F-15 AFM air intake remodeling

[bkthunder]

Hi,

with the AFM for this great bird coming up, I'm wondering if the behavior of the air intakes will be corrected.

 

Apart from being realistic and visually pleasing, I'd suspect having air intakes acting the wrong way (as they do now) would affect the AFM.

 

These are pretty large surfaces that are aerodinamically important, especially at high AoA. Imagine the increased drag generated at high AoA if the intakes were in the "up" position as opposed to streamlined with the airflow.

 

On top of that, behavior of the engines certainly depends on correct positioning of the air intakes.

 

 

 

Thanks

[Revelation]

I agree with what you are saying, though I don't know if that would make it into a FC3 level AC. If this was a DCS level release then I'd be confident that it'd be done right.

 

p.s. Need to go look and see they are in fact wrong - haven't paid attention prior to this post.

[VanjaB]

Im pretty sure that making an AFM doesnt make sense on an aerodynamicaly wrong 3d model. Unless they arent simulating airflow over the entire model. Which would be kind of a cop out? Right?

[NineLine]

No, I dont think that the 3D model affects the aerodynamic properties of the FM, the aero stuff is taken (I am assuming here now of course as I dont work on the FM, from flight and wind tunnel testing) That data would be based on correct real world shapes.

 

Now that doesnt equal a cop out as you said, I think to simulate airflow over a 3D model like you are talking about would melt most of our computers, just with one aircraft, let alone a number of them flying around with missiles flying.

 

Again, I could be wrong, but I dont think an error in the 3D model affects the aero properties.

[NRG-Vampire]

Again, I could be wrong, but I dont think an error in the 3D model affects the aero properties.

 

i hope you are wrong :)

watch the airflow around the mustang in this video

[NineLine]

i hope you are wrong :)

watch the airflow around the mustang in this video

 

That isnt showing what is being described above... as far as I know aero properties are taken fro test data and not from the 3D model in game.

[streakeagle]

The only flight sim engine I know of that can generate a flight model from 3d geometry is X-plane. The flight model consists of fixed look-up tables like almost any other consumer grade sim, but they are generated from the geometry of the model rather than hand typed. The flight models resulting from this very simple "blade element" approximation are horrible for anything flying above a 200 kts. X-plane allows the modder to override/replace the calculated flight model with actual real-world measured data as well. Computational fluid dynamics takes some real pc horsepower and still can't perfectly predict compressibility/transonic behavior.

 

So DCS is not modeling air flowing over the 3d model. Any 3d animations/shapes are purely for eye candy. Now if the flight model is actually modeling the variable intake, then it should be calculating the position of the intake to know its impact on thrust, lift and drag, which could easily be ported to drive the 3d model. Hopefully, that eventually will be the case for a full DCS release. I don't know if they are going that deep for the upcoming AFM. But if the AFM is supposed to be up to DCS level, then it would be embarrassing for them not to model both the FM effects of the intake and the 3d animation.

[VanjaB]

No, I dont think that the 3D model affects the aerodynamic properties of the FM, the aero stuff is taken (I am assuming here now of course as I dont work on the FM, from flight and wind tunnel testing) That data would be based on correct real world shapes.

 

Now that doesnt equal a cop out as you said, I think to simulate airflow over a 3D model like you are talking about would melt most of our computers, just with one aircraft, let alone a number of them flying around with missiles flying.

 

Again, I could be wrong, but I dont think an error in the 3D model affects the aero properties.

 

But thats exactly how the scripted SFM works. So whats the difference between a 'table' or 'chart' SFM and the AFM then? The AFM is supposed to calculate effects on the fly, I.E. as the airflow changes during flight dynamics the aircraft responds not according to a preset table but acording to sum vector forces acting on the different surfaces.

 

I understand that its simplified somewhat and that they do use wind tunel data as a base from which to pull the force vectors, but the sim should be pulling these out of a database on the fly so to speak, and on every surface. At least this is how its advertised on their main site.

[aaron886]

But thats exactly how the scripted SFM works. So whats the difference between a 'table' or 'chart' SFM and the AFM then? The AFM is supposed to calculate effects on the fly, I.E. as the airflow changes during flight dynamics the aircraft responds not according to a preset table but acording to sum vector forces acting on the different surfaces.

 

I understand that its simplified somewhat and that they do use wind tunel data as a base from which to pull the force vectors, but the sim should be pulling these out of a database on the fly so to speak, and on every surface. At least this is how its advertised on their main site.

 

The way you're using the terms "table," "force vectors," and "pulling from a database" shows a misunderstanding of how flight modeling generally works. Like the SFM, ED's AFM almost certainly uses tabulated data to represent the aerodynamic characteristics of different parts of the airplane.

