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Realistic FM?


bkthunder

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The last few exchanges I had on this subforum regarding VNSL landings made me think that, apart from straight and level "conventional " flight, it will be hard to see realistic beahvior during slow speed vectored thrust operations. Why?

 

Well, for starters, just skimming through the AV-8B manual you'll see how the interactions between the engine exhaust, the ground and the airframe are responsible for a huge portion of how the aircraft behaves.

Most of these effects are given by air cushioning, the jet blast bouncing off the runway and hitting the wings or stabs (e.g. IRL this causes a pitch down moment when approaching close to the ground).

 

So the real question is: does DCS model fluid dynamics required to simulate these effects? I don't think so. How can we then expect Razbam (or anyone) to model a realistic Harrier FM with such limitations? The only way is that they find some ways to "trick" the simulation engine, but it doesn't seem very promising to me...

 

So we'll probably have a realistic FM in forward flight, but during VTOL, I have big doubts we can see it with current sim technology (and computing power on a home PC).

Windows 10 - Intel i7 7700K 4.2 Ghz (no OC) - Asus Strix GTX 1080 8Gb - 16GB DDR4 (3000 MHz) - SSD 500GB + WD Black FZEX 1TB 6Gb/s

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Everything in all flight models is a try to get nearly real flight behaviors.

 

Nothing is "real" simulated. If u would try to simulate a flying aircraft, one frame would take e.g. 2 days to be calculated.

U would need a fluid simulation with all properties of air and the particles in it.

 

So, of course the flight models in DCS are not perfect. And I agree, there are in some situations strange behaviors.

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The last few exchanges I had on this subforum regarding VNSL landings made me think that, apart from straight and level "conventional " flight, it will be hard to see realistic beahvior during slow speed vectored thrust operations. Why?

 

Well, for starters, just skimming through the AV-8B manual you'll see how the interactions between the engine exhaust, the ground and the airframe are responsible for a huge portion of how the aircraft behaves.

Most of these effects are given by air cushioning, the jet blast bouncing off the runway and hitting the wings or stabs (e.g. IRL this causes a pitch down moment when approaching close to the ground).

 

So the real question is: does DCS model fluid dynamics required to simulate these effects? I don't think so. How can we then expect Razbam (or anyone) to model a realistic Harrier FM with such limitations? The only way is that they find some ways to "trick" the simulation engine, but it doesn't seem very promising to me...

 

So we'll probably have a realistic FM in forward flight, but during VTOL, I have big doubts we can see it with current sim technology (and computing power on a home PC).

 

Form now on my answer to all the FM thread is ;

Have you heard Wags interview in the Fighter pilot potcast? In it he talks about how close all the models where/are/will be and why they will never be 100% on purpose.

https://forums.eagle.ru/showthread.php?t=222192

Edit

FM talk starting at 19:00


Edited by mvsgas

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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Oh good. More armchair pilot opinions.

 

So, bkthunder, besides the fact that - whilst there *might* be a slim chance - I doubt you've actually flown a Harrier, what else makes you qualified to say it's unrealistic?

 

In this forum there are probably 95% armchair pilot opinions. Then probably everyone should shut up and the forum could be closed. Great contribution :thumbup:

 

Form now on my answer to all the FM thread is ;

Have you heard Wags interview in the Fighter pilot potcast? In it he talks about how close all the models where/are/will be and why they will never be 100% on purpose.

https://forums.eagle.ru/showpost.php?p=3653024&postcount=1

 

And instead of atributing to him, he'd demand a 100% realistic FM,

one should exchange experiences about the VNSL landings and then come to a judgement whether the criticism is justified (the developers are of course also welcome, but well, you know)

 

Wasn't that EA where the users can contribute to the development?

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Oh good. More armchair pilot opinions.

 

So, bkthunder, besides the fact that - whilst there *might* be a slim chance - I doubt you've actually flown a Harrier, what else makes you qualified to say it's unrealistic?

 

I'm sure you attack others who say the FM on any module is realistic. After all, they'd be equally unqualified to pass comment and of course you're not just posting the above to try and shut down any criticism. After all, that'd make you a hypocrite...

Would like to see:

Panavia Tornado

Panther AS565

English Electric Lightning.

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Oh good. More armchair pilot opinions.

 

So, bkthunder, besides the fact that - whilst there *might* be a slim chance - I doubt you've actually flown a Harrier, what else makes you qualified to say it's unrealistic?

