The F-35 and the Infamous “Sustained G” Spec Change: Part 4 (of 4)

Note: See Part 1, Part 2 and Part 3 for flight conditions, and important background information.

Let’s get into the hard numbers right from the start in this comparison as I’m sure no one expects our F-35A_H in any configuration to surpass the F-16A in the ‘lightweight’ (50% internal fuel, minimal armament) configuration we’re using:

F-16A Configuration Baseline

As a reminder, here is the sustained G difference between the two:



Sustained Gs: Spec F-35A_H vs. Lightly Loaded F-16A

In the chart above we find, the F-35A_H baseline shows the possible ‘high , ‘low’ and range in between of possible values compared to the much-vaunted F-16A’s performance.

The other expressions of turning ability that fall out from these values are Bank Angle:




Bank Angle: Spec F-35A_H vs. Lightly Loaded F-16A

…and of course "Sustained Turn Rate" expressed in Degrees per Second:



Sustained Turn Rate: Spec F-35A_H vs. Lightly Loaded F-16A

Clearly, the F-16A configuration's sustained turn performance, even compared to the best possible F-35A_H turn rate at 'spec' weight, goes beyond 'Superior' and  approaches the level of 'Dominance'. This indicates it would be pretty stupid for an F-35 driver to get in a protracted turning fight with an F-16A if each combatant needed to get behind the other to 'get the kill'. 

But what about a turning fight when the F-35A_H weight is at other likely F-35A_H values?

To parallel the process we followed in comparing the F-35A_H with the F-4, I’ve selected some feasible alternative loaded weight F-35A_H configurations to compare the possible performance of the F-35A_H with our lightly-loaded F-16A:

Three F-35A_H Load Configurations. (*Assumes fuel burn is constant when in reality as weight goes down fuel burn rate goes down in 'cruise'. Using this assumption, the F-35 is actually penalized: retaining more fuel than needed than the F-16.)

As you can see, Case A will be an F-35A_H with the same fuel load fraction (50%) as the F-16A’s. Case B adjusts the F-35A_H fuel load to yield an equivalent thrust-to-weight ratio as the same F-16A in the 50% Fuel Load configuration. Case C is a little more nebulous, but my ground rule assumption that fuel burn is constant though the weights are going down penalizes the F-35 more than the F-16, this is my ‘best estimate’ of what equivalent fuel loads would be for the airplanes to each be able to fly 500 nm at 30k feet to get back to their home bases. Even if my Case C range assumption is off, both fuel loads are realistic for a possible aerial encounter some distance from home base. Case B and C are remarkably close to each other, but still produce differences in performance:
Looking at all three cases, using the same formulas and physics as we've applied in Parts 1-3, the ranges of possible F-35A_H’s turn performance for comparison to the Lightweight F-16A are found:

Three F-35A_H Configurations: Three Ranges of Possible Turn Performance   

Here are the ranges of possible F-35A_H sustained turn rate performance for these three cases, when charted against the F-16A’s 50% Fuel Load Configuration performance:
   

Three F-35A_H Configurations: Closer to F-16A's Light Configuration Turn Performance   

Thus we can see that 30-40% of the possible values for the F-35A_H in Case A (50% Fuel Load) would place the F-35A_H in relative parity (less than 2 Degrees/Sec, see part 2) with the F-16A, and there are fewer, but still ‘some’ possible values for Cases B and C that would also make the F-35A_H in ‘parity’ with the F-16A. Obviously from what we’ve seen so far, the lighter the fuel load, more likely the F-35A_H could be on a par with the lightly loaded F-16A when it comes to sustained turn performance.

Every Pound of Load Difference Affects Lighter Aircraft More than Heavier Aircraft 

We’ve shown the F-35s sensitivity to fuel load in regards to turn performance, but the sensitivity of lighter aircraft to the proportionally similar fuel loads is even greater, as every pound of fuel represents a larger percentage change in the fuel fraction of the total weight. Observe the differences in performance between a full internal fuel load and a half-full internal fuel load on the F-16A:

Full Internal Fuel Reduces F-16A Sustained Turn Rate Considerably 

A full internal fuel load, even with our ‘light’ F-16A weapons load brings the sustained turn rate performance of the F-16A down significantly. What if our ‘Spec’ loaded F-35A_H was compared to the an “F-16A-like” aircraft carrying full internal fuel. Envision the “F-16A-like” aircraft had just jettisoned its external fuel tanks to do combat with our F-35A_H carrying 60% internal (and total) fuel.

How would the turn performance of the two aircraft compare? It would look like this:


Sustained Turn Rates: F-35A_H Baseline vs. Full Internal Fuel F-16A

Over half (~2/3rds) of the possible sustained turn rates for our F-35A_H Baseline aircraft are on a par with an F-16A carrying a full internal fuel load.

What does a comparison of our Case A, B and C configurations with a full internal fuel-loaded F-16A look like? That comparison looks like this:

F-16A 'Like' Aircraft With Full Internal Fuel Vs. Lighter F-35A_H Configurations (Fixed F-16A Label)

From this chart we see that ALL possible sustained turn rate values for the F-35A_H using any of the three selected fuel load cases are on a par with our ‘full internal fuel load’ F-16A.

Conclusion:

Depending on amount of fuel carried by each aircraft, the F-35A is capable of sustained turn performance on a par with the F-16A. Assuming the F-16A is still the ‘best’ in a sustained turn that there’s ever been at 15K feet and M.8, then that means the F-35A is capable of holding its own against all comers in a sustained turn when flown properly in competent hands.

