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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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