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ORIGINAL: ckfinite

I'm interested - how does adding a few cubic meters of volume to the front of the fuselage drive the entire aerodynamic design of the aircraft? The aircraft was always going to be about the same width and length (see following diagram), due to the size and volume of the internal stores and the need for serpentine intakes. How, exactly, did the lift fan's volume compromise performance?

Two issues:
1. Area ruling is messed up by that extra volume. Poor area ruling causes a large drag rise in the transonic region of flight, which happens to be where most air combat takes place.
2. The wing is too small. This came from keeping the weight down for the STOVL version. If you look at the equations that define many of the important aircraft performance parameters (rate of climb, cruise range, sustained turn rate, etc.) they all contain wing loading (weight/wing area) as a parameter. In general, high wing loading = poor fighter performance. (There are some exceptions.)

It's the poor aerodynamics that result in the EM deficiency of the aircraft highlighted by the recent "F-35 versus F-16D" report recently leaked

Have you actually read the report that Axe selectively quoted from? The pilot was addressing the effects of some absolutely brand new control law changes, not trying to dogfight. Specifically, he found that in high AoA positions, positions only made possible by control law changes made weeks before, a lot of energy was bled off. Furthermore, another complaint Axe highlighted was insufficient yaw control - control that, if you read the actual report, was available but inaccessible thanks to the control laws. These changes culminated in other BFM testing, where the F-35 performed much better. The F-35's flight control software needed evolution, and tests like that inform the software development.

This was essentially telling the pilot "go out and test the software," in a plane that isn't really representative of the final product (AF-2 is limited to just 5G, for just one example). It didn't have much bearing on the F-35's overall performance at all.

I sure did read those reports. To me, the most important section is the "Energy Management" section.

Overall, the most noticeable characteristic of the F-35A in a visual engagement was its lack of energy maneuverability.

There are several other places where the issue is the control laws, such as the high-AOA regime. Great. That can be fixed with software. But you can't fix EM with software.

As for the aircraft not being representative of the final product, that cuts both ways. It was lacking much of the low-observable materials, which reduced its weight, which improves its EM performance. So there were advantages and disadvantages to using that pre-production model. Personally, I think those effects all wash out and are lost in the noise.

Someone else mentioned Dr. Kopp as a source for more information. You have to be careful with his writing because some of it is good and some is garbage. However, he does understand basic aerodynamics, and his early writings accurately predicted the poor EM performance of the aircraft. Of course, so was everyone else who was capable of basic math and knew the right equations.

By the way, I tried looking for Wikipedia entries for the equations so you can run the numbers yourself, but for once Wikipedia let me down. I was using Raymer's aircraft design book, but it's a bit on the expensive side.

1. Area ruling is messed up by that extra volume. Poor area ruling causes a large drag rise in the transonic region of flight, which happens to be where most air combat takes place.

However, the internal stores really make up for most of that.

2. The wing is too small. This came from keeping the weight down for the STOVL version. If you look at the equations that define many of the important aircraft performance parameters (rate of climb, cruise range, sustained turn rate, etc.) they all contain wing loading (weight/wing area) as a parameter. In general, high wing loading = poor fighter performance. (There are some exceptions.)

Not really - if this had been a problem the F-35A could have had the F-35C's wing. The C's wing has the major issue of limiting the aircraft to 7.5g instead of 9g, but this is missing the point a bit. The F-35 uses large internal bays for munitions (look at the cutaway diagram to see them) so the fuselage width is much wider than the traditional wing loading metric accounts for. When you add this in, the wing loading then becomes comparable to that of the F-16. As it turns out, E-M doesn't really account for a lot of modern aerodynamics, since it assumes that the wings of the aircraft describe a very simple shape and that there are no other lifting surfaces.

I sure did read those reports. To me, the most important section is the "Energy Management" section.

