Wednesday, April 17, 2013

The F-35 and the Infamous “Sustained G” Spec Change



I can’t remember when I saw so many media outlets, bloggers, and just general ‘people’ with their panties in a wad over something they didn’t really understand. Honestly, who among those ‘critics’ bemoaning the Sustained G turn requirement changing from 5.3 to 4.6Gs even know everything they need to know as ‘inputs’ before they could even begin to formulate an informed opinion on the topic? There is STILL insufficient information in the public domain to come to any conclusions, but there’s a heck of a lot of presumption and assumption in spite of it.

I was neck deep in a home improvement project when Dave Majumdar at FlightGlobal’s DEWline blog put up his “What's the operational impact of reducing the F-35's performance specs?” piece. Majumdar gave a pretty big voice to an ‘anonymous’ but ‘highly experienced ‘ fighter pilot that had all sorts of negatives to pass on to the public. (I believe if you view the Majumdar/Flight Global posts and articles that followed on the topic you’ll conclude the voice in question almost certainly was coming out of a Boeing F-18 test pilot). This ‘anonymous’ fighter pilot’s views were counter-balanced at the end by someone who had actual knowledge of the F-35.

Note: I now notice that the entire post I commented on has been rewritten (down the memory hole, eh?) but the parts I lament in my comment seems to have been ‘reformulated’ elsewhere in an article here. (If I have a major complaint about FlightGlobal’s reporting, it is just this kind of ‘rewriting history’ stuff.)

I commented, in part, the following at that time:

The really disturbing aspect of this 'story' is how an anonymous "highly experienced" fighter pilot somehow is able to gin up some world-class doom and gloom from a few insufficient data points. The doom and gloom gets quoted here, and now this article is being passed around by the anti-JSF crowd-- zipping around the globe as some sort of authoritative source. However, all there is really are two insufficient sets of information that in all honesty should prevent someone from reaching any conclusion other than the program is being 'managed'.

First, let's deal with the changing of the maximum sustained g-force value for the different variant turn performance criteria.

All we know is that 1) The sustained turn g-force objective is a proxy for overall aircraft maneuverability (not even a KPP) and 2) For the particular speed, bank angle, weight, and altitude data points selected, the program office lowered the ‘g’ value after extensive testing and analysis.  

Aside from no real reason to assume the Program Office could or would do so without a good reason, or belief that the differences would not significantly impact operational performance, we should also consider the very real possibility that the F-35’s best sustained-g turn performance may have been found to lay outside the pre-selected test conditions. Perhaps by a little, perhaps by a lot. We don’t know. Not even “highly experienced” fighter drivers, unless they’re on the program, can divine the answer or what it means without more data.

On the one hand, I could have placed more emphasis on the ‘unknowns’: if only so a few of the others who were/are willfully ignoring them would have been perhaps a little less eager to jump on the ‘Lowering the Bar’ bandwagon. I COULD have typed “For the particular speed, bank angle, weight and altitude data points selected, [which are unknown to us] the program office lowered the ‘g’ value after extensive testing and analysis. On the other hand, I shed no tears for the self-identifying ignorami, and find the duping of a small number of media types merely... ‘unfortunate’.

Of course, the usual suspects took exception to my comments, and now the whole thread looks pretty silly as the article that I was critiquing, and others were defending (and OBTW also attacking me), is no longer even IN the article we all commented upon. But my prediction of the obvious, that the doom and gloom sound bites from a rather dubious source would spread like wildfire throughout the 'interwebs' while the factual counterarguments would remain alone and unloved proved all too true.

Fortunately, we have a few other data points that we can add to other information, including knowledge as to how programs and requirements ‘work’ and at least form one or more hypotheses as to what the relevance Sustained G spec change ‘means’-- based upon physics, publically released (vs. leaked) information (vs. unsubstantiated suspicions) about the F-35, and more importantly how everything relates to modern fighter design requirement priorities. We can make some assumptions (and identified as same) and use parametric modeling to more thoroughly understand the impacts of the changes, especially as they might either reflect or affect HOW the F-35 might ‘fight’ in the Within Visual Range (WVR) environment.

This post, as the title reflects, will concern itself with F-35 Sustained G-performance. I’ll get to the ‘Supersonic Dash’ spec change later.

“Sustained Flight”

Let’s begin with defining Sustained Flight, Sustained Level Flight, and what makes a ‘Sustained G’ Turn... a ‘Sustained G’ turn. If you know basic aerodynamics you can skip this section, and I don’t want to hear about anything I ‘leave out’ or ‘over-simplify’ in the comments if you don’t skip it. I’m not trying to ‘dumb it down’, I’m trying to leave out stuff I don’t need to explain to get to the larger points I’m trying to make.

