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"So which performance characteristics of the almost universally lauded RA planes are not realistic in your estimation? I'm intrigued. What do you mean by "bending the laws of physics?" We're talking here about a computer simulation. There really are no physical "laws"." As a physicist by profession, I can't let this one go by ;-) . It comes down to what one means by a 'simulation'. My dictionary says for 'simulator': "device that enables one to represent conditions likely to occur in actual performance". To me, that means putting in the actual physics and getting the plane to fly as it should. What I've recently found out is that MSFS actually responds to the "real physics" much better than I had previously thought. i.e. the flight engine in the simulator is actually pretty good in doing the physics. Therefore, if one is "bending the laws of physics" and doing unrealistic physics things to get a certain behavior, then one is engaging in computer fantasy, not computer simulation. A common poster around physics departments is a very funny one depicting "Road Runner Physics"; some very humorous stuff from the cartoon, but again just fantasy physics. I know one can respond that they are just overcoming the limitations of the flight engine, but again, my point is that the MSFS engine seems fairly well-developed in handling the real workd conditions. Mike

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You should realy look into your joystick / yoke settings in X-Plane.It took me hours of tinkering but when you finaly get it right 90% of the X-plane models instantly feel right. Much of it has to do with default X-plane being developed for the big / realsize yoke stuff.I think its funny when you discuss X-plane tweaking the spec based models and after that you reccomend heavely tweaked MSFS modesl.MSFS is also heavely based on realworld figures tweaked to get the right feeling.The only difference is tables flight model versus blade element theory model. Both flightmodels work on the basis of garbage in = garbage out. Both models need tweaks to get the more complicated models right the only real difference lies in the data used and the level of expertise in the tweaking.FS has the great talents of people like Rob Young and Alexander M. Metzger whose FDE's never dissapointed me. The only problem is the lack of fluidity 'on rails' feeling in FS9 9even wih the best models its still obvious). FS-X improved this though (if your hardware can run it).X-planes pool of flight model designers is much smaller so as a result its top flightmodels might be a little bit worse then the top FS ones. In return you get more fluidity at a consistant 30+ fps (it looks great, is smooth + its needed for the flightmodeling so don't you dare to choose graphics over fps....)


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Guest Zevious Zoquis

That's fine. Whatever works works. All I'm saying is that for me the end user, I couldn't care less about whats going on "behind the scenes" so to speak. All I care about is the end result. And afaic the R/A planes are marvelous end results...Theres alot of problems inherent in PC simulations like FS9. Issues related to translating 3 dimensions onto a 2D display, endless varieties of controller interfaces, a pretty complete lack of tactile feel. The fact is, if you had a PC flight sim that managed to incorporate all the laws of physics with pretty much 100% fidelity it would likely seem really screwy unless we were flying in fully 3D VR motion sims or something. Frankly, what this ends up being is more of an art than a science. All a developer can hope to do is come up with some reasonably convincing trickery. I'd imagine the devs of some of these models you are referring to had specific reasons for using the numbers they used. Whether you agree with those reasons or not is one thing, but I wouldn't just assume they are lazy or stupid. I mean you don't think you guys are the first to think of looking at schematics and using r/w data do you?

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>Have you performed your magic on any of the default planes? >If so, have you considered distributing the modified files? >I'd be interested in experiencing these improvements but I'll>probably not put in as much work as you have. Congrats on>your accomplishments.>Alex the guy that designed FDE's for Flight 1's Beechcraft Bonanza and Baron and the ATR72 has some tweaked default FDE's you can find them at:http://www.metzgergva.de/default_e.htm


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>> As a physicist by profession, I can't let this one go by>;-) . It comes down to what one means by a 'simulation'. My>dictionary says for 'simulator': "device that enables one to>represent conditions likely to occur in actual performance". >To me, that means putting in the actual physics and getting>the plane to fly as it should. What I've recently found out>is that MSFS actually responds to the "real physics" much>better than I had previously thought. i.e. the flight engine>in the simulator is actually pretty good in doing the physics.> Therefore, if one is "bending the laws of physics" and doing>unrealistic physics things to get a certain behavior, then one>is engaging in computer fantasy, not computer simulation. I already knew that MS responds to real physics that some seem to think. However, there is no "real air molecules", and any physical laws/theory by Bernoulli, Newton, and a host of others won't apply, when it comes right down to the guts of the matter. It's just electrons on a screen, and our eyes fool us into the sense of movement and feel.You just quoted: "device that enables one torepresent conditions likely to occur in actual performance". And that's exactly what some of these better designer/programmers do! Just as a full motion "sim" uses hydraulics to "simulate" axis of movement through the air, the best sim designers use their own bag of tricks to get the most desired and believable results!No one is bending laws of physics, because they simply don't apply to that fictitious aircraft that's moving across my monitors screen.Yes, someday we'll have super computers that will do away with wind tunnel testing completely. In the meantime, we'll be faking ground effect and using hidden wings in X-Plane, while making "sometimes weird" adjustments to get the desired results in Flight Simulator.What I care about is the final results! I would just as soon use hydraulics in a full motion, rather than directing a jet engine's exhaust across some moveable surfaces to twist and turn the sim module..............even if it's faking it!L.Adamson

