The Preliminary Research Aerodynamic Design to Lower Drag, or Prandtl-D was a series of unmanned experimental glider-aircraft developed by NASA under aerodynamicist Albion Bowers.

[1] The acronym is a reference to early German Aerospace Engineer Ludwig Prandtl, whose theory of the bell-shaped lift distribution deeply influenced Bowers.

The Prandtl-D's designs also drew on glider concepts of the German Horten brothers Reimar and Walter, and incorporate the conclusions of NASA aerodynamics pioneers R.T. Jones and Richard T.

A key difference in the Prandtl-D full scale model is an addition of a University of Minnesota developed Data Collection System (DAC).

Detailing the aerodynamic properties and mathematics associated with the project, Bowers discusses in depth the science behind altering the span load distribution on aircraft wings and the data gathered from experiments that demonstrated validation of its critical principles.

[7][6] In 2019, two of the aircraft, D1 and D3, were transferred to the Smithsonian National Air and Space Museum in Washington, D.C., and California Science Center, Los Angeles, respectively, for their display following a successful review of the program.