‘Sustained Flight’ is the condition where airspeed, altitude, and load factor ”n” are all held constant. To be ‘constant’ Thrust “T” and Drag “D” must be equal, and Lift ‘L’ has to equal Weight ‘W’ x n. Straight and level (coordinated) flight is ‘sustained’ when lifting surfaces are level, the aircraft is not climbing or descending, neither is it accelerating or decelerating, and the bank angle is ‘zero’.

When flying straight and level, the load factor equals 1 (n = 1) as in “1 gravity” or 1g. But it is not actually ‘gravity’. The load factor n is dimensionless: the ratio of Lift to Weight (L/W), and each component has the same unit of measurement and are thus cancelled out (pounds/pounds, etc). Load factor is referred to in terms of ‘g’ because it is "perceived" as some ‘multiple’ of the acceleration of gravity on board the aircraft.
 A ‘sustained turn’ is a turn where not only airspeed, altitude, and load factor ”n” are all held constant, but also the (non-zero) bank angle of the turn is held constant. To be ‘constant’, Thrust “T” and Drag “D” must still be equal, and Lift ‘L’ has to equal Weight ‘W’ x n, but ‘n’ is no longer equal to 1 because of the vector change of the aircraft as the turn is being executed.

For this exercise (and simplicity’s sake), we’ll assume the earth is ‘down’ and the aircraft is right side up (not inverted) and just say n is now greater than 1 (n > 1). For an aircraft with a typical wing-body-tail planform to enter and sustain the turn, the pilot/controller provides input to the control system that deflects the control surfaces to induce and then hold a bank angle, while increasing angle of attack needed to increase lift generated per unit of wing reference area to keep it equal to the load factor times the weight (n x W).

This means for any given set of airspeed, aircraft weight, and density altitude values, the load factor- accounted for in terms of ‘g’s- is a function of bank angle. Specifically, g= 1/cosØ. For example, a 60 degree bank angle (Ø=60), the ‘g’ value would be 1 divided by the cosine of 60, or 1/.5 = '2gs'.

60 Degrees Bank Angle and Resultant 2g Load Factor 

Increase the bank angle, and in a steady level turn the  g’s increase as well. As the use of a trigonometric function implies, and the chart below illustrates, the relationship is NOT linear.

'G's as a Function of Bank Angle
From the shape of the curve, we can easily observe that ‘g force’ begins increasing at a faster rate than the bank angle is increasing at around 3gs (if you remember your math, it is the point on the curve where the slope (m) of a line that is tangent to the curve = 1). By the time the typical ‘hot-fighter’ max g rating of 9g’s is reached, the bank angle has increased to about 82.6 degrees.

So what is ‘happening’ in the specific region of the curve where the F-35s ‘Sustained g’ spec change occurred?

Bank Angles: 4.6 vs. 5.3 Gs

The Only Conclusion: Bank Angles.

After that we need to start making 'assumptions'.

It doesn’t look like the airplane is doing anything too ‘different’ (minimal y axis delta) on the curve to get that ‘g’ difference, does it? That’s the first surprise waiting for people who haven’t thought much about ‘Gs’: the difference in bank angle between the two ‘levels of performance’ is about 1.6 degrees bank.

The difference looks like this:

Depending on airspeed, the bank angle could translate into a ‘small’ or a ‘large’ difference in turn rate and turn radius. Without knowing for certain what the weight, speed, and altitude is for the ‘performance standard’ at either 5.3g or 4.6g, the difference in bank angle between the two figures is all we can conclusively determine. Everything else depends on the missing data.


A Couple of ‘Likely Truths’

I’m pretty comfortable making some low-risk assumptions on top of the one conclusion. The first one is that F-35 max sustained G capability for the unknown flight conditions and configuration is actually somewhere in between the 5.3g and 4.6g values. I’m comfortable in doing so largely because the program has already demonstrated conservative programmatic behavior with the B model’s “take-off roll” spec change.

[History Sidebar: For the takeoff roll spec change, the JSF Program Office didn’t just ease the requirement to make it so the B model would ‘pass’. They changed the requirement such that it both met military need AND would allow for further ‘bad’ surprises without having to revisit the issue. Because of this past program decision, and just using common sense, I suspect the actual performance difference between the original spec and current performance is even less than all the complainers realize.]