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Guest Tom Goodrick

I started my engineering career working for the US Army back in the 1960's when most of us ordinary engineers had trouble getting to a computer. (They were used only for important things like payroll and class scheduling.) But as time-share terminals, mini-computers and various "desktop" computers became available, I ended up writing and using many specialized computer simulations of various motion problems that involved parachutes, airdrop payloads, aircraft and spacecraft before retiring in 1997 from NASA. The entire idea of a simulation is to enter the equations of motion that conform to the laws of physics so you can see what happens and make discoveries and predictions about the real world problem you are modelling. Gliding parachutes was my primary field early in the 1970's as we struggled to find what they would do for the Army, how you could control gliding cargo systems, how they landed and what payloads did when landing under a gliding parachute. With my gliding parachute 6 degree-of-freedom simulator, I was surprised to find it showed a spin very accurately. I verified the authenticity with comparison to data measured on board test parachute systems in dynamic flight including spins. This work was reported and published within the AIAA organization. My last published paper dealt with hypersonic aerobraking for modification of Lunar-return orbits. My last sim was written for NASA and involved transitioning from orbital flight to aerodynamic flight with a planet's atmosphere and landing accurately on the surface. If you pay attention both to all the laws of physics that pertain and to using the proper mathematical techniques to get an accurate solution, then you can have confidence in the result. With most motion problems involving dynamic flight, there is no way to predict the outcome of motion using simple equations. You have to get the forces and moments at one instant, sum them and integrate them to get the changes over a small time increment in velocity and integrate the velocity equations to get translational and rotational displacements. That is what happens in this neat little sim we call FS9, as well as in the other flight simulations.The problem is that you can only believe the outcome if the basic supporting laws of physics are employed. Many people in "fixing" the FD fiels for flight models in FS9 have mangled the MOI's thinking that was the quickest way to get what they wanted. That violates the laws of physics that dictate a relation between the distribution of mass throught the aircraft and the result of application of various moments to make the aircraft rotate about various axes.Many people think the effect MOI's have on rotation is the same as the effect of mass on translational or linear acceleration. It is not. There is a very complex relationship between rotational motion about various axes. pitch motion is the only one that is resonably independent. Try a little yaw input and you will soon see resulting motion about the roll and pitch axes. The nature of this result is dependent on the magintudes of the MOI on the primary disturbed axis and the differences in the MOI's about the two other axes.But, of course, MOI's are only part of the solution. You must have all the geometry, the weight and balance, and the control sensitivities correct. Then, in FS9, you must have the stability parameters correct and the variation of those parameters correct with respect to angles of attack and sideslip, and the pitch, roll and yaw rates. Only when all these things are done correctly for the entire range of attitude the aircraft can possibly encounter can you "fly" with confidence that you'd experience similar things in a real aircraft. In many cases it is possible to achieve a particular result by tweaking the wrong things. But when this is done, something else in the flight mechanics will get screwed up. We all start with the assumption that everything should go right in normal flight. Then we look at the special motion problems.

