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Those pesky contact points again!!!

S

Sid2008

Charter Member
Hello, Finally I got my Northrop Delta tail-dragger airliner done in FSDS 3.5. It is a beautiful, fat, single engine monoplane airliner, with a nice VC and virtual cabin. I chose to use the Jens Kristensen's Lockheed Orion model as a guide to compile this plane.

Now I have ACM (aircraft container manager), woo hoo, this is a terrific program. I used it to get the correct static pitch angle, bring the CoG closer to the front engine, and place the 2 wheel contact points under the wheel and 4 scrape points at the ends of the main wing and at the ends of the fuselage.

One shortcut I had taken, which I probably shouldn't have was not animating landing gear drop when it is not under load. Its landing gear is similar to the Stuka, so I just kept is without any vertical travel due to springs. I will fix that tonight.

This airplane dances all around when parked. I am simply at my wits end to fix this problem. Please someone help.

Thank you.
Sid
 
Sparks did a nice graphic explanation on contact points; really helps understanding.

http://www.mudpond.org/contact_points.pdf

As to the dancing, there are many possibilities from CoG placement, MOI's not correct, or contact points and CoG not correct with respect to each other.

Contact points static compression and max-static ratio need to be reasonable as well.

Be sure to allow plenty compression from "hanging gear" to static compression on the tarmac and 1.5 to 2.5 on the ratio. The sum of the two distances must equal the total animated distance.

In your design program, find you total movement distance. Determine how much of that you want as static compression. Divide total movement by static compression to get the ratio.
 
Milton Shupe, thank you very much for your response. I have decised that what I can do is
1) animate the "springiness" of the landing gear and derive the appropriate compression ratio; repeat this for the 2 main wheels and the tail wheel
2) go back to ACM and enter the compression ratios, move around the CoG along the centerline of the aircraft and check to see at what point does it stop dancing around while parked.

I will keep notes on what i have done, and if I succeed to sabilize the airplane, I will indicate my steps in a post on this site to help others as well.

Thanks,
sid
 
Sid2008 said:
Milton Shupe, thank you very much for your response. I have decised that what I can do is
1) animate the "springiness" of the landing gear and derive the appropriate compression ratio; repeat this for the 2 main wheels and the tail wheel
2) go back to ACM and enter the compression ratios, move around the CoG along the centerline of the aircraft and check to see at what point does it stop dancing around while parked.

I will keep notes on what i have done, and if I succeed to sabilize the airplane, I will indicate my steps in a post on this site to help others as well.

Thanks,
sid
Click to expand...

The CoG should be 25% MAC of the wing, or 25% rearward of the wings leading edge (typically the thickest part of the wing).

Don't forget to verify the MOI's are correct. Too light/low and it will give these symptoms as well.
 
Ok Milton, Its time for me to show my ignorance: what is a MOI, how do I find it and correct it?
Sid
 
Milton, what is MOI and how can I modify them? Sorry for the ignorance here.

Thank you for the guidance. As you can imagine, i am getting a bit frustrated at my own ignorance, but I am learning..........the hard way. What is MOI, and what role do they play? How can I change them and what values should I look for (i.e., any rule of thumb here?)
Thanks,
Sid
 
Always plenty to learn when it comes to design, modeling, flight models, panels and gauges, and textures.

MOI's as explained in the Aircraft Container SDK:
-------------------------------------------------------------------------------------------------------------------
Moments of Inertia
A moment of inertia (MOI) defines the mass distribution about an axis of an aircraft. A moment of inertia for a particular axis is increased as mass is increased and/or as the given mass is distributed farther from the axis. This is largely what determines the inertial characteristics of the aircraft.
The following weight and balance parameters define the MOIs of the empty aircraft, meaning that the value should not reflect fuel, passengers, baggage, or expendable armament (Combat Flight Simulator aircraft). Flight Simulator determines the total MOIs with these influences within the simulation. The units are slug - ft^2. Omission of a parameter will result in Flight Simulator defaulting to the value set in the .air file, if one exists.

Parameter Example Description
empty_weight_pitch_MOI empty_weight_pitch_MOI=1400.0 The moment of inertia (MOI) about the lateral axis.
empty_weight_roll_MOI empty_weight_roll_MOI=1137.0 The moment of inertia (MOI) about the longitudinal axis.
empty_weight_yaw_MOI empty_weight_yaw_MOI=2360.0 The moment of inertia (MOI) about the vertical axis.
empty_weight_coupled_MOI empty_weight_coupled_MOI = 0.0 The moment of inertia (MOI) about the roll and yaw axis (usually zero).
These values can be estimated with the following formula:
MOI = EmptyWeight * (D^2 / K)
Where:
Pitch Roll Yaw
D = Length (feet) Wingspan (feet) 0.5* (Length+Wingspan)
K = 810 1870 770

This formula yields only rough estimates. Actual values vary based on aircraft material, installed equipment, and number of engines and their positions.
------------------------------------------------------------------------------------------

There is lots of reading available on the concepts.

