  Flapping wings enable flying animals and biomimetic robots to generate
elevated aerodynamic forces. Measurements that demonstrate this capability are
based on tethered experiments with robots and animals, and indirect force
calculations based on measured kinematics or airflow during free flight.
Remarkably, there exists no method to measure these forces directly during free
flight. Such in vivo recordings in freely behaving animals are essential to
better understand the precise aerodynamic function of their flapping wings, in
particular during the downstroke versus upstroke. Here we demonstrate a new
aerodynamic force platform (AFP) for nonintrusive aerodynamic force measurement
in freely flying animals and robots. The platform encloses the animal or object
that generates fluid force with a physical control surface, which mechanically
integrates the net aerodynamic force that is transferred to the earth. Using a
straightforward analytical solution of the Navier-Stokes equation, we verified
that the method is accurate. We subsequently validated the method with a
quadcopter that is suspended in the AFP and generates unsteady thrust profiles.
These independent measurements confirm that the AFP is indeed accurate. We
demonstrate the effectiveness of the AFP by studying aerodynamic weight support
of a freely flying bird in vivo, which demonstrates that its upstroke is
inactive.
