1. Project Sunrise

    2. Los Angeles California 1974

    3. Flight of the Worlds First Solar Powered Aircraft

    4. by

    5. Roland Boucher

The excess power required to climb is the product of the Aircraft weight and the climb rate as shown in equation 8:


8 Climb rate hc = ΔP/ W  =  44 ft per minute per watt/lb

DARPA instructed Astro Flight to select a wing span of 32 feet for the demonstration model of the proposed 100 foot span Solar powered high altitude drone. The proposal called for a wing loading of 4 oz  per square foot and  an aspect ratio of 12:1. This yielded a wing area of 85.33 square feet and a gross weight of 21.33 pounds.  A lift coefficient of 0.9 was selected for both climb and glide.

From equation 6    minimum power  = 5.5 x( 98.5 / 32 ) = 16.92 ft lbs /sec

From equation 7    minimum power =  22.94 watts

From equation 3    Velocity = 15.28 fps  =  10.4 mph

From equation 2    Thrust = 16.92/15,28 = 1.107 pounds

Solar radiation at noon on June 21 in the Los Angeles area reaches a maximum of 900 watts per square meter. At noon our 85.33 square foot wing would receive about 7850 watts of solar radiation. If all this power were converted to useful thrust through perfect 100% efficient solar cells, motor, and propeller, the climb rate from equation 8 would be 16,193 feet per minute and our perfect plane would reach 73,000 feet in less than 5 minutes.

From equation 8    Climb Rate = (44 /21.33) x 7850 =  16,193 feet per minute

Today, in 2009, Solar cells can approach an efficiency of 40%, brushless motors and modern propellers can each approach 90% efficiency. Our nearly perfect plane would generate 3140 watts of electrical power, the resulting 2543 watts of thrust power would produce a climb rate of 5245 feet per minute.

In 1973 Economical and available solar cells were only 10 percent efficient, Solar cell roughness dictated that they be restricted to the aft 2/3 of the wing surface,and these inexpensive circular Solar cells lost another 26% of available area.

Our 7850 watts of solar radiation would now produce only 445 watts of electrical power. Project Sunrise, in the initial flight test on battery power, used two commercial servo motors to drive the propeller through a 6:1 speed reducer.  All flights under solar power used a single motor which used high energy Samarium Cobalt magnets. This breakthrough motor produced the same power at half the weight and operated at 85% efficiency.                                                                            

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