 

The difference between the SFM and a more detailed flight model is the number of point forces applied to the model. ED's SFM, for example, produces only one lift vector for a whole wing. Their AFM breaks the wing up into parts in order to model a range of forces across the span. This gives a more dynamic response to various disturbances, and if you apply that concept to the entire flight model you will have an idea of how it works. However, something still has to determine the strength and direction of the force applied at each point. Aerodynamic equations need coefficient data, and to my knowledge ED also draws this data from tables. Data + equations = performance

 

There ARE other ways to do flight modeling, like Navier-Stokes equations, but the amount of calculation required to make that possible in real-time would require computers of the future. That's more the territory of Computational Fluid Dynamics (CFD) simulations for engineering. Here's what I'm getting at: http://secretofflight.files.wordpress.com/2012/03/volvocar.jpg

 

With regard to the "blade-element" nonsense that X-Plane brags about, this guy has it right:

The flight model consists of fixed look-up tables like almost any other consumer grade sim, but they are generated from the geometry of the model rather than hand typed. The flight models resulting from this very simple "blade element" approximation are horrible for anything flying above a 200 kts

Edited December 4, 2013 by aaron886

[leafer]

Uh-oh. You two did not just call x-plane out like that. :shocking:

[VanjaB]

The way you're using the terms "table," "force vectors," and "pulling from a database" shows a misunderstanding of how flight modeling generally works. Like the SFM, ED's AFM almost certainly uses tabulated data to represent the aerodynamic characteristics of different parts of the airplane.

 

The difference between the SFM and a more detailed flight model is the number of point forces applied to the model. ED's SFM, for example, produces only one lift vector for a whole wing. Their AFM breaks the wing up into parts in order to model a range of forces across the span. This gives a more dynamic response to various disturbances, and if you apply that concept to the entire flight model you will have an idea of how it works. However, something still has to determine the strength and direction of the force applied at each point. Aerodynamic equations need coefficient data, and to my knowledge ED also draws this data from tables. Data + equations = performance

 

There ARE other ways to do flight modeling, like Navier-Stokes equations, but the amount of calculation required to make that possible in real-time would require computers of the future. That's more the territory of Computational Fluid Dynamics (CFD) simulations for engineering. Here's what I'm getting at: http://secretofflight.files.wordpress.com/2012/03/volvocar.jpg

 

With regard to the "blade-element" nonsense that X-Plane brags about, this guy has it right:

 

So the vector forces are calculated on the fly based on real world coefficient data, but at different points instead of the entire surface of an air-foil? (which would be almost impossible to calculate without a super-computer) And the difference between the SFM and the AFM is simply the number of points that are used to simulate the behaviour over the different surfaces of the aeroplane. Ok this makes a lot more sense. Thanks for clarifying that up.

 

So to get back on topic, since they are using real-world coefficient data the 3d model used would really be irrelevant to how the plane behaves in the sim.

[aaron886]

Sounds correct. Again, I'm not completely certain of ED's methods, but that is the general idea. Regardless, the model has nothing to do with it.

[Robin_Hood]

In regards to the original post, the bottom line is that the 3D model is independant from flight model.* Thus, although chances are good (<= probably an understatement here) that intake (or more to the point, it's effects) will be modelized correctly in the AFM, it could be that the 3D model stays the same (of course, maybe they'll do the favor to fix it).

 

* Test it yourself by replacing the A-10C 3D model by that of your choice, and see that it is still flying the same.

 

PS: I'm joining VanjaB in the thanks for the insight at what AFM is about

[streakeagle]

Uh-oh. You two did not just call x-plane out like that. :shocking:

 

The fact that X-Plane can generate a flight model from a user designed 3D model is pretty cool. You can get an idea of the relationship between various wing geometry choices and overall aircraft performance. But, it can't even predict the response of a Cessna 152 flying at 150 knots very accurately, much less even get in the ball park of the very complicated flight parameters of an F-4 Phantom.

 

But, X-Plane allows designers the option to override the calculated data tables with user-provided data. A good start is providing the lift curve for the airfoil used on the wing, but there is so much more than that is needed to replicate the dynamics. So, when X-Plane brags about how it is used to design and test real world aircraft, understand that the engineers are using the X-Plane engine, but filling in their own calculated/measured data for the flight model. It is still quite an achievement to have a flight engine that can use their data well enough for them to test real-world performance... but the "blade-element" theory utilized by the in-game design tool is almost worthless for real-world flight performance/behavior predictions.

[NineLine]

But thats exactly how the scripted SFM works.

 

I see a number of people responded to this, but will add that I didnt describe how any FM works, or doesnt work, I only tried to make the point that the 3D model (.edm file), that I know of, doesnt affect the performance of the FM.

[VanjaB]

So, when X-Plane brags about how it is used to design and test real world aircraft, understand that the engineers are using the X-Plane engine, but filling in their own calculated/measured data for the flight model. It is still quite an achievement to have a flight engine that can use their data well enough for them to test real-world performance... but the "blade-element" theory utilized by the in-game design tool is almost worthless for real-world flight performance/behavior predictions.

 

This doesnt make much sense. Why would they 'test' their designs by filling in calculated/measured data from real world planes/models? I think you got it backwards.

[NineLine]

Lets keep the discussion on DCS and leave the other sims/games alone.

