bum steer

[mue]

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>                        


If "h-stab_left/AoA-deg" represents the aerodynamic angle alpha then I think "h-stab_left/uv-fps" should be changed to "h-stab_left/u-fps".

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 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>                        

Your downwash correction for u and w I don't understand. It seems your downwash model is not just a simple rotatation of the free airstream velocity vector by the downwash angle. Anyway, I would assume that after applying the downwash correction (ignoring any rotational rates) u and w should satisfy

Code: Select all

u/w = tan(alpha - downwash_angle)

And I have the feeling your calculation does not. But maybe my assumption is already wrong. I'm not sure.

[bomber]

If "h-stab_left/AoA-deg" represents the aerodynamic angle alpha then I think "h-stab_left/uv-fps" should be changed to "h-stab_left/u-fps".
[/code]

Changes in beta would reduce the velocity if you only used the u-fps.... the horizontal velocity used is a vector of both U and V velocities

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>                        

Your downwash correction for u and w I don't understand. It seems your downwash model is not just a simple rotatation of the free airstream velocity vector by the downwash angle. Anyway, I would assume that after applying the downwash correction (ignoring any rotational rates) u and w should satisfy

Code: Select all

u/w = tan(alpha - downwash_angle)

And I have the feeling your calculation does not. But maybe my assumption is already wrong. I'm not sure.

That's the very first code I showed.

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>

[mue]

Changes in beta would reduce the velocity if you only used the u-fps

That I don't understand. How does changes in beta reduce the (and what) velocity?

the horizontal velocity used is a vector of both U and V velocities

Then the angle you compute is not aerodynamic alpha but some kind of alpha total. If you indeed want alpha total and not aerodynamic alpha, then OK. But I thought you use some airfoil tools to compute aerodynamic forces and moments. I would assume that those tools expect aerodynamic alpha.

Regarding your downwash calculation. What I meant is the following:
My assumption is that the downwash reduces alpha exactly by the downwash angle: alpha_hstab = alpha - downwash_angle. That means u_hstab/w_hstab = tan(alpha - downwash_angle).
If I ignore rotational rates than you computation is as follows: u_hstab = u * cos(downwash_angle) and w_hstab = w + u * sin(downwash_angle). Then (u * cos(downwash_angle))/(w + u * sin(downwash_angle) = tan(alpha - downwash_angle) with u/w = tan(alpha) must be satisfied. And my feeling is that it's not.

[bomber]

That I don't understand. How does changes in beta reduce the (and what) velocity?

aero=u-fps or any u vector velcoity is specific to the wind travelling in the u direction, so any value of beta has a effect of reducing this value by cos beta.

In the real world the free stream can flow over the wing from a beta angle and it will travel from the leading edge to the trailing edge at this beta angle... If you were to cross section the wing following the airstream and compare it with the cross section in the u direction we'd see that the thickness of the airfoil remains constant but the cord length changes with beta either greater or lesser.

I wanted to model that...

Then the angle you compute is not aerodynamic alpha but some kind of alpha total. If you indeed want alpha total and not aerodynamic alpha, then OK. But I thought you use some airfoil tools to compute aerodynamic forces and moments. I would assume that those tools expect aerodynamic alpha.

yes I use the airfoil tool to create coefficients for the airfoil at zero beta and -32 and 32 beta and then use a 3d table.

[bomber]

Regarding your downwash calculation. What I meant is the following:
My assumption is that the downwash reduces alpha exactly by the downwash angle: alpha_hstab = alpha - downwash_angle. That means u_hstab/w_hstab = tan(alpha - downwash_angle).
If I ignore rotational rates than you computation is as follows: u_hstab = u * cos(downwash_angle) and w_hstab = w + u * sin(downwash_angle). Then (u * cos(downwash_angle))/(w + u * sin(downwash_angle) = tan(alpha - downwash_angle) with u/w = tan(alpha) must be satisfied. And my feeling is that it's not.

