Aerodynamic Strip Theory

Aerodynamic Strip Theory is the process of looking at a wing or lifting surface as a series of independent strips.  Each strip has its own aerodynamic characteristics that are dimensionalised using the strip geometry to influence the airframe as a whole.

A lifting surface can have a range of geometrical characteristics such as twist and dihedral that means the axis system of a section of the strip differs from that of the aircraft as a whole.  Thus the strip will have different velocities which in turn means it will have a different angle of attack/sideslip to the aircraft.  When we then consider the aircraft moving through space, as it rotates, so there is a velocity distribution that is a function of the speed of rotation and the location relative to the centre of gravity.

If we break the surface down into a series of small strips, so the problem can be discretized. The changes in velocity due to the geometry and the motion of the aircraft result in each strip having its own local angle of attack.

When calculating the changes in velocity on the strip, j2 Elements not only considers the location of the strip but also its attitude  to the airframe (twist and dihedral) and any additional velocities (downwash, propwash).  The complete set of velocities are reoriented into the strip’s local body axis and then the local angle of attack is calculated.

By using this value on the strips’ aerodynamic characteristics, so a local set of coefficients for each strip can be found.

Using the strips’ geometry and orientation, these can be changed into force and moment contributions to the aircraft as a whole.  The coefficients in the strip’s local axis system are reoriented back into the airframe’s axis system before being summated to give a resultant set of forces and moments.

What this means is that it is possible to calculate the dynamic response of the aircraft from the airfoil characteristics. This is especially useful when there is limited data available or the aircraft is in the early stages of design.