Sign in to follow this  
Guest maddoccanis

land speed car

Recommended Posts

i have built a rudimentary bonneville salt flats and am working on a jet car. i am having trouble getting it to stay stable. i have a few times exceeded mach 1 before it takes off, but can never deccelerate without crashing. i want to have a small wing with negative lift. i have tried to turn off ailerons and steer by rudder. any suggestions...doc

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

As far as I know, there's no way you can model negative lift given the FDE's limitations.Two things to try--make sure your wing area is zero, or close to it. Also, make your wing length and chord close to zero.Next, try to ajust your COG by moving weight fore or aft. If the COG is too far forward, that may cause problems.You don't say what exactly is happening prior to the crash--is the Car becoming airborne, nosediving into the ground, going into an uncontrolled roll? All these things can be treated as symptoms and resolved, but the term "crash" is a bit too broad to be of real help.-John

Share this post


Link to post
Share on other sites

thanx, what you suggest has been where i started. i have had all of the described failures. i think a really big moi for roll may help side rolling. the ultimate problem seems to be becoming airborn, which is of course the problem n reality. i really need negative lift. the odd thing is it goes unstable when i cut power. can i change the point of the spoiler action so it is well behind cog? what is the relationship of the .air file to the aircraft.cfg file. the .air file seems to have some airfoil properities you can edit....doc

Share this post


Link to post
Share on other sites

If it becomes unstable after you cut power, the answer may be with your engine location. It could be placing down force on the aircraft, and cutting it creates a sudden release of that pressure.Try moving the engine location up and down, and fore and aft of the COG...-John

Share this post


Link to post
Share on other sites

For air file editing, there is support at:Air File Board, post March 2003http://www.avhistory.org/scripts/MegaBBS/logon.aspYou may need a real aerofoil over the rear wings with negative incidence. Don't use the tail plane as only the elevators produce down force.I assume you use a prop or jet for propulsion. Where is it relative to CoG and what is the pitch angle of the thrust? This is a throttle effect you are describing.Ian

Share this post


Link to post
Share on other sites

ok i just registered for the bbs. in the meantime how do you make a negative lift airfoil? how do you change thrust angles for jets or props? what exactly does the engine location in the cgf file represent? if only elevator give downward force, what do incline angles mean? is any of this documented anywhere?.....doc

Share this post


Link to post
Share on other sites

"I assume you use a prop or jet for propulsion. Where is it relative to CoG and what is the pitch angle of the thrust? This is a throttle effect you are describing."What I said :)

Share this post


Link to post
Share on other sites

doc...As I mentioned before, there's no way to create a negative lift airfoil--zero lift is the best you can hope for. Nor is there a means to create a second airfoil.One other thing that may help is a section in the aircraft.cfg called "flight tuning". There's a parameter there called "elevator effectiveness" change it to zero, and the elevator should produce virtually zero upward force on the car.As far as engine location, open up the default 737 aircraft.cfgMicrosoft did a favor to aircraft designers--it heavily commented the entries there for budding FDE designers. You should be able to locate the engine location parameters and start playing with them.It is outside the scope of this forum (only from a personal time standpoint) to walk you through FDE design. Considering what you're trying to do, you'll want to know exactly where to go when we suggest changing a parameter, and that takes some study. Don't try playing with the .air file. If it gets to that point, I'm willing to help you. Always try to tweak with the aircraft.cfg first, and the .air file last. Tweaking the .air file is much more challenging and considering you're a novice, start easy and go from there.-John

Share this post


Link to post
Share on other sites

ok, thanx again. i will go study the 737 file. while i am a novice at this i do have a phd in physics and spent 25 years simulating nuclear explosions,coupled global ocean/atmospheres and high power magnetohydrodynamic implosions.how about putting an elevator at the cog and controlling trim vs airspeed with and xml guage?later...doc

Share this post


Link to post
Share on other sites

"while i am a novice at this i do have a phd in physics and spent 25 years simulating nuclear explosions,coupled global ocean/atmospheres and high power magnetohydrodynamic implosions"Interesting--my father was a physicist and worked with the NRC, and prior to that with the Navy. Virtually all the reactor specs used in the Navy today were first drafted by my father, and when he went to the NRC, Rickover tried to intervene and keep him from going through with the transfer. It was only when my father threatened retirement than he was allowed the transfer. Sadly, I think it was his profession which caused him to pass on--he had no less than three different cancers raging in him at the end.However, my father would never have touched an MSFS FDE with a ten foot pole (although he was prior to MSFS's time)--a PHD in physics is great for physics, but doesn't help much for FDE's, other than proving one's smart which sometimes contradicts the logic in the FDE :)Bless my father, he didn't force any of us to follow in his footsteps. I first went into accounting and business management, and later I.T. where I've stayed put for fifteen some odd years. I think that makes me pretty smart meself, but when it comes to FDE's--sometimes MSFS still makes me look pretty stupid.-John

Share this post


Link to post
Share on other sites

we used to have a saying among the greybeard designers of nukes."run about 1000 hours of CRAY time and make a guess." but todays PCs could blow the doors off a CRAY1. anyway simulation is as much an art as a science. we used to "trick" the weapons design codes into doing all sorts of things the code writers never intended. we were able to do pretty damn well at predicting tests in Nevada.anyway i am very aware of the limitiations of simulations, but i am very impressed with MSFS. i really just need some more info on how it does what it does.......doc

Share this post


Link to post
Share on other sites

"i really just need some more info on how it does what it does.......doc"I think the best approach about MSFS FDE design that I can think of is that MSFS simulates how it thinks aircraft should act in the environment, vs. simulating the environment and the actual airflows and nuances that environment has with objects in its space. From what I read on X-plane, it uses FDE algorithms that are probably more in line with what someone with your training, or my fathers, would expect.The best example I can think of is my own little flight sim I wrote years ago while studying C++. I simply modeled the movement of the aircraft based on input, but most of it was a fudge. The tables and parameters in the MSFS FDE's are the same, although I think they do a very good job of simulating flight so I'm equally impressed. But it doesn't model the shape of surfaces--you can't create a high efficiency airfoil with MSFS and play with its shape as you could in XPlane (assuming Zplane can do that). The best you can do is play with reaction tables.If you're willing to send in the aircraft.cfg and .air files (I wouldn't need the mdl file since I can toss together a simple (but ugly) test model in FSDS in a few seconds), I can take a look at them over the weekend, and if I succeed I can document the settings that made the difference. It certainly would be a fun thing to try, provided my wife and daughter will allow me to lock myself away for a few hours under the guise of "product development". If you're interested, send me a PM and I can provide an email address for you to forward what you've done..-John

Share this post


Link to post
Share on other sites

i think someone has actually done this with x-plane. i haven't checked. i will upload the whole ball of wax to my download page. you can get it any time. i am at maddoccanis@earthlink.net my web page is possumkingdommunitions.com/index1.html the download page is fsdownload.html doc

Share this post


Link to post
Share on other sites

Actualy doc,MSFS models the ground effect. That is like a Lloyd's mirror. The reflected image of the car is added to the real aerodynamic flow to deflect the downwash to run horizontally over the ground. This reduces the vortex induced drag. See AirEd.ini for details.Ian

Share this post


Link to post
Share on other sites

thanx,this sounds like something i can sink my teeth into....doc

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this