Side

[bomber]

Side for the v-stab is easy.... just use it's area, the dynamic pressure, beta angle and the co-efficients for the airfoil being used...

Now the wing's another kettle of fish.... I'd say have two wings a left and a right and work out what AoA each wing is seeing with regards to the beta angle and it's inherent dihedral.

You then have a lift for left and right wing and from this you could work out the sides for each wing.

Simon

[IAHM-COL]

sounds hard. Looks fun. I Can see the plane spinning outa control till you get all those multivariate functions under control.

[bomber]

why do you think it's taken me 4 years.... I've my wings split up into 8 sections each side, never mind just a simple left and right split..

this is the make up of the side forces for the beagle on sanhozzays.

Code: Select all

   <function name="aero/force/Side_beta">
       <description>Side force due to beta</description>
       <product>
           <property>aero/qbar-psf</property>
           <property>metrics/Sw-sqft</property>
           <property>aero/beta-rad</property>
           <value> -0.4194 </value>
       </product>
    </function>

    <function name="aero/force/Side_roll_rate">
       <description>Side_force_due_to_roll_rate</description>
       <product>
           <property>aero/qbar-psf</property>
           <property>metrics/Sw-sqft</property>
           <property>aero/bi2vel</property>
           <property>velocities/p-aero-rad_sec</property>
           <value> -0.0036 </value>
       </product>
    </function>

    <function name="aero/force/Side_yaw_rate">
       <description>Side_force_due_to_yaw_rate</description>
       <product>
           <property>aero/qbar-psf</property>
           <property>metrics/Sw-sqft</property>
           <property>aero/bi2vel</property>
           <property>velocities/r-aero-rad_sec</property>
           <value> 0.3339 </value>
       </product>
    </function>

    <function name="aero/force/Side_rudder">
       <description>Side_force_due_to_rudder</description>
       <product>
           <property>aero/qbar-psf</property>
           <property>metrics/Sw-sqft</property>
           <property>fcs/rudder-pos-rad</property>
           <value> 0.2554 </value>
       </product>
    </function>

[Richard]

Why would side force depend on wing area? Is it just a way of estimating overall dimensions of tail and fuselage for an aircraft of conventional shape?

The coefficients that we are using are normalized; i.e. they have no units. To turn the coefficient into a force you need to denormalize it - i.e. multiply it by something which is scaled into an appropriate value. They're called coefficients because that's the mathematical term for a factor used in a multiplication.

To get the coefficients the forces that were measured (or calculated) are simply divided by a figure; in fact the formula for Cl would be

Code: Select all

Cl = (Lift lbs) / (1/2 * rho * v*v * wing surface area)


It doesn't have to be done this way (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3357408/), and I don't really fully understand the mathematical reasons why these factors were chosen. I suppose if you really wanted to you could use the size of your grandad's allotment as a normalization.

Usually the forces are denormalized with wing surface area, and pitch uses wing surface area and wing chord, roll and yaw use wing surface area and wing span.

This probably means that you could change the surface area and the result might be consistent - but I'm not sure this is an accurate use but it would probably generate a stable model.

[bomber]

Code: Select all

<function name="aero/force/Side_beta">
  <description>Side force due to beta</description>
  <product>
    <property>aero/qbar-psf</property>
    <property>metrics/Sw-sqft</property>
    <property>aero/beta-rad</property>
    <value> -1.0907 </value>
  </product>
</function>

so when we look at this....

what we could do is change out the wing area for fuselage side area...
then we'd have to change out the -1.0907 to a flat plate value.... nearer 1.28
and we'd then need to swap out the aero/beta-rad to be sine aero/beta-rad.

[sanhozay]

Usually the forces are denormalized with wing surface area, and pitch uses wing surface area and wing chord, roll and yaw use wing surface area and wing span.

Ah OK, I understand. It's just a convention for scaling the coefficient based on a broad brush measure of aircraft size. So the numeric coefficient, which is presumably estimated based on htail area and a guess of the fuselage area, is the same order of magnitude for large and small aircraft.

Now I think about it, there are very few very large or very small numbers as coefficients in FDMs I've looked at, and this is probably the reason.

[bomber]

And I think what you'll actually have worked out is the form drag of the fuselage.

it's a lot easier to link side to the lift vector multiplied by the tan of the dihedral.

[Richard]

[code]<function name="aero/force/Side_beta">
...

what we could do is change out the wing area for fuselage side area...
then we'd have to change out the -1.0907 to a flat plate value.... nearer 1.28
and we'd then need to swap out the aero/beta-rad to be sine aero/beta-rad.

The big problem with that equation is not the lack of a sin(beta) it's the constant value of -1.097. This will result in something that is almost, but not quite entirely unlike sideforce; i.e. a linear response that doesn't vary with alpha or elevator. You would be better off just estimating a sideforce table based on CR-1756, or any other sideforce table for a similarly shaped craft..

[bomber]

well yes but it is only one part of what makes up the side force in this example.... bear in mind I don't agree with this type of flight modelling.... personally I scraped the lot as total bunkum and started from fresh..

Alpha or elevator having an effect on side force.... well yes but that's because in reality lift side and drag are just vectors of the REAL force ie the Resultant of the airfoil.... as that increases with AoA or decreases for that matter so does it's vectors...

Thanks Wright brothers for being so thick !

Simon