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mnmon

another side of the hobby that's rewarding

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About 2 months ago I got a freeware airliner. generally it's very good, but the CG mark on the Fuel and Payload page was sitting on the nose. This kept bothering me. I spent a lot of time with Tom Goodrick and Ron Freimuth ;-) Not literally! Just reading all their posts and tutorials. I also spent time reading over carefully the MS aircraft container SDK. The work paid off: I was able to modify both the cfg file and some tweaks on the air file and not only fixed the CG location, but the plane flies great. I then tackled a payware RJ that I could never hand fly on approach without the AP as it would Dutch Roll like crazy. Fixed it in about 1 1/2 hours time and didn't even touch the air file. Did the same for some another planes. What I've learned, and haven't seen discussed much, is that a large number of planes out there (payware included) seem to have the basic parameters of the planes wrong. The major offenders are the location of the CG, the values of the MOI's, station load positions, and the entire airplane geometry section in the cfg file. Want to have some different fun with FS? Here's what to do: Get some scale drawings of the plane usually from the manufacturer's web site. They don't have to have anything but top, side and front views with the basic dimensions. Print them out. Take a ruler and get the scale for the drawing, ft/inch. Find the geometric center for the plane, This is the ref_datum_point in the cfg file if it's set to 0,0,0. EVERYTHING else is measured off this point. You can then place the CG, station load points, fuel tanks, and everything else in the geometry section properly. How bad are things out there? Well, the payware RJ I worked on has a T-tail. Guess where the horiz stab. was set in the cfg file? In the middle of the fuselage like a 737. That was just one error, there were more such as listing the wing sweep as 0 degress when it's actually a 30 degree swept wing. The crew was positioned 10 ft in front of the nose of the plane! The payoff, after doing about 4 planes now, is that you get something that actually flies very well. In fact, I now think that if done carefully, the FS FDE can come close to what x-plane does. It's rewarding to do that work and then go fly the result! Mike

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> The payoff, after doing about 4 planes now, is that you>get something that actually flies very well. In fact, I now>think that if done carefully, the FS FDE can come close to>what x-plane does. It's rewarding to do that work and then go>fly the result!>Don't get too close to what X-Plane does (or doesn't do)... :DThe reality of X-Plane, is that many models require "fine tuning" by using hidden flight surfaces, etc. An X-Plane model might be created using actual specifications, and perhaps it get's the model in the ballpark, but it still doesn't perform as the known aircraft does. At that point, it requires those "tweaks". Personally, I prefer the "feel", as well as simulated flight characteristics, that I get from some excellent models created for MSFS over X-Plane. I'll often find that X-Planes will have a lack of dampening, mass, and tend to react too quickly to joystick/yoke imputs. They often remind me of a puppet tied directly to a joysticks strings.IMO, the "flight dynamic" superiority of X-Plane is overblown, and was more correct about 15 years ago when X-Planes blade element theory was introduced. In today's world, I usually prefer MSFS.But now, that I got that issue of my chest..... :)Keep up with the experimenting, as some MSFS models need it! L.Adamson --- FS9,FSX, X-Plane 8.60 w/global scenery

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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.R-

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I haven't looked at the the default planes. There's definitely an issue of copyright infringement in terms of distributing the modifications, even with some freeware. I'm doing these mods for my own enjoyment and benefit. Doing the modifications is easy once you have the drawings and basic specs on the plane. It's really a matter is just taking measurements with a ruler and inputting into the cfg file. There's one rule for placement of the CG (1/4 of MAC) but that's easy also. Calculating the MOI's is also easily done using a table Tom Goodrick has published. If you have a particular plane, email me and I'll look at it. Larry: you're probably right about x-plane. I haven't used it in about 7 years. I do note that Austin has gotten x-plane to be used to drive some full-motion sims I believe which may or may not say something about his flight model. It's the rest of the x-plane "world" that eventually frustrated me. Mike

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Agree with that. Trying to understand flight dynamics and how to modify and tweak FDEs is a very interesting thing. It can also be very rewarding.About the x-plane vs. MSFS - I think, it's like comparing apples and oranges. These 2 sims use very different approaches to calculate flight dynamics and I would not say that one of them is superior in any way. Both approaches have areas in which they can work very nicely and other areas where they may fail miserably (compared to the real thing).A good FDE designer can create nice flight dynamics with MSFS. Same is probably true for x-plane. In any way, we should understand that we are talking about software which was basically created for entertainment purpose (yes, I know the thing about x-plane being FAA approved, but that only applies to a very special configuration, including special hardware and not to the standard PC version).

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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.

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Thanks Tom. I was unaware of your site until today. R-

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Tom,In general X-Planes won't spin. Several aircraft designers keep working on the problem, but RealAir Simulations for FS9/FSX has done a much better job. The simulation FLY was modeled with separate wing surfaces, but tended to "dilly dally" around for a few seconds before deciding to drop a wing or not. It drove me nuts!Since that time, it's been RealAir's mission to develop MSFS aircraft with excellent rudder control that works well for slips, aerobatic maneuvers such as hammerheads, tail-slides, and snap rolls, as well as the spin. You can get a variety of intentional spins, as well as "accidental" cross controlled spins. Airspeed does not build as in a spiral dive, and aileron isn't required to "fake" the looks of a spin. The previously menntioned slipping ability is about pure perfection! My favorite is the RealAir SF260, but they also have a Decathlon.L.Adamson

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IIRC, FS5.1 was the last version of MS flightsim that had inherently modelled spinning tendencies (ie the default aircraft 'out of the box' could be spun). Since then, I believe the default flight models have not supported spinning. I recently had a go at the Realair SF-260 on a friends PC and it performed spins and snap-rolls so well, it amazes me what good FDE experts can do with the inherently limited physics modelling in FS9.

