Sign in to follow this  
Guest

MOI values etc

Recommended Posts

Hi all!two questions:can anyone here point me to a website or pdf ebook or something that has information about the MOI (moment of intertia)? It'd be great if you could please share any good source of info with solid and complete knowledge of aerodynamics in general (online or offline) :)I have a few books but I couldn't find anything related to MOI, and google brought a bunch of information that is more physics-related than anything (well, MOI has to do with physics obviously!).Wich brings me to question number two:How do FDE developers calculate the MOI for their aircrafts?Using the suggested numbers included in the SDK seem unrealistic, and I've been calculating these base numbers for many aircrafts I've downloaded and they're all different!As a bonus it'd be great if you could also point me to any good Flight Model or FDE design page with teorical information mostly, since I've already downloaded and tried the AirED and Abacus' tools.Thank you all in advance!!kind regards,-Enrique

Share this post


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

Given that I am attemtping to build a homebuilt aircraft that does not exist in real life, I have this question also. I will need to set the correct various MOIs without any data for the aircraft (since I am desinging it!). It is based on several existing and history ultralights, so there is some data, but I have not see any on MOI. There are some impressive spec sheets for the RAN's (Stinger, Airaile) and others with a wealth of detail, but not MOI.I do not know if you have FS Ed, the aircraft editor that comes with FS2002 Pro. I do and it offers suggested MOI based on the weight and I assume wing data you enter for the aircraft. Although it does say actual MOI may vary quite a bit from this estimate. I cut my MOI down a little to reduce shaking in the aircraft at speeds I consider normal.My wing data is close to the Stinger in dimension, but uses the Clark-Y airfoil that the Aviat Husky does. I could probably use values close to that. But I have settled on the ones suggested by FS Ed for now.Steve

Share this post


Link to post
Share on other sites

I adjust MOI values largely by feel.My explanation is hardly that of an aircraft designer, but think of the MOI as the determinant of the "force" it takes to move the aircraft on an axis, and the momentum it maintains after that force is stopped. The MOI is also closely tied into the aircraft's weight as well.Aircraft with high MOI's (in FS) take more stick force to move, and take more stick force to stop the movement. The best example of this is the roll MOI...if it is high relative to the aircraft's weight, the aircraft will still roll a bit even after the stick is centered. If it is low, the aircraft will stop its roll in an instant. If MOI values are too low, the aircraft will get the "shakes" once a certain speed is reached.Adjusting the MOI in FS is more an art than a science. And in tandem with it, the pitch_stability, roll_stability, and yaw_stability entries in the flight_tuning section can create an interesting mix of results.As for the right value, it all depends on the aircraft weight. I use an approx fifteen to one ratio to start with. For ultralights, a model with an empty weight of 254 lbs usually gives me an MOI of about 3800. For a 100,000 lb 737, I start with MOI values of about 1,500,000. I usually fly the aircraft, make edits to the MOI, fly it again, until I get it just right. An out of the box aircraft that seemed to be ideal from the start was Dreamfleet's Cardinal. Hope this helps a bit...none of it is scientific. My MOI preferences are based on personal likes, and based on the way I've "felt" aircraft react that I've flown throughout the years..

Share this post


Link to post
Share on other sites

Hi William,thanks for the info!I downloaded that pdf file on my lunch break today and was able to understand a few things :)I have a few new questions for you about the flight model,here they come:-I'm trying to tweak a 757's flight model but I've only managed to alter it's performance using the aircraft.cfg. Whenever I modify any value in it's .air file, I don't see the change reflected on the performance (even as I've tried closing fs2002 and opening it back!).Has this ever happened to you?What I'm acctually modifying in this 757 are the loading stations, it's fuel capacity and the flaps configuration so I can achieve more realistics landing speeds etc.So, I've been playing with a few variables but still no luck, which led me to modify the .air file. I'm playing with the gear drag and a few other variables but like I just told you, the changes I make to the .air file are not reflected in the 757's performance.So...I've been wanting to ask this question for some time,does FS2002 make any kind of flight model calculation based on the visual model file , the .mdl?I ask this because I don't seem to find the lenght of the fuselage, for example, in neither the aircraft.cfg or the .air file.Because if this is true that the visual model is taken into account by FS2002 to do any kind of calculation, I'm basically out of luck :(Thank you for your time! :)cheers,-E

Share this post


Link to post
Share on other sites

E,First, yes the FS knows the critical measurements of your aircraft. There are measurements from the CoG (reference_datum_position=0.000, 0.000, 0.000) to the tail and nose sections, wingtips, landing gear, etc.Secondly, all the changes you made to the .air file were overridden by entries in the aircraft.cfg, at least all those you mentioned above. Loading stations, fuel capacity, and flaps need to be updated in the .cfg.Hope this helps.Milton

Share this post


Link to post
Share on other sites

Hi Milton! thanks for your reply :)I've been trying to find in the aircraft.cfg file any variable regarding the effectiveness of the spoilers and the drag induced by lowering the gear, but I've only seen those in the .air file!I've tried to modify those values in the FDE editor but I don't see any reaction or changes in the way the aircraft flies.Any clues?Many thanks in advance! :)cheers,-Enrique