Code: Select all

   <fcs_function name="T4T/aero/h-stab_right/AoA-deg">
      <function>
         <todegrees>
            <sum name="T4T/aero/h-stab_right/AoA-rad">
               <atan2>
                  <property>T4T/velocities/h-stab_right/w-aero-fps</property>
                  <property>T4T/velocities/h-stab_right/uv-fps</property>
               </atan2>
               <product>
                  <sin><property>T4T/aero/h-stab_right/beta-rad</property></sin>
                  <value>0</value>         <!-- dihedral rad -->
               </product>
               <product>
                  <cos><property>T4T/aero/h-stab_right/beta-rad</property></cos>
                  <value>0</value>         <!-- rigging rad -->
               </product>
            </sum>
         </todegrees>
      </function>
   </fcs_function>

If we remove the effects of beta on any dihedral and the rigging angle of the h-stab.... which are both zero on the plane.

Code: Select all

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

That means u_hstab/w_hstab = tan(alpha - downwash_angle).

apart from it being
tan theta = o/a
tan h-stab angle = w/u

that is what I have
(although it also contain v velocity as explained above)

[mue]

That means u_hstab/w_hstab = tan(alpha - downwash_angle).

apart from it being
tan theta = o/a
tan h-stab angle = w/u

Ooops...my bad. You are right. It should read w_hstab/u_hstab = tan(alpha - downwash_angle) and w/u = tan(alpha).

[Richard]

I am not understanding what you meant.

Simon calculates moments at run time (Force x Distance) whereas in a standard aero model the forces and moments are measured and normalized either in a wind tunnel, in the air or using CFD. The reference point is clearly important for this and it is important to ensure that coefficients from different sources use the same reference point.

Does an AERORP actually exist or is it a convention of the JSBsim simulation?

AERORP is the JSBSim name for it. There are other names such as datum, reference point, Xref, CG. See http://zaretto.com/sites/zaretto.com/fi ... 101648.pdf page 52 for the way that they specific it for the F-14. This document is a fairly good illustration of how aero modelling is usually performed.

The only reference point I've ever seen in reports is the CG at the time the measurements were taken; often expressed in percentage mean aerodynamic chord (%MAC).

FG and JSbsim documentation is not good enough to make any of these clear.

The JSBSim documentation and modelling guidelines I found useful; although it is outdated. However I usually refer to the source code as that to me is the definitive documentation for all opensource projects.

[bomber]

Ooops...my bad. You are right. It should read w_hstab/u_hstab = tan(alpha - downwash_angle) and w/u = tan(alpha).

No problems I knew what you meant... we all make these little mistakes writing equations every now and again.

If we can allow each other to make mistakes without jumping down each other throats conversations would be a lot less flamefesty.

Simon

[IAHM-COL]

AERORP is the JSBSim name for it. There are other names such as datum, reference point, Xref, CG.

?

I thought Aerorp datum and CG were all different places.

I believe, the datum to be imaginary point of reference that one can place in a vertical-axis most anywhere, even forward the nose.
it does not seem to match my expectations of the aerorp or the CG.

[mue]

I still don't fully understand your approach.

In the real world the free stream can flow over the wing from a beta angle and it will travel from the leading edge to the trailing edge at this beta angle... If you were to cross section the wing following the airstream and compare it with the cross section in the u direction we'd see that the thickness of the airfoil remains constant but the cord length changes with beta either greater or lesser.

I wanted to model that...

My understanding of this is that you "convert" the original airfoil according to beta into an airfoil with a cross section along the airflow (i.e. with a greater cord length,...). Then indeed you need your "alpha total" and not aerodynamic alpha. And you implicitly set beta = 0.

yes I use the airfoil tool to create coefficients for the airfoil at zero beta and -32 and 32 beta and then use a 3d table.

But here you use beta for creating coefficients for the (original or converted?) airfoil. If you use the original airfoil then I think you need aearodynamic alpha and not "alpha total". If you instead use your converted airfoil then why you need beta, because with the converted airfoil beta is always zero?