LM Aerodynamic Wizardry, CL_max, and Vapes

[steve2267]

I believe it accurate to state that probably all "us regulars" here on F-16.net know and understand that the F-35 was designed to perform, kinematically, somewhere between the F-16 and the F/A-18. It has been described as being able to rate like the Viper, yet radius like the Bug. I am most interested in the Lightning's turning ability in this thread. It's acceleration characteristics having been discussed extensively elsewhere. The Viper is known as a rate machine. The Bug is renowned for its nose pointing ability and high alpha prowess. Somehow, LM combined both in the F-35 without resorting to thrust vectoring.

The equation for turn radius is given by

Turn Radius Eqn.PNG (5.96 KiB) Viewed 182844 times

while the equation for turn rate is given by

Turn Rate Eqn.PNG (5.15 KiB) Viewed 182844 times

The principal variables available to be toyed with by the aircraft designer are:

1. lift coefficient, Cl
2. wing loading, W/S
3. load factor, n

The F-35 wing loading is comparable to the F-16, perhaps higher, so there is no "benefit" there. Max load factor for the Stubby is 9g, like the F-16, so again, no "benefit." If one takes various statements by LM personnel (VP Bumbrage?, test pilot Billie Flynn) at face value, then LM may have found a way to increase lift coefficient above that achieved by 4th gen aircraft. The lift coefficient, or max lift coefficient, Cl_max, is unknown as far as I can tell, and likely classified. Cl (and Cl_max) can obviously be calculated if one knows all the other variables in the above equations. Perhaps Spurts has perused and collected enough numbers from everywhere, that he has an idea what Cl_max is, and if the F-35's Cl_max is, indeed, higher than these 4th gen fighters.

(I am not discussing thrust-to-weight ratio here, although that is related. T/W comes into play when determining sustained turn ability, but the F-35 does not seem to have any magic up its sleeve regarding T/W -- there are no reports of a magic turbo-boost switch, water injection etc.)

Billie Flynn has also made statements that the performance of the F-35B and F-35C, both with load factors lower than the F-35A, will not be noticeably different. This performance could possibly be achieved through a higher Cl_max, despite a lower load factor, (n in the above equations). The Marine pilot with whom I corresponded, whom I believe is a Bee driver, stated he could rate with a Viper (though an airshow Viper might be a bit faster in rate), or radius like a Bug (though an airshow Bug might be a bit tighter), but that he would out-radius the Viper, or out-rate the Bug to kill them. IMO, this all suggests a potentially higher Cl_max.

But what I have noticed is the vapes in so many F-35 photos. I recall vapes streaming off the Viper's strakes / LERX, also from the F/A-18. But those Viper / Bug vapes mostly seem to follow the tight, spiraling core of the vortex street spilling off the LERX. The Lightning doesn't really have a LERX... but LM seems to have generated similar effects with the nose / forward fuselage chine, and the smaller lip running back from the leading edge of the air intakes.

Rather, photos of Lightnings really pulling some pitch rate or turn rate, show vapes pouring across the entire wing surface.

I will also note the work LM performed, and which was extensively flight tested, to enable the F-35 to retain control throughout an extensive flight envelope. No matter where it finds itself, the F-35 is able to recover to a stable flight attitude. Even "uncontrolled" flight appears to be "controlled." (Seems a bit of a misnomer to me.) The F-35's high alpha capabilities have been compared to the Bug's high alpha ability, though I think there are enough quotes out there to make the argument the Lightning exceeds the Bug in this area as well, or in parts of it. The F-35 and the F/A-18 both share the twin, canted vertical tails. However, the F/A-18 experienced some teething problems related to structural strength as a result of the severe buffeting the LERX-shed vortices imposed on the vertical tails. I surmise the F-35 avoided those buffet-induced loads problems by both design of and location of the vertical tails relative to the forebody fuselage / air intakes, but also by avoiding such large forebody-shed vortices, yet retaining more than enough control authority through fastidious flight control design (e.g. horizontal stabilors, independently controllable flight surfaces etc). In addition, the F-35 planform / layout seems to be a further refinement of the F-22 design -- the F-35 appears to be the (ultimate?) refinement of this twin canted vertical tail, relaxed static margin (i.e. unstable) design, with horizontal stabilators pushed way to the rear, that LM has been working with since the mid 1980's, perhaps earlier.

