bum steer

[bomber]

I decided to make this topic to highlight the bum steers in flight modeling that goes on...

https://forum.flightgear.org/viewtopic. ... 490593a00c

Now Alan, the question was how to adjust the value, not that it can be done.... why not simply tell the person where the value is ?

fdm/jsbsim/metrics

His next problem is writing to it... if he wrote in a system file he'd be able to use an output..

So something like..

Code: Select all

      <scheduled_gain>
            <input>T4T/one</input>
            <table>
               <independentVar>velocities/mach</independentVar>
               <tableData>
                      <!-- table data -->
               </tableData>
            </table>
         <output>fdm/jsbsim/metrics/aero-rp-x-in</output>
      </scheduled_gain>

Stop bum steering people... if you can't answer the question, don't post !

[bomber]

But an even bigger bum steer is the post by AndersG

The aero reference point in JBSim is not equal to the centre of pressure. It is just a reference point for the aerodynamic forces and moments - it can be (almost?) anywhere (e.g. at the nose).

It's laughable !

aero is the point on the airframe that the forces, Lift, Side and Drag are applied... putting it at the nose with a long armature to the CoG will result in JSBsim calculating large rotation moments and watch your plane spin.... The moment calcs BTW are calculated by JSBsim around the COG in case your're wondering.. and the CoG moves.

[mue]

But an even bigger bum steer is the post by AndersG

The aero reference point in JBSim is not equal to the centre of pressure. It is just a reference point for the aerodynamic forces and moments - it can be (almost?) anywhere (e.g. at the nose).

It's laughable !

aero is the point on the airframe that the forces, Lift, Side and Drag are applied... putting it at the nose with a long armature to the CoG will result in JSBsim calculating large rotation moments and watch your plane spin.... The moment calcs BTW are calculated by JSBsim around the COG in case your're wondering.. and the CoG moves.

No, AndersG is right and it's actually your post that's laughable.
I assume you have never heard of or at least didn't understand "Equivalent Force Systems".

[IAHM-COL]

Not only there's a big issue on how the FG "community" answers questions == By monkey's through the branches instead of simply giving the factual answer if they have it.
The problem is compounded by lack of useful software documentation.

[IAHM-COL]

I assume you have never heard of or at least didn't understand "Equivalent Force Systems".

I am glad you are knowledgeable about this (no pun intended, today)

This discussion is actually interesting to me.

Now, does this mean that the AERORP -- alledgedly on of the most important points in the FDM, where the forces will be actually applied -- can be set arbitrarily on the tip of the nose, and still since there are equivalent force systems, then the distance AERORP - GC does not matter, nor it will create momentums (in a lever-like manner)?

This is mind blowing to me, but I can accept that the software can do all sorts of computing to account for this.

However this is what still confuses me big time:

If aerorp can be set arbitrarily at any point (in space, be within the aircraft frame or even outside), why do we need to specify this point at all? Any effort we do to try to identify its location will be null as it does not really matter the location we had set, the equivalent force system will nullify the aerorp's location effects.

Am I missing something? cause if I am, then that I am missing is quite the elephant in the plane.

[bomber]

What you're missing is that mue et al are not correct...

You cannot apply the equivalent force at the nose....end of.

JSBsim does work out additional moments based on the distance between the aerorp and the cog on top of moments calculated via the moment section of the XML.

Simon

[bomber]

No, AndersG is right and it's actually your post that's laughable.
I assume you have never heard of or at least didn't understand "Equivalent Force Systems".

The aero reference point in JBSim is not equal to the centre of pressure. It is just a reference point for the aerodynamic forces and moments - it can be (almost?) anywhere (e.g. at the nose).

I just thought I'd quote it so as it don't get lost... Back sliding not allowed.

aerorp although not the centre of pressure of the wing, it is the centre of pressure of the whole plane, normally referred to as the neutral point and should be located a few inches behind the cog.

Simon

[Richard]

Now, does this mean that the AERORP

AERORP is simply the centre of gravity when the aerodynamics were measured (or calculated from).

It is somewhat misleading to say it could be anywhere on the airframe; but equally it has nothing to do with the centre of pressure.

[bomber]

AERORP is simply the centre of gravity when the aerodynamics were measured (or calculated from).

Looks like a duck, quacks like a duck.... Why not call it a duck.

Richhard, you excel yourself in linguistics slight of hand... Why not call it the neutral point ?

That position on the airframe where the h-stab moments are equal to the wing moments... And thus there is no moment generated by lift. Except for the small amount of nose up moment generated from this point to the cog and which thus helps with longitudinal stability.

It is somewhat misleading to say it could be anywhere on the airframe; but equally it has nothing to do with the centre of pressure.

Read my signature.... I think Mitchell said it best

So it either can be on the nose with JSBsim or not....which one is it....get off that fence !

Simon

[IAHM-COL]

I believe @Richard already agreed upon the AERORP makes no sense being the nose tip