Orbiter's "Atmo" (Aero) capabilities need study!

[emarkay]

I see the fantastic orbital physics and physical realizations in "outer space" , but have also see n a few "inner space" vehicles - a 5 year old B-52/X-15 add on and a much newer X-35 VTOL plane. Apparently there's no "aero" vessel designed specifically for Orbiter 2010, but the burning question is:

What are the capabilities of flight models in Orbiter with atmospheric (aerodynamic) parameters?

X-Plane boasts that it has the most accurate aero modeling parameters, but has poor graphic model capabilities (the planes basically look like cartoons), and it emphasizes cockpit views(as a "true flight simulator") but does not allow for much variety in more "fun" modes like "chase plane view" and the like. However, it is multi OS capable, including Linux, which is a major positive factor.

Microsoft's Flight Simulator 9 and 10 ("X" or AKA 2004) are both the established standard in operation and support by third parties. It is possible to go near orbital with poorly modeled aircraft (an XB-70 comes to mind), or near-perfect models like a few X-15s, but has no "outerspace" capabilities.

FlightGear is another multi OS flight aero-only simulator, and it has many of the best and worst of the above. It is one I am evaluating more in detail, but has many "quirks" to get accustomed to.

YS Flight, another multiplatform sim, which is even more limited, but will work in near DOS environments, so it's usable "everywhere".

Orbiter's graphics capabilities exceed a number of these "Flight Simulator" programs, and it appears that it is possible to make a realistic "air-breathing" flight model.

What can it not model?

What prevents it from matching MS in realism and preformacen (air drag, wing flex, CG effects, etc.)?

I propose that someone who is familiar with Orbiter, MSFS and X-Plane design a "cross-cross-platform" air breather test model that can be built and used in as many of the flight simulators listed above, and can then be used to find limitations and "Gee, I wish I could do that in..." features.

Of course, this would be focused on documenting the limitations of Orbiter, and to add legit requests for enhancements and features here.

While I am not anywhere near an expert in creating aircraft/vessel/models, I do know a lot about the parameters and compromises needed to make a realistic performing vehicle. I also have all the above sims in Linux and WIN XPSP3 available to test.

This could be both fun and valuable as I see it. What do you all think?

Thank you,
MRK

 

[RisingFury]

In general, the tools available to developers for the atmospheric flight in the API are very rigid...

 

[emarkay]

In general, the tools available to developers for the atmospheric flight in the API are very rigid...

I presume that means well defined and accurate? So it is possible to make the flight modes "as real as it gets"with a well designed model?

What about atmospheric stresses, turbulence, weight and drag from icing, density changes in subfreezing temps and of course obscured visibility due to clouds and all that?

Fascinating...

 

[Wishbone]

JSBSim might be what you're looking after. Of course, getting your own license is an option, too.

 

[hribek]

It appears to me that the aerial flight limitations of orbiter are in some areas of the atmospheric model - weather, chemical composition and a few other parameters that are approximated for now. However, it is possible to create atmospheric model addon modules for Orbiter that are better.

Each vessel has as complex flight model as it was coded to have. There is just very few vessels in Orbiter whose authors can/wish spend their time and effort on a very detailed and accurate flight model.

In short, I think there is a lot of freedom and little limitations. I'm not totally sure what RisingFury meant by "rigid". From what I can see, the options are there. Perhaps other simulators have some more advanced API functions or libraries built upon those. I don't know.

Related documents:

Doc\Technotes\Earth_atm.pdf (and others)
Orbitersdk\doc\API_Guide.pdf

 

[C3PO]

I haven't trid this out, but how real is the atmosphere on Mars or Venus in those flight sims?
You should realize that flight sims have only one important task, and that's to model Earth's atmosphere. The Orbiter engine has to do so much more.

 

[Tommy]

Orbiter's aerodynamic flight model is fairly simplistic - nowhere near as accurate (at low speeds) as the other sims you mention. However, none of those other sims are very accurate at extremely high speeds often encountered by spacecraft. Earth re-entry speeds exceed Mach 28, the other sims start to "fall apart" much over mach 6. They just aren't designed for hypersonic flight and the "shockwave" effect. None of the fligh simulators mentioned (or Orbiter itself) can model a hypersonic "Waverider".

