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G is for Groundspeed


Is it just me, or this thing going backward?

Many years ago as a student pilot, I discovered first-hand that a single-engine Cessna is capable of flying backward.

My instructor and I were working on my first cross-country flight from Clow Airport – a small, but fairly busy local airstrip – to Bloomington, just over 50 miles to the south. The winds aloft were howling out of the north, providing a push that made our first leg almost three times faster than our return flight into the wind. As we inched steadily closer to our final destination, my instructor took advantage of the wind to illustrate the difference between airspeed and groundspeed by having me practice slow flight.

Slow flight is one of the maneuvers that students must master in order to earn their Private Pilot Certificate. The pilot slows the aircraft by reducing the power and maintaining the plane’s altitude by gradually increasing its attitude – or pulling the nose up – relative to the horizon. Once the plane reaches a certain speed through the air as shown on the airspeed indicator, power must be reintroduced in order to avoid a stall – which is simply the point at which the wings no longer produce lift – and maintain level flight at the new airspeed.

On this particular day, he had me slow the plane to about 50 knots – 10 knots or so above stalling speed – and perform slow flight into the wind. Then he had me watch our flight path over the ground, and sure enough, we had slowed enough so that the wind was pushing us backward over the ground, producing a negative groundspeed.

Another way to imagine this is to think about a powerboat going upriver. If you slow the boat enough, the river’s current will be stronger than the boat’s engine output, carrying the boat downstream regardless of the boat’s indicated speed through the water.

Not only was his demonstration a success since I obviously have never forgotten the lesson, it was also a whole lot of fun, and a great way to distract me from the fact that we would have lost a race with cold molasses going uphill. As someone who adores speed in all its forms, I considered this a very welcome distraction indeed.

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