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JSBSim GroundReactions: Difference between revisions
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Several updates of force calculations, strut_force usage, etc.
(→Physical Characteristics: added info about square damping) |
(Several updates of force calculations, strut_force usage, etc.) |
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===Physical Characteristics=== | ===Physical Characteristics=== | ||
#Name and type. BOGEY refers to contact points associated with the landing gear while STRUCTURE represents contact points associated with the airframe. | #Name and type. BOGEY refers to contact points associated with the landing gear while STRUCTURE represents contact points associated with the airframe. | ||
Both of these contact types basically result in a force which resists the penetration of the | |||
ground by the aircraft. The main difference between the two types of contacts is how the ground | |||
reaction force is computed--BOGEY types exert forces only in the vertical direction in the aircraft's frame of reference, whereas STRUCTURE types can exert force in any direction. | |||
Furthermore the BOGEY type includes features which are typical to landing gears such as brake and steering. | |||
#X, Y, Z location, in inches in structural coordinate frame | #X, Y, Z location, in inches in structural coordinate frame | ||
#Spring constant, in lbs/ft | #Spring constant, in lbs/ft | ||
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===Operational Properties=== | ===Operational Properties=== | ||
#Name | #Name | ||
BOGEY contact points have these properties available--they do not apply to STRUCTURE contact points: | |||
#Brake Group Membership {one of LEFT | CENTER | RIGHT | NOSE | TAIL | NONE} | #Brake Group Membership {one of LEFT | CENTER | RIGHT | NOSE | TAIL | NONE} | ||
#Max Steer Angle, in degrees. If negative, the steering will be in the opposite direction. 360 represents an un-steered caster. | #Max Steer Angle, in degrees. If negative, the steering will be in the opposite direction. 360 represents an un-steered caster. | ||
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#Since the friction force takes place solely in the runway plane, the Z coordinate of the normalized wheel velocity vector is set to zero. | #Since the friction force takes place solely in the runway plane, the Z coordinate of the normalized wheel velocity vector is set to zero. | ||
#The gear deflection force (the force on the aircraft acting along the local frame Z axis) is now calculated given the spring and damper coefficients, and the gear deflection speed and stroke length. Keep in mind that gear forces always act in the negative direction (in both local and body frames), and are not capable of generating a force in the positive sense (one that would attract the aircraft to the ground). So, the gear forces are always negative - they are limited to values of zero or less. The gear force is simply the negative of the sum of the spring compression length times the spring coefficient and the gear velocity times the damping coefficient. | #The gear deflection force (the force on the aircraft acting along the local frame Z axis) is now calculated given the spring and damper coefficients, and the gear deflection speed and stroke length. Keep in mind that gear forces always act in the negative direction (in both local and body frames), and are not capable of generating a force in the positive sense (one that would attract the aircraft to the ground). So, the gear forces are always negative - they are limited to values of zero or less. The gear force is simply the negative of the sum of the spring compression length times the spring coefficient and the gear velocity times the damping coefficient. | ||
#*BOGEY forces act only in the z direction (vertical plane) whereas structure forces work in any/all directions (that is, the total force is calculated for both BOGEY and STRUCTURE, and then for BOGEY elements only the z component in the aircraft frame of reference is used). This is one reason it is important to surround BOGEY contact points with STRUCTURE contact points so that BOGEY contact points act only at angles near the vertical. | |||
