On Monday 08 December 2008, gerard robin wrote: > On dimanche 07 décembre 2008, LeeE wrote: > > On Saturday 06 December 2008, gerard robin wrote: > > > On dimanche 07 décembre 2008, gerard robin wrote: > > > > On samedi 06 décembre 2008, Martin Spott wrote: > > > > > gerard robin wrote: > > > > > > With the c172p i have included the following: > > > > > > > > > > [...] > > > > > > > > > > > To me that is perfect, [...] > > > > > > > > > > This is the sole point I'm talking about: Apparently, > > > > > even though 'we' have "original" drawings of the entire > > > > > airframe, still none of us has authoritative information > > > > > at his hands how it is supposed to be properly positioned > > > > > 'at level'. This is the issue which I'd was trying to > > > > > sort out. > > > > > > > > > > Cheers, > > > > > Martin. > > > > > > > > Yes it is a guess, how many models here are drawn with a > > > > guess ? not only the landing gear :) > > > > > > > > Giving it a pitch of -3 deg is not so bad. > > > > Or extend more the nose gear which will be ugly. > > > > > > AND > > > The question isn't it , only: > > > Which is the less stupid :) > > > to keep the model floating above the ground when not in air > > > ? or > > > to modify the offset ? > > > which won't shock anybody using that FG Reference Model. > > > > > > Easy to do, giving the possibility, latter on, to update, if > > > somebody is able to bring the right blueprint of that > > > Aircraft ( same model, same equipment .....). > > > > > > cheers > > > > Once you've aligned the reference point between the 3d model > > and the FDM model you should never have to add offsets to the > > 3d model; if there's a discrepancy it means that something is > > wrong. > > > > The best way, for modellers, is to make the landing gear in > > it's maximum extended position - the position it would be in > > without any weight upon it - and use those coordinates for the > > gear contact points in the FDM. Then you vary the gear spring > > and compression rates in the FDM so that the aircraft sits on > > the ground at the right height and attitude, and then finally > > adjust the model's gear compression animation so that the two > > match. > > That is right but only theory. > That means, too, that you include an animation compression > extension of the gear (which is not the case with that c172p). > With it , yes, > => a drawing (blue print detailed) which gives you the right > position of the gear when extended > + => the know how about the right values of the landing gear ( > damping_coeff , spring_coeff, pos min pos max, ) > + => the right weight and force applied , on each contact point, > we can do the perfect Aircraft, with a perfect FDM regarding the > landing gear reactions. > > Unfortunately it is very rare to have all of these informations, > and in many case we must be pragmatic, and the less stupid (if > possible). > > I remember when i did the F-8E , i had some very high detailed > drawing, but nothing else regarding the landing gear, but the > usual position on ground. In order to get it, in a correct > position when it is in air fully extended, and to get it, > retracted correctly in the box without cheating , i had to > calculate the size and the geometry of each components , it was a > lot of work :( > I had the same difficulties with an other aircraft done for a > friend (SU-34). > > > While it's usually impossible to get exact data on what the > > height and attitude on the ground should be, with reference to > > photographs etc. it should be possible to get it correct to > > within an inch for small aircraft, and perhaps several inches > > for large aircraft. > > > > One of the checks that every modeller should do is to check the > > gear compression under different loads. This will amount to > > testing different fuel and passenger loadings, including > > asymmetric loadings. Military aircraft can also be checked > > with different weapon loads. Regardless of aircraft type > > though, once you've got it right the gear will sit on the > > ground whatever the loading, even with asymmetric loading. > > > > LeeE > > Cheers
All of FG is "only theory" - it all takes place in your computer:) The C172P FDM includes spring and compression rates, not only for the main and nose gear, but also for the tail skid. Although the main gear is fixed in position it flexes and both this, and the nose gear compression, need to be modelled/animated if the gear wheels are to stay on the surface of the ground and not sink through it on touch-down, or rise above it during the take-off roll as the wings start to generate lift. This isn't difficult to do and doesn't need exact drawings or data to achieve an acceptable result. A bit of thought about the landing gear design and reference to lots of photos will give you a very good idea of the limits to the flex and movement, or oleo compression, in landing gear. Too much flexure in the fixed C172P main gear and they will break, and the oleo damper arms limit the maximum extension in the nose strut. Once you've established plausible limits to the gear flexure/extension it's just a question of putting in the time and effort to narrow down those limits and tune the animation to match them. The C172's main gear flexure is, after all, just about the simplest type of gear animation to do. Although I had pretty good drawings of the Comper Swift that I did for FG, it wasn't clear whether the drawings showed the gear in compression, or not, but this did not matter too much in the end because common sense told me what the plausible limits to the main gear flexure would be - too much and they would break:) It was then just a question of putting in the time and effort, trying different rates and narrowing down the range of possible values, until the FDM was consistent with the 3d model and behaved plausibly. While I'm sure it wasn't _exactly_ right, I know that it's in the right ball-park and works. You seem to argue that you need exact data to do any of this and as we don't have it, we shouldn't bother trying. But this applies to the entire FDM; we never have _all_ the data we need, but with the data that we can get, plus a bit of thought, it's possible to work out plausible limits for the data that we don't have and then, when we tune the FDM, we refine the limits for the data we don't have. If you start changing values for data that is known, to correct a problem in an aircraft, instead of refining the limits to the data that is unknown but extrapolated, you're just introducing new problems, not fixing old ones. As you pointed out, when you did the F-8E, it was a lot of work. This is correct - it _is_ a lot of work but if you want to get it as good as possible you have to do that work. If it's any consolation to you, the people who make the real thing have to do a lot more work;) Perhaps it just comes down to how much effort you are prepared to put in to it. Personally, I used to find that I would spend much more time waiting for FG to load than I actually spent checking that the animations were correct once FG was actually running, and I've spent nearly a thousand hours test flying some of the aircraft I did, constantly tuning and refining the FDMs, trying to narrow down the limits for the data I couldn't get. LeeE ------------------------------------------------------------------------------ SF.Net email is Sponsored by MIX09, March 18-20, 2009 in Las Vegas, Nevada. The future of the web can't happen without you. Join us at MIX09 to help pave the way to the Next Web now. Learn more and register at http://ad.doubleclick.net/clk;208669438;13503038;i?http://2009.visitmix.com/ _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