 

Whilst there *might* be a slim chance, I am pretty sure you haven't applied basic reading & comprehension skills :music_whistling:

 

Thankfully some other members have understood the OP is not an attack on Razbam or the current FM (if anything, it's the opposite). I think with such a unique aircraft, we all have to come to the realization that our beloved sim and our beloved PC don't have the horsepower to properly compute a VTOL flight model to the same level of accuracy we see in other modules / flight regimes. But

 

1. I don't know what the sim is capable of calculating in terms of fluid dynamics

2. I don't know how realistic of a result can be obtained by "bypassing" the problem somehow.

 

Hence the thread. For discussion, as you do in a, you know, forum.

Windows 10 - Intel i7 7700K 4.2 Ghz (no OC) - Asus Strix GTX 1080 8Gb - 16GB DDR4 (3000 MHz) - SSD 500GB + WD Black FZEX 1TB 6Gb/s

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I guess the title of the thread could easily wind up some folks, it's the nature of the game. I for one would always try to be a little more circumspect, and not jump in feet first.

 

I guess what it all comes down to is what you feel defines a realistic flight model. Is it close? Probably. Could it get closer - possibly? It all boils down to a few fundamental factors. How much does anyone know about real-world behavior? How much time and resources can be put into getting it closer? Can one aspect of the flight model be enhanced without destroying another?

 

 

The bottom line will always be that this is a bunch of ones and zeros trying to replicate what happens to the prototype AV-8B. Razbam are definitely in the game to make things as realistic as humanly possible given their available resources.

 

 

Fundamentally though, what you have is the best currently available, so it can't be all bad. The key is not to convince yourself it is wrong, but to fly what you have to the best of your abilities until something better comes along.

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I did wonder how it'd be possible to model the exhaust choking the engine inlets (say when VIFFing backwards) which is a major issue with VTOLs.

 

There is no way you'd get exhaust gasses into the inlets by VIFFing, your forward speed and the associated airflow will stop that, it's only an issue in the hover, even a few knots of forward speed will cause the hot exhaust to tend to be near the tail not the nose, not to mention you have the cold exhausts between the hot and the intake.

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Just reading the NATOPS, the Harrier has a huge number of quirks to be aware of. Especially when using vectored thrust close to the ground. I guess these could be scripted to a certain extent?

I'd personally rather have a scripted behavior than nothing at all.

Windows 10 - Intel i7 7700K 4.2 Ghz (no OC) - Asus Strix GTX 1080 8Gb - 16GB DDR4 (3000 MHz) - SSD 500GB + WD Black FZEX 1TB 6Gb/s

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the FM is not too bad at all, except for the big proviso of the issue of too low a drag value and possibly too high a thrust level at moderate RPMs. The response in ground effect could be modelled with simple algorithms to reduce engine power if you sit in a very low hover (sub 30 ft) to mimic hot gas ingestion.

 

However, if you practise / mimic the correct VSTOL techniques, then you should rarely, if ever, need this element of a FM, since the techniques are designed so that you don't loiter in the height band where significant ground effect / reingestion become an issue.

 

I'm all for improving the FM though!

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Oh good. More armchair pilot opinions.

 

So, bkthunder, besides the fact that - whilst there *might* be a slim chance - I doubt you've actually flown a Harrier, what else makes you qualified to say it's unrealistic?

 

That is exactly what we are. Don't come to a forum full of armchair pilots and say "Oh good. More armchair pilot opinions." because that is exactly what you are going to get.

 

This has to be the best forum post of 2018.

 

:thumbup:

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There is no way you'd get exhaust gasses into the inlets by VIFFing' date=' your forward speed and the associated airflow will stop that, it's only an issue in the hover, even a few knots of forward speed will cause the hot exhaust to tend to be near the tail not the nose, not to mention you have the cold exhausts between the hot and the intake.[/quote']

 

He said BACKWARDS! But hey, why let the facts get in the way of a good yarn!

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He said BACKWARDS! But hey, why let the facts get in the way of a good yarn!

Is it still VIFF if you are travelling backwards, though?

 

Fundamentally though, what you have is the best currently available, so it can't be all bad. The key is not to convince yourself it is wrong, but to fly what you have to the best of your abilities until something better comes along.

I like how you put that. I should print it out and look at it once in a while when browsing through all the flight model discussions on these forums.