You can get the SDK here: http://download.microsoft.com/download/9/f/5/9f57691b-7c5f-4031-aa29-2ab00f676ba6/aircraft_container_setup.exe
 
I am not a designer and I am sure Milton has forgotten more than I know about sim aircraft. One thing I have noticed during all of my tweaking...if the contact points are very close to being under the COG, the plane will bounce while on the ground. Moving the contact points forward a bit, even if it means that the contact points are no longer under the tires, will stop the dancing.

OBIO
 
OBIO said:
I am not a designer and I am sure Milton has forgotten more than I know about sim aircraft. One thing I have noticed during all of my tweaking...if the contact points are very close to being under the COG, the plane will bounce while on the ground. Moving the contact points forward a bit, even if it means that the contact points are no longer under the tires, will stop the dancing.

OBIO
Click to expand...

If the issue is caused by CoG being too close to the main gear contact points, then the appropriate thing to do is move the CoG (which then was obviously mistated in the first place).

This can be done in the aircraft.cfg Geometry section with this statement.

wing_pos_apex_lon=2.08

Increase the number to set the CoG rearward. Do this in small increments (tenths) as the CoG impact is large percentage-wise.

If you get a FD monitoring tool like AFSD, you can set CoG perfectly with a percentage of MAC readout.

You can get AFSD here: http://pagesperso-orange.fr/hsors/index.html
 
My Contact Points Experience

Thank you Milton for your help. I am trying to understand what you wrote and it wil take me some time.

I want to tke this opportunity to share my contact point experience step by step for the benefit of other compleat idiots like me.

1. I made an airplane in FSDS 3.5 and it is a single engined airliner not too different from the Lockheed Orion.
2. I cheated by compiling the airplane using Jens Kristensen's Orion model as a standard for contact points (wheels and scrape points), weights and flight dynamics.
3. When I designed my FSDS model I made sure that the (0,0,0) coordinates was 25% behind the leading edge of the wing and along the line of thrust so as to put the CoG over there.
4. I initially did not model any springiness to the fixed landing gear and that was a big mistake. So I went back to the FSDS model and allowed the main gear units to compress by 1 ft each.
5. I have ACM ( Aircraft Container Manager?) which I used to modify the static pitch of the airplane and the height of the CoG from ground level so all 3 wheels are touching the ground.
6. I moved the wheel contact points to be slightly below the 3 wheels and saved the airplane in my FS9 directory.

As a consequence of this, my airplane now sits beautifully and taxies well. The problem I now see is upon touchdown the main wheel units tend to sink in the runway. I plan to tweak the contact points so that this does not happen.

Sid
 
Sid2008 said:
Thank you Milton for your help. I am trying to understand what you wrote and it wil take me some time.

I want to tke this opportunity to share my contact point experience step by step for the benefit of other compleat idiots like me.

1. I made an airplane in FSDS 3.5 and it is a single engined airliner not too different from the Lockheed Orion.
2. I cheated by compiling the airplane using Jens Kristensen's Orion model as a standard for contact points (wheels and scrape points), weights and flight dynamics.
3. When I designed my FSDS model I made sure that the (0,0,0) coordinates was 25% behind the leading edge of the wing and along the line of thrust so as to put the CoG over there.
4. I initially did not model any springiness to the fixed landing gear and that was a big mistake. So I went back to the FSDS model and allowed the main gear units to compress by 1 ft each.
5. I have ACM ( Aircraft Container Manager?) which I used to modify the static pitch of the airplane and the height of the CoG from ground level so all 3 wheels are touching the ground.
6. I moved the wheel contact points to be slightly below the 3 wheels and saved the airplane in my FS9 directory.

As a consequence of this, my airplane now sits beautifully and taxies well. The problem I now see is upon touchdown the main wheel units tend to sink in the runway. I plan to tweak the contact points so that this does not happen.

Sid
Click to expand...

Make sure that the contact points static compression and ratio equal 1 foot or your animation distance..

e.g. .5, 2.0 or .40, 2.5, or .60, 1.67
 
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