#The lateral/directional force acting on the aircraft through the landing gear (along the local frame X and Y axes) is calculated next. First, the friction coefficient is multiplied by the recently calculated Z-force. This is the friction force. It must be given direction in addition to magnitude. We want the components in the local frame X and Y axes. From step 9, above, the conditioned wheel velocity vector is taken and the X and Y parts are multiplied by the friction force to get the X and Y components of friction. | #The lateral/directional force acting on the aircraft through the landing gear (along the local frame X and Y axes) is calculated next. First, the friction coefficient is multiplied by the recently calculated Z-force. This is the friction force. It must be given direction in addition to magnitude. We want the components in the local frame X and Y axes. From step 9, above, the conditioned wheel velocity vector is taken and the X and Y parts are multiplied by the friction force to get the X and Y components of friction. | ||
#The wheel force in local frame is next converted to body frame. | #The wheel force in local frame is next converted to body frame. | ||
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===Additional function: Strut Force=== | ===Additional function: Strut Force=== | ||
JSBSim now allows inclusion of an arbitrary table or function to calculate strut force, rather than using JSBSim's built-in functions to calculate forces and damping on a contact point. This should be operational in Flightgear 2.8 and forward. Include a section like this within your contact definition: | JSBSim now allows inclusion of an arbitrary table or function to calculate strut force, rather than using JSBSim's built-in functions to calculate forces and damping on a contact point. This should be operational in Flightgear 2.8 and forward. Include a section like this within your contact definition: | ||
<syntaxhighlight> | <syntaxhighlight> | ||
<strut_force> | |||
<strut_force> | |||
<function> | <function> | ||
<sum> | <sum> | ||
<product> | <product> | ||
<value> | <value> -5400. </value> | ||
<property> gear/unit[ | <property> gear/unit[2]/compression-ft </property> | ||
</product> | </product> | ||
<ifthen> | <ifthen> | ||
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</strut_force> | </strut_force> | ||
</syntaxhighlight> | </syntaxhighlight> | ||
The strut_force section above would replace and replicate the function of this section of a contact definition: | |||
<syntaxhighlight> | |||
<spring_coeff unit="LBS/FT">5400</spring_coeff> | |||
<damping_coeff unit="LBS/FT/SEC">160</damping_coeff> | |||
<damping_coeff_rebound unit="LBS/FT/SEC">320</damping_coeff_rebound> | |||
</syntaxhighlight> | |||
[http://forum.flightgear.org/viewtopic.php?f=49&t=20599 More information and example usage here.] | [http://forum.flightgear.org/viewtopic.php?f=49&t=20599 More information and example usage here.] | ||
[https://gitorious.org/jsbsim/jsbsim/commit/2f03c48f8e5d7e56ca2f134c0a63efc477616c19 JSBSim example usage here.] | |||
===How ground reaction forces are calculated=== | |||
The ground reactions are computed as forces that support the aircraft above the ground, and affect the motion over the ground. These forces can thus be split into two components: | |||
#the ground normal reaction, ie, support of the aircraft above the ground | |||
#the ground tangential reaction, ie, friction of the aircraft along the ground | |||
JSBSim computes the ground normal reaction by a spring/damper model. The normal reaction | |||
force is therefore obtained by the formula | |||
Fn = k*u + b*v (linear damping) | |||
Or | |||
Fn = k*u + b*v^2 (square damping) | |||
where k is the spring stiffness, b is the spring damping, u is the displacement due to compression, and v is the rate of compression. Spring damping, b, is | |||
Parameters u and v are computed by JSBSim and k and b are given respectively by the <spring_coeff>, | |||
<damping_coeff> and <damping_coeff_rebound> properties in the <contact> element definition. If v is positive then <damping_coeff> is used; if v is negative then <damping_coeff_rebound> is used. | |||
If the strut_force element is included, then your calculation of the strut force is substituted in placed of this formula, and parameters <spring_coeff>, <damping_coeff> and <damping_coeff_rebound> are ignored. | |||
Note that there is no upper limit to amount of compression of a contact element allowed--and thus the amount of force generated under, for example, extreme ground collisions. This is one reason for the extremely elastic ground collisions sometimes observed when JSBSim aircraft strike the ground at high speed. | |||
([http://jsbsim.sourceforge.net/JSBSim/FGLGear_8cpp_source.html See the actual code that implements this under function ComputeVerticalStrutForce here] | |||
===Other resources=== | ===Other resources=== | ||