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hawker111

Flight Modeling - How Can We Be Sure it is Accurate?

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First: Aerodynamics cannot presently be modeled to 100% accuracy on a PC. I would argue that even the most powerful CFD software (computational fluid dynamics) used by the aerospace industry and/or NASA still must be validated by extensive wind tunnel testing and full-scale development test flights with extensive instrumentation.

Within the limits of what a PC is capable of doing, Strike Fighters uses the most practical approach: Interpolated look-up tables. The SFP1 and SF2 flight model engines' look-up tables can be customized to the resolution desired by the person filling in the data. Out of the box, SFP1 lookup tables that were based on Mach number were based on 0.4 Mach intervals:  0.0, 0.4, 0.8, 1.2, 1.6, etc. If I recall correctly, SF2 went to a 0.2 Mach interval. The problem is that around transonic speeds of Mach 0.75 to Mach 1.2, the changes are extreme over a very small interval. So even with a 0.2 Mach interval, you can't show the subtle but significant differences in how aircraft handle in that region. So, if you fly/fight around that Mach number, you will find odd behavior inconsistent with published performance data. Unfortunately, from the F-86/MiG-15 and on, this is exactly where most air combat starts.

Out of the box, the flight models generally aren't too bad by the time SF2 was released. TK responded to customer feedback and greatly improved the F-4 and F-105 flight models compared to published performance. He never spent a whole lot of time developing refining flight models for AI aircraft. When you set the game to use "HARD" flight models, the full strength of the game's flight engine is unleashed for player flown aircraft, but all AI aircraft still follow a simplified system that is more like the "NORMAL" flight model. Some of the data needed for accurate "HARD" flight models is simply not available and TK fills in "reasonable estimates", particularly in the area of the stability and inertia coefficients. Over the years, TK constantly tweaked the flight models. Not necessarily to make them more realistic, but to make them "flyable" for the fun "lite" sim quality he aiming to deliver. I am an F-4 Phanatic. While TK greatly improved the drag numbers for the F-4 in the SF2 version of the game (the level flight/zero lift drag was always to low in SFP1/WoX), the unslatted F-4s never displayed the nasty high AoA spin/departure behavior that downed around 200 F-4s in Vietnam and the "lite" approach of the SFP1/SF2 series on handling high AoA flight made it difficult if not impossible to edit the flight model to reflect this problem. But aside from high AoA stall/stability behavior, SF2 flight models can generally be tweaked to almost perfectly replicate real-world performance charts.

Even if you had access to all the real-world aerodynamic coefficients tabular data needed to create an SF2 flight model, the tables still aren't extensive enough to replicate what actually happens. For instance, t-tail aircraft like the F-104 and even the F-4 have control/stability issues caused by the wings disturbing the airflow to the horizontal stab/elevator at high AoA. There is no way to model the changes in tail surface effectiveness based on the AoA of the wing. If you "bake" the numbers to reflect high AoA limitations, then the aircraft won't fly correctly at normal level/low AoA conditions.

Despite all of limitations, I don't know of another PC flight sim engine that is more capable of replicating real world flight performance than SF2. The out-of-the-box flight models were generally dumbed down a little to make the airplanes easier/more fun to fly. But if you have the time and patience to research real world aerodynamic coefficient tables and port them to SF2, you can end up with flight models that can only be bettered by hand-coded FMs such as the Professional Flight Models of the best DCS World aircraft modules. But even DCS World aircraft have buggy/inaccurate flight models due to insufficient real-world data and/or not enough look-up table data to properly replicate the real world aerodynamics physics of any one particular aircraft.

In the end, unless you actually have flown the aircraft in question, all you can really test in flight models are the peak chart numbers: ceiling, max speed for a given altitude, instantaneous g, sustained g, sustained climb rate for a given altitude, etc. For the most part, SF2 is reasonably close to those real numbers, most especially on a relative scale: i.e. the MiG-17 should turn like a bat out of hell compared to the F-4, and it does. So, play the game. If you don't like the way the flight models work for a particular aircraft, try doing some research and tweaking the FMs yourself, or find another game where you do like the flight model for that same particular aircraft. How close the numbers are to the real world is insignificant compared to your own subjective enjoyment of the game. The less you dig into how accurate the numbers are, the happier you will be with any of the flight sims that are available for the PC, because none of them are truly accurate.

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I guess I never directly answered the original question: "How Can We Be Sure it is Accurate?"

