Shaking Air Tractor

hurricane3

hurricane3

SOH-CM-2023
I just downloaded the Air Tractor 802 F both wheels and float versions.
During the first third of the takeoff run ,on land, both versions shake so bad it's hard to keep them on the runway. The float version (it's an ampihb) takes off fine on the water but on land both versions shake very badly.
I;am assuming it's a contact point issue but don't know what to change.
Anyone have any ideas on how to fix these planes?
 
It might be a MOI(moment of inertia) issue. That can also maifest as an oscilation on the ground, as well. It a suprisingly easy fix- recalculate the MOI values... I came across an expanation and the data to recalculate MOI in another forum... this seems to apply to FS98 through to FSX. Hope it helps...


"Probably the most esoteric aspect of setting up an aircraft model for FS98 is
the estimation of Moments of Inertia or MOI's. You need to enter an MOI for
roll, pitch and yaw referenced to the three standard aircraft axes. In theory,
these are calculated by summing the product of the mass of each component with
the square of its position radius about the axis. This is a very daunting task.
Yet the MOI's play a very important part in the flight characteristics of the
aircraft. The roll MOI determines how much the aircraft resists the roll
control input, for example. Also, the relative values for the different pairs
of axes determine the coupling between axes. A simple roll input will typically
produce secondary yaw and pitch rotations as well as a roll rotation. The
direction of rotations is determined by the difference between the pairs of
MOI's: (Ix-Iy), (Iz-Ix), (Iy-Iz).
Hence, to model a plane's behavior well, it is important to nail down these
MOI's with reasonable accuracy. It is helpful to think a little bit about how
the relative size of the MOI’s is determined. To find the pitch MOI, we look at
the position of masses relative to an axis passing through the wing. Thus the
wing itself and any engines mounted on the wing do not contribute much to the
MOI. Any heavy weight in the nose or tail would contribute significantly. To
find the roll MOI, we look at the position of masses relative to an axis
passing through the center of the fuselage. Here the wing and any engines
mounted on the wing make big contributions but anything in the fuselage does
not. Modern jets with engines mounted near the fuselage have a low roll MOI. To
find the yaw MOI, we look at the position of masses relative to a vertical axis
through the wing/fuselage joint (centered on the fuselage). Almost everything
on the aircraft that is offset from the CG contributes strongly to this.
Therefore we would expect the yaw MOI to be the largest.
Dr Jan Roskam, has published an 8-volume set of texts on aircraft design. He
has taught courses in aircraft design for some time at the University of Kansas
and has developed a method for estimating MOI's that uses only basic aircraft
weight and dimensions combined with a set of coefficients of radii of gyration
for several specific aircraft. We simply determine which aircraft is similar to
the design we are concerned with, take the coefficients for each axis and the
approriate dimension and calculate MOI's. The formulae are:
Ix=(W/g)*(Rx*b/2)^2
Iy=(W/g)*(Ry*d/2)^2
Iz=(W/g)*(Rz*e/2)^2
where g=32.2 (gravity acceleration)
W=max takeoff weight (MTOW, lb)
b=span (feet)
d=length (feet)
e=(b+d)/2
and Rx, Ry and Rz are chosen from the table for the aircraft type. Some of Dr
Roskam’s dimensionless radii of gyration for several aircraft are given below
(used by permission). All are set at full fuel and MTOW.

AIRCRAFT ROLL Rx PITCH Ry YAW Rz
Low Wing Single (Beech N-35) .248 .338 .393
High Wing Single (Cessna 182RG) .242 .397 .393
Light Twin (Beech 55) .260 .329 .399
Medium Twin (Cessna 402) .373 .269 .461
Light Jet (Cessna 550 (Cit II)) .293 .312 .420
Medium Jet (Lockheed Jetstar) .370 .356 .503
Twin Turbo Prop (Fairchild F-27) .235 .363 .416
Four Eng T Prop (Electra) .394 .341 .497
Jet Airliner 4 eng (Convair 880) .322 .339 .464
Jet Airliner 3 aft eng (B 727-200) .248 .394 .502
Jet Airliner 2 eng wing (B737-200) .246 .382 .456
Jet Airliner 2 aft eng (DC-9-10) .242 .360 .435
Prop Airliner 4 eng (DC-6) .322 .324 .456
Prop Airliner 2 eng (Conv 340) .308 .345 .497
Jet Fighter (F-86) .266 .346 .400
Jet Fighter (F-104) .224 .392 .563
Jet Fighter (F-102) .295 .386 .520
Prop Fighter 1 eng (F4U Corsair) .268 .360 .420
Prop Fighter 1 eng (P47) .296 .322 .428
Prop Fighter 2 eng (Bristol Beauftr) .330 .299 .447
Prop Bomber 4 eng (B-29) .316 .320 .376
Prop Bomber 2 eng (Martin B-26) .270 .320 .410
Jet Bomber 4 eng (B-47) .346 .320 .474
Jet Bomber 8 eng (B-52) .346 .306 .466
Flying Wing (RB-49A) .316 .316 .510
These data were taken from Airplane Design Part V by Jan Roskam. ISBN 1-
884885-50-0 published by DARcorporation, Lawrence, Kansas, used by permission.
The author started using this method with Flight Shop for FS5.1 because he
noticed that the estimation method used within Flight Shop was way off making
many aircraft behave strangely. The values they suggested did not vary with
aircraft weight and basic dimensions as they should. Also, for a while with
FS95 and FS98, there was a bad problem with landing dynamics that many
developers “solved” by increasing the MOI’s. However, the landing gear inputs
in FDEditor fix the landing gear appropriately making it possible once again to
use correct MOI's for a considerable improvement in handling realism."
 
