCRAFT, Vol. 41, No. 3; 2004.

The ‘red’ colors highlight the position, volume and amplitude of areas with REVERSE airflow and stall. What this phenomenon looks like on an actual F-18E is illustrated below in these snapshots taken from a video (Hall & Woodson):



Source: “Introduction to the Abrupt Wing Stall Program”; Hall, R.M., Woodson, S.H.; JOURNAL OF AIRCRAFT, Vol. 41, No. 3; 2004.

The lower photo shows a gap in the normal low pressure condensation making an intermittent appearance near the “wing drop angle of attack” (Hall & Woodson).

The ‘wing drop’ issue was probably the best known challenge the F-18E/F overcame, but there were some others “interesting” behaviors that had to be wrung out that the public never really heard about.

“Upright Coupled Departures”

Predicted modeling of a ”step left-and-full-aft stick command at 200 KCAS predicted a sharp left roll and pitch up into stall, a benign outcome”. The actual result provided the "most significant surprise of the program".

Within 3s[econds] of the input, the airplane rolled left, pitched up, and then departed nose right, going flat plate to the airstream in the negative direction. The resulting 3.7-g Nz exceeded the structural design limit, and the pilot’s helmet struck the canopy with sufficient force to leave an audible crack on the cockpit recorders….
…The next point was to have been 300 KCAS, doubling the dynamic pressure. Whereas the aircraft had tracked the simulation predictions earlier, this departure was completely unexpected (5 min earlier, the team had told the pilot that this would be a benign event). Departure testing was suspended for four months to allow for analysis, redesign, testing, and fielding of new FCS software (Heller Et Al).

During the four month downtime, the F-18E/F team discovered that the “aircraft aileron power had been significantly underestimated” (by about ~20%), but most disturbing was that with this new information, the team could see that if they had proceeded with the 300 KCAS airspeed test point, they would have “exceeded the ultimate strength of the airplane” (Heller Et Al).
The ‘fix’ to this surprise was to make software changes that would ensure the roll authority would never exceed a value that would trigger the departure varying the roll rate possible by airspeed and pitch rates (upward). I think it was a rather elegant solution, even though it set the stage for further interesting behaviors. [As an aside, I also note here that while F-18E/F ‘Fans’ and the Boeing PR machine frequently tout the lack of an AoA limit on their ‘favorite’, they rarely mention, if they are even aware of, the flight control limits that ALLOW the unlimited AoA.]
   
“Our pilot laughed aloud at the sight of his own exhaust through the front windscreen.”

Engineers in the lab exploring the effects of the software ‘fix’ above found a “kink in the armor":

To avoid unnecessarily limiting roll rate, only a rapid aft-stick input in the presence of roll rate activated the clamp [Ed. a ‘software imposed limitation’]. Slower aft inputs were regarded as nonthreatening because of reduced susceptibility to inertial coupling. This assumption proved to be inaccurate. The simulator indicated that, in the presence of a full lateral stick roll, if the stick was brought to the aft limit in 3–5 s, an abrupt departure would occur. This prediction was passed to the test team to validate. The simulation was correct, with the most disorienting departures yet seen. Our pilot laughed aloud at the sight of his own exhaust through the front windscreen. (Heller Et Al)

This artifact of the previous ‘fix’ required the program make a decision: limit the plane or limit the pilots:

Because slow-aft departures were all occurring well after 360 deg of roll, and a fairly precise stick trajectory was required, the pilots decided that the conditions required to provoke this departure were sufficiently isolated such that degrading the roll performance would be unwarranted. A flight manual limit was imposed, identical to that on the heritage Hornet, restricting full stick rolls to 360 deg. (Heller Et Al)

“Flip-Flops” – A Spin on the Wild Side

Finally, A mode that manifested in a “significant minority of spins” was “a change in polarity” during recovery. An upright spin would suddenly flip to ‘inverted’, and inverted spins would suddenly flip upright. Viewed directly from above or below, the spin direction would not change, but from the pilot’s perspective, “an upright spin to the right was now an inverted spin to the left.” Or vice versa (Heller Et Al). The ‘fix’ was made in the Flight Control System software to first recognize the ‘flip’ and then provide the pilot with correct display information to assist in the recovery.