Orbiter doesn't model airfoils, rather it uses a fairly simple lift-drag calculation. Aircraft can't "spin", even stalls aren't modelled with any real accuracy (aero control surfaces continue to work effectivly when they shouldn't, etc.)

As mentioned above, most of the "missing" fuctionality could be added. There is already a "WeatherMFD" that adds turbulence, wind, and visibilty loss, etc. However, making a vessel that was accurate in it's flight characteristics would be very difficult, and the number of calcualtions required during flight would bog down the sim significantly. As things stand, this would have to be done in the vessel code to be realistic (a "generic" module wouldn't have access to the data required to make realistic calculations for each vessel's specific design - airfoil shape, weight distribution, etc, would all have to "approximated" - mostly by analysing the mesh, and these meshes aren't designed for true realism - they are predominantly cosmetic.

In short, it wouldn't be difficult to add some realism - such as weather, etc, but there is a limit to how realistic you could get and still have a flyable sim. Also, there is a limit to how much time and effort the dev's are willing to put into aerodynamic realism - since most vessels are designed primarily for spaceflight. Also, the knowledge required to create that realism (a thorough understanding of fluid dynamics at a wide range of speeds, etc) are a bit beyond most people. While we have several extremely talented devs, not many have Phd's in aeronautical engineering.

 

[theflyingfish]

Is there any possibility of increasing the accuracy of the spin/stall/high-alpha manuevers simulation in orbiter, possibly in the wish list for Orbiter 20XX? I think it would be useful during re-entry (high-alpha), as well as transitions from hovering to forward flight and back again, and also landing. The DG and XR2 have very high landing speeds, but it is possible to make them go so slow that they simply fall out of the sky, like a plane with a C of G that is too far back. That would probably not happen in real life.

 

[C3PO]

Actually the CoG in most deltawinged craft in Orbiter is too far forward to make realistic reentries. If the CoG was correct the craft would be too unstable for manual control.

So after a developer has spent a lot of time making the craft realistically unstable, he has to add some kind of dampening autopilot like rate control to make it "flyable" again.
You will not find many people with that much dedication. So most add-ons have the the CoG in a more pilot-friendly position.

 

[emarkay]

Orbiter doesn't model airfoils, rather it uses a fairly simple lift-drag calculation. Aircraft can't "spin", even stalls aren't modelled with any real accuracy (aero control surfaces continue to work effectively when they shouldn't, etc.) While we have several extremely talented devs, not many have Phd's in aeronautical engineering.

I agree the devs are fantastic and they are giving away all this expert labor. Thanks again!

While I agree this is a "spaceflight" sim, the few atmospheric craft I have encountered are oh-so close enough as considered, so that maybe FWIW a Dev Conference should be considered for the next release to get the next "level" of atmospheric enhancements for Orbiter. Yes, the Shuttle and DG's "fly" OK, but how much better could they be - or, what do they have in this realm that other "lesser developed" add-ons don't? (Clues, my friend, clues!)

What about the other sims, and the mechanics behind them - is there any GPL or open source "engines" that can be added to Orbiter.

What about a 2 mode unit that "when the sky turns blue the other engine kicks in" type of thing - that could make the atmo environment separate from the space environs.

I am surely not complaining, nor am intending to bother, but darn it, I am liking Orbiter more and more, even in the air, than the "true-to-life" MSFS. More bizarrely, even knowing there's no force-feedback, and no "surround sound"! I just like the interface and teh ease of "flow" that the program has in flight.

Thanks again and let's keep this thread open for ideas and suggestions.

Oh, and I don't think a Doctorate is needed, but maybe just, as one said, some study to the "obvious" fallacies in reality, and what can be done to make them more real for now - then one can rewrite and meld the molecular physical and and thermal functions of the "other" sims here.

 

[hribek]

Orbiter's aerodynamic flight model is fairly simplistic - nowhere near as accurate (at low speeds) as the other sims you mention. However, none of those other sims are very accurate at extremely high speeds often encountered by spacecraft. Earth re-entry speeds exceed Mach 28, the other sims start to "fall apart" much over mach 6. They just aren't designed for hypersonic flight and the "shockwave" effect. None of the fligh simulators mentioned (or Orbiter itself) can model a hypersonic "Waverider".

I disagree with that. It is not a problem of orbiter simulation core, but of vessel developers. The tools are there.