Verifying accuracy is fairly easy. Find a reliable source for extremely detailed/accurate performance of the aircraft in question, then test the sim aircraft under the same conditions.

For most aircraft, you can buy a pilot manual online that will include performance charts. Some manuals are better than others.

Fly the sim aircraft in the same conditions used to generate the chart data and see how closely they match.

The total weight is a critical component. I prefer to investigate clean configurations as it is doubtful that any game will get the drag penalty of various stores loads anywhere near correct.

Simple tests are the 1g flight envelopes: i.e. the minimum and maximum speeds for level flight at any given altitude as well as the maximum ceiling. If a game can't get these right, there is no point in going any deeper.

SFP1 and SF2 has a debug view that can be enabled in the ini files. This provides data that lets you verify the g-load, lift, drag, etc.

If you are good on a stick, you can learn to test instaneous and sustained g-loads.

Another easy test is acceleration. The F-4 flight manual has extensive tables on how long it takes to accelerate from one mach number (say 0.5) to another (say 1.2) for a given altitude, weight, and drag configuration.

Rate of climb can be harder to measure, but can be done as well.

The key is deciding on an accurate source for performance data, replicating the conditions used to generate that data, and then comparing in-game results.

But I don't care what PC flight sim you fly, if you dig deep enough, you will be disappointed in the results. Learn to appreciate the look and "feel" of the game rather than the absolute accuracy of the physics and you will be a lot more satisfied.

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Streakeagle,

Thank you very much. I really enjoyed reading all that you wrote.

I would like to ask you a question about something you had wrote:

"For instance, t-tail aircraft like the F-104 and even the F-4 have control/stability issues caused by the wings disturbing the airflow to the horizontal stab/elevator at high AoA."

Could you explain exactly what happens when the F-104 and F-4 exceed a certain angle of attack? I know that the F-101 Voodoo had a "pitch up" problem. Here is a film:
 

 

Thanks,

hawker111

 

 

 

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9 hours ago, hawker111 said:


Streakeagle,

Thank you very much. I really enjoyed reading all that you wrote.

I would like to ask you a question about something you had wrote:

"For instance, t-tail aircraft like the F-104 and even the F-4 have control/stability issues caused by the wings disturbing the airflow to the horizontal stab/elevator at high AoA."

Could you explain exactly what happens when the F-104 and F-4 exceed a certain angle of attack? I know that the F-101 Voodoo had a "pitch up" problem. Here is a film:

Thanks,

hawker111

 

 

 

I'm going to throw in a few comments here because I've been tinkering with a VooDoo flight model for quite some time and the VooDoo pitch up issue was common to some other types as well. From my research, the VooDoo pitch up is initially caused by wing tip stall. With the loss off lift associated with the tip stall, and keeping in mind the swept wing,  the center of lift moves forward to the inner unstalled section of the wing. This would generate an initial pitch up but would also contribute to the tail blanketing issue resulting in loss of stabilator effectiveness. The result was a potentially uncontrollable pitch up. The tip stall could be aggravated by high roll rates so ultimately, the VooDoo was equipped with a roll limiter as well as a  pitch limiter system. From anecdotal sources, at least one VooDoo pilot said the stability systems could be "outsmarted" probably from overly aggressive control inputs. Some other aircraft that would pitch up from tip stall include the F-100 [even with the low tail] and the early Hawker Hunters, which had a G limitation at higher mach numbers due to the risk of pitch up coupled with over G possibilities. The problem with the Hawker Hunter was addressed by the dogtooth/sawtooth /whatever it's called wing that was used on the later version.

This info is all from memory.  The F-104 Starfighter pitch up might be a bit different but it's been a while since I looked into it. Streakeagle made some intersesting comments a long time ago regarding the TW F-104 FM. He suggested it was based on the theoretical behavior for the low aspect ratio wing. This made a lot of sense to me as the TW F-104 seems to over perform in some areas and under perform in others. I "think" the stick kicker system in the F-104 kept the AoA somewhere in the 15-16 deg range while the low aspect ratio wing could probably operate effectively at significantly higher AoA. The pitch up issue with the F-104 "may" have been caused more by tail blanketing and a significant amount of lift being generated from the forward fuselage/canopy area coupled with the long moment arm of the forward fuselage/canopy.

These comments are more of a quick overview over some potential causes of pitch up and, like I said, all from memory.

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