To Summarize

There were a few other aero performance challenges the F-18E/F engineers and test teams discovered and overcame, but IMHO none as spectacular as those we’ve just covered. The F-18E/F was perceived as a ‘low-risk’ program containing evolutionary and NOT revolutionary advancements in Warfighter capabilities.
Yet incremental ‘tweaks’ to the design as it progressed added hidden departure modes, some of which at the time could only have been ‘discovered’ and not ‘predicted’.
While the F-35 benefits from advancements that High AoA research produced in the wake of the F-18E/F, the F-35 is a ‘clean-sheet of paper’ design and not a derivative of something else. The potential for unknown-unknowns is seen to be higher AND seen to be worth the effort.

Keep the F-18E/F lessons learned in mind the next time the legions of uninformed and/or irrational F-35 critics begin their silly chants. As my Aero lecturer was fond of saying: there’s a surprise hidden inside every airplane.



REFERENCES:

“Operational Lessons Learned from the F/A-18E/F Total Flight Control Systems Integration Process”; Hanley, R.J., Dunaway, D.A., Lawson, K.P.; NAVAL AIR SYSTEMS COMMAND PATUXENT RIVER MD, June, 2001.

 “Transonic Unsteady Aerodynamics of the F/A-18E Under Conditions Promoting Abrupt Wing Stall”; Schuster, D.M., Byrd, J.E.; JOURNAL OF AIRCRAFT, Vol. 41, No. 3; 2004.

 “F/A-18E/F Super Hornet High-Angle-of-Attack Control Law Development and Testing”; Heller, M., Niewoehner, R.J., Lawson, K.P.; JOURNAL OF AIRCRAFT, Vol. 38, No. 5; 2001.

“Naval Aviation: F/A-18 E/F Acquisition Strategy” (Letter Report, 08/18/94, GAO/NSIAD-94-194).

 “Introduction to the Abrupt Wing Stall Program”; Hall, R.M., Woodson, S.H.; JOURNAL OF AIRCRAFT, Vol. 41, No. 3; 2004.

"Barack Obama Told the Nation"



This was sent to me by a friend who is also in the 'Defense Industry'. Given the latest news on the Defense Sequestration found here, and here  for examples– I thought it apropos.

Barack Obama told the nation:
Have no fear of sequestration!
From EVERYONE, a Corp-o-ration ROBS!
Though I cannot say ‘twas really smart
RIF hundred thousands? - Just a start!
To ‘save’ Americans from... their jobs?

Now a homeless shelter resident
I oft’ wonder ‘bout the President,
Yeah I know Barack, he ‘loves’ me so.
Yet how sadly I remember
Way back yonder in November,
When he said my job would “never go”

Barack Obama told the nation
Have no fear of sequestration!
From EVERYONE, a Corp-o-ration ROBS!
Though I cannot say ‘twas really smart
RIF hundred thousands? - Just a start!
To ‘save’ Americans from... their jobs!!!!!?
 
C'mon and SING it!

NO apologies to JUST another aging Hippie that I’m waiting for to die off and who wrote the original "Lyndon Johnson told the Nation"

And then-- the 'idjiit' updated it with this:

Dooooooosh.

 

The F-35: What Will Happen While Exploring ‘High Angle-of-Attack’, Part 2

 “Heritage F-18: Surprise!”

Part 2 in a series of posts where we document 'The Profound Truth' of High Angle-of-Attack (AoA) flight testing of high performance aircraft.

 The Profound Truth:

Discovery and rectification of undesirable aircraft behaviors during High Angle-of-Attack testing of High Performance Aircraft is not only the ‘Norm’, but those behaviors needing rectification/mitigation are usually complex, sometimes bizarre, and often ‘spectacular’.

For this post, we will avoid mentioning all other problems the F-18 program dealt with that did not have to do with the High AoA performance, behaviors and testing. They would be helpful highlighting in yet another way, how the F-35 program isn’t as ‘concurrent’ as some would lead us to believe: but I’ll resist the temptation to beat that dead horse (this time).

