Whether you view Google as a benevolent Internet overlord or the new 'Evil Empire', there's no arguing that the search giant at least devotes some of its squazillions towards environmentally beneficial causes. Earlier this year the company invested US$168 million in what will be the world's largest solar power tower plant and now it has partnered with NASA to sponsor the Green Flight Challenge that offers a prize purse of $1.65 million for the design of quiet, practical and energy-efficient aircraft.

Announced at AirVenture 2009 in Oshkosh, Wisconsin, the Green Flight Challenge was originally scheduled for July 2011 but has now been rescheduled for September 25 to October 3, 2011. It will be held at the CAFE (Comparative Aircraft Flight Efficiency) Foundation Flight Test Center at Charles M. Schulz Sonoma County Airport in Santa Rosa, California. Registration for the competition closed on December 31, 2010, with 13 teams from across the U.S. entering to compete for what is the largest ever prize for civilian aviation.

Competing aircraft must be able to maintain an average speed of at least 100 mph (161 km/h) with a fuel efficiency of 200 passenger MPG on two 200 mile (322 km) flight - or one gallon per passenger. The aircraft must also keep the noise down to under 78 dBA at full power on takeoff and clear a 50 foot obstacle in under 2,000 feet (610 m) on takeoff.

While most of the prototype aircraft competing, such as the e-Genius, will be propelled by batteries and electric motors, some are bio-fuel or hybrid aircraft. The electric aircraft will have their batteries charged using clean geo-thermal based electricity from The Geysers geo-thermal fields in the Mayacama Mountains North of Santa Rosa.

The winner of the main competition will be awarded $1.3 million, with four next placed runners up receiving $80,000, $60,000, $40,000 and $20,000 respectively. A Bio-fuel prize of $150,000 will also be awarded but with only one bio-fuel aircraft in the running, it comes down to a question of whether it can achieve at least 80 mph and at least 160 passenger MPG. If not, the $150,000 will be split and added to the other prizes.

All competing aircraft will be shown to the public at the Google Green Flight Challenge Exposition hosted by NASA at Moffett Field-NASA Ames Research Center, from 9 AM to 4 PM on October 3, 2011. Advance online tickets will be available through the CAFE website.