Silent Flight: PV Powered Airships

It's surprising there aren't more solar powered blimps for tours of canyons. The forest service could use them to count big horn sheep,
etc. If the weather was fair, wouldn't you like to take a quiet ride in an airship? Maybe sneak up on a mule deer . . .
There's no question conventional PV would get a blimp going over 25 mph. Boeing's new 40% efficient PV would move an airship at 30 - 35 mph.
Assume a football shaped blimp 30 meters long 10 m in dia.
The volume is about 1500 m^3 or over 1 ton of lift using He.
The area is about 250 m^2 on a side which is enough space for 100 kW of
PV at 40% efficiency.
Use this calculator to estimate the drag force:
http://www.ecs.syr.edu/centers/simfluid/redder/dragforce/DragForce.html
10 meter dia ball going 9 m/sec => 700 Newtons drag force.
A football of the same dia will have somewhat more drag so, to be conservative, assume it's 1000 N
Multiply this by 9m/sec for the power = 9000 N m /sec = 9 kW
Assuming prop efficiency is 70% and propulsion efficiency is 60% -- the
props must be big -- only 41 kW will end up as thrust.
So we have 4.5 X more power than what we need to go 20 mph!
Using the propeller rule (p ~ v^3), top speed is closer to 55 km/hr -- cruising speed for a Sonoran pronghorn!
The first PV airship should be named the "Pronghorn."
Bret Cahill
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Canyons = funky winds = unhappy airships.

I'm not sure those new 40% PV's are practical for an application like this...Spectrolab's main business is space cells. To get enough for a blimp might be horribly expensive. Give it a little time, though, and the price ought to come down out of the stratosphere (literally).
Tom.
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You don't need to be down in the wash. Moreover, there are other scenic places.

They claim $3 - $4 / watt. The problem may be it needs to be focused.
At $1/watt the thin copper alloy PV may be better suited.
The power cubed propeller rule gives you a pretty predictable speed.
Bret Cahill
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