04 August 2006


There were six of us packed into a 1960 Ford Falcon........
a car only slightly larger than a VW Beetle.
Grandparents, Mom and Dad, Sis and I, mighty cozy with one another.
Grandpa at the wheel, it was raining cats and dogs outside the car.
I vividly remember him turning to me with a smile, saying, "The engine runs better in the rain."

Do you believe that? I always did.
And it's one of those things we believe that's dead wrong.

The only thing about rain that is good for an engine is that rain normally comes with a drop in air temperature. The temperature drop makes the air more dense, and denser air does make an internal combustion engine perform better.

But the rain doesn't help. Rain brings higher humidity. Higher humidity means water vapor in the air, and water vapor is made of two parts hydrogen, one part oxygen. The hydrogen molecules make the air less dense, which works against engine efficiency.
(It also impairs airfoil efficiency.)

Another misconception is wind.
My first instructor asked, "If you take off on a round trip flight with a 20 knot wind, does the tailwind on the return flight cancel out the headwind on the outbound flight?"
I answered "yes", and again was dead wrong.

To illustrate why, imagine I'm flying an aircraft that cruises at 100 miles per hour, and I'm flying to a heliport 50 miles away.
With no wind, the outbound and return legs will both be 30 minutes: 50 miles at 100 miles per hour = .5 hours, or 30 minutes. Therefore, with no wind the round trip flight will take one hour.

Now, let's assume a 50 m.p.h. headwind on the outbound leg. With this wind we'll cover the ground at 50 m.p.h. on the outbound leg, and 150 m.p.h. on the return flight. The outbound flight will take an hour: 50 miles at 50 m.p.h.. The return flight will take 20 minutes: 50 miles at 150 m.p.h. = 1/3rd of an hour, or 20 minutes. Total flight time with the wind: 1 hour 20 minutes. Any wind lengthens the overall flight-time. The wind has more time to impede the aircraft on the outbound leg than to assist the return flight. The stronger the wind, the worse the impact.

Learning to fly exposes you to a great deal of physics. I'm continually amazed at how many "light bulb" moments I have, where I realize I've known something all my life, and didn't know why what I knew was true. But there are a few things I have "known" all my life that simply aren't true. As I'm faced with those things in the future, I'll try to remember to bring them to your attention.

1 comment:

Jason said...

I had one myself earlier this week. I was always told, and believed, that chiggers burrow into your skin and stay there, causing you to itch.

But my wife went sleuthing online (after she was bit up this weekend) to find that they don't do that at all and that putting nail polish on the bites only helps reduce the itching because it keeps air out of the wound.