## Tuesday, January 08, 2013

### Filling the sky

kw: trends, transportation

I received the January 2013 issue of Scientific American two weeks ago, and immediately read their feature article (several short pieces), "50 100 150 Years in the Future". Two pages of graphics begin on page 26, but the substance begins on page 28: "A Drone in Every Driveway", by Mary Cummings. Her prediction is that millions of us will have small airplanes within 50 years. To avoid a traffic control nightmare these planes will be required to fly themselves. This presumes some kind of radar control grid they can all detect. The article's closing sentence is, "The George Jetson of 50 years from now will be riding a drone."

Now, depending on season and time of day, as many as 12,000 planes are aloft above the U.S. Can you imagine 12,000 planes aloft above just one town of 50,000 during rush hour? So, I got the point. But I was thinking, "What is Ms Cummings thinking?" So what if we do solve the problem of keeping 100 million airplanes from crashing into one another? How much extra fuel is this going to require?

The mileage of the current crop of small to midsize autos is about 25 mpg in town, and 30 or more on the highway. The President has urged automakers to strive for 50 mpg (highway) within 12-20 years. By contrast, the most economical small 1- and 2-seat airplanes achieve 15-22 mpg at cruising speed (75% of full throttle). Does anyone expect a "driveway aircraft" to hit 40 mpg, even in 50 years? I don't, and here is why. Slipping through the air, while maintaining enough lift to stay up, requires a certain minimum power expenditure that depends in a complex way on velocity. Go too slow, approaching stall speed, and drag increases; go too fast above the "sweet spot" of 100-150 knots for a small craft, and drag also increases. The sweet spot for big jets is 450 knots or so because they fly above 80% of the atmosphere. A driveway airplane won't fly much above 5000 feet, and seldom that high (I remember when you couldn't fly a Piper Cub to Denver because it couldn't fly above 4,500 feet).

I have an alternate prediction for 2063: Flying car-type automatic drones are very likely to be developed, but will remain the plaything of a rich few who can afford a vehicle that gets somewhere 2-3 times as fast as a road car, even if it uses twice the fuel (at \$20-\$30 per gallon) getting there.