RRRIP

This is not about flying, but it is about the air, which has gotta be worth something. It’s a natural phenomenon you never hear of that I know is real because I’ve experienced it directly, more than once. Surely someone else somewhere, some time, has witnessed this phenomenon too, so I claim no discovery. Yet decades of admittedly casual research have unearthed zero mention of any such thing in publication or conversation. One more proof that being ‘unheard of’ doesn’t make something impossible.

I refer to it as atmospheric thunder for lack of any better notion. You could say ordinary thunder is atmospheric too, but it’s caused by an electrical phenomenon. The thunder in question here has no electrical component, static or otherwise.

Before starting this piece, I looked up ‘thunder’ just to be sure of my terms and, naturally, learned something. As children we were taught that thunder is the sound of air normal slamming together after air within a lightning bolt turns to plasma. Made plenty of sense at the time. But according to Wikipedia, the current theory on ordinary thunder is some kind of shock wave caused by extremely rapid expansion of superheated air. Fine. Now lower your eyes, because here’s where I revive that earlier theory to account for this other, less popular kind of thunder.

I lived for years in a ski lodge tucked below the peak of Vermont’s highest mountain. Between the lodge and the summit loom a pair of near vertical cliffs, really just two facets of one cliff divided straight down the middle by a very sharp edge. One November evening I was standing out front of the empty lodge on the flat below those cliffs and heard a sudden boom that sounded like a shotgun. From up there! And while I hastened toward cover, scanning the skyline, there came another boom, dissimilar and not as loud… Then nothing else.

This raised several questions. If it was a gun, or guns, had the shooters(s) ever seen me? Even if they were crazy enough to shoot at me, a shotgun at that distance was nothing to fear — was it? Also, two different reports implies two gunners, which is another worry…

Or were those sounds caused by something else? And if so, what? Should I climb up there and snoop around — or not?

Thick bouldery woods obstruct the base of those cliffs and dusk was nigh, so I left that for another time and wandered inside… and locked the door to my quarters, which I rarely did, and grabbed a big scary butcher knife on the way to bed. Next morning, I supposed I’d never know what or who was the source of that unnerving noise, and didn’t really care either, so long as it was never heard again.

Then some other late fall, a different roar came from the same cliffs. This time instead of a shotgun blast, it sounded more like coarse sailcloth being torn with great force, RRRIP. And where before one different report followed, this time the same RRRIP was repeated again and again, sporadically. I studied the skyline closely as before, and watched for any kind of movement in trees below the cliff. Nothing. Then RRRIP!

So having simply no choice and ample time before sunset, I scrambled up to take a look. As usual, from calm fifty feet down the trail, wind across the ridge was a robust thirty something, gusting with a punch common only to mountaintops. I stepped, uneasy with that wind at my back, to the verge of those cliffs, went down on my belly and crawled until my upper torso was out over the edge, right between the facets.

Bingo.

The air was flowing straight over the cliffs, more or less. Hard one minute favoring this face, then jibing to the other, then back. The first RRRIP tore my hat off, to never be seen again. Not just hundreds of yards closer to the roar, here my head was inside it. At a gut level it was terrifying, but I was too enthralled to turn away. Every next RRRIP though, made me more anxious to leave. Then came the coup de gras.

An especially intense gust formed a cloud of condensed vapor that lasted only a second or so, more than long enough to sere the image in my mind forever. The cloud shot out perpendicular to the cliff in the shape of a medieval jousting lance, twenty feet or so, right below me. Visual evidence of a powerful vortex that was probably there all along. And that RRRIP was more explosive too.

Good time to get my scrawny tuchus off them battered rocks before AEOLUS reached again for the shotgun…

gremline.com

After our winch clinic this past weekend I stumbled across an organization in Great Britain that you may have already known about, but I didn’t. Gremline.com is a non-government online magazine dedicated to aviation safety. (Why it’s named gremline isn’t clear, but it might be an interesting story.) 

John Stewart-Smith is one of the principles, with a heavily stacked resume. Presumably he wrote the essay excerpted below, though there was no attribution. In any case, these snippets from a much longer piece offer further discussion of winch launching to augment the information we soaked up this past week.

A stall during the transition from takeoff to the main climb on a winch launch may result in the glider rolling uncontrollably… A stall during pitch rotation can result in one wing losing lift marginally before the other, causing it to drop. The stalled wing has an increasing angle of attack as it drops, keeping it stalled. The rising wing has a reduced angle of attack, moving it away from the stall and allowing it to produce lift. This induces a rapid rolling moment and can lead to autorotation and a spin.
The stall speed of a glider increases during rotation in pitch as a larger angle of attack is required to achieve more lift. More lift is required to balance the other forces on the glider and to provide a vertical acceleration into the climb. There are three reasons for this:

1. As the nose pitches up the lift force is inclined away from the vertical and must be increased if the vertical component of lift is to balance the weight of the glider.

2. The pull force on the cable is large, typically 80% of the weight of the glider. At takeoff this force is horizontal, providing the glider’s initial horizontal acceleration. As the nose pitches up during rotation the lift force becomes increasingly opposed to the pull force. The lift must therefore increase if it is to balance this pull force and stop the horizontal acceleration.

3. At the end of the rotation the glider is climbing at about 55 kt, which gives a vertical velocity of about 35 kt. The vertical velocity of the glider must therefore increase during rotation from zero at takeoff to about 35 kt. This requires a force which comes from an increase in lift generated by the wing.