Very interesting...

I had to read through a couple times to stop my eyes glazing over halfway through.

Math has never been my strong suit, in the equations what is "*" and what is "^" ?

Looking it over again let me guess: * means multiply and ^ means the expression is over 2 (divided by 2) ?
 
my eyes started to glaze over too

Since math isn't my strong suit,it took a couple of readings for me to grasp what lazarus was talking about and still not a hundered percent sure.I the past when I've had this problem ,someone would just tell me to change a few lines in the config file, or send a new config file altogether.Didn't realise how complex it could be.
Guess I'll live with the shakeing,once I get off the ground they fly fine, just make sure you have a wide runway,at least at first,hate to run into hangers and parked planes.
Thanks Lazaruz.
 
Look for Milton's contact point tutorial. Since the airplanes fly fine I doubt it is an issue with the MOIs. More likely you will need to play with the static and max compression numbers in the contact points section.To understand which number does what start by opening the cfg for the default Cessna because MS explains every column there while an add-on might not have those comments.Stefan
 
Both

I got the AT 802's at Flightsim.com,by Brian Gladden and it's both planes ,ON THE GROUND, the one with floats flys fine off the water,very smooth, but since it's an amphib ,on the ground it shakes the same way the wheeled version does.
I tried different aircraft realism seetings with no difference. I did find it's possible to taxi but there are times it shakes so bad you can't read the taxiway signs.
I also found it has a short takeoff run so it's controllable over a short distance.I believe the wheeled version is called Fire Boss, and the Float version is called Fire Tanker.
 
Yeah, I ran that one in FS8/9, and it ran fine. Tail wheel lock? FireBoss is the tradename for the floater, the wheely is just the Tractor, some times 'Air cow' or 'sea cow' in homage to its agricultural origins, cantankerous water manners, sluggish hot and high performance ,poorly harmonized controls, and spotty quality and maintenance manuals...but its relatively inexpensive, and it has the aspect of a yellow Stuka!... I'll have a look and see if anything leaps out.
 
"*" is for multiplication, in other words, 2*2 is the same thing as 2x2. The "^" symbol is for exponents, such as 3[SUP]2[/SUP]. The previous example would be entered as 3^2. The forward slash ("/") is the same thing as ÷, so 8÷2 is typed as 8/2.

Microsoft has different formulas for calculating MOI's. I've taken those and coded them into an Excel spreadsheet to make my life a little easier.

Remember that the calculations are just starting points. Roll and yaw may need to be increased for aircraft such as a B-17 with engines stuck out on the wings, and heavy electronic equipment in the main cabin (P-3 Orion) would increase the MOI's for pitch and yaw, but have little effect on roll.
 
don't think it's the tailwheel lock

I tried both shift +G on and off which is the default tailwheel lock and didn't make any difference. I'am getting used to the bounching which ,now that I found out it takesoff rather quickly,is mainly a problem when taxiying ,but it's controllable.The bounching seems intermentant when taxying ,hard to predict when it will bounce and when it won't. It may be just the nature of the beast. Thanks for your help
 
hurricane3 said:
I tried both shift +G on and off which is the default tailwheel lock and didn't make any difference. I'am getting used to the bounching which ,now that I found out it takesoff rather quickly,is mainly a problem when taxiying ,but it's controllable.The bounching seems intermentant when taxying ,hard to predict when it will bounce and when it won't. It may be just the nature of the beast. Thanks for your help
Click to expand...
:icon_lol:
Just like the RW Fireboss... the between castoring nose wheels and glassfiber springs, and skull crunching roll over structure, on a rough strip you need to cage eyeballs before takeoff!
 
It would be helpful to know where you got this package, maybe a link;
And, to post the Empty Weight MOI's;
And, to post the contact points data.
 
got both planes at FlightSim..com

The bounching is managable and may be the way the real planes act. isn't really somthing I can't live with, from whats been said in here it acts fairly normal. So I'll live with it.
Thanks for the reply( right now I'am running a fever of 102.4 with the flu,) so sorry if I just wanna make the whole problem go away.
I'll keep both planes and learn to taxi them.
 
