Sunday, June 30, 2013

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.


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?


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.


energo said...

Again, great work. I've talked to many seasoned F-16 pilots who like to point out that the early F-16A Blocks still is the "sports car" of the Blocks. Newer Blocks are heavier and putting heavier equipment in the nose moves CG forward, affecting performance under some conditions.


energo said...

Again, great work. I've talked to many seasoned F-16 pilots who like to point out that the early F-16A Blocks still is the "sports car" of the Blocks. Newer Blocks are heavier and putting heavier equipment in the nose moves CG forward, affecting performance under some conditions.


TOMARI IV said...

To compute the sustained load factor you multiply the T/W ratio with the L/D ratio. The L/D ratio varies with speed, the T/W ratio not very much so. This explains the phenomenon of low T/W fighters being able to match the sustained performance of other jets possessing a much higher T/W ratio. The F-86, with a peak L/D of 15 or the F-5 with an L/D of 10 could out-turn the PHANTOM, which had an L/D of about 8, if I remember correctly.

Fuel state is a factor that implies the defending fighters will be at a disadvantage versus the attackers in terms of turn rate, who have flown a longer distance to the battle area, but enjoy a advantage in terms of persistence. Clearly, as a strike fighter the F-35 will join battle with less fuel than the opposition.

The question is who's gonna be turning with the F-35? No other a/c type can do such a thing. And a second question, why on earth should the F-35 drop a wing to start a turn? Get this wrong and your assessment of the F-35 will be off by miles.