This would be a lot more exciting news if Sustained Turn Rates were still as important today as they were before the advent of the All-Aspect missile attack. I wonder what technical performance metric the critics will criticize next?

Closing:

Why This Topic and Why Now? The impetus for this mini-series of posts springs from all the things that seemed to be hitting the headlines while I was out having a life (of sorts) away from the web. Things like the usual POGO Clown Posse faux alarm protestations that arose in response to the recent F-35 performance spec changes: Protestations that were then flung like monkey-you-know-what by the usual suspects for consumption by the innumerate and ill-informed around the globe. Further, once industry and defense pilots who’ve actually, y’ know, FLOWN one or more of the three F-35 versions-- probably in response to the flingin’—got a little vocal with positive comments on the F-35’s maneuverability, the ‘Anti’ crowd and business scavengers went DEFCON1 trying to drown them out.

I see the next couple of years as make or break for the F-35….critics.

  The number of F-35 pilots will reach critical mass in this timeframe, and the Anti-JSF'ers know that once that happens, unless the plane really IS a bust, no amount of hyperbole and lying coming from POGO and its fellow travelers will be able to sustain a false 'image of failure' for the F-35.

The increasing desperation of the anti-defense, anti-JSF action-network is palpable…and delicious. But their outrageous claims must still be contested with facts, and their Misleading Vividness must be undone by sober observations. They must be contested lest their deceptions gain any real traction or (equally undesirable), their deceptions provide cover to others who would kill the F-35 for their own evil, or more likely, ignorant and misguided motives who would use the F-35 misinformation as a useful tool.

What Next on the F-35 Front?

With the F-35 numbers in the field climbing, expect the ‘Reform’ types to focus on generating “Cost! Cost! Cost!” misconceptions more than on technical performance topics. It doesn't mean every little glitch won’t get a snarky article posted someplace in the manner of targets of opportunity, but that won’t be their primary focus.

The F-35 and the Infamous “Sustained G” Spec Change: Part 3

(Links to Part 1 and Part 2)

F-4E vs F-35: The F-35 does not have F-4 'like' Sustained Turn Performance
I selected the F-4E for one of two comparisons for a reason beside the fact that it represents the low end of aircraft data in hand: This is to be an Anti-(Not to be confused with ‘Counter’) Propaganda post. When I read a Golden-Armed Meat Servo quoted claiming the F-35 had 1950s ‘F-4 like’ turn performance based solely on the sustained turn spec change, I had to chuckle at the use of ‘misleading vividness’ and ‘damning with faint praise’ in reference to a comparison with the venerable F-4 . I wonder if such bluster was scripted for him by another meat-servo (generic) working in the Boeing Business Development Office? I wonder, because it is a good ‘hook’ of a sound bite for the ‘low-information’ public…but a pretty stupid one when you get down into details.

The F-4 in Our Analysis

The F-4E in our comparison isn’t a product of just the 1950s, But one of progressive improvement from the 1950s through the 1970s. The F-4 design was constantly tweaked throughout its operational life. It has been observed that the F-4 had seven different wings over its evolution through about 1980 if you count the test canard-equipped F-4 configuration (Bennett and Rouseau, 1980), and I believe the AF’s leading edge slat (LES) configuration was wing #4 or #5.

The ultimate USAF F-4E dogfighter IMHO, was the ‘slat-bird’ with TISEO, a far different aircraft than even the Non-Slat F-4E configuration that my late Father-in-Law flew in Vietnam (or the non-slat F-4Es we in the 57th FIS were still flying out of Keflavik Iceland in the early 80’s).

Eventually, all serving USAF F-4Es would get the LES treatment, and it was for good reason. Wind tunnel data for the Post-Vietnam LES wing had predicted a remarkable improvement in controllability (Hollingsworth and Cohen, 1971):

From "Determination of F-4 Aircraft Transonic Buffet Characteristics"; E. G. Hollingsworth, M. Cohen, Journal of Aircraft; Vol 8, No. 10; October 1971.

…and when the design was first executed on the YF-4E, flight test validated those significant improvements (Hollingsworth and Cohen):

From "Determination of F-4 Aircraft Transonic Buffet Characteristics"; E. G. Hollingsworth, M. Cohen, Journal of Aircraft; Vol 8, No. 10; October 1971.

 

Col Robin Olds smote “highly maneuverable” MiGs while flying comparatively less agile and capable F-4Ds. Think about what he could have done in a F-4E ‘Slat Bird’.

From the charts above we find the basic aerodynamics of the late model F-4E are clearly ‘superior’ (even using our grading methods) to the 1950’s or 1960s F-4 image that still resides in most people’s minds. From history, we know that it was lethal when in capable hands against far more 'agile' aircraft of its day.

Now we will take note of the specific weight and drag circumstances used for the F-4 in our comparison.


Source: GD fighter Weapon Symposium "Fighter Performance" Handbook, Circa 1986

The configuration of our F-4E (50% internal fuel, 2 AIM-9s and 2 AIM-7s, and a nose load of 20mm) closely describes a fighter that is either 1) on a short range point defense mission, 2) or has dropped its wing tanks a “half-internal fuel load” ago. Either way, it seems the F-4 we describe had better be on its way home or to a tanker pretty soon on ‘Mil’ power if it hopes to make it back to his base.