The ‘Heritage’ F-18A/B/C/D provides an excellent exhibit of 'The Profound Truth'

The Heritage Hornet (F/A-18A thru D) was one of the ‘first-generation fly-by-wire (FBW)’ aircraft  developed in the 1970s. While other notable 1stGen FBW aircraft of the era (such as the F-16 and the Mirage 2000) employed AoA limiters “within their control laws to avoid out-of-control-flight (OOCF) losses due to departure, spin, or deep stall”(Heller, Et al, 2001), it was found the Heritage F-18 did not need one….but only in a ‘clean’ (and therefore nearly useless militarily) configuration. High AoA testing revealed a design that would let the Blue Angels boggle John Q. Public’s mind with precise aerial displays, if you hung a weapons load with almost any real asymmetry the Max AoA allowable for the Heritage F-18 is reduced and other bad things happen:

Modest asymmetries increase the departure and spin susceptibility and come with undesirable fight manual limitations on the maneuverability. Large asymmetries impose severe limitations, which must be rigidly observed, thereby reducing the airplane’s safety and operational flexibility (Heller).



Well 'Connected' Vortex Flow
at Moderate AoA

The other really ‘big’ thing discovered in the YF-17/F-18A development effort was that the leading edge extension (LEX) has all sorts of advantages (up until it doesn’t). It generates vortices over the top of the wing and fuselage that increases lift at higher AoAs until it reaches a point where the AoA is so steep the vortices break down and turbulent flow takes over. The LEX was modified, ‘fences’ were added to help, but at Max AoA the sudden onset of turbulent flow beats up the vertical tails and knocks controllability out the window. Later, it was discovered that the tails took such a beating that the attachment points were reinforced with additional structure. (Aerospaceweb has an excellent short summary here.)    

As an aside, I must add that a detail design engineer on the F-18 program once noted in a lecture I attended that the early F/A-18 (Correction: YF-17) wind tunnel models shed verticals like crazy at high AoA. The model makers assumed it was the models’ fault, so they just built the models stronger. In retrospect, the wind-tunnel models were telling them something.

'Broken' Vortex & Turbulent Flow at
High AoA (NASA HARV Program

NASA got involved with fixing some of the Heritage F-18 ‘controllability problems’:   

In 1979, an F/A-18 test aircraft at Patuxent River suddenly and unexpectedly departed controlled flight during a wind-up turn maneuver at high subsonic speeds. None of the baseline wind-tunnel data predicted this characteristic, and the F/A-18 Program was shocked by the event. The fact that the free-flight model had also exhibited such a trend did not go unnoticed, and a joint NASA, Navy, and McDonnell Douglas team was formed to seek solutions with the free-flight model at Langley. Following exhaustive wind-tunnel tests in the Full-Scale Tunnel, the team recommended that the wing leading-edge flap deflection be increased from 25 deg to 34 deg at high angles of attack. Following the implementation of this recommendation on the test aircraft (via the flight control computers), no more departures were experienced, and the flap deflection schedule was adopted for production F/A-18’s. (Chambers, 2000)

Between late 1979 and end of Full Scale Development (aka FSD --closest corollary is today’s SDD) there were FIVE different series of F-18A/B’s control law changes. These major changes “…were incorporated in each of the major PROM series. Control law changes have been incorporated to improve handling qualities at all flight conditions (including high AOA and out-of-control), improve roll performance, reduce structural loads, improve departure resistance characteristics, incorporate and refine pilot relief modes, and provide an active oscillation controller to suppress undesirable in-flight oscillations.” (Kneeland et al)
Fortunately, these changes mitigated or eliminated most of the Heritage F-18’s early untoward behaviors, but one in particular remains to this day: the ‘Falling Leaf’ departure mode (aka ‘alpha hang-up’). The mode remains “suppressed”, but as the video below illustrates, still remains a threat to all but the most wary Heritage F-18 pilots.

 

Many of the design objectives behind the F-18E/F were focused on eliminating the extant problems and limitations of the Heritage Hornets.

Keep in mind the Heritage F-18’s discoveries when the rabid army of F-35 haters start sounding off.

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REFERENCES:

“The Impact of the F/A-18 Aircraft Digital Flight Control System and Displays on Flight Testing and Safety”; Kneeland, B. T. , McNamara, W. G. , White, C. L.; NAVAL AIR TEST CENTER PATUXENT RIVER MD; 1983.

“Partners in Freedom: Contributions of the Langley Research Center to the U.S. Military Aircraft of the 1990's”; Chambers, J.R.; NASA SP-2000-4519; 2000.

“F/A-18E/F Super Hornet High-Angle-of-Attack Control Law Development and Testing”; Heller,M., Niewoehner, R.J., Lawson, K.P.; JOURNAL OF AIRCRAFT, Vol. 38, No. 5; 2001.