Wednesday, August 14, 2013

Critical wind, LTE, and me

Winter's over here in Southern Africa. And apparently, we don't bother with Spring, just crank the heat and get down to it. Twenty or more species of birds returned on Sunday with more following every day. Now when the wind blows, it's refreshing in the way a hair dryer on low is refreshing while standing in full sun wearing overalls and a ski mask.

It is nice to have some wind again to fly in. The breeze cranks from the south until about halfway into my first scenic flight of the day when it switches and comes from the north, usually while I'm turning over a crocodile.

North winds make landing at the Xaxaba pad interesting, especially with a full load and DA over 6000ft. I came in on approach day before yesterday with a 20kt wind from two o'clock and did a go-around before I ran out of pedal. I had to go to 800 TOT and Gregory Hines the pedals to pull off the tailwind landing.

I thought about loss of tail rotor effectiveness(LTE) while doing my post-flight. The term was literally invented for the helicopter I fly. The U.S. Army did extensive tests and created the first literature on the subject. I still have lots to learn and because I went right into "real" helicopter work as opposed to instructing, I don't have a support group to discuss theory with over a cup of coffee while waiting for the next student to show up.

But this wasn't LTE, cuz the wind was right quartering headwind, not left. It had to be Critical Wind Azimuth. The Bell pilot operating handbook talks about the critical wind azimuth occurring at high altitude, be aware of slow turns to the right, out of ground effect hovering, etc. I can tell you that it's not just an issue in thin air or while hovering.

Those trained in American helicopters are taught to fear wind from the left and directly behind via the LTE diagram. Here's the thing, if you lay a critical wind azimuth diagram over an LTE chart, there is almost no good wind direction. Maybe a better way to think about it is wind speed relative to the power of your tail rotor. If the wind is cranking, take it on the chin or up the poop-shoot.

I didn't have time to over think why I was losing control of my tail, the cool thing is that no matter which issue you're dealing with, recovery is the same. Lower the collective push into forward flight and give it another go.

On another note, I saw a vervet monkey kill a page 47(you were right, Bridget, it was a crested barbet). Who knew they ate birds?

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