bum steer

[mue]

not what I asked... it was a simple question... so here another...
This is the pitch moment section of the C172.... point out this m_aerorp as it's got to be there or as you say JSBsim will be producing wrong kinematics

You are kidding, right?
The moment at aerorp m_aerorp is a three dimensional vector. In JSBSim it's defined by it's three components: pitch, yaw and roll moment. The pitch moment section you posted describes one of the three components of m_aerorp.

You even don't know that....

[bomber]

wow, I even made it simple for you by only asking you to point out the pitch component..... which happens to be the primary one we're talking about.

It can't hard for someone as educated and skilled as you are.

[mue]

m_aerorp is just the aggregate of all pitch, yaw and roll moments defined in the <aerodynamics> section.

[bomber]

So if you completely deleted all the pitch yaw and roll moments in their sections there'd be no pitch, yaw and rolling happening.... is that correct ?

Simon

[mue]

So if you completely deleted all the pitch yaw and roll moments in their sections there'd be no pitch, yaw and rolling happening.... is that correct ?

Simon

No.
It's all in the moment (at CG) formula: M_cg = M_aerorp + (CG - AERORP) x F_aerorp.
I assume with "...completely deleted all the pitch yaw and roll moments in their sections..." you mean M_aerorp = 0. Then the formula is reduced to M_cg = (CG - AERORP) x F_aerorp.
If AERORP is not at the CG and the acting force F_aerorp is not parallel (or antiparallel) to the vector (CG - AERORP) then M_cg will be not zero and the aircraft will experience angular acceleration, i.e. the aircraft will rotate.

[IAHM-COL]

not what I asked... it was a simple question... so here another...
This is the pitch moment section of the C172.... point out this m_aerorp as it's got to be there or as you say JSBsim will be producing wrong kinematics

You are kidding, right?
The moment at aerorp m_aerorp is a three dimensional vector. In JSBSim it's defined by it's three components: pitch, yaw and roll moment. The pitch moment section you posted describes one of the three components of m_aerorp.

You even don't know that....

No mue,
Bomber is not kidding.

Instead, I believe he is kindly trying to lead you to realize that the moments are applied to the CG (as it should be, as no object rotates around any point other than their GCs), but the forces are applied to the AERORP. (it is something along those lines, as I am not FDM'expert)

[Richard]

And I don't mean that the AERORP is ignored by JSBSim. Of course the AERORP is used for computing the moment at the CG

With all due respect the discussion about AERORP has gone beyond anything useful.

show me where 99% of JSBsim flight models are more complex in downwash

What other models do is irrelevant to this discussion; everyone is aware that there are a lot of simplistic aero models in FGAddon (and elsewhare).

yes it does (take into account pitch rate and the damping due to pitch rate)

It is better if you explain the mathematics that you are using so that we can understand what you're doing and review it. Pasting XML snippets doesn't really help me understand what you're doing. For example it is unclear how you convert the angular velocities which are in rad/sec into the ft/s values.

Explaining the formulas is all part of an in depth discussion of flight dynamics - pasting XML is not really conducive to peer review.

Angle is an angle... how fast the air is moving is a product of the qbar calculation.

Except when the angle is changing - and you're not taking into account the speed of the airstream and this can be significant; especially at slower speeds. I suspect that this is something that can be solved with a differential equation or integration - but I don't know how to do this because I don't understand the maths properly.

The tailplane doesn't rotate through any freestream as the wing downwash effects the freestream

Freestream as in the airflow arriving at the tailplane; and if the aircraft is changing pitch angle then the tailplane will be moving in the airflow that arrives at the leading edge of the tailplane.

...the moments are applied to the CG

Aerodynamic moments (except for Simon's method) already have the offset to the AERORP baked into them; which is why you can't mix coefficients from two different reference points without changing them to use a common reference point. The difference between AERORP and the actual CG then generates another moment in each axis.

[IAHM-COL]

...the moments are applied to the CG

Aerodynamic moments (except for Simon's method) already have the offset to the AERORP baked into them; which is why you can't mix coefficients from two different reference points without changing them to use a common reference point. The difference between AERORP and the actual CG then generates another moment in each axis.

@Richard

I am not understanding what you meant.

Do you mean how JSBsim operates and the simulation assumptions of bomber and others? or do you really mean about the behavior of physical bodies moving through viscous fluids? Because on the latter, I don't think there are really two competing physics laws to choose from.

