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Uncommanded Pitch Oscillations

V

VulcanB2

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177
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unitedkingdom
Hi,

On an FM I'm working on, when the aircraft accelerates above 100kts or so, the aircraft starts to pitch up/down slightly, as if riding on very rough ground. The aircraft does respond to pitch inputs, but this is offset slightly by the uncommanded pitching. It only settles down again when travelling VERY fast at high altitude, or when below 100kts on the ground.

Any ideas as to the cause?

Best regards,
Robin.
 
Hi Robin,
Try increasing pitch_MOI in the cfg file a bit.
Hope this helps,
Elias
 
Hi,

I've tried as you've suggested, and I'm presently working with a value of 50,000,000 (without the commas). It has ertadicated the bouncing that was being experienced, but now the aircraft feels like it is attached to the end of an undampened spring! If you command pitch down, it will go, but then you have to wait for it to stop "springing" before you can pitch up again.

The controls feel like you're holding the other end of the sprint, then pulling, stopping, and waiting for the mass on the other end to cause the spring to just fully shrink.

I hope that makes sense. I've tried extremes of values in both the .air file and in the aircraft.fg file to see whether the value I'm looking for is somewhere in the middle, but that isn't working. :(

Best regards,
Robin.
 
Hi,

I've finally got it sorted out, but I'm using some crazy values for the pitch MOI!

empty_weight_pitch_MOI = 5000000
Click to expand...

5000000 ?! :eek: It seems to work alright though!

Best regards,
Robin.
 
It depends from the mass and dimensions of the aircraft you are modeling.
To have an idea, the default 747 pitch MOI is 24223159, B737-400 is 3172439, C172 is 1400.

If your model is not something approx to a B767-200 mass/dimensions you should check the airplanes geometry data in the cfg file. Sometimes a positive value where it must be negative or vise-versa create this kind of problems. Also check the CoG and engine positions to be correct.

regards,

Elias
 
Elias said:
It depends from the mass and dimensions of the aircraft you are modeling.
To have an idea, the default 747 pitch MOI is 24223159, B737-400 is 3172439, C172 is 1400.
Click to expand...
How do I know which should be negative/positive? I've used a default aircraft.cfg file as a guide. Is that MGW or EW?

If your model is not something approx to a B767-200 mass/dimensions you should check the airplanes geometry data in the cfg file. Sometimes a positive value where it must be negative or vise-versa create this kind of problems. Also check the CoG and engine positions to be correct.
Click to expand...
Where can I see/set the CoG? I need to move the engines for sure! :o The aircrafts MGW is 190,000 lbs.

Best regards,
Robin.
 
From the FSX SimObject Container SDK/Aircraft Configuration files:


[weight_and_balance]
reference_datum_position :
Offset (in feet) of the aircraft's reference datum from the standard Flight Simulator center point, which is on the centerline chord aft of the leading edge. By setting the Reference Datum Position, actual aircraft loading data can be used directly according to the aircraft's manufacturer. If not specified, the default is 0,0,0.

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 or baggage. Flight Simulator determines the total MOIs with these influences within the simulation. The units are slugs per foot squared. Omission of a parameter will result in Flight Simulator defaulting to the value set in the .air file, if one exists.
These values can be estimated with the following formula:

MOI = EmptyWeight * (D^2 / K)


[generalenginedata]
engine.0
to
engine.n
Offset (in feet) of the engine from the Reference Datum Position (longitudinal, lateral and vertical). Each engine location specified increases the engine count (maximum of four engines allowed)



This, and much more useful information can be found in the SDK which is the best so far and explains (almost) everything in simple words.

Creating a flight model from scratch is not an easy task as it requires a really good knowledge of how MS Flight Simulator works, mathematics, aerodynamics and programming.
You may use a default air/cfg file so you can check the visual model in the sim, but if you want a good and accurate fm you must spend a lot of time (unfortunately).

I highly recomend Jerry Beckwith's tools:
www.mudpond.org
Using his FlightDynamicsWorkbook and/or AirWrench you will be able to create a descent fm as a basis which you can then customize and give it a "character" if you wish.

regards,

Elias
 
Hi,

Thanks for the info! I just hadn't thought that moving thee engines might affect it. :o I'll do that today and let you know the result.

Best regards,
Robin.
 
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