Tuesday, August 26, 2008

Express elevator, going down

The other day my instructor and I flew over the shoulder of the volcanic ridge that crowns the island of Hawaii. It was a spectacular flight that took us to the heavens at five thousand feet. The chilly air dimpled our skin, wisps of cloud fluttered at our feet, and stark sunshine streamed into the cockpit. Cows in the fields below looked like pepper spilled on moss.

On the other side of the ridge, I was happily filming some video when he hit me with the first of two surprises that day.

"Autorotation in three-two-one."

Without even giving each count a full second, he flattened the rotor blades and cut the throttle to enter an engineless descent. I was caught a bit by surprise, and you can see the lurch in my footage as gravity is momentarily lost.

"You asshole," I jabbed, jokingly. I tend to curse when I get suddenly spooked by heights, so much so that my first instructor instituted a three-expletives-per-flight rule.

Besides being used for emergency landings, autorotation is the express elevator down in a helicopter. He was using it to quickly lose altitude down the long slope to our next objective, the seaside cliffs of the northern shore. Halfway down, it became apparent that our descent was too steep to clear the cliff edge and glide out over the sea.

Rather than re-engage the throttle, he chose to demonstrate a peculiarity of the helicopter's design. By allowing rotor rotation to slow to 90% of normal RPM, a Robinson R22 pilot can maximize the glide angle in an autorotation, buying more horizontal distance per altitude lost.

It's a maneuver not to be taken lightly. At any value below 97%, the Low RPM Horn begins wailing, alerting the pilot of the vulnerable condition. At any value below 80%, the rotor stalls, and you fall out of the sky.

So there we were, falling almost 2000 feet per minute without any engine power, the rim of a cliff looming to cut short our approach to the sea, and the Low RPM Horn lamenting our imminent demise. I'm proud to say I wasn't really fazed; I just kept filming.

My instructor's tactic worked, and we cleared the cliff with room to spare. I've shared a short clip below. As you watch it, notice how our altitude changes relative to the clouds as we float down from a mile high to nearly sea level.



I'll share my instructor's second surprise that day in another post.

2 comments:

sundayrider said...

I just wanted to say how much I enjoy reading your blog. I had my first lesson in an R22 last month and was immediately hooked, but I share all your reservations and fears! It's so reassuring to read of someone else going through the same thing. Most flying blogs are very technical and miss out the whole emotional element of learning to fly.
I've done 3.5 hours now, not sure how far I'll take it, but I'm sticking at it so far!

Jen

Ehud Gavron said...

Great comments.

In a Robinson R-22 the minimum %age of survivable rotor RPM is 80% PLUS 1% for every thousand feet of density altitude.

Those of you who fly around Hawaii and LA don't need to take that into consideration. Those of us who fly in Tucson Arizona (2640MSL and in the heat of the summer up to 6500ft dens. alt.) do :)