Monday, July 25, 2005

Bikes Don't Fly!

But I do fly. Again. You're all correct. I haven't forgotten how to fly. After half an hour I forgot about my back. After two hours I felt my back again, but it didn't stop me from doing my job.

I encountered some performance increasing windshear on landing today. That means that the wind was blowing more strongly towards me close to the runway than it was at a higher altitude, so that as I descended, I suddenly hit a stronger headwind.

Now, if you were riding a bicycle, working against a headwind would decrease your progress. The same is true of an airplane: the stronger the headwind, the slower the progress over the ground. If an airplane that can fly at 200 miles per hour in still air had a wind from behind at 20 miles per hour, its speed over the ground, called groundspeed, would be 220 miles per hour. The groundspeed is indicative of time to destination: stronger headwind, longer time to get there, stronger tailwind, get there more quickly.

So why did I say that suddenly encountering a stronger headwind was performance increasing? Despite all the encouraging comments attached to the last entry, an airplane is not a bicycle. It doesn't push backwards on the ground as it travels, so as it flies through the air, it doesn't know about the ground going by. Although the steady tailwind described above carries the airplane over the ground at 220 miles an hour, the speed of the air passing by the airplane is still only 200 miles per hour. And for the purpose of how the airplane handles in the air, how it feels to the pilot, and whether it is within its safe speed limits, the speed relative to the air, the airspeed, is what matters.

In preparation for landing, the pilot slows the airplane to the correct approach aispeed, based on weight, flaps and other considerations. If there is a strong headwind, this may give a slower than usual groundspeed, but that's okay. We actually would like to have the slowest possible groundspeed for landing, because it makes it easier to brake to a stop. We can't go too slowly, however, because the lift requireed to hold the airplane up depends on airspeed. The approach speed is therefore a compromise between maintaining a high speed through the air and obtaining a low speed over the ground.

Now back to the bicycle, because I have devised a way to continue talking about bicycles. A bicycle speedometer measures how fast the wheels are going around, i.e. the rate the bike is passing over the ground. Imagine that you rigged a bicycle speedometer worked by having a little propeller on the front, so that it measured not how fast you went over the ground, but how fast you went through the air. And imagine that you came around a corner, and suddenly encountered a blast of headwind. Your propeller would momentarily speed up to the sum of your speed PLUS the speed of the oncoming wind, and your eyes would water with the sudden onslaught of air. The added air resistence would then quickly slow you down, but for that moment, the speed of the bike through the air is increased.

So the same thing happens to the airplane encountering a sudden increase in headwind. Momentarily the air flowing over the surfaces is increased. This increases lift, resulting in a combination of a lower descent rate (or a climb) and increased airspeed. And that's what happened to me. I found myself higher than I wanted to be, and faster than I wanted to be, but with a long runway, so the landing was long and ugly but safe.

I should have realized that the winds aloft were lighter than the winds at the field and expected it, but I didn't come straight in: I'd been turned for that runway at quite short notice. I'll do a better job tomorrow.


Lord Hutton said...

At least you got back in one piece.

sweavo said...

Beats anticipating a gust and not getting it!

Ah, I'll just cut the engines and point at the ground, nice gust of headwind should give a bit of lift and reduce groundspeed!

I was having "fun" with airspeed at the weekend with a kite. Got it aloft on a beach but the air was turbulent and people on the beach not very aware. They'd wander into the area beneath the kite and the kite would be pointing, and travelling, directly at their heads... I'm holding the control lines in the "do not crash into person's head" position, but it was up to the wind whether it was going to fill the kite and cause it to follow my instructions.

Damn beach was deserted when I started rigging the kite, too