The F4-E in our static ( i.e. “snapshot”) comparison has a sustained G turn rate slightly above the F-35A_H (Low) boundary configuration.   How might that change if we lowered the F-35A_H fuel load to something comparable to the F-4E’s? By ‘comparable’ I do not mean ‘the same’, I mean a fuel load that will allow the F-35A_H to fly as long or far as an F-4. This "apples to apples" comparison can be reasonably estimated.

While the F-4E in its modeled configuration is no doubt 'lightweight', it does have a slight but measurable sustained turn advantage compared to our representative F-35A_H(Low) but is at a greater disadvantage against the F-35A_H (High) boundary configuration. It must be remembered the later-generation fighters have tailored-airfoil wing designs developed using Computational Fluid Dynamics to wring as much efficiency as possible and with blended fuselage-wing profiles would be superior to a wing using a catalog airfoil or airfoils (In the case of the F-4, it uses a modified NACA 0006.4-64 airfoil at the wing root transitioning to a modified NACA 0003-64 airfoil at the tips, Source) The F-35A_H in our example now assumes a far higher fuel fraction of total weight in fuel is onboard than the F-4E example. What if we reduced the amount of fuel on the F-35 to be more operationally equivalent to the F-4E’s fuel load?

Normalizing the ‘Fighting Weight’

We can adjust the fuel fraction of the F-35 by adjusting fuel carried (downward) to approximate the equivalent fuel of the F-4. What is important is to closely approximate the fuel on board the F-35 that is an ‘equivalent’ needed by the F-35. I’m going to avoid quantifying the ‘time element’ up front and use it later as a "punch line". In making the adjustments, let us give as much benefit of the doubt as possible to the F-4E example. Two of the biggest benefits of first building a ‘worst case’ scenario in parametric modeling for studies that I perform are 1) If a solution is an obvious one, even with everything ‘going against it’ it is easy to get even Management on board with the solution and 2) If the solution is an obvious one I don’t have to do any more excursions – it’s a great time saver.

Our Assumptions:
1) We assume the F-4 has no higher drag/’total thrust’ ratio than the F-35A_H while maneuvering, though the F-4E is heavier and the overall F-4 design was originally optimized for intercepts at higher altitudes and supersonic speeds and has a less efficient (for maneuvering at lower altitudes) ‘catalog’ higher aspect-ratio wing (modified NACA 0006.4-64 at the wing root and NACA 0003-64 at the wingtip ).
2) The Specific Fuel Consumption (SFC: pounds of fuel per hour-pound of thrust) at all throttle settings is assumed to be equivalent between the two aircraft. Besides having a much older J79-17 engine design in the F-4E, there is no FADEC as in the F-35A_H. Though the J79 engines in the F-4 may have a very slightly lower SFC in afterburner than the F-35A_H’s F135 engine, they will have significantly higher SFC in Mil Power (Max throttle no Afterburner)—these are typical differences between turbojets and turbofan engines.
3) Since the F-35A_H has higher thrust-to-weight in Mil Power AND Afterburner and a much higher Mil Power/Afterburner ratio, the higher efficiency of the F135 is even more apparent, but we will ignore that higher efficiency in our analysis and call it ‘a draw’.

So if we assume the drag/total thrust ratio and fuel efficiency (SFC) are equivalent between the two aircraft, all we now have to do is account for the disparity in non-fuel weight and thrust ratings. The F-4s fuel fraction in this modeled configuration is about 14.5%. To get the same fuel fraction in the F-35A_H, we would reduce the fuel carried from about 11800 lbs to about 5120 lbs. But since we assume we need all that extra thrust in the F-35A_H to do the same amount of ‘work’ as the F-4E, we need to add enough fuel back into the F-35A_H to ensure it has the same relative endurance as the F-4E. The F-35A_H has slightly more than 26% more thrust than the F-43 (43000 lbs vs 34000 lbs), so we will increase our assumed F-35A_H fuel load by adding that same percentage to the intermediate 5120 lb value to arrive at an “F-4 equivalent” fuel load for the F-35A_H of 6474 lbs.

Note that by using the higher 43K pounds of thrust figure vs. any lower thrust value, we are requiring our F-35A_H to carry more fuel than otherwise. Again, this is done to give the F-4E every possible benefit before deriving sustained G turn rates in our comparison. Our two contender’s configurations in this excursion are therefore as identified in the table below.

Normalized Fuel Weights and Configurations for Comparing F-4E and F35A_H

F-35A_H Sustained Turn Performance at F-4E Equivalent Fuel Loads

We can now use these numbers to derive the estimated changes to the range of possible “Sustained-G Turn” capability for the F-35A_H.  The nice thing about 'sustained turns' for doing this kind of extrapolation is that we are dealing with states of equilibrium. We can take the 'g' load and multiply by the aircraft weight to arrive at the total Load Factor and equivalent lift required. At a lighter weight, the same lift and total load is achieved at a higher bank angle and the same load divided by the new lighter weight yields a higher g-rating. As the aircraft is still flying at same speed and altitude, which requires the same wing performance/ efficiencies which have the same drag effects in our region of operation.

Use the figure from Part 1 (left) to visualize the force vector relationships and how they change as the weight is decreased. If the dark blue weight 'vector' is smaller, to get the same equivalent lift and load factor found at max turn and heavier weight, the bank angle is greater.

The only assumption I am making in this case is that the Center of Gravity (CG) shift is not a factor. If the CG shifts forward or aft because of the weight difference, there may be some variation, but modern aircraft are designed to optimize the CG as fuel burns down, so this factor is ignored.

The F-35A_H Sustained turn bang angle and 'G' boundaries we used earlier for the higher weight assumption thus shift noticeably higher when the F-35A_H fuel load is reduced to something comparable to the modeled F-4E configuration. 