Just so that I say it clearly, Orbiter doesn't have anything for gas flow modelling or so. That's best done by dedicated software and is computationally intensive. However, the output data from that (or actual flight data) can be used to make realistic craft. The realism levels depends on the extent to which the characteristics are identical to the measured ones.

 

[Tommy]

I disagree with that. It is not a problem of orbiter simulation core, but of vessel developers.

The method Orbiter provides to calculate lift is quite limited compared to other sims, IIRC vessels need to provide a fuction that returns a lift and drag ration based on AoA and velocity - and the core determines the actual forces to be applied to the vessel. To make a vessel with a more realistic flight model, the vessel dev would have to calculate the forces themselves - accounting for the forces already applied by the core (which, since they don't have Orbiter's source code, they can't accurately predict). This is more work than most dev's are willing to take on. Most devs put in hundreds, even thousands, of hours making the vessels we have - its hard to ask them to spend a few hundred MORE hours (and purchase software capable of providing the data required to model the vessel's flight characteristics - and learn how to use it, etc) to make the aero flight accurate.

Most dev's have done a great job "faking it". They have created vessels that behave in reasonable realistic manner by tweaking COG location, the lift/drag calculation, etc. They focus on making a vessel that "feels right", an illusion of accuracy, rather than one which is technically correct.

To make the vessel technically correct would likely more than double the development time for a vessel. Since, in reality, most deltaglider type vessels (including the XR series) would be aerodynamically unstable, a "fly-by-wire" system would also need to be implemented by the vessel. For such a system to work, it would need to perform it's calculations at least 100 times per second. That means it would have to be based on simstep, not framerate (as AP's are now). That's a lot of overhead. Conventional flightsims can handle it because they aren't spending cycles on things like precession, multiple gravity sources, non-spherical gravity sources, gravity gradient torque, etc. Most flight sims don't even actually model the Earth's rotation or coriolis force. A vessel which did use such a complex aerodynamic model (and the fly-by-wire to make it actually flyable) would only work on a very top-of-the-line multicore PC, making it unuseable to the vast majority of us. It's hard to see a dev spending that much time and money on an add-on that perhaps 2% of Orbinauts could actually use.

We have many add-ons that behave in a reasonable realistic manner under most conditions. Not all conditions - none can spin or stall realistically, but since good pilot avoids situations where you would spin or fully stall, is that really so big an issue?

 

[Linguofreak]

Is there any possibility of increasing the accuracy of the spin/stall/high-alpha manuevers simulation in orbiter, possibly in the wish list for Orbiter 20XX? I think it would be useful during re-entry (high-alpha), as well as transitions from hovering to forward flight and back again, and also landing. The DG and XR2 have very high landing speeds, but it is possible to make them go so slow that they simply fall out of the sky, like a plane with a C of G that is too far back. That would probably not happen in real life.

It actually would: For one, delta wings tend to be able to achieve very high angles of attack. For another, the ability to sustain high angles of attack is needed for reentry. For another, these spacecraft have RCS, retro, and hover thrusters, which makes them able to do things that an aircraft can't do on aerodynamic surfaces alone.

 

[Grover]

well, ive just thought of a couple of problems:

orbiter is young compared to quite a few of them, there is no weather system worthy of mention (though it is a parameter somehow...), so it lacks a dynamic atmosphere

orbiter is a more a physics simulator than a flight simulator, so it sacrifices any advanced aerodynamic simulation to make way for its very accurate gravitational simulations and the such. didnt you know that you experience gravitational forced from all bodies in the solar system at the same time? now you do

when martin makes this, he probably thinks of the space capability of the simulator, and hardly considers the atmospheric models and such, they work dont they? and since you spend most of your time out of the atmosphere, it would be a waste to develop the atmospheric simulations when you can develop the orbital simulations and features which you will use more

do you know how difficult it is to simulate aerodynamics accuratley? to add it to orbiter yould massivley increase the computer hardware requirements, and even my 3 month old computer struggles when i have an XR5 and an Energia in play at Wideawake with some SSBBs


best bet: make do with what we have. unless you think it possible and worthwhile to make an addon which could redo aerodynamics, for the orbiter engine, the simple version we have is complex enough for what we need.

of course, if you had a briliant idea, dont let me put you off, ive flown a few vessels and thought about their aerodynamics, its just not the main focus of orbiter