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I understand. Both you and Zevious are more interested in the "bottom line". As the title of thread suggests, there's another aspect of the hobby for those of us who like to dig into the "guts" of the FDE. AND I am no where near the experience level of Tom. Case in point: as was suggested previously, yesterday I played with the default C182. Got the scale drawings, did some adjustments, loaded FS and laughed all the way as the plane promptly tipped over onto its nose sitting on the ground :( I think I know what I did wrong and the hint was in the original cfg file where MS had shifted the ref datum by 3.6 ft forward and the Cg 3.6 ft aft. I don't think I was using the right model center. I'll have some more fun tonight fixing it. As Tom mentioned though, knowing that the physics is at least in the correct ballpark, makes me at least more confident that what I am experiencing is a correct flight model. Doing the 182 is going to be important, as its a plane I have actually flown. Can't say that about an 737 ;-) Mike

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Guest PARADISE

I'm no physicist, but as I see it,does it realy matter where the CG or refference datum points are in a simulator? As far as I know there is no real gravity in a simulator like there is in real life. So a sim developer is not limited by laws of gravity.John M

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Guest krswen

>I am glad to hear someone has found my writing helpful. Yes,>indeed, the main problem with 90% of the airplanes I download>are simple things like CG, MOI and the weights.>>It is unfortunate that there does not seem to be much of a>market for an update of the FD text I wrote several years ago.> The fundamental things discussed in that are still good but>there are now many new things to take into consideration. For>example, there are stability derivatives in the air file now>that can be tweaked to modify the steadiness in pitch and many>other things including the capability to do intentional>spins.>>I played with X-plane a little about 10 years ago and found>some useful things in it. But FS has remained better for>general purpose flying when flight models are fixed up. The>main thing X-Plane does that FS cannot do is snap rolls and>unintended spins. These do not happen in most general-purpose>flying. An accidental spin will occur when the rudder is>poorly used in a low-speed turn. An excessive yaw rate can>cause the lift on one wing to be much greater than on the>other wing and you are suddenly spinning. This cannot happen>in FS because the lift is computed for a central angle of>attack as for a single wing. By distributing the lift along>the wing, X-plane accurately models this phenomenon. However,>by setting up aircraft to be able to do intentional spins, you>can see what it is like to be in a spin and can learn how to>recover as best you can.>>I have about 40 aircraft listed on my web site for free>downloading. That site is at>http://home.hiwaay.net/~goodrick/Downloads.html and includes>several revisions of the FD for default aircraft.Hi Tom ....I'm finding your Learn to Fly the USA very helpful. Did you take any IC Engines classes from Tom Murphy when you were at the U of M? I was two years behind you, Mech Engg. He was my favorite prof.

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>You should realy look into your joystick / yoke settings in>X-Plane.>It took me hours of tinkering but when you finaly get it right>90% of the X-plane models instantly feel right. Much of it has>to do with default X-plane being developed for the big />realsize yoke stuff.>With many years of 2" long joysticks flying R/C, I found that I easily adapt to different real life joystick/yokes, as well as those for sims; without over controlling.L.Adamson

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Guest Tom Goodrick

On my web site I have handled this problem in a way that I think is legally correct yet still gives everyone the oportunity to fly aircraft that perform close to real specs. I started out doing just FD fixes for aircraft models in the Public Domain and a couple that I did myself (Those are on this site as well). Then I decided to just post the FD files on my site where they can be downloaded for free and identify the model and you can download to put those files in if you wish. (Most of those downloads are from this site). If you pass along my "improved" FD files, I don't mind and I don't think anyone else should have any grounds for complaint. If you use my files to make another designer's aircraft more fun to fly, it seems to me that's your prerogative. No one is uploading a complete aircraft and no one is taking credit for anyone else's work.By the way, we have to remember that people who claim copyrights, have the legal right to be wrong.I have noticed that even with "major brand names" in the FS aircraft business, some simple things need fixing. I forgot the particular names but I downloaded a beautiful prop plane the other day that was great except you could not set the power correctly because the prop control simply did not work. It took five minutes or less to adjust the max blade angle and get that control working. Now the plane flies great.

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Guest Tom Goodrick

I can probably answer this as well as anyone, having written a flight sim code for NASA that handles regular atmospheric flight all the way up to orbit and back in a 3D universe. We put gravity in the machine the same way we put all other elements of the unverse in the machine that apply to our simulation. We put it in the equations the code processes each delta T during your flight. I set that program up to run in real time when in the atmosphere and in accelerated time during orbits. (A standard Earth robit in low altitude where the shuttle flies takes 90 miutes and I was impatient.) With this code you can see the changing parameters and can input controls such as rolls and thruster burns as needed to stay on a flight profile. One very interesting thing was the display of total aerodynamic force ratioed to weight (nominal, as weighed on the surface). You could G's go from zero in orbit to 1 when in steady aerodynamic flight. Of course, getting it to 1 without seeing 2 or 3 first is a trick.If you don't think gravity is in there, try this. Set only one pilot in the left front seat of the Cessna 172 and set all other seats to zero weight as well as the baggage. Then load a full tank of gas into the left wing and leave zero in the right wing. Then go flying. You will probably notice something different.That example I used may wake up some who wonder about gravity in space, but, that's a topic for another day and another forum.