Sorry for the long winded lead-in... here is the crux of my question (and wonderment):

Has the F-35 achieved it's kinematic mastery through an aero design that results in a Cl_max higher than fourth gen aircraft, and are wing vapes the proof in the pudding, so to speak?

General Dynamics used the NACA 64A-204 airfoil on the F-16, and some reports have the F-22 using NACA 64A?05.92 at the root and NACA 64A?04.29 at the tip. It seems reasonable to assume (dangerous!) that LM stayed with an airfoil or airfoil family with which it is familiar. So I don't think a Cl_maximization came through airfoil selection. IMO, if I am correct, LM maximized lift coefficient through careful forebody design, and control law design, that enabled them to push the wing right to it's limit, which is why photos show vapes spilling across the entire F-35 wing, and not just concentrated in one region.

(NOTE: I am not referring to vapes shed in obviously detached flow situations.)

So am I all wet, or do you think LM has achieved a higher Cl_max than 4th gen aircraft through careful design, and entire-wing-shedding vapes in photos are evidence of that?

[steve2267]

Often times I find it difficult to distill what I am thinking down into an elegant, succinct statement such as I often read from Gums or .

While I am not suggesting LM has somehow made a drastic "breakthrough" in aerodynamic design, the qualitative difference of the vape pics I have seen of the Stubbee compared to Vipers and Bugs, cause me to wonder if, somehow, LM is not pushing the wing to the edge or taking more advantage of the wing developing lift compared to 4th gen aircraft, which seem to be more dominated by vortex flow at the wing root / LERX. As johnwill has noted, a wing is far more efficient at developing lift than a fuselage (i.e. wing lift vs body lift). As a result, I wonder if the F-35 is possibly able to fly to a higher Cl_max, and if that might possibly explain some of its performance, even if wing loading is no better than a Viper.

[quicksilver]

And, so what might this pic tell us?

[zero-one]

Okay, not an engineer but I always had a personal theory that since the F-35's wing loading and T/W ratio runs around the Viper's ball park, the secret sauce could be higher body lift. I'm trying to look for Sprt's calculation for lift loading for the F-16 vs the F-35, but since F-16.net's search function is well....you know.
I think thats the reason why a combat configured F-35 performs closely to a clean F-16 as per Maj. Dolby Hanche.

[steve2267]

QS, thanks for that. Have you by chance the same photo but in much higher resolution?

To answer your question, that photo would seem to be a simple and quick rebuttal of my hypothesis. Can you say anymore about the flight conditions / flight attitude of that shot? It *appears* to be during a pitch up? But it could also be at high subsonic / transonic flow over the wing.

[quicksilver]

The point I make is that we should not infer too much about technical details from the layman’s CFD of dramatic airshow visuals.

IMO, anecdotal comments about “rates like...” while having “radius like...” from those who fly the jet were intended to communicate how the jet actually performs in the firestorm about the “bfm report” some years ago. For those who have flown lotsa bfm and know what kind of fight one typically sees from well-flown Vipers and Hornets/SHs we know exactly what that means, and exactly what that looks like. But, up to a certain point, the collective ‘we’ hadn’t ever seen an F-35 even fly, much less perform maneuvers that generally confirm the anecdotes about its bfm performance.

Back in the day, a well-flown F-4 could do things in a slow speed scissors that defied the anecdotes about flying bricks. It was impressive to see after lotsa canned (similar type) bfm training that one was exposed to in flight school, the FRS and ones initial fleet squadron. When the Viper first showed up, it was all but impossible to beat it down to a slow speed fight; it was like fighting an F-4 on steroids, but the fundamentals remained the same.

When the Hornet first showed up at Lemoore it began to redefine bfm performance — visually — in ways that an E-M diagram just didn’t communicate. Comparatively speaking, it could seemingly ‘point’ at will. Can’t begin to tell you how impressive it was visually the first time you fought it. It a way, it seemed the fundamentals had changed, but that really wasn’t true. Its aerodynamic features simply allowed exploitation of a part of envelope that had been avoided because of its proximity to OOCF events.

What the Viper generally did best was sustain its energy; what the Hornet did best was ‘point’ — with seemingly carefree handling qualities. What each did less-well was what the other did supremely well; ‘celebrity grudge-matches’ emerged as the two communities learned how to best fight the other type. Over time, the result was an updated ‘book’ on how to fight those types, generically speaking.

With F-35 you get both, and hence it’s a much tougher type to beat. In short, it doesn’t have a conspicuous shortcoming to exploit.