The High and Low Boundaries of the F-35A_H Sustained G Turn Capability Increases
Dramatically When Aircraft is Loaded Comparable to the F-4E Configuration.

This higher bank angle at the same .8 Mach at 15K feet altitude operating conditions thus increase the range of possible Sustained Turn rates of our F-35A_H accordingly:

F-35A_H Sustained Turn Rate Range at F-4E Equivalent Fuel Load.

F-35A Sustained Turn Capability: Clearly NOT ‘F-4 like’

It should be obvious to even those most critical of the F-35 that I could have made a slew of small errors at the margins in this analysis and it wouldn’t have significantly changed the end result. When configured at “apples to apples” operating weights, the F-35A_H clearly outclasses the F-4E. We can say that at these configurations, the F-35A_H is "better" to "superior" in comparison to the F-4E. 

Nails in the F-4E’s Coffin

The F-35 airfoil efficiency is also not reduced by external stores in its ‘Day 1’ configuration. With its superior fuel load the F-35 can 1) pick and choose the time to engage, probably without the F-4 ever knowing it was there and 2) capitalize on its superior thrust/weight ratio and better controllability at higher angles of attack (AoAs) .
In addition, if you refer to the E-M diagrams in the previous post and above, you will observe that the bleed rate ‘isobars’ for the F-4 in a turning condition are much closer together than for aircraft in later generations. I suspect the F-35’s E-M diagram looks much more similar to later generation fighters than the F-4’s.

The Final Nail: Fuel Consumption

If we refer to the ‘Dash-1” flight manual for the F-4E and view the combat fuel consumption plots, we find several important bits of information.

1. The plots assume supersonic wing-level flight. 

2. The plots show a range of fuel consumption: from minimum afterburner to maximum afterburner and for an ICAO ‘standard day’ as well as a 10 degree warmer day.  

3. The plot for the closest configuration to the F-4E configuration we are using is the for 4 x AIM-7s instead of the 2 x AIM-7 and 2 x AIM-9. But that’s OK, because the AIM-7 carriage is lower drag than the AIM-9 on the F-4 (Pylons and launcher on wing vs. semi-conformal on fuselage) so we have another negative we ignore to give benefit to the F-4E in the comparison.

This is the plot:

F-4E Combat Fuel Burn Plot

The Punch Line

Now, while our F-4E versus F-35A_H ‘engagement’ wouldn’t be straight and level and above Mach .8 much less Mach 1.1 (the lowest speed at 15k Ft with Min Afterburner In the plot), the fuel burn rate in afterburner is still relevant, and likely still very optimistic for our purposes, as the F-4E would certainly be using higher-rate afterburner settings in such an engagement. So to giving more benefit of the doubt to the F-4E, and assuming it was using only the minimum afterburner to maneuver against a more powerful and lighter F-35, we get to the ‘punch line’ I alluded to earlier:

720 lbs/minute fuel burn rate and 6020 lbs of fuel on board = about 8 minutes 20 seconds before the F-4 crashes, hits a tanker, or lands.

That’s assuming all the fuel on board is usable, though it is not. From this fact alone, it should be obvious to the reader that the F-4E configuration that could theoretically compete with an F-35 in a “sustained turn” competition was “infeasible”—an F-4E “strawman” that was good for highlighting that even with every advantage, and flying as light and slick as possible, the F-4 sustained turn rate doesn’t quite match up to more modern aircraft. I suspect that was the entire purpose of including it in the Fighter Weapon System Symposium materials in the first place: a benchmark to compare against the F-16 that was designed to make the F-16 ‘look good’.

Next in Part 4: F-35A_H vs. the F-16A Sustained Turn Performance. What will we find?

The F-35 and the Infamous “Sustained G” Spec Change: Part 2

(Part 1 Here)
I want to reiterate that this series of posts is for the purpose of developing an understanding of aircraft performance parameters and the factors that influence them. It is not at all about providing answers (we don’t have enough relevant information about the aircraft we’re surveying, much less how they were/are or will be employed) but it is about how to THINK about the questions in the first place.

Let’s begin this post with an updated version of the table I showed the last time:

Selected Legacy Aircraft Configurations

I’ve added an ‘F-18C Light’ configuration to the mix because I found enough information graphed in a General Dynamics handbook on Fighter Performance from a 1985 Fighter Weapons Symposium (FWS). The book is a recent find of mine from a used book store very near the old GD, now Lockheed Martin plant: the same Plant 4 where F-35 output is being ramped up and boutique batches of F-16s still issue forth form time to time. From what I can tell, General Dynamics hosted this event and distributed material to operational F-16 squadrons around the world for years.



GD fighter Weapon Symposium "Fighter Performance" Handbook, Circa 1986

We’ll take the sustained G numbers from the charts for the aircraft configurations above and then crank out the sustained turn rates and similar numbers for a hypothetical F-35A ‘Bowman’ configuration and operating conditions. We'll use the upper and lower bounds of possible F-35A Sustained G values (between 4.6 to 5.3) to give us a range of possible sustained turn rates for that F-35 configuration/set of conditions/assumptions. The formula is:

Turn Rate =Velocity Squared /(G *tan Ø)*
(G *tan Ø)/Velocity; where Ø is still, as shown in the previous post, the “bank angle”.

*originally mis-typed formula for turn radius, but all rate values presented used correct formula (I checked the spreadsheets). Hazards of changing direction in middle of analyses, I guess.

