Simviation Forum - I need help with B757

	Search the archive:

Simviation Main Site | Site Search | Upload Images

Simviation Forum › Design Forums › Aircraft & 3D Design › I need help with B757

(Moderators: Mitch., Fly2e, ozzy72, beaky, Clipper, JBaymore, Bob70, BigTruck)

‹ Previous Topic | Next Topic ›

Pages: 1

I need help with B757 (Read 211 times)

Sep 27^th, 2004 at 4:21pm

Pen-15 Offline
Colonel
OH-LGA

Posts: 20

I have the POSKY's Boeing 757-200 and i would like to do some changes to it. The problem is that in approach, if i want to maintain that 0 degree pitch and still have 700 - 900 fpm descent i have to drop speed to about 110-120 kts with full flaps and that's not realistic. Now i need some professional help to work this thing out, do i have to change the .cfg file or .air file, and what changes do i need to do? I've also noticed that in aircraft "weight and balance editor" shows that the point of gravity (don't know if that's the right term) is almost in the radome of the aircraft and it can't be changed. Another issue is that it will taxi very fast even when thrust is in idle position, need to brake frequently.

IP Logged

Reply #1 - Sep 29^th, 2004 at 1:47pm

garymbuska Offline
Colonel
I would rather be flying
Jacksonville, Florida

Gender:
Posts: 2850

You could edit both the .air file and the aircraft cfg file.
Point of gravity not bad but it is called the center of gravity. But hey you were on the right track.
What I do is after selecting the plane is to go to the change fuel/payload & make note where the center is if it is at the very end of the nose then it is nose heavy. First completly empy all fuel and any cargo this way you can check to see where the empty weight balance is if it is still nose heavy then check out the weight&balance section of the aircraft cfg file if you see any lines that read station_load 0=999887 Then try lowering some of the first few lines or make them a comment by putting // at the start of the line. After editing a line or two save and reload the aircraft to recheck the empty balance point. Once you get it toward the center then reload the fuel and cargo and see where the balance goes. If it goes back to the front then there is a problem with how the fuel is set up not quite sure if this can be changed but it might be you have to first figure out which tank is causing the problem and try to correct the aircraft cfg file for that tank. This can be really tricky because what you have to do is to change where the weight is put at not how much weight and you have to know where the orignal balance point is. I usually do not mess with these because a little change can have drastic results.
If the other method does not correct it then I junk the plane unless you feel that it is worth the effort it can be done it just takes time and a ton of patience.

IP Logged

Reply #2 - Sep 29^th, 2004 at 2:22pm

juri Offline
Colonel
Finland

Posts: 74

my brain is smoking for thinking about this but let me see if i got it right. You know that at certain speed and rate of descent you should have pitch of 0 deg. something is wrong because you have to lower your speed (lowering speed means your pitch increases =nose goes up). therefore your plane has to be nose heavy and you should move center of gravity backwards.

backup your plane's aircraft.cfg and whatever.air file and start messing around with them (not the backups, ofcourse Wink

).
you can try to change the a in (i think you should increase the value) :
empty_weight_CG_position = a, b, z
and / or the first value of the tank positions in "fuel" section.
as i understand it, station loads are ment to model cargo or people inside the plane, and they can certainly be moved too. make small changes and test them...

modifying .air file is a bit trickier and you'll need for example aired -program. you'll find lots of parameters affecting center of gravity there. good luck...

(kohteliaasti englanniksi ?)

-juri

"Persistence and determination are omnipotent." -Calvin Coolidge

IP Logged

Reply #3 - Sep 30^th, 2004 at 10:02am

Pen-15 Offline
Colonel
OH-LGA

Posts: 20

Thank you for help. The airplane is indeed very very very noseheavy, the center of gravity is at the end of the nose, and it can't be changed in weight and balance 'cause there's only one load station. I copied the "[WEIGHT_AND_BALANCE]" section to .cfg from FS9's default 737 and the aircraft just flipped backwards at the runway and the center of gravity was not even close to the center of aircraft (it still was supposed to be noseheavy). I will test some other things because this is one of my favourite aircrafts, it has beautiful FINNAIR livery and i have very good cockpit for it too.

IP Logged

Reply #4 - Sep 30^th, 2004 at 11:18am

juri Offline
Colonel
Finland

Posts: 74

in aircraft.cfg there is also a line that defines a reference point:

reference_datum_position = 0, 0, 0

the empty_weight_CG_position is defined relative to this point, and to make things even more confusing, i think this all is dependent on the "pivot point" of the 3d model of the aircraft... this may confuse one to think that the COG is somewhere it actually is not at.

check the position of the main gear of your 757, they are in contact_points section of aircraft.cfg file:

point.0=1, x,xx .... longitudial position for center gear (x,xx)
point.1=1, x,xx ....longitudial position for main gear 1(x,xx)
point.2=1, x,xx ....longitudial position for main gear 2(x,xx)

etc.

for example in default 737, the empty_weight_CG_position is 7.75 feet in front of the main gear, and this should be about the same situation in your 757. i think your center fo gravity was pretty good from the start (before any modifications), and it only takes a couple of feet to get it right. revert back to the original values and move the weights (including tanks) one feet at a time.

-juri

"Persistence and determination are omnipotent." -Calvin Coolidge

IP Logged

Pages: 1

‹ Previous Topic | Next Topic ›

« Home

‹ Board

Top of this page