“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Wednesday, April 14, 2010

Not to pooh pooh NASA, MIT, et al, but...

In my blog list is a link to a site entitled "Altternative Energy. It's an interesting site and frequently has items well worth reading and other times more silly or trivial. But today I saw an article about an aircraft called the "Puffin. This is a concept aircraft designed by one Mark Moore, a NASA aerospace engineer. The article quotes briefly from and links to the NASA site about the Puffin.

It's a single person vehicle ostensibly capable of vertical take offs and landings and a top speed of 150 m.p.h. (thought it's stated that the Puffin is more efficient at lower speeds). It's 12 feet long with a wingspan of 14.5 feet. It's stated to weigh 300 pounds empty, with 100 pounds for batteries and 200 pounds of payload (pilot and baggage). It's electrically powered and its motors develop 60 horsepower. Its range "with current battery technology" is stated to be about 50 miles.

Well. First, it doesn't exist, even as a prototype. But Analytical Mechanics Associates has produced an animation that can be seen in a YouTube video. According to the video, a one third scale validation model was to have been tested in March for hover capability, with transition to forward flight demonstrated after that. I have been unable to determine if these test flights have actually taken place.

But what about the plausibility of such a craft? Let's start with battery capacity. I'm going to assume the Puffin will achieve its 50 mile range at 100 m.p.h. I'll also assume that the 60 horsepower are required to achieve the top speed of 150 m.p.h. Since power required varies generally with the cube of speed for forces, such as aerodynamic drag, that vary with the square of speed, I can roughly estimate that the Puffin will require 60/1.5^3 (1.5 is the ratio of 150 m.p.h. to 100 m.p.h.) or about 17.8 horsepower. I'll be generous to the claims and assume zero reserves of energy. Thus, the batteries must supply 17.8 horsepower for 30 minutes to go 50 miles at 100 m.p.h. Googling (17.8 horsepwer)*30 minutes in kilowatt hours returns the conversion to the 6.64 kilowatt hours that are required.

Moving on to this excellent battery site I find that Lithium-ion batteries have the highest energy density available currently at 128 watt-hours/kilogram. Thus, I'll need 6,640/128 or 51.9 kilograms of batteries. This mass weighs 114 pounds here on Earth. This 6,640 watt-hour battery pack will cost an estimated $28,000.

For fun, I ran through the same calculation at 50 m.p.h. (the stalling speed in forward flight is not indicated, nor are such aerodynamic characteristics as flat plate area, propeller efficiency, lift/drag ratios at various speeds, etc.) and determined that, with no energy to spare the batteries must store 1,660 watt-hours of energy weighing 28.6 pounds and costing $7,090. But this battery pack could only maintain 150 m.p.h. for a little over two minutes.

OK, let's finally figure a 100 pound Li-ion battery pack. This 45.4 kilogram pack should provide about 5,810 watt hours. That capacity will provide 60 horsepower for just slightly under eight minutes. And remember, this is to "dry tanks" and includes no increment for such things as vertical takeoff and transition to level flight (very energy intensive phases for aerial vehicles with which I'm familiar). Finally, let's realistically assume that it would be nice to have a 10% reserve. How fast can you fly to have a 50 mile range with the 100 pound battery pack and have such a reserve? It's kind of a nitpicky problem in algebra and units, but the answer is about 89 m.p.h.

So my conclusion is that, while the claims may not actually be false, they seem quite misleading. You may be able to go 50 miles (and glide, powerless, to a landing); you may be able to go 150 m.p.h.; you may be able to take off vertically with a payload of 200 pounds; but you won't be able to load up 200 pounds, take off vertically, and fly 50 miles at 150 m.p.h. to your destination.


EMONEY said...

It sounds a lot like a miniature version of the V-22 Osprey. If it has half of those problems, i'm not impressed.

King of the Road said...

It doesn't look like the rotors tilt and delivering the spectacularly high power through the gears and transmissions in the Osprey and the complexities of the tilt rotor mechanism have been the V-22's Achiles' heel.

They say that "the design would be powered by a total of 60 horsepower through electric motors, which are designed to be able to fail any two powertrain components on either side and still produce the required power to hover."

I'm guessing that this vehicle will never be produced.