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Is it still VIFF if you are travelling backwards, though?.

 

Well, technically - VIFF means vector in forward flight, but you can fly backwards in a Harrier, and you vector the thrust in order to do so. So I guess it was just an incorrectly spelt VIF. The key was that he did explain viffing backwards, so it should have been pretty apparent.

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Just to clarify: honestly when first reading I thought it meant to vector the nozzles backwards (well, as in: as far as they'll go) during forward flight.

 

Kind of this, I read it to mean pushing the nozzles fully forward while flying forward to use the thrust to break, anything other than very slow speed in this regime will not cause the hot exhaust to re-enter the inlet.

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Just done some experimentation.

 

On the ground, 33% 10N keeps a steady taxy speed in the teens. Normally, idle and 50N is required to control speed on the ground, since the aircraft dashes off like a scalded cat at idle & 10N. This indicates a ground friction model which is too high.

 

In the air, with a clean aircraft and around 4.5k fuel and water, 39% maintains 230 kts and 41% 300 kts. That suggests that idle plus 12% can overcome 300kts worth of airborne drag. This suggests that either the RPM vs thrust model gives too high a thrust in the air, and/or the drag model is set too low. I'd suspect the latter is definitely involved somehow (if not a bit of both), since the flight model is very reluctant to lose speed, and the AV8B is very draggy compared with 'normal' jets. All that VSTOL gubbins has a big drag penalty! For example, in the smaller engined version (and RPM tended to be about 10% lower for the same thrust as the big engine), around 60% is a good figure to hold 230 kts.


Edited by viffviff
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I think AV-8B FM is realistic enough. It is , IMHO, the most demanding module of fixed wing modules in DCS. However I do not own F/A-18C or YAK-52. In VTOL regime, any error is magnified, and if not arrested results in simulated mishap. In DCS , when flying AV-8B, you have to fly first, navigate second, and fight thirdly. In VTOL regime, you have to fly 100%, total concentration. Especially when landing. Even more challenging at night.

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paradoxically, a more accurate thrust/drag model would make it easier to fly. But I echo your comments; it is a good 'beta' representation of the VSTOL flight model. The aircraft for real is demanding to fly in the VSTOL environment. In fact, without the feedback you get in RW (acceleration forces, great visuals and peripheral) some manoeuvers in simulation are more difficult than for real! Particularly true for VLs and AAR.

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Six months ago a Harrier pilot was doing an AMA (Ask me anything) on Reddit. So I took the opportunity and asked; How does it actually fly with asymmetric loads?

 

His answer differs somewhat from DCS, it's been a few months since I flew the Harrier, so I ask, how's the Harrier behaving nowadays with asymmetric loads? It used to be very noticeable before...

 

This is what he said, which isn't compatible with RAZBAM's FM:

 

BIb8Awe.jpg

 

Just done some experimentation.

 

On the ground, 33% 10N keeps a steady taxy speed in the teens. Normally, idle and 50N is required to control speed on the ground, since the aircraft dashes off like a scalded cat at idle & 10N. This indicates a ground friction model which is too high.

 

In the air, with a clean aircraft and around 4.5k fuel and water, 39% maintains 230 kts and 41% 300 kts. That suggests that idle plus 12% can overcome 300kts worth of airborne drag. This suggests that either the RPM vs thrust model gives too high a thrust in the air, and/or the drag model is set too low. I'd suspect the latter is definitely involved somehow (if not a bit of both), since the flight model is very reluctant to lose speed, and the AV8B is very draggy compared with 'normal' jets. All that VSTOL gubbins has a big drag penalty! For example, in the smaller engined version (and RPM tended to be about 10% lower for the same thrust as the big engine), around 60% is a good figure to hold 230 kts.

 

We have performance charts available everywhere for the Harrier, so why don't we compare it with those?

 

Unfortunately, I am not able to do that at the moment.


Edited by Vitormouraa
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My Harrier is very responsive in a hover. So responsive I had to add curves to get it under control. There's probably something wonky with your controls setup.

 

On the subject of drag/power modeling, the other night my climb rate seemed way too good to be true:

 

Payload: 3xGBU12, 1xTGP, 1xA9M, 7100lbs fuel

Weather: 30°C, 29.77

 

I was able to maintain 309kts in a 20° climb with 99.8% RPM. With three bombs and a TGP I would expect a shallower climb ability. Is this normal?

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