Note that I include 5.3gs as the upper boundary partly because we do not know how close the F-35 came to meeting that spec, but more because we know the ‘grade’ must be against some weight that includes some fixed aircraft dry weight, and we do not know how ‘light’ or ‘heavy’ the F-35 will be until after the final weight accounting that is sometime in the future. Right now, per the latest DOT&E report, we know as late as December the F-35A was nearly 1% below the projected weight needed to meet performance specs. If it comes in below spec weight, it will have sustained turn performance higher than what is currently ‘predicted’ based upon the spec weight. 


F-35A_H Sustained Turn Rates Derived From Possible Range of 'Sustained G' Capability of 'Bowman Configuration' (Corrected**)

**Table corrected to show only difference between 'High' and 'Low' F-35A_H is assumed Sustained G performance boundaries

We now have the high and low boundaries for the possible raw sustained turn rate for our hypothetical F-35 (To keep things concise, let’s call it the ‘F-35A_H’ from now on in this series) and can compare it to the derived Sustained Turn rates for our selected legacy aircraft configurations:


Sustained Turn: F-35A_H vs Legacy Aircraft

Don’t Go There

I presume the above is the kind of raw comparison that sets simple minds down the ‘F-35 can’t turn’ path. Don’t go there – you don’t know enough about what matters…yet. The next thing we need to do is highlight the relative importance or unimportance of the differences shown.

Since we are using the boundaries of our F-35A_H performance in covering the range of possible performance, I’m going to present the ‘low’ boundary evaluation first (Remember, among other things, we don’t really ‘know’ the altitude at which this spec change was applied, we are ‘assuming’ for 'learning' purposes only):

Sustained Turn: F-35A_H (Low) Estimation Vs. Legacy Aircraft

Determining Turn Rate Parity, Superiority and Dominance

The rules used for assignment of comparative parity, superiority, and dominance ranges above and to follow are not mine. Consulting Raymer* (page 105) provided me with my first indication:

An aircraft designed for air-to-air dogfighting must be capable of high turn rate. This parameter dѱ/dt or ѱ, will determine the outcome of the dogfight if the aircraft and pilots are evenly matched otherwise. When air-to-air missiles are in use, the first aircraft to turn towards the other aircraft enough to launch a missile will probably win. In a guns-only dogfight, the aircraft with the higher turn rate will be able to maneuver behind the other. A turn rate superiority of 2 deg/s is considered significant.

*Aircraft Design: A Conceptual Approach 3^rd Edition; D.P. Raymer;AIAA Education Series; 1999.

I had originally intended to use the ‘2 deg/s’ standard to evaluate which aircraft had ‘significant’ advantage over others when, in reviewing my ‘Fighter Performance’ handbook, I found an expansion on Raymer’s observations:




Sustained Turn Rate Equality, Superiority and Dominance

I can’t find the basis for the yardstick stated by Raymer and expanded upon in the FWS handbook, but I imagine it has to do with the typical engagement segment duration where a sustained turn difference would typically yield a ‘significant’ or even ‘dominant’ advantage.

Using the same methodology for our upper bound F-35A_H (High) configuration we find the F-35 fares quite a bit better in the comparison:


Sustained Turn: F-35A_H (High) Estimation Vs. Legacy Aircraft

So in closing Part 2, we see that the possible range of the 'Bowman' F-35A_H's Sustained G performance is broad enough that if the actual F-35 performance is just a little better than the new Sustained G spec value, it will yield turn rates on a "par" with all but the F-15C and F-16A. If the actual performance is closer to the 'old'  spec, the F-35A_H configuration we have modeled comes much closer to "parity" with our F-15C and F-16A configurations.

"What If"? (Parts 3 and 4)

These comparisons are rather static and one-dimensional and the relationships can change dramatically with changes to the armament and fuel carried. It is instructive to note the very large difference in F-18C sustained turn-rate performance based on the variation in weight (which is why I included the 'F-18C Light' data in the first place). Now consider the 'Bowman Configuration' assumption of 60% fuel on board for the F-35 also means that the F-35A_H's fuel load, as a fraction of the total weight, appears to be significantly larger than any of the other aircraft we are comparing. This hints that there may be equally 'realistic' if not more realistic F-35A configurations with far higher sustained turn capability than is attainable at the 'Bowman' weights.

I think it will be worthwhile (and fun) to look at things from a ‘1 v 1’ perspective with the two extremes of possible comparisons in our selected group: F-35A_H vs. the F-4E, and F-35A_H vs. the F-16A, and exploring the 'what ifs' of having slightly different configurations in our comparisons. I anticipate (but won't know until I get there) that I will be illustrating the F-4E at very light weights is not to be trifled with, and that the F-35A, when using comparable fuel weights, based upon equivalent fuel needs will be seen to have solidly 'respectable' sustained turn performance in comparison to other modern aircraft.

Part 3 will look at the F-35A_H vs F-4E.

BONUS GRAPHIC

From the Fighter Weapons Symposium Handbook, we see that the F-16A is/was about the 'cream of the crop' when it comes to Sustained G turns:

Sustained G: Thrust to Weight Matters Too 

So do you think the latest Sukhois do any better? IMHO they're probably more to the right on the X-axis but not any higher on the Y-axis compared to the 'old' Su-27s.

A Minor Note Concerning F-35B 'Bring Back' Weight

The UK's National Audit Office has a report out called "Carrier Strike: The 2012 reversion decision". In the report, the findings are generally positive: The UK switching back to the B model is seen overall as a "good thing".  That should be the end of the story. But.....

