RE: Tough call at Chehalem

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ma...@mgforbes.com

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Nov 25, 2014, 8:17:39 PM11/25/14
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[Steve wrote...]
Sat on launch for an hour debating and then decided if it's
that hard of decision it would be best not to fly. I would bet
that more experienced pilots would have had a great day. Winds
were steady about 10-12 sometimes gusting to 14-15. Locals were
parked up high but easy penetration down lower on the ridge. Was
thinking about launching a lull and playing down lower but the the
thought of getting plucked into the higher winds above launch won
out. Very calm down below in Newberg. Wish I would have gotten
out a little earlier.

= = = = = = = = = = =

Your observation about Newberg is the key here. Conditions are
inverted; cool, stable air on the valley floor, warmer and
windier conditions aloft. The transition between these can
be VERY abrupt. In that transition zone you'll find significant
turbulence. Some of that turbulence and rotor is mixing down
to launch level, contributing to the gusts you observed.

From a ridge soaring perspective, the risk is that you're going
to launch into marginally-soarable conditions on the ridge, and
as soon as you climb a little bit, you're going to be in high
speed air getting blown over the back into rotor. It all depends
on just how high the inversion is. This sort of weather is
typical in the valley during winter, particularly in the north
where cold air pours off the plateau, down the gorge and into
the metro area. You may remember the forecasts talking about
this last week; an approaching warm front overriding the cold
pool, but the cold air being deep and stable enough to resist
scouring out for a while, resulting in freezing rain and icy
conditions during the transition.

You'll see this at the coast during the summer, when the cold
ocean surface chills the air. Aloft, it's blowing strong,
but down on launch it's barely a zephyr, and at the LZ it's
dead calm. There's a deep layer of cold air stalled against
the coastline and the warm winds aloft are blowing over the
top of it. The transition zone can be as little as a dozen
feet deep, from "too light to soar", to "AAAHHHHHGGG" as
you get dragged over the top of Kiwanda dune.

This sort of thing becomes really noticeable when flying the
trike. I often see light winds on the surface, 40mph indicated
airspeed and 36-37 on the GPS. As I climb above treetop height
there's some bumping around and now the GPS is indicating 25.
Climb to pattern altitude and I'm showing 15-20 on the GPS,
the runway is barely moving underneath me and it's warmer
and smooth. Claw my way out to the crosswind turn, then
downwind and I'm doing 55-60 over the ground. Moments later
it's time to turn base, crab toward the runway and line
up for final. As I get down to 150AGL, I get thrashed
around pretty good, and by the time I'm on the deck and
flaring to land, it's nearly calm.

MGF

Steve Carpentier

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Nov 25, 2014, 11:08:03 PM11/25/14
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Thanks Mark,  That makes a lot of sense and it was interesting to learn how it can have a similar effect at the coast.  It was really interesting to watch and after reading your explanation I am glad I didn't decide to launch.  Getting blown back at Kiwanda is one thing, Chehalem is another!
- SC


ma...@mgforbes.com

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Nov 25, 2014, 11:46:10 PM11/25/14
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Some nice, inverted day this winter, we should go flying in the trike
and
explore the different layers in the atmosphere. HG, PG and trike are all
different, and all good in different ways. It's really cool to be able
to
fly in what feels much like a glider, but with the ability to climb to
a chosen altitude and location without the limitation of finding thermal
or ridge lift.

If I'm headed out to fly, I'll post to the list. The trike is based at
Lenhardt Airpark, three miles east of Hubbard at the corner of Whiskey
Hill and Meridian Roads.

MGF

Steve Carpentier

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Nov 26, 2014, 12:02:34 AM11/26/14
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Cool, I would love to tag along on the trike if you have a spot open.  Thanks mark.


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