The forces on a glider during rotation may be modelled and the load factor (g) estimated for different rotation rates, pull forces on the cable, climb angles and other variables. This modelling shows the stall speed during rotation is very dependent on the RATE of rotation, i.e. the higher the rate of rotation, the shorter the time in which the glider has to be accelerated vertically from zero to 35 kt. As this requires a greater force from the wings, there is an associated increase in the stall speed.

The Safe Winch Launch is an ongoing initiative by BGA. It is recommended that all glider pilots get a copy of the “Safe Winch Launching” leaflet (downloadable from the excellent BGA website) and study it carefully.

CALL IT MELLOW YELLOW

We all should know better than to think we’ve seen everything, but I confess sometimes I begin to feel that way. Spring of 2008 in central Oregon is one example. The landscape there is expansive like ours in the Mojave, yet very different. To the west lie a string of 10-K volcanoes snow capped all year, and to the east, high stony desert. In between, the Deschutes River valley is a carpet of vivid green from aloft, billions of ponderosa pines so uniform they look like a thousand square miles of newly mown grass. Grass a hundred feet high.

Down beneath that canopy an open air cathedral, sweetly shaded, disappears beyond sight in every direction. In spring, soft clouds of pollen drift from boughs at the slightest puff, wafting everywhere the full bouquet of God’s own candle shop. Downright heavenly unless folks have allergies; for them it’s just a gorgeous version of hell with extra mosquitoes. Hatchoo.

There’s also a tantalizing visual effect you’ll never see anywhere round here. Early in the day before thermals are soarable, thousands of baby sized pollen devils, bright yellow instead of brown, rise all across the forest. Their multitude does not mean more or better lift, it’s evidence of incipient convection with the strength to raise only pollen, not dust. Gliding over them low, you feel nothing. I proved this, to considerable dismay, more than once.

Real dust devils are totally different. Typically miles apart or in furious clusters at the base of a shear line, they occur mostly during the hottest hours of the day. Dust is comprised of superfine rocks, relatively heavy stuff that requires great energy to be carried high aloft. Pollen is nearly weightless, designed to float in the slightest movement of air. Once above the trees, this super light particulate disperses so rapidly, a dense pollen haze develops below 2000 AGL at the onset of a thermal day. That low-level obscuration is not dense enough to greatly diminish sunlight reaching the surface, but neither does it help.

At the end of one sneezy week, a heavy thunderstorm washed unnumbered tons of pollen from the air and trees and everywhere else it had fallen, all together onto the ground. By dark that evening cars looked as if kids had Halloweened them, and for several days afterward, low spots that collected rainwater held puddles of bright yellow paint. Ah, so much for allergies!
But the mosquitoes? They just fed on that like they feed on everything. Price of doing business.

AND ALL HECK BROKE LOOSE

One day I was asked to take our Baby Grob up on a quick return-to-service hop. Nothing special, just once around and back down so a renter could have the bird for the afternoon. It had been a calmish morning, but by noon the air was rustling more each minute. Situation normal.

The takeoff started sleepy, then by mid-field was barely manageable. During liftoff I noticed a sudden dustup a couple hundred yards to the immediate right — keeping pace with us! What? That by itself seemed worthy of analysis, but things were changing so fast I had no time to think. It would have been wise to release right then, less than fifty feet up, but in fact I welcomed the unique experience.

After two recoveries from slack I knew more was coming, so when another devil bloomed on the right I released and rolled toward it, not 800 feet up. Just then one more sprang from below on the left. Seconds off tow I was diving through sink between two whirlies with zero altitude to spare. No problem, still essentially over the runway with no traffic, I could land whenever necessary. Yet, hazardous and challenging as the launch was, why go back down in the same chaos if I could stay up and avoid it?

A tight turn in just the right spot regained more height than I’d lost, and for half a circle it felt like the real thing. Then came that sucking feeling as my parcel of air was overpowered by a bundle of other pressures and pulled into a sharply different wind direction. Devils were forming and dissipating everywhere, moving opposite directions and changing directions so quickly, there was no keeping track of them.

I watched the towplane disappear moments before touchdown as a jack-in-the-box plume of dust exploded in its path, and sighed relief when it emerged in full climb. A minute later someone came on the Unicom to announce flight operations temporarily suspended. Good, the pilot waiting to use this bird wouldn’t be needing it right away.

For what felt like an hour I darted from one plume to the next, seldom more than a thousand feet above ground while this hyperactive interface between hot air masses swirled across the field. Flying continually from one rotational wind into another, and then another – low – was dizzying. At any point I could have climbed quickly and soared away, but that was not the point. I needed to know more about what was happening here and now, for next time.

As the storm moved on and abated I began to comprehend a bigger picture. This was one of those huge megadevils, the kind you normally see only in the middle of wide lake beds, a rotating ring of ordinary whirlies a half mile across.

Why did it boil up right at Crystal airport, and seemingly out of nowhere? Got me. But since you asked, discrete shear lines intersect often in these parts, quite benignly. On rare occasion though, they powerfully collide in a kind of atmospheric triple whammy. When I’ve been already aloft and that happened, the causes were obvious. Prior to this freak show however, I was on the ground and there were no clouds anywhere until…

Things settled out for a routine landing as the day’s only cumulus formed not far downwind, thank you very much. The flight was brief, never got a mile from the field, but I learned more in those twenty minutes than many whole days sailing high and dry in ‘ideal’ conditions. What more could anyone want?