[sprstdlyscottsmn]

I second what QS said about trying to read the vapes. All they can tell you is where low pressure is occurring and the nature of the low pressure flow (steady vortex, low and attached, separated, etc) but even then it is 100% dependent on local atmospheric conditions.

Now, about the math of lift generation. My last (several years ago) attempt to estimate the CL max of the F-35 (based on things like planform shape, high lift devices, body lift, vortex generation, and lifting tail) resulted in an estimated CLmax of 1.91. I guessed based on several CL/alpha curves I had seen that said lift occurred around 35 degrees AoA.

Here's the kicker...

That point falls inline with an F-16 plot generated by NASA. I thought it important however to highlight on the plot where the AoA limits for the F-16 are, as it helps highlight some things we see in reports, pilot anecdotes, and airshow performances.

The F-35 has a much higher CL max, especially in the high G region, but this comes with greatly increased lift induced drag. This supports the E-M plot that was released some time ago, where the -Ps lines imply much greater deceleration than the F-16. I have never seen an E-M plot with Ps lines for a Hornet so I can't speak to that.

[quicksilver]

For steve — while the ability to generate a given alpha is impressive, IMO, making it useful is a consequence of achievement in stability and control. You may wanna look at the AIAA papers dealing with the CLAWs and flight test of same.

[steve2267]

I do appreciate your comments, QS. Kinda puts matters in perspective, as it were.

Quotes from LM bigwigs about the F-35 being no worse, or identical, or better than 4th gen aircraft, combined with Billie Flynn's rather emphatic statement about overlaying E-M diagrams led me to consider taking their statements at face value. As my engineering curiosity got the better of me, and I got to wunderin... the only unknowns in those equations left to play with are lift coefficient. If LM managed to bump up CL_max 10-20% over the 4th gens, that could explain how the jet, with potentially higher wing loading, could stay right with the Bug, Viper et al.

However, as I type this, I recall discussion hereabouts regarding seemingly counter-intuitive motion and position of F-35 control surfaces: rear stabs positioned in a way that strongly suggests positive lift at the aft end, flaperons positioned trailing edge up, where conventional wisdom leads one to expect flaperons should be down. So maybe LM figured a way to increase "effective" wing area (thereby decreasing wing loading)? That, AND/OR increase CL_max through control dynamics sorcery -- e.g. drooping LEF combined with reflexed TEF perhaps increases the stall angle of attack, thereby enabling a higher CL_max?

I don't know if the Marine pilot was referring to the Bee, or the F-35 in general, in regards to his statement comparing rate-of-turn to an "airshow" Viper. I note, however, that if the F-35A turn rate really does sit right on top of the Viper, then a reduction of load factor from 9g to 7g would result in a turn rate that is close to that 2°/sec no-practical-difference-in-turn-rate that SgtMac has discussed at length on his blog.

[marsavian]

So that chart is saying that F-35 can pull 9g at 25 degree AoA when the F-16 only pulls 1g ? Or it pulls 9g at 20 degree more AoA ? I suppose that's why Dolby said he can be more aggressive in the F-35 with his positioning.

[steve2267]

Spurts, thanks for sharing that CL-vs-alpha plot. It makes a lot of sense to me. This CL-vs-alpha plot, combined with your noting the rapid rise in (lift induced) drag, makes perfect sense and explains to me how the F-35 can so quickly move back-and-forth within it's E-M diagram, so to speak. To me, anyway, it explains Dolbe's comments about slowing down faster than a car, but also other pilot comments about how it regains energy so quick once you unload it -- that big lift induced drag penalty evaporates and that big F135 motor, all the pilots rave about, takes over.

With regard to the infamous "F-35 can't dogfight" meme, as I recall, the test pilot made some comments about "sluggish" pitch rate. I think we established (or he stated) this was in the mid 20° alpha range? (I think "we" (here @ F-16.net) deduced that subsequent CLAWs improvements reduced or eliminated that sluggishness.) If the jet wants to fight up around 35° alpha, but they kept trying to fly it around 20-25° alpha (like a Viper), that would explain a lot. It would also explain Col. Gunn's reply to his (F-16 pilot) friend about "we learned how to fly the jet" after his friend exclaimed "what did you do to your jet?"

QS, I think I am tracking with both you and Spurts. Spurts chart, if accurate, confirms my suspicion about a higher CL_max. I agree this was most probably achieved through control dynamics (stability and control) "magic."