What was particularly notable to me was the reasons stated for the conclusion, namely that the pitch and yaw rates were insufficient. When you read back in the report, these reduce to control law problems, as the FCS was able to reach high pitch/yaw rates when challenged.

But you can't fix EM with software.

You can screw up EM with software, which is what they seemed to do according to the report.

ORIGINAL: ckfinite

1. Area ruling is messed up by that extra volume. Poor area ruling causes a large drag rise in the transonic region of flight, which happens to be where most air combat takes place.

However, the internal stores really make up for most of that.

That article is looking at drag counts and is not talking about area ruling at all.

Speaking of external stores, they are (almost universally) hung under the wings, near the middle of the aircraft, so they have a smaller impact on area ruling. The airplane is already "fat" there, so a little more "fat" doesn't make much of an impact. The problem with the F-35 is that it goes from "pointy" to "fat" really quickly since it has to make room for the lift fan right behind the cockpit.

2. The wing is too small. This came from keeping the weight down for the STOVL version. If you look at the equations that define many of the important aircraft performance parameters (rate of climb, cruise range, sustained turn rate, etc.) they all contain wing loading (weight/wing area) as a parameter. In general, high wing loading = poor fighter performance. (There are some exceptions.)

Not really - if this had been a problem the F-35A could have had the F-35C's wing. The C's wing has the major issue of limiting the aircraft to 7.5g instead of 9g, but this is missing the point a bit. The F-35 uses large internal bays for munitions (look at the cutaway diagram to see them) so the fuselage width is much wider than the traditional wing loading metric accounts for. When you add this in, the wing loading then becomes comparable to that of the F-16. As it turns out, E-M doesn't really account for a lot of modern aerodynamics, since it assumes that the wings of the aircraft describe a very simple shape and that there are no other lifting surfaces.

The F-35C's wing is still too small. It's wing loading is still too high.

As far as considering the F-35 a "lifting body" (as the article you cited argues) and therefore the EM methodology is bunk, well, we would also have to consider all the other fighters with a flat bottomed fuselage as a "lifting body" as well, right? So does anyone think the traditional EM computations for the F-14, F-15, Su-27 are wrong? No. Why? Because while it is true that a flat-bottomed fuselage does generate some lift, it turns out to be a very inefficient way of producing lift. (It tends to be draggy.) So designers usually try to minimize the lifting aspect of the body since it causes drag. (The pitching moment created can also cause issues with high-AOA pitch stability.)

In summary, the simple computation of wing area that ignores the fuselage's contribution is not as accurate as one that does take it into account, but it doesn't actually change the answer very much. That's why people have continued to use that method to compare F-104s to F-15s and the comparisons still come out pretty accurate.

I sure did read those reports. To me, the most important section is the "Energy Management" section.

What was particularly notable to me was the reasons stated for the conclusion, namely that the pitch and yaw rates were insufficient. When you read back in the report, these reduce to control law problems, as the FCS was able to reach high pitch/yaw rates when challenged.

More from the report:

The EM of the F-35A is substantially inferior to the F-15E with PW-229s due to a smaller wing, similar weight, and ~15,000 lbs less in afterburner thrust.

No mention of yaw or pitch rates in that statement, which is speaking directly to the cause of the EM deficiency.

Insufficient pitch rate exacerbated the lack of EM.

I read that to say the EM was bad, but the lack of pitch rate made it terrible. Fixing the software will improve it from terrible to bad. Yay?

But you can't fix EM with software.

You can screw up EM with software, which is what they seemed to do according to the report.

The report seemed to say the software made the EM deficiency worse, not that it was the cause of it. EM is a product of aerodynamics, and software can't change the laws of aerodynamics.

Yokes

The F-35C's wing is still too small. It's wing loading is still too high.

I really find that hard to believe, especially given the g-loading limits mostly imposed by the size of that wing. Making it bigger would only serve to further restrict maximum turn rate due to structural issues, and that size was only chosen in the first place to allow a sufficiently low stall speed.

