Monday, February 18, 2013

Flying the Prius

I've been driving a Toyota Prius for a little over a year.

Being a hybrid, it is a quirky car.  And, no, it doesn't fly, although the salesman tried to impress me by talking about its coefficient of drag.  I called his bluff; he didn't really know what a coefficient of drag was, but he knew it sounded good.

But even though this is as close as the Prius comes to flying, it still can teach us a lot about my two favorite aerodynamic parameters, essence and finesse.

First the finesse part: finesse is the ratio of lift to drag, what glider pilots call L/D.  Due to a quirk in the mathematics of aerodynamical forces, when you write L/D as a formula everything cancels except the coefficients: the coefficient of lift, and, Nick the Toyota salesman's favorite, the coefficient of drag.  When the latter is small, the ratio is big.

Now the Prius's coefficient of lift is about zero, and dividing zero by a smaller number still leaves zero, but at least morally the Prius's finesse is increased.

Now for essence, a compendium of the various kinds of energy available to the craft.  As a hybrid, the Prius has a gas motor and an electric motor.  It starts on the electric motor, sounding like a MontrĂ©al subway train, but then the gas motor kicks in.  The gas motor is pretty puny, and the thing holds less gas than my 1946 Taylorcraft.  But the Taylorcraft was a 25mpg machine, while the Prius is a 50mpg machine.

What the Prius has instead is a big battery with its own charge meter hidden somewhere in the display menus.  (It's hard to photograph, being a raster scanned projection.)  As you climb the hill, you can watch essence flow through the gas motor and through the traction motor to the front wheels.  As you descend, you can watch the gas motor turn off and the regenerative braking send essence from the front wheels to the battery.

Essence is malleable: chemical essence becomes kinetic essence becomes potential essence which can become kinetic essence again.  In the Prius, potential essence becomes electrical essence and you can watch it happen.

Still, no matter what you do, the gas gauge only goes down.

Friday, February 15, 2013

Asteroid DA14 and Diamond DA20s

As I write, the asteroid DA14 is passing its perigee, its closet approach to Earth.  I am not afraid of it hitting me, nor am I afraid of it actually hitting a GPS or other navigational satellite directly.

I was worried about the perturbation of a satellite's orbit due to a 1.9E8 kg asteroid passing nearby.  As you are aware, GPS depends on having a super-accurate model of the satellite's orbit, and any error in computing the satellite's orbit translates directly into an error in your GPS estimated position.

My first thought about flying today was to make sure that I am prepared to revert to non-GPS navigation.  No, I take that back, not just prepared, but I need to actually make non-GPS navigation primary.

But after a back-of-the-envelope calculation of the gravitational attraction between a 1.9E8 kg asteroid and 2E3 kg GPS satellite I am not so worried.  Still, the lesson remains: many things beyond human control affect the accuracy of GPS navigation, and we as pilots need to maintain our ability to navigate without GPS.

Tuesday, February 5, 2013


Here's an interesting story from Skybrary about a Boeing 737 that was given an altitude below the Minimum Vectoring Altitude.  I fly in mountainous terrain, so this is a particularly important topic.

The 73 got a "Pull Up!" warning from whatever terrain system they had.  It was an -800, so presumably they had some kind of visual terrain warning available (my $700 Garmin handheld and my FlightGuide EFB and my SkyCharts EFB all provide this).  Did they not monitor the terrain?

And what if this had been on a cold day?  With an airport elevation of 639', an aircraft indicating 7800' on a -20C day is more than 800' lower; this airplane passed less than 700' above the terrain.

Back before these great terrain awareness tools were available, I used weather radar to provide a terrain picture.  I set the radar in MAP mode with a short scale (10 or 15 miles).  Approaching Salmon, Idaho on the RNAV approach from the North, I could see the mountain to the left of the course, and the valley straight ahead, on the radar set.

And it has happened to me, more than once.  "Cleared as filed, maintain 9,000" didn't make any sense with 12,000' terrain so close, so I asked.  Whoops!  I wasn't cleared as filed, I was cleared on a victor airway that went the other way, over lower terrain.

The pilot (or crew) is always responsible for terrain clearance.  Always!  Use all the tools you have.