If someone didn't know squat about the science of flight, military flight operations, and the relevant physics involved, that someone-- and I'm not saying who -- might be tempted to cherry-pick a certain paragraph in a lame attempt to paint a new operational fact of life (now being dealt with) that comes with increased STOVL capabilities as a 'problem' with the F-35B. The "offending" report paragraph that 'might' be distorted 'might' be (emphasis mine):

3.10 An important enabler of the UK’s STOVL Carrier Strike capability will be the ability to conduct Ship-borne Rolling Vertical Landings (SRVL). This landing technique will be necessary where a conventional vertical landing is less likely to be possible without jettisoning large weapons or fuel load when in hot, humid or low pressure weather conditions. At present the technology is not proven with redesigns required to the carrier deck and aircraft software. The capability will be required for operations by 2020 and the Department included a provision to complete development as part of the cost of reverting to STOVL. The Department is confident it will develop the technique within the required timescale.

Fine (and 'Dandy')!

If someone were to pervert the above into something like: "The report says that the F-35B will have no vertical landing ability in hot, high density altitude, low pressure situations “without having to jettison heavy loads”, it might -once again- be helpful to provide some perspective showing that it is something new the F-35 program has to deal with because the ability to bring back a significant weapons load in all but the most benign conditions by a STOVL aircraft has never been possible before .

Let's take a look at the performance of the highest performing STOVL aircraft the F-35B is replacing: the AV-8B. From the AV-8B's Standard Aircraft Characteristics publication NAVAIR 00-110AV8-4 (1986), we first find the important 'weights' for the AV-8B:

The first key weight we'll note is the 'operating weight': 13,086 lbs. Now let's look at the maximum landing weights versus temperature chart for the AV-8B. The 'wet' thrust is assuming the water injection system was not used on takeoff, but on a hot day, we'll see later that this is pretty much a non-factor:

 The first thing these two charts tell us is that you aren't going to be vertically landing so much as vertically crash-landing the AV-8B on a 'hot' day unless you are on fumes with NO payload. Even then it is going to be 'sporty' to say the least:

But the really wild thing here is if you are operating off a deck afloat in the 'tropics' you had to do a rolling takeoff, using about 90%+ of the available flight deck, and you were only able to do it if you had 40 knots of wind over the deck. With less deck or wind you weren't leaving with what you wanted to take with you in the first place. You'd have to leave fuel behind and 'tank up' en route.

 From this last graphic, we can see chances are that on a heavy-hot-high mission, the AV-8B probably used its water injection system just taking off.

We can also infer that the current rolling takeoff spec for the F-35B of "600 foot" allows growth for much higher takeoff weights. This should make development of the "Ship-borne Rolling Vertical Landings" (SRVLs) an irresistibly attractive option for the USMC: If you can takeoff with 'more' you want to be able to land with 'more'. I suspect that the USMC will probably be on board with the concept before the Brits even begin operations. Pursuit of an SRVL recovery method is clearly more about eventually fielding MORE capability than currently planned and NOT about preserving current projected capabilities.     

The whole idea of vertical 'bring back' weight is mostly about meeting a 'cost' objective by lowering operating costs incurred by jettisoning stores and is NOT and never has been an 'operational' problem. The weights under what conditions were selected almost certainly on a cost/benefit basis. I would assume either the number of days in a F-35B's operating life where temperature and humidity would conspire to affect the normal 'bring back' weights as negligible or the cost to allow for them exorbitant. Otherwise, the requirement would have been factored into the specs in the first place.

Colin Clark: "Cost Estimates" and the F-35

Great article by Colin Clark at AOL Defense. Clark captures what weapon system 'cost estimates' really "mean" better than I've read anywhere else in the press.

If you can't visualize 55 years of Operations, don't pretend to be offended by the costs.

The 'F-35 H8ers' are no doubt dissing the F-35 and the article (and probably my comment on the thread come to think about it) as I type, but go, as they say, read it all here.

,,,and I can't post this graphic too often:

How the Anti-Defense 'Reformers' Practice 'Slash and Whine'

P.S. I'm almost done with Part 2 of the "F-35 G-Spec change" posts. Research is done. Crunching numbers now and will write it up soon.


Update 4 May: If I seem a trifle rough on some of the commenters at the AOL link, so be it. I cannot abide regurgitators of myths, poseurs, or what the Soviets used to refer to as the "govnoed".

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

PART 1

Introduction

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 (not 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 F-16.net 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  

2 AMRAAMS  

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  

Charter Cable: Media Malpractice

Charter News, 'Isn't'

(Still working on a lengthy 'aircraft/F-35 maneuverability' post, but this HAS to go up tonight.)

Charter Cable is my cable provider. NO complaints about the internet speed or connectivity, not even though I suspect their move to 'all digital' last week wreaked havoc with signals (off and on) as thousands of users finally added even more thousands of cable boxes and cards to the network in just a few days. It now seems to have stabilized, so 'no problem'.

But Charter Cable's 'homepage' has a section with rotating 'news' headline pictures and captions. All too often the caption and photo make it appear that some tragedy has happened in the US or even just the 'Modern' world, and you click on the link talking about a school being bombed with what appears to be a typical American elementary school (they've done school buses too if I recall correctly) and the story is about a school in some war zone in a 'turd world' country. The Chief and I just chalked it up to lazy web content developers and editors.

Today, they went beyond 'lazy' and deep into 'media malpractice' . I got home and booted up the laptop to check the web and this is what greeted me (left headline):



Charter Home Page 9 April 2013 ~1920 Hrs CST

 



WTFO? "Veteran Kills 13"?

