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.