I have been a fan of Brian Gladden's planes since my CFS2 days.....converted a number of them to use in CFS2...though they all flew like Hellcats and Zeros since I used the flight dynamics from those two CFS2 planes to get a lot of FS2002/2004 GA planes flying in CFS2. I thought I had all of Brian's planes installed, but had overlooked the AirTractors. Thanks for reminding me about them.

OBIO
 
your more than welcom

Glad you found somthing new OBIO. Your more than wecome. Now if I can just shake this flu.
 
Hurricane...

For your flu....get a big round glass...not a tall one, but a really wide round one. Fill it with water. Put it in the microwave for about 10 minutes....really super heat that water. Once super hot, drop in 4 or 5 tea bags, then add a nice long pour of honey (the darker the honey the better). Set the glass on the edge of your table. Sit in your chair, drape a thick towel over your head and lower your face over the super hot glass of tea...using the towel to contain the steam. Inhale those steam vapors deeply into your lungs...this will break up any congestion you have. Keep inhaling the steam until the tea has cooled enough to drink. Drink the tea. It may be way strong and way sweet...perhaps not the best glass of tea you have ever had. But the honey and the tea will have you feeling right as rain the following morning. I have used this "cure" for colds and flues my entire life.....handed down to me by my paternal grandpa. It works!

Back to the Air Tractor:

I took the Fire Boss for a flight out of KBFI and had absolutely no bounce or shimmy during my take off run. Smooth as glass.

Will do more testing to see if I experience any shimmy shake with the planes.

OBIO
 
Ahhh, yet another one I forgot to move over to my new rig. Just installed all three versions and did a few circuits with no adverse events: on the ground taxing, take offs or landings. Down side if I can't reproduce your problem I can't do much to help.

I'll be posting the Shockwave bits later today. Got to get ready for the Formula 1 race in Valencia, Spain.
 
Oh ya must be somthing about my computer

I just don't understand why I'am the only one who have the bounceing with the Air Tractor ,has to be somthing on my computer.
 
I forgot our wonderful US broadcast networks must ensure everyone gets their dose of useless trivia in the morning vs showing the Formula 1 race live. They're delaying the showing 4 frigging hours.

Anyway, it allowed me to do the Shockwave bits sooner.

The wheeled version of the tanker and the duster are the same.

Aircraft.cfg Entries:

[lights]
//Types: 1=beacon, 2=strobe, 3=navigation, 4=cockpit, 5=landing
light.0=1, -19.11, 0, 9.01,fx_shockwave_beacon,
light.1=3, 0.93, -28.32, 0.38, fx_shockwave_navred
light.2=2, 0.36, -28.31, 0.38, fx_shockwave_strobe
light.3=3, 0.97, 28.26, 0.40, fx_shockwave_navgre
light.4=2, 0.36, 28.27, 0.38, fx_shockwave_strobe_2
light.5=3, -20.32, 0, 0.30, fx_shockwave_navwhi
light.6=4, -4.97, 0, 5.10, fx_shockwave_vclight,
light.7 = 6, 13.20, -0.6, -1.0, fx_Shockwave_landing_light_small,
light.8 = 5, 13.20, 0.6, -1.0, fx_shockwave_landing_light_tail,

The Amphibian version is slightly different in that the "taillight" is a bit further back.

Aircraft.cfg Entries:

[lights]
//Types: 1=beacon, 2=strobe, 3=navigation, 4=cockpit, 5=landing
light.0 = 1, -19.11, 0, 9.01, 9.01,fx_shockwave_beacon,
light.1 = 3, 0.93, -28.32, 0.38, fx_shockwave_navred
light.2 = 2, 0.36, -28.31, 0.38, fx_shockwave_strobe
light.3 = 3, 0.97, 28.26, 0.40, fx_shockwave_navgre
light.4 = 2, 0.36, 28.27, 0.38, fx_shockwave_strobe_2
light.5 = 3, -23.10, 0, -1.30, fx_shockwave_navwhi
light.6 = 4, -4.97, 0, 5.10, fx_shockwave_vclight,
light.7 = 6, 13.20, -0.6, -1.0, fx_Shockwave_landing_light_small,
light.8 = 5, 13.20, 0.6, -1.0, fx_shockwave_landing_light_tail,

Since the Amphibian has retractable gear, you can also add this to the Panel.

Panel.cfg Entries:
[Vcockpit01]
.
.
.
gauge31=shockwave_lights!SW Lights_gear, 1,1,1,1
gauge32=shockwave_lights!SW Lights_taxi_gear, 1,1,1,1
 
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