I clicked on the link, and this is what popped up:

Charter 'Article' 9 April 2013 ~1920 Hrs CST

Oh. A Serbian 'vet' in Serbia loses it and goes on a rampage. Tragic in it's own right. Why the 'trick' headline?

You would have to be either incompetent or agenda-driven to put this one up.  Either way it doesn't 'inform' but misleads and distorts several issues in one nice swoop.

Besmirching veteran's mental health? Check!

'Tragedy' as background for upcoming 2nd Amendment legislation? Check!

The Chief likes to try and calm me down when some unthinking slug nearly kills us because they're doing something clueless in traffic. She says something like "I'm sure they didn't see us". She forgets what makes me the MOST angry is the fact that they probably were clueless as to what was going on around them. If I assume Charter was just being 'brain dead' in this, it just p*sses me off more. tell me again: What business are they in? Do they have any standards?
 
Either way, Charter's website is Media Malpractice writ large in a Low Information Consumer world.

Are There ANY Adults At Charter Cable?

A Backgrounder on Energy-Maneuverability

Because if you learned all you know about E-M from Boyd, you’re only about a half-century behind the learning curve

Housekeeping

The first two things to remember in this kind of discussion are:

1. Performance metrics are used as PROXIES for what is important in a Weapon System under acquisition or already fielded. The metrics are NOT important in and of themselves. They are only as important as the degree to which they inform developers and operators on the system’s true versus desired capabilities.

and...

2. Weapon system specifications are initially established based upon what is believed to be needed and what is believed to be feasible within the projected budget and schedule before the development is given the ‘go-ahead’. There are only varying degrees of assumed confidence in the ability to achieve what is seen as feasible, and this depends much on perceived technology maturity. Only after the project is underway will the need and/or feasibility, given the actual maturity and budget, begin to be revealed, and it may be only truly knowable towards the end of development. Adjusting the specifications as new information is acquired, while still meeting Warfighter needs is sound engineering and management. It is NOT (as someone in the POGO crowd or ‘low information media’ might claim) “cheating” or a sign of “failure”.

Beyond Boyd: A Quick Survey of the Evolution AFTER the E-M Revolution

I’ve picked two AIAA papers on the subject to illustrate the post-Boyd reality: one from the 1980’s and one from the 1990’s. There are a fairly large number of scholarly papers concerned with the inadequacy of the traditional Boyd ‘E-M diagram’ that lead right on up to the present day, but as T. Takahashi observed just last year in a paper proposing yet another ‘better way’ to visualize E-M:

For aircraft whose missions demanded combat maneuverability, the United States Air Force required production of Energy-Maneuverability (E-M) plots. The design of the F-14, F-15, F-16 and F-18 was tailored by designers working to better the E-M capability of Soviet aircraft. In this context, a government oversight driven tradition developed where industry was required to plot many aerodynamic and aircraft performance parameters as functions of speed and altitude.

To begin our review, I think one of the better overviews of how E-M factors evolved post-Boyd is encapsulated in a paper by Skow, Hamilton and Taylor (1985). They observed:

“In the late 1940's and early 1950’s, with the advent of jet propulsion, radical new wing designs and greatly expanded flight envelopes, a corresponding need for more definitive measures of merit for aircraft performance comparisons was generated. When the "century series" fighters were developed and rear-aspect IR missiles became the principal air-to-air combat weapon, point performance comparisons were found to be inconclusive and insufficient to predict superiority. Out of this need, energy-maneuverability (E-M) concepts were formulated and developed. In the 1960's. E-M came into widespread use by aircraft designers and fighter pilots. E-M provided an analog picture of a fighter's performance capabilities over a range of velocities and altitudes. It gave quantifiable credit to measures of merit which allowed the advantages of speed (energy) and turning (maneuverability) to be balanced. These measures of merit were shown to be dominant in determining the outcome of an air battle at that time.  

 ...

But, as they say, time marches on, and in the past 10-12 years, several significant advancements have been made in the capabilities of fighter aircraft and air-to-air weapons. Three of these advancements; the all-aspect IR missile, greatly improved weapons delivery systems, and high thrust-to-weight engines have dramatically altered the character of the air battle, especially the close-in fight. The modern air battle is characterized by (1) time compression – shorter duration maneuvering required and (2) harder maneuvering - nose position at the expense of energy vs. nose position with energy conservation…

Air combat trends have expanded to ever increasing altitudes and speeds for beyond visual range (BVR) combat and conversely have tended to a lower and sometimes slower arena for close-in, within visual range (WVR) combat. … …This changing complexion of air combat, primarily due to the all-aspect IR missile, has altered the relative significance of the various performance characteristics with which we judge relative merit. Table 3 depicts the more common agility characteristics with some relative rankings.”  

Table 3 Reconstruction from “Advanced Fighter Agility Metrics”

In 1992, Cox and Downing featured Befecadu (BF) Tamrat’s 1988 metric of Combat Cycle Time in the proposed system of “functional agility metrics” they evaluated for use in measuring aircraft maneuverability: Combat Cycle Time, Dynamic Speed Turn Plots, and Aircraft Energy State. The impetus for searching for these new metrics?

