Angle of Attack is KING

[bomber]

Understanding the angle of attack and what changes this be it the wing wash out/in, roll or yaw rate is an important part of flight modelling. Regardless of which method you're using.

This is an acceptable example of a planes lift derived through what I suspect is a datcom or areomatic approach.

Code: Select all

    <function name="aero/force/Lift_alpha">
      <description>Lift due to alpha</description>
      <product>
          <property>aero/qbar-psf</property>
          <property>metrics/Sw-sqft</property>
          <table>
            <independentVar lookup="row">aero/alpha-rad</independentVar>
            <tableData>
              -0.20 -0.4365
               0.00  0.2500
               0.23  1.0288
               0.60  0.5615
            </tableData>
          </table>
      </product>
    </function>

if however like me you've split the wing up into sections then you have to do this calculation for each section airfoil area taking into account any angle of attack variation due to washout and then just total all these values up.

An advantage of doing this is that knowing each sections lift you can calculate their roll moment... knowing the total wings lift you can calculate the wings centre of lift and roll moment.. this comes in handy later on.

[legoboyvdlp]

Regarding AOA, I was just thinking of what it means. If your nose is up and you are dropping, you probably need to shove your nose down, since you are in a deep stall. If you are pushing your nose down to a dive and climbing like a rocket... do you have negative AOA? I've been reading Ernest K Gaan's book 'Fate is the Hunter'. He was an American Airlines pilot in the days of DC-2's; he remembers one time entering a thunderstorm with Captain Ross, and seeing how Ross was pointing the nose at the ground... while climbing at 1500 feet per minute. He remembers that Ross ordered gear down, and remarks that if the gear were not down, the aircraft would be climbing a lot faster.

[bomber]

No you're not nesseccarily in a negative angle of attack..

The leading edge of a thunderstorm has a lot of fast upward moving air hiting the airfoil at a high AoA..if you're flying straight and level the resultant AoA is naturally not as a high as the rising air but it's neither zero. So to reduce AoA and lift you've got to point the nose into the airflow which would give the impression of a negative AoA but in reality isn't.

[bomber]

Which brings up the AoA of the propwash hitting the wing..

Here's an example of propwash.. ok the dynamic pressure is increased by the thrust coefficient however the lift is being calculated as though it's hitting the airfoil/flap at alpha rad.

Code: Select all

    <function name="aero/force/Lift_propwash">
      <description>Delta lift due to propeller induced velocity</description>
      <product>
         <property>propulsion/engine[0]/thrust-coefficient</property>
          <property>aero/qbar-psf</property>
          <property>metrics/Sw-sqft</property>
          <table>
            <independentVar lookup="row">aero/alpha-rad</independentVar>
            <independentVar lookup="column">fcs/flap-pos-deg</independentVar>
            <tableData>
                     0.0     60.0
              -0.01  0.000   0.000
               0.00  0.003   0.007
               0.24  0.011   0.028
               0.48  0.000   0.000
            </tableData>
          </table>
      </product>
    </function>

If like me you've worked out the coefficients for the section of the wing that's within the radius of the propwash the lift for this section is the addition of the propwash lift at zero AoA based on prop induced velocity plus the lift for this section at it's current AoA based on dynamic pressure.

[bomber]

And then there's these calculations

<function name="aero/force/Lift_pitch_rate">
<description>Lift_due_to_pitch_rate</description>
<product>
<property>aero/qbar-psf</property>
<property>metrics/Sw-sqft</property>
<property>velocities/q-aero-rad_sec</property>
<property>aero/ci2vel</property>
<value> 5.3514 </value>
</product>
</function>

<function name="aero/force/Lift_alpha_rate">
<description>Lift_due_to_alpha_rate</description>
<product>
<property>aero/qbar-psf</property>
<property>metrics/Sw-sqft</property>
<property>aero/alphadot-rad_sec</property>
<property>aero/ci2vel</property>
<value> 4.3685 </value>
</product>
</function>

I have no understanding of their worth.....

[bomber]

this is the list due to flaps..

<function name="aero/force/Lift_flap">
<description>Delta Lift due to flaps</description>
<product>
<property>aero/qbar-psf</property>
<property>metrics/Sw-sqft</property>
<property>fcs/flap-pos-deg</property>
<value> 0.0123 </value>
</product>
</function>

however if you've calculated the flaps wing sections coefficients, it's already covered in your main wing lift calcs.

[bomber]

And finally the h-stabs...

<function name="aero/force/Lift_elevator">
<description>Lift due to Elevator Deflection</description>
<product>
<property>aero/qbar-psf</property>
<property>metrics/Sw-sqft</property>
<property>fcs/elevator-pos-rad</property>
<value> 0.3929 </value>
</product>
</function>

But we know a planes h-stab area... and we can work out the co-effiecients of it as easily as we can the main wing so why not work it out accurately rather than using a rad number, the area of the wing and a 'magic number'....

[bomber]

if angle of attack is king..... what does dihedral with changing beta angle do to alpha AoA ?

[IAHM-COL]

Queen? Bishop? Rook? Knight?

[jwocky]

Oh oh ohhhh, a bit careful here, please ...

First of all, you can't just skip "lift due to flaps" and just assume it is in the main lift ... because there are two different things to keep in mind with flaps (and some other dynamic things)

1.) They go in and out ... obviously. Means, they change dynamically their relevance in lift and drag. Which leads to the effect that is is really dynamic ... as in, the flaps move from one position to the other. Now imagine you have all this in your main lift routine. Every function is internally worked as kind of a little chukn of software inherently blocking other parts of the software from getting to work while this thing runs. So you really want to split it up in smaller chunks.
Another example for such effects (not aerodynamically, but software-wise) is a JSB-fuel system. Looks quite nice on the paper and the theory is so elegant. And of course it runs faster in JSB than it would run in an extra.Nasal module ... however, the cost in blocking core cycles are higher

2.) Aeromatics calculates flap lift and drag by an estimated "share" of the total wing area. Since in most planes the exact information is not available, that is based on an estimation by aeromatics and that one is usually not good. It gets worse, if you have planes for example with oversized flaps, like the A310. And because this is such a pain and you need to play around till you get the data points right (a certain VR and V1 at certain weights), I prefer to have the flaps, slats and spoilers separated to tweak them easier.

And yes, that's all about programming, your aerodynamics is of course impeccable.

And then, there is this "lift due to pitch" ... well, yeah, a lot of planes have that because it is an approximation because they have no "lift due to AoA" function. A certain AoA at a certain wind will lead always to a certain pitch at a given time t. They are not independent. So, since you went already with the AoA (you could because you did some extra miles on the math), the "lift due to pitch" would make only sense if it takes the aerodynamical behaviour of the fuselage in consideration. And I am not even sure if that wouldn't be also rather based on AoA since that is the angle to the wind. However, that would definitive put your plane development on a new level as far as FG is concerned.