The second assumption I’m willing to make--with only slightly less confidence--concerns which ‘limit’ was hit going for 5.3 sustained ‘g’s under those mystery (can’t repeat it enough) conditions. Max Sustained Turn limits are either “lift limited” or “thrust limited”. I believe the ‘thrust limit’ was hit versus lift limit, for a couple of reasons, and it is important to note now that ‘thrust limited’ can be viewed as either insufficient thrust for the drag experienced at the specified conditions (weight, speed, and altitude) or higher than expected drag at the specified conditions.

I would almost bet, but have no information to confirm, that the drag rise was higher than expected for the selected set of 'spec conditions'. I remind readers again, we have no direct information as to what those flight conditions were.

Exercise: Exploring Comparative Sustained Turn Rate Performance

If we are to do ANY comparison of the JSF performance with any other aircraft benchmarks we are going to have to make what some (no me) might call a small leap of faith and presume the weight, armament, and fuel loads as well as the altitude and airspeed are the same as what is commonly referred to at as the ‘Bowman' Paper or Brief.

[Personal Note: If there is any Cosmic Justice in this world I predict it will befall CDR Bowman for ‘phoning it in’ on his Air Command and Golf paper and the superficial analyses and ‘pronouncements’ within. If he is still active duty when the time comes, I look for him to standup one of the early F-35 squadrons, if Navy assignment desks are half as evil as Air Force assignment desks.]

For the purposes of our exercise therefore, let us ASSUME (and we all know how that word parses) that the flight conditions and the aircraft configuration for the Sustained G spec is as follows:
60% Internal Fuel  
15000 Ft Altitude
Mach .8 Airspeed

I will proceed only in evaluating the ‘G-Spec’ change for the F-35A model and let others make their own analyses on other variants and other comparisons than the ones I will make. I also caution against assuming that the results for those conditions and aircraft configuration translate linearly to any other set of conditions (they don’t) and against assuming that the actual performance at that set of conditions/configuration was the ‘best’ possible at any one of the given conditions. Just one of the weight, speed or altitude parameters could vary slightly and sustained turn performance could go up or down in a manner out of proportion to the change. I’ve also noticed that while the specs usually look at a .8M sustained turn, from at least the ‘F-15 forward’ the best sustained G for US fighters at 10,000-20,000 feet altitude seems to reside somewhere between .85M and .9M.

 Keep these curves and data in mind when we move the discussion forward in the next post:

F-15 Turn Rates

These diagrams come from around the web and a personal reference I picked up at a used book store near Carswell JRB/Plant 4. The web sources are of uncertain provenance, but I found a good ref for the F-15 at a little different weight at 10K ft that correlates well to the F-15 data above. The F-18 data smells like public relations and is more nebulous. I can't tell you how many empty weight values I found for the F-18s, and early in the program the Navy was absolutely anal about couching internal fuel weights as fuel fraction percentages instead of just how much fuel would be carried internally.

The F-16 Blk15 is a good reference-probably the best available-as it has all that vaunted maneuverability the 'reformers' bemoan as ruined with later, heavier versions. The F-4 is a good data point because we have a distinct configuration attached to the performance numbers, and some disparage the F-35 as 'F-4 like'. The Mirage was interesting to me because it is a contemporary of the F-16 and represented the pinnacle of the delta-winged dogfighter (I remember reports of its debut at the Paris Air Show quoting a USAF General as saying "The French have finally perfected the F-106") until the Euro-canards started rolling off the line. In a perfect world, Sukhoi and Eurofighter will e-mail me their E-M diagrams before I go to press on the next post.

 I expect the next post to cause a furball all of its own.  

Since we’re working with ‘pictures’ and NOT real data, I’ve had to do some translating which may have brought associated minor errors with it. I don’t see anything remarkably out of place at the moment:

Aircraft Performance and Configuration Data Translated From Curves 

If anyone has problems with the table I’ve assembled below using the charts above, and can come up with either better authoritative released data or good reasons why I shouldn’t use this data, I’ll leave the door open to changes for a couple of days while I finish doing the 'turn rate' and other math.

I predict that analysis of the data, combined with certain ‘truths’ about what level of significance should be attached to differences in sustained turn rates, and other things we know about the F-35 and have already covered concerning Energy-Maneuverability in an earlier post will clearly indicate we should be thinking of the F-35 as probably being a ‘competent’ if not ‘solid’ “kinetic” dogfighter and definitely NOT ‘a Dog’ (as people who have 'agendas' or little understanding of 21st Century air combat and aircraft design would lead us to believe).

Next: Part 2  


NukeFromOrbit said...

Great article, do you plan on writing about the acceleration issue with the F-35C?

SMSgt Mac said...

Sorry for not responding right away, but no, I'm not planning on doing anything for the C model at least for quite a while. There's not enough info out yet, probably because the C model is the last bird in the program priority list and only a very few examples are flying so far.