During the Korean War and the Vietnam conflict, jet fighter aircraft emerged with greatly expanded altitude and Mach ranges. This era also saw the advent of the short range heat seeking missile. These missiles required maneuvering to achieve a rear-aspect firing position. The measures of merit, altered to match the increase in aircraft capability, were advanced point performances (thrust-to-weight ratio, maximum Mach number, sustained turn rate, etc.) and the energy-maneuverability performance method. In recent years, the level of fighter aircraft and weapon system technological sophistication has reached new heights; the most critical advance being the development of the all-aspect infrared missile. This missile negates the requirement of maneuvering to achieve a rear-aspect firing position and concurrently has caused the traditional point performance measures of merit to become deficient for determining the combat effectiveness of a fighter aircraft. To remedy this deficiency, new measures of merit are being investigated which examine aircraft maneuver and control capabilities not previously quantified.

We could dive deep into discussion on each metric proposed in this paper, but I would like to focus on Tamrat’s Combat Cycle Time, as it is what differs most from the pioneering but simpler Boyd-Era POV of E-M, and the authors ably describe how Tamrat builds on same:

Traditionally, the need to achieve a rear-aspect position for a gun or missile firing opportunity led fighters to engage in battles lasting several minutes. These engagements were characterized by sustained maneuvering. This type of combat made the turn rate verses Mach number diagram (doghouse plot) useful for determining one fighter's advantage over another. When maneuvering for a rear-aspect firing solution, the interior of the doghouse plot is important. This interior region is represented by the sustained turn rate line and is typically dictated by available thrust and the lift to drag ratio of the aircraft. The compressed time scales of today's air combat arena have made sustained maneuvering less critical. The desire for a first shot opportunity leads to fights dominated by transient load factor/lift limited maneuvers. The emphasis then is shifting toward more dynamic maneuvering or the exterior of the doghouse plot. Figure 1 shows these critical regions on a conceptual doghouse plot.



From "Evaluation Of Functional Agility Metrics For Fighter Class Aircraft"

The first region is the dynamic pressure limit line (A) along which pitch rate is used to load the aircraft to enter a turning situation. Next, flight path and nose pointing maneuvers occur along the limit load factor and lift limit boundaries (B). The time to unload the aircraft is shown as the region in between the lift limit line and the 1-g line (C). The last critical region shown is the 1-g acceleration (D) in which an aircraft regains lost energy to leave the engagement or enter a turn to pursue another target. The Combat Cycle Time metric, developed by B.F. Tamrat combines these four critical regions into a single parameter. The time for each maneuver segment is calculated and then summed to obtain a time for the entire maneuver cycle. The critical regions, segment times, and corresponding maneuvers are:  

A t, pitch up to load factor limit  

B t, turn along load factor and lift limit 

C t3, unload from elevated AOA/load factor  

D t4, regain original energy level  

The Combat Cycle Time metrics offers an advantage over traditional measure of merit for turning maneuvers. The doghouse plot shows the sustained and instantaneous turn rates for only one flight condition (i.e. specified power setting, mach, altitude, altitude, weight, etc.). The Combat Cycle Time, however, is characterized by a continually changing flight condition constrained within structural, lift, and power limits.

What it All Means

If there are recurring themes over the years in all the proposed metrics that move beyond Boyd/ Christie’s E-M diagram paradigm, it is 1) All-aspect missile attack changed the game and 2) the proposed metrics try to account for the missing values of the speed of ‘transition’: the time needed to point the aircraft in a desired direction from another, the ability to decelerate as well as accelerate in vertical and the horizontal, to change pitch, roll and yaw.

 The lingering deference to standard E-M charting in the operational world springs more from their relative ease of understanding by non-engineer aircrew and government bureaucrats than from their utility. The E-M diagrams benefit from the fact that what really counts, is harder to explain. So we lumber along with the deficient E-M charting and rely on the ability of the untrained/uninterested to recognize the important nuances.

One of the biggest impacts on aircraft E-M performance that isn’t in the vehicle’s dynamics vis a vis the external environment (but sometimes dealt with in the OODA loop) is the man-machine interface: how fast the pilot perceives a change/threat and reacts to or preempts the effect of the change/threat by changing the vehicle dynamics. How well can the crew execute the mission in light of all the factors that could diminish the pilot’s ability to manage change and threats? Factors such as: How much attention does he have to pay to not bending or breaking the airplane or keep from departing controlled flight while engaged in the fight?

 

Limitations of Boyd-Era E-M Proxies and the F-35

A good example of the limitations of the traditional metrics is found in the caterwauling over the recent change in the F-35’s sustained-g turn standard from 5.3 to 4.6gs at some pre-selected altitude and speed. So much hang-wringing and finger-pointing—coming from those who not only do not know how important or unimportant the change was to needed/desired overall aircraft performance, but for the most part also have no idea what the change meant in and of itself, in ‘performance’ terms.

That, friend Reader, is a good lead-in for my next post (or two). Examine the reconstructed ‘Table 3’ above and ponder how the F-35 (especially the F-35A) performance stands up in the post-Boyd air combat world.

We'll soon engage in some speculative analysis on the F-35, but the speculation will be based upon logic, facts, physics, and experience.

References:

AIRCRAFT CONCEPT DESIGN PERFORMANCE VISUALIZATION USING AN ENERGY-MANEUVERABILITY PRESENTATION; Timothy T. Takahashi; AIAA 2012-5704

ADVANCED FIGHTER AGILITY METRICS; Andrew M. Skow, William L. Hamilton, John H. Taylor; AIAA-A85-47027

EVALUATION OF FUNCTIONAL AGILITY METRICS FOR FIGHTER CLASS AIRCRAFT; Brian W. Cox, Dr. David R. Downing; AIAA-92-4487-CP