  This paper discusses the conceptual design and proof-of-concept flight
demonstration of a novel variable pitch quadrotor biplane Unmanned Aerial
Vehicle concept for payload delivery. The proposed design combines vertical
takeoff and landing (VTOL), precise hover capabilities of a quadrotor
helicopter and high range, endurance and high forward cruise speed
characteristics of a fixed wing aircraft. The proposed UAV is designed for a
mission requirement of carrying and delivering 6 kg payload to a destination at
16 km from the point of origin. First, the design of proprotors is carried out
using a physics based modified Blade Element Momentum Theory (BEMT) analysis,
which is validated using experimental data generated for the purpose.
Proprotors have conflicting requirement for optimal hover and forward flight
performance. Next, the biplane wings are designed using simple lifting line
theory. The airframe design is followed by power plant selection and
transmission design. Finally, weight estimation is carried out to complete the
design process. The proprotor design with 24 deg preset angle and -24 deg twist
is designed based on 70% weightage to forward flight and 30% weightage to
hovering flight conditions. The operating RPM of the proprotors is reduced from
3200 during hover to 2000 during forward flight to ensure optimal performance
during cruise flight. The estimated power consumption during forward flight
mode is 64% less than that required for hover, establishing the benefit of this
hybrid concept. A proof-of-concept scaled prototype is fabricated using
commercial-off-the-shelf parts. A PID controller is developed and implemented
on the PixHawk board to enable stable hovering flight and attitude tracking.
