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?