[whartsell]

The fact that X-Plane can generate a flight model from a user designed 3D model is pretty cool. You can get an idea of the relationship between various wing geometry choices and overall aircraft performance. But, it can't even predict the response of a Cessna 152 flying at 150 knots very accurately, much less even get in the ball park of the very complicated flight parameters of an F-4 Phantom.

 

But, X-Plane allows designers the option to override the calculated data tables with user-provided data. A good start is providing the lift curve for the airfoil used on the wing, but there is so much more than that is needed to replicate the dynamics. So, when X-Plane brags about how it is used to design and test real world aircraft, understand that the engineers are using the X-Plane engine, but filling in their own calculated/measured data for the flight model. It is still quite an achievement to have a flight engine that can use their data well enough for them to test real-world performance... but the "blade-element" theory utilized by the in-game design tool is almost worthless for real-world flight performance/behavior predictions.

 

Its easy to predict the flight dynamics of a 152 at 150 kts. It would have to be flying straight down ;) thats also past vne

[GGTharos]

How are you going to test your model if not against real-world data?

 

You plug in the real-world data and see what the flight model does with it. There's no other way to test an FM.

 

This doesnt make much sense. Why would they 'test' their designs by filling in calculated/measured data from real world planes/models? I think you got it backwards.

[leafer]

The fact that X-Plane can generate a flight model from a user designed 3D model is pretty cool. You can get an idea of the relationship between various wing geometry choices and overall aircraft performance. But, it can't even predict the response of a Cessna 152 flying at 150 knots very accurately, much less even get in the ball park of the very complicated flight parameters of an F-4 Phantom.

 

But, X-Plane allows designers the option to override the calculated data tables with user-provided data. A good start is providing the lift curve for the airfoil used on the wing, but there is so much more than that is needed to replicate the dynamics. So, when X-Plane brags about how it is used to design and test real world aircraft, understand that the engineers are using the X-Plane engine, but filling in their own calculated/measured data for the flight model. It is still quite an achievement to have a flight engine that can use their data well enough for them to test real-world performance... but the "blade-element" theory utilized by the in-game design tool is almost worthless for real-world flight performance/behavior predictions.

 

I gave up on x-plane very long time ago. I'm no pilot but the feeling of flight just wasn't there for me. People still rave about how accurate the fm is so on and so fourth, and I was never quite sure if it was just me. I remember flying some of the heavies and they didn't feel heavy. Thanks for your explanation.

[VanjaB]

How are you going to test your model if not against real-world data?

 

You plug in the real-world data and see what the flight model does with it. There's no other way to test an FM.

 

Im not sure we are on the same page here. Correct me if Im wrong but isnt the 3d model of a brand new design (Im talking about NEW designs) usually built before any scale models, or even 1:1 models?? I.E. the feasibility of the design is tested in a simulated environment and only once it passes those tests does it go into scale model production, wind tunnel testing, etc. etc....

[mvsgas]

Im not sure we are on the same page here. Correct me if Im wrong but isnt the 3d model of a brand new design (Im talking about NEW designs) usually built before any scale models, or even 1:1 models?? I.E. the feasibility of the design is tested in a simulated environment and only once it passes those tests does it go into scale model production, wind tunnel testing, etc. etc....

 

Things to keep in mind

- Aircraft manufactures may do some testing with computer models, but they use specific computer program that only design for those things.

- ED is not making "new designs", is trying to copy or mimic existing ones, impossible without the Actual aircraft information.

- The Information is out there for some aircraft like the F-16 ( here is a direct link to one of those papers, http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19800005879_1980005879.pdf )

- If we can find something similar to that for the F-15, then we can help.

- We would also need to know how does the Flight Control System (ARI, ACI, PRCA, etc.) work, in detail.

 

Without all this information, probably need more, we are just speculating if an AFM is accurate or not.

[VanjaB]

Things to keep in mind

- Aircraft manufactures may do some testing with computer models, but they use specific computer program that only design for those things.

- ED is not making "new designs", is trying to copy or mimic existing ones, impossible without the Actual aircraft information.

- The Information is out there for some aircraft like the F-16 ( here is a direct link to one of those papers, http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19800005879_1980005879.pdf )

- If we can find something similar to that for the F-15, then we can help.

- We would also need to know how does the Flight Control System (ARI, ACI, PRCA, etc.) work, in detail.

 

Without all this information, probably need more, we are just speculating if an AFM is accurate or not.

 

We werent talking about DCS or ED, we were talking about NEW aircraft designs. Which I guess is not really on topic. So Ill stop now. :thumbup:

[KillarZ]

Interesting read on flight models. Does anyone know what sources and what information one would need to create an AFM or even an SFM. Where could you find these sources? Don't mean to get off topic but just a quick question.

[GGTharos]

Unfortunately there is no quick answer. NASA has a lot of data regarding F-4, F-16, F-18, and F-15, where exactly you could get them, I'm not sure.

 

You can also try to model an FM with the -1 data, but those don't tell you everything either.