Because while it is true that a flat-bottomed fuselage does generate some lift, it turns out to be a very inefficient way of producing lift

There were a number of successful lifting bodies that flew for no other reason, and the F-35 has a much more heavily curved fuselage than the other aircraft you mention.

No mention of yaw or pitch rates in that statement, which is speaking directly to the cause of the EM deficiency.

So how did the STOVL version alone drive the single engine requirement (and not cost, reliability, and size)? Also, the F135 is rather similar in performance to the 2xPW-100 configuration for the F-15 - and is, with full fuel, 10,000lbs lighter.

ORIGINAL: ckfinite

I really find that hard to believe, especially given the g-loading limits mostly imposed by the size of that wing. Making it bigger would only serve to further restrict maximum turn rate due to structural issues, and that size was only chosen in the first place to allow a sufficiently low stall speed.

Exactly. To build a bigger wing would require more structure and therefore more weight. They sized the wing to support the STOVL requirement and that limited the maximum size and strength. The A model kept the same wing for cost savings (commonality), but the Navy wanted more range, so they spec'd a bigger wing, but at the cost of maximum G loading.

So yes, even the C model wing is too small.

There were a number of successful lifting bodies that flew for no other reason, and the F-35 has a much more heavily curved fuselage than the other aircraft you mention.

So I taking from that statement that you familiar with what a lifting body looks like. Are you suggesting that the F-35's fuselage was designed to be a lifting body? Do you have any evidence that it was designed to be a lifting body? I have never heard anything along those lines, so I would love to see any references you have.

So how did the STOVL version alone drive the single engine requirement (and not cost, reliability, and size)? Also, the F135 is rather similar in performance to the 2xPW-100 configuration for the F-15 - and is, with full fuel, 10,000lbs lighter.

I'm not sure if I understand the question, so if I missed your point please correct me.

The STOVL version drove the requirement for a single engine because of the difficulty in cross-linking two engines (in the case one fails). People have tried to solve this, but the consensus is that its better to use a single engine. Plus there was the whole "affordability" aspect of the project that drove the single engine as well.

I'm not sure how the PW-100 is germane to the discussion. The pilot in the report quoted above was using the F-15E with PW-229s as a comparison.

Yokes

I am not sure a modern fighter has been developed in the last 10 years where the aircraft body was not designed as a form of lifting body. It is standard practice.

ORIGINAL: thewood1

I am not sure a modern fighter has been developed in the last 10 years where the aircraft body was not designed as a form of lifting body. It is standard practice.

Maybe it's an issue of semantics.

I agree that the lifting affects of the fuselage are taken into account with all modern fighters. But I would estimate that the lift generated by the body accounts for something like 5%-10% of the total lift. I wouldn't consider that a "lifting body", and I don't consider it enough to cause a meaningful change in the wing loading analysis of the aircraft.

If anyone has numbers on the fuselage's contribution to lift I would love to see it. I'm just spitballing, so I'd love some hard info.

Yokes

I am an aerospace engineer by schooling and my nephew works for Boeing as an airframe structural engineer. I discussed it with him a few minutes ago and he said 25% is the expected contribution to the lift vector on a modern fighter. 5% would more than likely be for a round body like an airliner.

That sounds reasonable.

Stealth in this game is really well modeled, but you have to understand how signature reduction works in real life as well as how its modeled within the game.

If you look at the aircraft in the DB under the signature section, you'll see it has a range of IR and RCS values for front, side, rear, top of the aircraft.

You will notice that the frontal aspect is the lowest for each too. This is the same as real life. The frontal aspect has no flat surfaces pointing in the direction of an illuminating radar, so frontal radar cross section (which is one of the factors in the radar maximum range equation) is the smallest.

In real life, the frontal aspect also has the lowest IR signature due to the exhaust plume at the rear of the aircraft being masked by the aircraft's fuselage.

