Thursday, September 22, 2011

Transonic Airfoil Analysis for Jet Aircraft, Propellers and High Speed Blades

Whether you design blades for propellers or airfoils for high speed executive jets, as an engineer you must have an efficient tool that is capable of screening and designing custom airfoil shapes over all speeds of operations. An aerodynamic problem that arises (and this is a potential show stopper) is the tendency of the pressure drag to increase by an order of magnitude through the transonic regime of operation. This sudden drag rise depends on such factors as airfoil thickness, camber and angle of attack.  For a propeller design, the efficiency can decrease dramatically after reaching a certain helical tip mach number (this limits forward airplane speed and/or available power).  For a wing design, the performance of the airplane can be severely limited due to airfoil shapes that were poorly designed and tested across the transonic regime of flight.  The figure below shows how the profile drag can suddenly increase on an airfoil past a given speed or Mach Number.  This Mach number is known as the drag divergence Mach number or Mdd.

Profile drag can suddenly increase by a factor of 10.

As the power of the PC increases, engineers now have access to tools to screen blades that operate poorly in the transonic regime of flight.  VisualFoil Plus has the capability to analyze airfoils in subsonic, transonic and supersonic flows. The above figure shows the pressure drag rise for a NACA 2412 airfoil as predicted using VisualFoil Plus.  The NACA 2412 is a popular airfoil for the wings of general aviation aircraft.  It is also used in some propeller designs.  Currently, propeller companies design and test propeller blades based on modern (often proprietary) airfoil shapes. 

 VisualFoil Plus can be used to design an airfoil that pushes the drag rise towards the right side of the graph resulting in a significant performance boost.  The software features automatic grid generation, a built-in airfoil library and tools to run the analysis from an external program (to facilitate custom airfoil design and analysis).

Transonic Flow Past an Airfoil.

To find out more about testing airfoils in high speed, please read the following article "High Speed Blade Screening" (.pdf file). 

More information about VisualFoil Plus can be found at