[ACE-OF-ACES]

Originally Posted by swift

My point is that you will have difficulties to derive reliable coefficients and you seem to agree on this.

Key word being reliable..

Now ask yourself..

How do you prove to yourself they are reliable?

Answer is you compare the results (outputs, such as tas, roc, roll-rates, turn rates, etc) of the 6DOF using these coefficients to the real world data..

Which is pretty easy to do when you have the real world data!

But what do you do when you don't have any real world data?

You guess it.. You rely on, aka trust, the math!

That is my point

That being we do NOT have real world data on each aspect of the WWII plane.. So unless we come up with a time machine, we are going to have to rely on calculated results (the math) for not only simulation but validation.

Originally Posted by swift

I do know a little about flight mechanics and trajectory computation (where the 6dof equations intervene) and about coefficient determinations (it is my daily business).

You too? Here as WSMR we use a lot of trajectory math to calculate the launch to impact site of the missiles we test here. We also have our own flight simulation software that we call RAGE

http://www.csc.com/public_sector/suc...tual_landscape

I work with the guy who wrote that software on a daily bases.. I also write plug-ins for RAGE but he is the true gu-roo of the software. All in all a great job, on my way to work I may see anything from a F22 to a UAV fly by at tree top level (landing or taking off from holloman).. It can be dangerous sometimes.. In that I am always looking up in the blue instead of looking forward at the road!

Originally Posted by swift

At work we frequently use a simplified tool to calculate the aerodynamic coefficients for subsonic and transsonic flight conditions and I can tell you I would not trust them for applications such as CoD. We use them for different applications where the impact is minor so we can live with it. But CoD would rely heavily on these coefficients and I'd say to obtain something that is halfway close to reality such a tool is not sufficient.

Well than I guess we will have to agree to disagree on that point

Originally Posted by swift

And from experts working for years in the aerospace business using modern and highly sophisticated cfd tools I know that using these methods for subsonic regions is far from trivial. And it would take hours to days to calculate just one flight point for one configuration

Note I never said it was easy or trivial.. My only point is it is doable! Many have for many years now.

Originally Posted by swift

and probably would take longer if one would take into account the viscious terms instead of relying on simplified Euler calculations.

I know of only one PC flight sim that implemented a real-time computational fluid dynamics flight model (what you call 'viscious') and that was FLIGHT Unlimited

http://en.wikipedia.org/wiki/Flight_Unlimited

It was done back in 1995.. And was not only overkill IMHO but too much for the PCs of that time. Maybe even today, I don't know in that not many make use of it in that the 6DOF (what you call simple euler) has proven itself to be more than adequate for military applications, thus more than adequate for PC games IMHO.

Originally Posted by swift

But what we need in CoD does not stop at the determination of lift, drag and lateral force coefficients and the moment coefficients about the three axis. We also need the derivate coefficients to obtain a believable flight model.

Key word being believable..

Now ask yourself..

How do you prove to yourself they are believable?

Answer is you compare the results (outputs, such as tas, roc, roll-rates, turn rates, etc) of the 6DOF using these coefficients to the real world data..

Which is pretty easy to do when you have the real world data!

But what do you do when you don't have any real world data?

You guess it.. You rely on, aka trust, the math!

That is my point

That being we do NOT have real world data on each aspect of the WWII plane.. So unless we come up with a time machine, we are going to have to rely on calculated results (the math) for not only simulation but validation.

Originally Posted by swift

Up to now the means with which these are "determined" is more than crude and very little reliable.

Well than I guess we will have to agree to disagree on that point

Originally Posted by swift

This gets even more complicated when one considers that each flap, rudder and aileron movement will have an impact on the aerodynamic coefficients (the 6 static coefficients and the derivates). You'd need a database set for several flap, rudder and aileron deflection combination. Then we have the trimmed and untrimmed flight conditions and other aerodynamic control surfaces such as flaps and airbrakes.

Which has all been done before and done for years!

Originally Posted by swift

Now let's talk about canopy open or closed and radiator and oil cooler openings ...

Better yet..

To drive my point home..

Let talk about you going out and finding the real world test data for each plane in the game of the 'effects' of the canopy open vs. the canopy closed on each aspect of the plane (tas, roc, roll-rate, turn-rate, etc)

Allow me to spare you that effort!

In that you wont find such data!

Thus, back to square one of my point

You will have to trust the math and how it says the canopy open vs. closed will 'affect' the flight

Oh sure you may find some anecdotical evidence for some of the planes.. For example we have all read the stories.. Like the Me262 that was stuck in a high speed dive, until the pilot popped the canopy and started to bail out, at which point he noticed that popping the canopy 'changed something' such that he was able to regain control, and thus didn't bail out. What is not 'clear' about such stories is the parameters to re-create that scenario in the game to see (validate) the flight model. For example..

What was his altitude when he popped the canopy?
What was his speed when he popped the canopy?
What was his dive angel when he popped the canopy?
What was his flap setting when he popped the canopy?
What was his trim setting when he popped the canopy?

The list goes on and that is only for the point in time of when he popped the canopy..

So all we know from that story is that 'something' changed.. For all we know his plane was slowing down and the popping of the canopy had NOTHING to do with it.. It could have just been a coincidence that he popped the canopy at the same time the plane had slowed down enough that he was able to regain control

Originally Posted by swift

And these are just the coefficients for the airframe. We'd also need reliable data for the propulsion set.

Ah, glad to see you agree with what I said in my last post!

So not all is lost, in that we can agree on some things!

Originally Posted by swift

I really do not believe in being able to obtain a full AEDB that will result in a flight performance that will be close to the real thing anyway, provided we even know where the real thing was.

Bingo!

So do we give up?

Or do we trust the math and move on?

Originally Posted by swift

I think it is smarter to take the bottom up approach by tweaking the used coefficients in such a way that they fit to the experienced behaviour including test results and, where values are missing, to anecdotical evidence as long as there is a bunch of anecdotes saying the same.

Which is exactly what I was saying minus the anecdotical evidence (aka pilot combat reports)

Just too many variables involved to make most if not all anecdotical evidence useful

As I noted early on

Many 'feel' the can derive some sort of statistical average of the anecdotical evidence.. Many have tried, all have failed! Which is not surprising when you consider the fact that the anecdotical evidence is not something that varies a 'little' As in one reports says the top speed is 305, another say 307, and another says 302, and another say 310. If that was the case it would be a simple mater of taking the average and calling it good! But that is not the case for anecdotical evidence, what we have there is Spitfire pilots saying they could out turn 109s and 109 pilots saying they could out turn Spitfires. So based on that it is not surprising that those who have tried have failed.

Originally Posted by swift

BTW: CoD is definitely using 6dof.

Ah, glad to see you agree with what I said in my last post!

So not all is lost, in that we can agree on some things!

Originally Posted by swift

What we are disputing is how they come up with the forces and moments they inject into the 6dof equations.

No, we know how they come up with them.. What we are disputing IMHO is how they validate the flight model.. For the aspects of which we have no real world test data to use in the validation. In such situations I am saying we have to trust the math to fill in the blanks, where as your saying we should use anecdotical evidence to fill in the blanks..

In summary

I put more faith in the 6DOF math than you do
You put more faith in anecdotical evidence than I do

Other than that I think we agree