What are the least powerful airplanes that ever flew? What manned airplanes have achieved flight with the least powerful engines (no gliders!), and what was their top speed? Im sure I've heard of an airplane with an 8hp engine capable of exceeding 120 MPH airspeed
There are a number of human-powered aircraft (list here).  For the Gossamer Albatross, we have


  In still air, the required power was on the order of 300 W (0.40 hp),
  though even mild turbulence made this figure rise rapidly.


As far as top speed:


  Allen completed the 22.2 mi (35.7 km) crossing in 2 hours and 49
  minutes, achieving a top speed of 18 mph (29 km/h) and an average
  altitude of 5 ft (1.5 m)