The same applies for visual size too. The front is the smallest visible aspect.

So some tips for using stealth aircraft that I've worked out are:

- When approaching to close range with the enemy, keep your nose pointed at them. Turning side on or away will give away your location and you'll be fired on.

- High altitude and speed gives you a better PK on your missile shots, but if you approach the target/s too fast you will have less time to get off a large volley before flying too close to the enemy and being detected. I have better success getting well within the NEZ on minimum thrust, then firing of 2/3rds of my flight's missiles in the first volley and splitting them between targets so they are attack from all angles. This approach is consistent with what one of the Australian exchange pilots said of his experiences being shot down by the F-22. He said that the F-22 wasn't using its high altitude, high speed capabilities at all to kill him.

Re: APA Analysis. Carlo Kopp is a very smart guy and his site is a very good source of data, unfortunately his bias against the F-35 (due to engineering and test contracts they missed out due to the RAAF's choice of the F-35 instead of a modernized F-111 + F-22 combo) has led him to publish conclusions that are completely inconsistent with the data presented on his site. This might be why they haven't published anything meaningful since their shallacking by people with REAL access to the F-35's capabilities in the Aus parliament in 2012.

Also, because he doesn't have access to software that can simulated rayleigh scattering, he makes some incorrect assumptions on its impact re: RCS of fighter sized objects.

If I could just edit the database, I'd reduce the F-35's agility to 4. It would also be good if I could lower the OODA loop delay time significantly without having to change the pilot skill level. F-35 will have some features to speed up the decision loop and sorting process.

Detractors and bloggers often omit the that it is still in development. Problems are blown out of proportion and presented sensationally.

The F35 is one part of a whole system designed to defeat integrated air defense systems. But they treat it like it's supposed to be the P-38 Lightning and it's 1938.

Anyway, in that moment the F35 takes off to fight Russian or Chinese forces we are going to have other problems than worry about dogfighting capabilities of aircraft. Like nuclear armageddon.

ORIGINAL: ckfinite

If anything, the CATOBAR requirement was actually the nasty one, too. This was the LM CALF (JSF predecessor, STOVL and conventional takeoff) proposal:
. . .
The F-35C's CATOBAR requirement was harder to fill than the STOVL's, driving a more conventional design. I think that this was because the F-35C is so much more radically different than either of the other two.

This is caused by a lot of things, including a much stronger fuselage, a larger wing, the totally different undercarriage, and the tailhook.

An interesting point that I had not heard made before. So you're suggesting that LM gave up their earlier canard-delta design largely due to the CV requirements? Do you know of a published source that alludes to this? Something I hadn't considered before.

Thanks for the insight.

ORIGINAL: Brent119
An interesting point that I had not heard made before. So you're suggesting that LM gave up their earlier canard-delta design largely due to the CV requirements? Do you know of a published source that alludes to this? Something I hadn't considered before.

Thanks for the insight.

This has a lot of very very interesting PDFs linked from it. Of particular note is

The addition of four new ground-attack missions from the MRF
program changed the design emphasis from a fighter with some strike
capability to a strike aircraft with some air-to-air defensive capability.
The development of stealth and long-range air-to-air missiles had
changed the nature of air combat, and so the emphasis was on
achieving a first-look, first-kill capability and reducing the need to
dogfight at close range. For these reasons, the two AIM 9 missiles
were removed and the aircraft was designed to carry two 2000 lb
bombs in the internal weapons bays, in addition to the two AIM 120
missiles. This increased the aircraft’s frontal area and wave drag. The
Air Force variant was derived, as before, by removing the lift fan and
thrust-vectoring nozzles and substituting a fuel tank and conventional
cruise nozzle. These aircraft are shown in Fig. 15.

From here. This suggests that the redesign was unrelated to the takeoff mode, and was instead driven by the size and 2x2000lb bomb requirements.

Thanks again. Some good reading.