But I'm more of a "flow" guy, and I do find myself returning to images of vapes coming off the leading edge of F-35 wings in a seemingly much more "uniform" manner than I have seen off of Viper and Bug photos. Of course, that is a rather anecdotal comment. On the other hand... perhaps the stability and control "magic" enabled the aero guys to take the wing performance much closer to that OOCF edge?

[lbk000]

Well if the F-35 matches F-18 envelope at the low end and F-16 envelope at the high end, you can have a situation where it doesn't exceed 4thgen performance yet still have the ability to dominate either individual aircraft because there is a part of the envelope that the F-35 can abuse against them while at best they can only break even.
Reports may differ but they don't necessarily conflict.

Controllability is also a really big thing that doesn't show up in E-M diagrams, you can have the best turner in the world but if it's trash at rolling at alpha it will die when you pull it into scissors.

@mars being more aggressive can be as simple as having better acceleration, you can trade airspeed for angles (slow down for a sharper corner) because you know they wont get to walk away from you afterwards.

[steve2267]

I second what QS said about trying to read the vapes. All they can tell you is where low pressure is occurring and the nature of the low pressure flow (steady vortex, low and attached, separated, etc) but even then it is 100% dependent on local atmospheric conditions.

I agree. I wasn't trying to draw flowfield conclusions from the vapes.

The part that caught my eye, is that the F-35 seemingly has a "larger" low-pressure region more evenly distributed across its wings. Sure, it still has steady vortices developed from the fuselage forebody chine and the air inlet "lips" (or chines?) But those voritices appear to be roughly an order of magnitude (at least) smaller (or less important) than the vortices shed off of Viper forebody strakes or Bug LERXs. Just yesterday I ran across johnwill's comment from a while ago that wings generate lift at least twice as efficiently as "body lift." No doubt the F-35 generates body lift, and the LM aero guys have worked that over to the nth degree, but those uniform vapes coming off the leading edge got me thinking if somehow the LM guys didn't make the F-35 wing somehow more efficient.

Back to the stability and control angle... It is very possible the ability to independently position the flight control surfaces (e.g. LEF down, TEF up) has enabled the LM guys to utilize a non-traditional airfoil shape that performs much better >> 20° alpha, say.

But, at this point, I consider my ramblings to be more of the semi-educated, old fart, wunderment variety.

[steve2267]

So that chart is saying that F-35 can pull 9g at 25 degree AoA when the F-16 only pulls 1g ? Or it pulls 9g at 20 degree more AoA ? I suppose that's why Dolby said he can be more aggressive in the F-35 with his positioning.

The chart says to me that the Viper achieves it's 9g turn performance at around 17° alpha, whereas in an unloaded configuration (e.g. 1g), it can reach 25° alpha. This seems to be consistent with statements I recall from Gums, johnwill, et al about the Viper's angle-of-attack being limited at higher load factors owing to stability and control issues.

The F-35, in contrast, appears to have no such limits and can pull upwards of 35° alpha (or more) irregardless of load factor (e.g. gees). But without the actual Dash One (or AIM), I take Spurts' plot to be of the best-guess variety, a very good guess nonetheless.

[hornetfinn]

I love this kind of threads which are highly informative and enlightening as I'm not very fluent with aerodynamics...

My take is that if F-35 goes against lightly loaded F-16 or similar (EF Typhoon, Rafale for example), it is very competitive in rating with it (high speed, high G) and in acceleration. F-35 is going to beat them badly if the fight goes slow as it has much better high-AoA manevuerability and nose pointing abilities.

Against high-AoA machine like F/A-18s, F-35 is clearly superior in high-G, high-speed fight having clearly better STR and acceleration. If the Hornet manages to get the fight to slow speed region, it might get more equal. But Hornet is still faced with aircraft that has at least equal high-AoA and nose pointing capabilties but much better acceleration.

One thing that really is awesome about the F-35 aerodynamics is the capability to turn 9Gs (in A), have 50 degree AoA or go Mach 1.6 when carrying 2-8 bombs, 2 AMRAAMs, targeting pod and enormous amount of gas. No other aircraft can do anything even close. F-22 being closest, but still not able to carry a targeting pod or over 1,000 lb bombs. Pretty much all other current fighters are restricted to something like 5G, subsonic and 15-20 degree AoA. Most fighters have trouble reaching A-G loaded F-35 performance when totally clean. The difference in real world is just staggering there. That must be huge advantage in A-G missions when facing enemy fighters or especially avoiding enemy missiles (SAMs especially).