Share this post


Link to post
Share on other sites

Hello Enrique,First let me say if you haven't downloaded the documents from th elink above, please do so. Flight Dynamics by Tom Goodrick is an excellent document that gives you the information necessary to understand the basics and facilitate your .air file/.cfg changes.Secondly, if you prefer not to get into the details too much, then try first adjusting the Flight Tuning scaler's in the aircraft.cfg file. If you really want to effect changes using the .air file then spend time there looking through all the parameters to familiarize yourself with the Sections. Keep in mind that the .cfg parameters used will override the .air file parameters.Regarding pitch, drag, lift for gear, use section 1101 for those parameters. Remember that there are always related parameters and effects when adjusting these.Sections 320, 327, and 340 reference spoilers as well as 1101 for moments. I do not know if FS2K2 uses them all.I have tried to get gear drag set effectively for the Dash 7 and we are happy with it. You might glance at it's .air file.Also keep in mind that for very large heavy aircraft with large MOI's, reactions for gear may be harder to see.Milton

Share this post


Link to post
Share on other sites

Hi Milton!thanks again for your great input.I gotta find out why exactly the changes I'm making in the .air file are not reflected in the aircraft's performance, I'm sure I'm using values that are not being overlapped by the aircraft.cfg.Anyway, thanks again! Hopefully I'll have something worth of uploading over the weekend since I've made some progress lately...thanks again Milton!cheers from Caracas,-E

Share this post


Link to post
Share on other sites

Back in '90, while returning to the U.S. on a business trip in Venezuela, I had to stay the night in Caracas. We stayed at a hotel with an odd name--the "Eurobuilding", but I remember it well. It was an ultra-modern hotel, very nice. Curious to know more about the hotel, I've tried to search for information on it, but I assume it must have changed names sometime since I stayed there. Does the name sound familiar? All I remember is that it was in the central city...-John

Share this post


Link to post
Share on other sites

it sure is familiar...it's in a very nice location in our city, I believe it's still called the Eurobuilding, but it was acquired by Radisson sometime last year.I hope you enjoyed your stay over here :)cheers,-E

Share this post


Link to post
Share on other sites

Without a doubt, the time I spent in Venezuela was the best business trip I ever had. I was sent along with a partner to Margarita for a training needs assessment on a System 36 network (at the Margarita Concorde), and to conduct the actual classes. I was there almost three weeks. Our hosts were very gracious. Our contract required at least eight hours of daily classroom training, but our hosts would have nothing to do with that. We'd start teaching around 8:30AM, ended by 1:00PM, and were told to go enjoy ourselves after that, and most often, our hosts would join us.It was really my first experience in Latin American culture, and it had such an impact that, when I met a nice young lady in Mexico four years later (same type of business trip), I knew I had found my wife. I often tell her about my trip to Margarita (during which I visited Canaima and Angel Falls). My image of Latin America was created from that trip, and it has never changed. My wife and I share a daughter, and have been married almost eight years. Sadly, I've not had the chance to return to Venezuela, but I hope to someday return along with my wife.And to this day, I hardly speak a word of Spanish (other than the things a husband should know to say to his wife).-John

Share this post


Link to post
Share on other sites

E,Use aired152 - http://members.dsl-only.net/~eagle/ and the attached updated *.ini file. Change the name to aired.ini, open the file for the credits of the individuals keeping the *.ini current as new items are found. If you use FS2002Pro Aircraft Editor, it will add sections 1539 thru 1547. Section 1540 seems to be drag. It is also interesting to note that none of the default fs2002 aircraft have these sections. They seem to override Section of 1101.

Share this post


Link to post
Share on other sites

Good stuff, William, many thanks!!I will start working with these values and the new .ini file this evening. Like I said, I still have to figure out why FS2002 is disregarding every change I make to the .air file.Once I'm done with that, I'll upload my flight model version for beta-testing ;)cheers! and thanks again,-Enrique

Share this post


Link to post
Share on other sites

hey John!that's a great story...sounds like you had a blast down here!I haven't been to Margarita for a few years, but it's a very cool place to spend a few days indeed!You won't believe this (and I really have no excuse!), but I've never been to Canaima or the Angel Falls!! That's one thing I've really gotta do sometime soon...funny how one's country has sometimes the lowest priority on our traveling plans! Latin America is a great place to travel, if you ever have the chance try going further to the south.Ecuador is beautiful, so is Bolivia and Peru.And from what I've heard Chile (except for Santiago) and Argentina are spectacular destinations!You'll find very different cultures but somehow they're inter-connected and share common values.Venezuela, regretfully, is going through very shaky political and economical times, so I must (sadly!) suggest that you avoid coming down here for the next 6 to 12 months.I don't want to go into details to avoid starting a political discussion here.But once things get back to normal you definitely gotta come to Los Roques Islands (there's a very nice FS2002 addon scenery somewhere in Avsim's library, I'm sure you'll enjoy it!) and to Merida, a very small "student" city to the south west, in the Andes.You can't ski there, but you can hike or drive to very beautiful remote locations.You'll love it!Anyway, thanks for writing!cheers,-Enrique

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