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".