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Guest Zevious Zoquis

Nobody is claiming that the effects of gravity aren't represented in the sim. What we are saying is that we don't really worry about what the dev has to do to get the plane to perform the way they want it to. As a guy who just wants to fly around in a flight sim in planes that perform reasonably authentically, I don't mind at all if the CG is off in the cfg by 11 inches as long as the end result is a plane that flies right. If there's something that is not functioning properly and you've been able to fix it and not had an impact on some other aspect of the model, fantastic. I'd be interested though in hearing from the devs of these models and getting their perspective on the reasons they did things the way they did...

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Guest ha5mvo

>I'd be interested though in hearing from the devs of these models and >getting their perspective on the reasons they did things the way they >did...Not claiming to be in the know, I would assume that it all boils down to the sims physics engine.if it was as simple as digging out the specs for the real thing and just putting it in, then dynamics modelling would not have gotten the reputation of being a "black art" of sorts.With this in mind I can see why developers may wish to bend things a little in order to compensate for the sims shortcomings.take the elevator trim for example. How come its so often more effective than the elevator itself? how come it can always trim out the plane regardless of the CoG? do autopilots really control the pitch using the elevator trim?

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Guest Tom Goodrick

I'll try to answer both #26 and #27.You should care about the CG if you like flying realistic aircraft just as the pilot of a real plane cares greatly about his CG. Load the plane wrong and you have problems. Somewhere in Cyberspace there is a guy flying one-wing-low if he took my advice and loaded the Cessna 172 on the left side. FS is realistic enough to show the same plane can behave differently if loaded incorrectly. Personally, I like flying with a CG gauge on the panel that weighs the plane on the ground and calculates the changes in weight during the flight so I always know where it is. That helps me a lot in developing the FD for an aircraft. But any pilot would like to know that too. For example a tyical aircraft can move its CG by several percent just by burning most of its fuel load. In a good design the CG moves forward as fuel is burned. But in some, the CG moves aft. The Bonanza is notorious for that. My V35B Bonanza (this site) does just that as you fly.Incidentally, there are very few aircraft that can have all the seats filled and all the tanks filled and still fly. If you ignore that aspect of FS flying, you are missing a big part of aviation education that this sim can provide.As far as the elevator trim is concerned, it has taken me a while but I can now adjust the trim and the elevator controls for more realistic deflection and result. This comes after making sure the geometry is reasonable for wings and tail, alweights are reasonable and properly placed so the CG is in a good position. Much of FS does not conform to aeronautical text books so some adjustments are where some black art or "trial and error engineering" comes in.The pitch trim is a standard aeronautical parameter though it is implemented in many different ways on different aircraft. Once while designing a "real" radio-control aircraft, a 14 foot span twin-engine plane that dropped instrumented test payloads for testing new parachute designs, I played with the tail design parameters on a PC. I set the incidence of the horizontal stabilizer as the trim (in the computation) and the angle of elevator deflection as the control. For fixed values of a incidence you could get a remarkable range of airspeed in level flight through the angle allowed for the elevator. What varied was the sensitivity of the speed to the elevator angle at certain "trim" settings. In some cases the speed would go up 2 fps for a deflection change of half a degree. Piper has consistently done their tail design that way. Trim sets the stabilizer incidence. Boeing also does it that way. Cessna and Beech use trim tabs. Autopilots may use some trim and some direct elevator control. It varies. Of course there is no reason to constrain FS like that. Letting the autopilot use the trim control is effective and has the advantage that the pilot can take over manual control with the trim already adjusted.Bear in mind though, however the trim on a real plane works, all serious pilots use trim all the time. They trim out the control forces in climb, then in cruise, and finally in descent so that they are not working against a force on the stick or yoke for very long.There is one thing you all should be aware of. The main thing most designers work on is the looks of a model. This is what YOU look for and criticize so that's what they work hardest on. Many of them, don't know much or care much about the FD aspect. You get a lot of jets that contain the FD of the Learjet 45 or Boeing 737 and many piston planes that are mainly C182's on the inside of the FD files. That's the way it is. In many cases just putting in proper weights and wing area will make the plane behave as it should.

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