Replies

• Murray Scott

  17 November 2007 | 05:23

  I came upon this post only recently so any comment will be old news but here goes anyhow.

  “If mastering flight is your goal, you can’t do better than to emulate a dragonfly.”

  This is only true if you are the size of a dragonfly and fly at the same speed. In identifying drag as fundamental to dragonfly flight (and that of other insects) the author clearly recognises the significance of Reynolds Number in differentiating the airflow regime for insects from that of larger, faster flyers. The air at low Reynolds Numbers behaves as a viscous fluid through which the insect swims as through honey, gravity being practically irrelevant.

  She must therefore recognise that data from this insect-scale flow regime is unhelpful in designing even model aircraft, let alone human-carrying ones for which the airflow is, to a first pproximation, frictionless and gravity greatly influences the power required for flight. Even if it were possible for aircraft to use drag for propulsion this is inherently inefficient compared to vehicles that exploit “lift” forces normal to the airflow. Contrast the efficiency of a rowing boat (propelled by drag opposing the direction of the oar motion) and a dolphin (pushing primarily up and down with its tail to move forward).

  In drawing lessons from nature for efficient flight, we should begin with migrating birds. Even here however we must beware attempting to copy boundary layer effects which do not translate into larger airframes.

  Murray Scott

• Chunlei Liang

  12 March 2008 | 00:19

  Murray,

  Great point!
  Maybe, it will be useful when nano-robots operate small-scale flapping-wing flights.

  Charlie Liang