[IAHM-COL]

And I don't mean that the AERORP is ignored by JSBSim. Of course the AERORP is used for computing the moment at the CG

With all due respect the discussion about AERORP has gone beyond anything useful.

I actually find the AERORP topic of utmost interest.
The difference. you already understand it well. i dont'.

While author X (say helijah) is making a JSBsim FDM-- he will define an AERORP somewhere.

Where?

And that 's the point.

Does an AERORP actually exist or is it a convention of the JSBsim simulation?
Can it be anywhere? [In the universe, in the airframe, by the nose, or tail?]
Does it need to be in the vicinity of the GC, or even coincide with the GC ? (and how about a moving GC and a static AERORP?]
Does the AERORP move as well? because of simulation conventions or real flying dynamics?
What happens when I move the aerorp to the flying behaviors meter by meter in every dimensional axis?

You see, there's a lot about it I dont know the answer for, or the reasons such answer must be given like they are. And we come to my typical banter about documentation. FG and JSbsim documentation is not good enough to make any of these clear. In the forums, when this topics come about they are threads and threads of flames and no one can be single out as given a knowledgeable asnwer.

[bomber]

It is better if you explain the mathematics that you are using so that we can understand what you're doing and review it. Pasting XML snippets doesn't really help me understand what you're doing. For example it is unclear how you convert the angular velocities which are in rad/sec into the ft/s values.

Explaining the formulas is all part of an in depth discussion of flight dynamics - pasting XML is not really conducive to peer review.

Wow what a turn around, everytime I want to have a discussion about an aspect of flight modelling people just throw equations at it.... usually with a smart arsed comment about how simple the physics is.

To determine the angle of attack on the h-stab we need to understand the two velocities uv and w

with these determines it's a simple case of Pythagoras tan Angle = O/A
Better yet use the atan2 function in xml as this doesn't give infinite results which would throw the AoA out.

Code: Select all

   <fcs_function name="T4T/aero/h-stab_left/AoA-deg">                        
      <function>                     
         <todegrees>         
            <atan2>            
               <property>T4T/velocities/h-stab_left/w-aero-fps</property>         
               <property>T4T/velocities/h-stab_left/uv-fps</property>         
            </atan2>               
         </todegrees>                  
      </function>                     
   </fcs_function>                        


using the angle of the airstream coming off the wing and the u-aero velocity to calculate the actual u-velocity

Code: Select all

<fcs_function name="T4T/velocities/h-stab_left/u-aero-fps">                        
   <function>                     
      <product>                  
         <cos><property>T4T/aero/h-stab_left/downwash-rad</property></cos>               
         <property>velocities/u-aero-fps</property>               
      </product>                  
   </function>                     
</fcs_function>                        

using the angle of the v-aero velocity and the r-rotation velocity to to calculate actual v-velocity

Code: Select all

<summer name="T4T/velocities/h-stab/v-aero-fps">                     
   <input>velocities/v-aero-fps</input>                  
   <input>T4T/velocities/h-stab/r_rotational-fps</input>                  
</summer>                     

using the w-areo velocity and the rotation speeds on the h-stab in both p & q to calculate the w-aero velocity

Code: Select all

<fcs_function name="T4T/velocities/h-stab_left/w-aero-fps">                        
   <function>                                 
      <sum>               
         <property>velocities/w-aero-fps</property>            
         <product>            
            <sin><property>T4T/aero/h-stab_left/downwash-rad</property></sin>         
            <property>velocities/u-aero-fps</property>         
         </product>            
         <property>T4T/velocities/h-stab/p_rotational-fps</property>            
         <property>T4T/velocities/h-stab/q_rotational-fps</property>            
      </sum>                              
   </function>                     
</fcs_function>                        

using the actual u and v speeds that the h-stab see's to calculate the uv velocity

Code: Select all

<fcs_function name="T4T/velocities/h-stab_left/uv-fps">                        
   <function>                     
      <pow>                  
         <sum>               
            <pow>            
               <property>T4T/velocities/h-stab_left/u-aero-fps</property>         
               <value>2</value>         
            </pow>            
            <pow>            
               <property>T4T/velocities/h-stab/v-aero-fps</property>         
               <value>2</value>         
            </pow>            
         </sum>               
         <value> 0.5 </value>               
      </pow>                  
   </function>                     
</fcs_function>                        

Simon