Tuesday, April 05, 2005

Keeping the Cows Small

Airplanes all work the same way. Push forward and the cows get bigger. Pull back and the cows get smaller. Pull way back and the cows get bigger again.

Yesterday Kris Johnson described what he termed a holy war over airspeed and altitude control. He perceives that "aviation experts are divided into two camps about The Right Way to learn how to do it."

I wrote a long, physics-filled posting in response, but I didn't post it because I saw David Megginson's posting. Go read it.

Here are a couple of accident reports that illustrate his point.

This pilot escaped with minor injuries.

These four people weren't so lucky.

An airplane has kinetic energy from its forward movement and gravitational potential energy from its altitude. One can be excahnged for the other. Energy is lost to drag. Energy is added with thrust. Pitching up exchanges airspeed for altitude, kinetic energy for gravitational potential energy, but it's like a credit card. You can only afford to buy altitude as rapidly as you can pay in thrust. Otherwise that altitude is going to be repossessed, and that might hurt.

You can use power to replace energy losses and maintain altitude for several hours, and then you run out of fuel. You can use pitch to overcome drag and maintain airspeed for several minutes, and then you run out of altitude. You can use pitch to maintain altitude for several seconds, and then you run out of airspeed. At this point, you need to be a few centimetres above a suitable landing area. And hope there are no cows in the way.

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