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I am aware of Real Simulations work. I have had big nasty arguments with those people. They have ignored the laws of physics and struggled through a lengthy trial and error process to make airplanes spin above all else. They turn the MOI's upside down and inside out to get what they want. They think those of us who take a conventional aeronautical engineering approach are full of hooey. I think they are full of hooey.On my site you'll find FD for the J-3 that enables it to do very reasonable intentional spins. I used to do those as a kid in the real world. I have made some other aircraft spinnable but I have made some aircraft. like the Baron go into a flat spin from which recovery is in doubt. That happens in the real world too, especially if you screw around with aft CG loading.My own aerobatic design, called the Aerobat on my site, can do just about everything you can imagine and a few things you probably can't. Toss it around the sky for some fun. But the thing is I have not taken any short cuts with the laws of physics. in some cases it is just a matter of giving a plane more rudder power. Most FS designs are short on rudder power. But also there is a set of stability derivatives set up in default modes by Microsoft that are intended to keep things from ever getting out of hand. microsoft wants you to think all aircraft are perfectly safe. Get them into trouble and take your hands off the controls and they will recover quickly at least to a condition from which you can easily regain control. Many default stabilty charts return to zero alpha conditions at + and - 180 degrees instead of going into strange and unrecoverable conditions. Flying most planes tail first is a "No, No!" Real planes are not easily recovered when you go far outside the envelop. I play with those settings and have found interesting results. Fly my planes like a good pilot would and they work fine and are very safe. But, if you push one of my planes too hard, you'll experience a fatal crash. Learn to fly first.It seems to me the only value in this sim is if it shows realistic flight characteristics and performance. That's what I work on.

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This is going to mess up the order of this thread but your remark about snap rolls deserves some discussion. Do you know if it is truly a snap roll or just a very fast roll induced by the rudder? I recently worked on flight models of a T-38 and an F-86 where I made tem do very fast rolls. The T-38 does an aileron roll so fast you blink your eyes and you miss it. The F-86 does reasonably fast aileron rolls. But few people know that it will snap roll and go out of control if you use too much rudder while at cruise speed. i found out with some embarassment when I was a CAP cadet sitting in an F-86 simulator at Ellsworth AFB. I was used to flying the Piper Cub (doing my spins) so when the instructor told me do ro a high-bank turn to the left, I through in plenty of rudder like I would in the Cub. It was in spin which quickly went flat and became unrecoverable (from 10,000 ft). The instructor could not save the situation. So, when I was working on the F-86 a couple weeks ago, I made it do that sort of thing using just rudder. You can still do all the good combat aerodynamics for which the Sabre is famous. But keep your feet on the floor!A snap roll happens in the real world when the right wing is developing lift at half the weight and the right wing is suddenly stalled, losing all lift. With proper control, that lift on the left wing can be maintained for a few rotations. I rode through a few snap rolls in a T-28 once that showed the violent nature of these forces. The best example I can give is the beginning of the act of The French Connection. The male pilot, who later was killed, used to do a snap roll with two or more turns on takeoff.

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I would love to see a site that contains a way to share edited .cfg and .air files. I know it is against the EULA to re-upload and whatnot, but maybe a site for others to request the files, and then recieve them when ever they could be sent. This would be a great help the not so savvy users like myself. I always thought some planes just didn't feel right, and I just dismissed it as good ole FS doin what FS does...

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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." Devs are free to bend things however they wish if it gets them to their desired end results. If that means they need to position the crew 10 feet in front of the cabin then so be it. Thats really only a problem if they also choose to graphically show the crew floating in space ten feet in front of the plane. To a certain degree, I agree that those who take a "conventional aeronautical engineering approach" are full of hooey. Don't get me wrong now, if putting the correct real world numbers in gets the desired result, great. But if it doesn't (and from what I understand of both FS9 and X-Plane, often it doesn't), and the numbers need to be fudged to do so then so be that too...

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It's always been my position, that sitting in front of a computer to "fly" is very much on the side of fakery to begin with! :-hah And I'm very aware that programs such as X-Plane that use some built in aerodynamic laws, just are not that powerful to spit out the desired results all the time. Maybe not even a 10th of the time! :7 Therefor, I could care less how RealAir get's the required job done, as I see no air whatsoever passing above and below my computerized wings. I believe that a complete aeronautical approach would be just dandy, but I don't see computers up to snuff as of yet. Examples of what a CPU isn't going to do, would be adding on small items such as vortex generators, wing cuffs, root cuffs, wedges, etc., to predict or simulate the changes in flight characteristics. Add these to X-Plane and no difference will be noted. So "fake" the differences for MSFS, as well as adding or subtracting a few invisible surfaces for X-Plane.Bottom line. I like RealAir products, even if they're best at faking, what is truly without doubt --- "fake" flying! :-hah I use to fly a Pitts S2B (as a two year aerobatic instructional program), and have had some good G-wrenching snap rolls as well as many spins, including inverted. I now hangar with a Pitt's M12, that has a 400 hp Russian radial and MT three blade prop. Looks like I need to go up again, as it's been 12 years (since snap rolling), and the owner is my old aerobatic instructor, as well as a commercial airline pilot. But, I'm always open to trying new simulated models, and especially if aerobatic capable outside the normal right side up envelope. I've seen your list, and will check a few out.L.Adamson

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Have anyone seen sort of FS9 FDE tutorial with a few examples?Thanks.

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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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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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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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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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>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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