Turbulence and airspeed

Based on the weather and the windshifts here yesterday, my guess would be
wind shear. Tailwind shears to a headwind and your IAS goes up. Wind shear
is also associated with bumps.

Worst I ever witnessed was when flying into a 50+ knot headwind in a C182
and it sheared to a tailwind. I saw my airspeed drop to 80, nose pitched
down, and the ground speed jumped. It sheared back to a headwind,
everything reversed, and we reset the hourglass. I remember watching cars
pass us on the interstate below.

Jim

As an aside: Is your Va close to, or at Vno? (In the old days we called
Va "Rough Air Penetration Speed" now it is called, rather inappropriately
I think; "maneuvering speed". I may be misnterperting your message and if
I am, my apologies. From your message it appeared that you are concerned
about staying out of the yellow arc vs staying in the green arc in rough
air. Can I then I assume your Vno and Va are close to each other?

Crap -- you made me dig out my POH. (That hasn't happened in a while!)

From Wikepedia:

a.. VA: design maneuvering speed (stalling speed at the maximum legal
G-force, and hence the maximum speed at which abrupt control movements will
not cause the aircraft to exceed its G-force limit).

- Va in our plane is 138 mph, and is not depicted on the ASI.

While Vo is:

a.. VNO: maximum structural cruising speed (the maximum speed to be used in
turbulent conditions).

On our plane, that's 156 mph (136 knots), which is the start of the "Yellow
Arc". Since our plane cruises at 142 knots (at 23 squared), we're always
aware of this airspeed.

Now that we've established all this, what did you want to know?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jay Honeck wrote:


- Va in our plane is 138 mph, and is not depicted on the ASI.

Isn't it usually placarded?

It can be, but remember it varies with weight, so it would be placarded at a
specific weight. Higher Va for a higher weight. It was placarded in our
182RG, it's not placarded in our Aztec.
Jim

"B a r r y" wrote in message
et...
Jay Honeck wrote:


- Va in our plane is 138 mph, and is not depicted on the ASI.

Isn't it usually placarded?

- Va in our plane is 138 mph, and is not depicted on the ASI.

Isn't it usually placarded?

Yep. (Well, on our Pathfinder and Warrior it is/was, anyway...)
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

The only part of turbulence I truly DON'T understand is the kind that
tips
one wing up violently. How the heck a "parcel" of air can be so
different
in the span of just 30 feet (our approximate wingspan) escapes me, but
I've
had turbulence push one wing up so hard that it took nearly full
opposite
aileron to remain level.


When a parcel of air is swirling around, the other parcel of air has to
go somewhere to get out of its way. Turbulence is the air (or water,
or any fluid) burbling around all over the place, up, down, left right,
clockwise, counterclockwise, and there are boundaries all over the
place. You crossed several boundaries in succession.

Jose

"Jay Honeck" wrote in message
news:6DIGf.756459$x96.534195@attbi_s72...
Interesting. So what we've always interpreted as an "updraft" is *really*
an increase in relative wind, which (in turn) increases (or decreases)
lift?

That's not an "either/or" proposition.

That makes a LOT more sense to me than the commonly labeled "UPdraft",
which implies a wind from below. True UPdrafts only make sense to me near
the ground, where wind over ground obstacles can create eddies and
currents, much like water in a stream burbles around rocks and other
obstructions.

For orographic uplift (updraft), your statement is true by definition.
However, it ignores convection, which can and does create updrafts that rise
tens of thousands of feet. In either case, what goes up must come down,
figuratively speaking.

A change in relative wind would also better explain the other common type
of turbulence, where the tail is "kicked" to one side or another, creating
that annoying "fishtail" feeling.

Since a change in relative wind and an up (or down) draft are not mutually
exclusive, I don't see how "a change in relative wind" can explain
turbulence-induced yaw better than any other description of turbulence.
They are part and parcel of the same thing.

I do agree that "a change in relative wind" provides a more clear
explanation of what's going on in turbulent air. But it's really just a
more general way of describing the various sources of turbulence that exist.
All turbulence involves a change in the relative wind, but that change can
result from a wide variety of causes.

The only part of turbulence I truly DON'T understand is the kind that tips
one wing up violently. How the heck a "parcel" of air can be so different
in the span of just 30 feet (our approximate wingspan) escapes me, but
I've had turbulence push one wing up so hard that it took nearly full
opposite aileron to remain level.

It's not necessarily the case that your airplane is "one foot in, one foot
out" so to speak. Since you already understand that the turbulence felt is
a result of a change in the relative wind, it should not take much for you
to understand this change can result in the dihedral (physical and design)
to induce a rolling force.

Just as the airplane will return to level flight in calm air if it's banked
a bit, due to dihedral, a change in relative wind can alter the point of
equilibrium, bank-wise. The resulting bank is simply the airplane trying to
follow this new point of equilibrium.

I'd say it's probably pretty rare for an airplane to actually be a little
bit in one parcel of air and a little bit in another (except for gliders,
the pilots of which go around intentionally doing this ).

Pete

Frequently turbulence is circular in motion. So one wingtip is in
the up air, the other is in the down air. It's usually incorrect to
think of turbulence just being an upsurge or downsurge of air. It is in
all directions. This is why Va, although a good idea, does NOT really
GUARANTEE no structural damage. Slowing down helps, but since you can
be hit with oncoming air that almost instantly raises your airspeed (as
well as violently moving the plane up or down), it IS possible to get
structural damage in extreme turbulence even if you are flying at or
below Va.

On Thu, 9 Feb 2006 09:19:53 -0600, "Jim Burns"
wrote:

It can be, but remember it varies with weight, so it would be placarded at a
specific weight. Higher Va for a higher weight. It was placarded in our
182RG, it's not placarded in our Aztec.
Jim


I have read that Va varies with aircraft mass, not weight. If this is
true then Va is not affected by added g-forces in turns, for instance,
but is affected by the people, luggage, etc., the aircraft is
carrying. This is apparently because Va is an acceleration limit, not
a G limit. Interesting if true.

"Jay Honeck" wrote in message
news:ScyGf.763242$_o.277999@attbi_s71...
....
En route we encountered mostly light to moderate turbulence, with long
periods of little "bumps" intermixed with some pretty good jolts. I hit
my head on the ceiling once, and my seat belt was plenty tight.

Having 1:45 to study this uncomfortable mode of flight, we discerned
something about turbulence that we'd not noticed befo Upon entering an
area of more severe turbulence, air speed invariably climbs.


Besides all the other stuff people have written herein, the increased IAS
reading may also be a function of the location of your static port, with
respect to the direction of the stronger gusts.

Recall that airspeed measurement is a function of the difference between
your static pressure and your pitot pressure. A sudden quartering wind
gust, with your static port in the lee of the gust, will increase the IAS
reading. Similarly, a sudden drop in the wind (with the static port on the
windward side of the aircraft) will increase the IAS reading. Until the
aircraft re-stabilizes its crab with the new airflow.