Turbulence and airspeed

Mary and I flew to West Bend, Wisconsin (KETB) today. It was supposed to be
CAVU all day, except along the Lake Michigan shoreline (where they were
getting stomped with lake-effect snows all day), but, of course, the
forecast was just a bit off, as it often is in winter.

From Dubuque (DBQ) to Madison (MSN) we ended up flying at 3500 feet under a
broken-to-solid overcast. With an artic cold front settling into the upper
Midwest, the winds were howling pretty good, flipping around from 020 to 310
at 19 knots when we landed in West Bend.

It was a good, but busy, landing...

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. Since our
Pathfinder (with all its speed mods) usually cruises well into the yellow
arc, this can be a real problem.

We would set the throttle and prop RPM so that we would be clearly out of
the yellow arc, and then -- just when you DIDN'T want it to climb higher
(i.e.: when hitting moderate turbulence) -- the airspeed indicator would
almost instantly jump into the yellow arc. Sometimes it would stay there
for a minute, and we'd have to reduce power to get things back in line.

We debated this phenomenon for some time, as it didn't seem to make sense.
Why would the airspeed jump when encountering turbulence? It *feels* like
an updraft, when it slams your head into the ceiling, which seems like it
should result in either a drop -- or no change -- in airspeed. And
wouldn't you think turbulence would be equal parts up- and down-drafts?

One possibility we considered: Perhaps, upon encountering an updraft, we
were instinctively pushing the nose over, to hold altitude. This would, of
course, result in an airspeed increase.

However, as soon as we postulated that theory (man, we have some
*interesting* husband/wife conversations, no?) we were able to test it, and,
no, that wasn't the case. With the yoke held rock-solid in place during
turbulence, indicated airspeed still increased.

Another likely possibility: The airflow over the pitot tube is being
disrupted, and making it LOOK like the airspeed is jumping into the yellow
arc. A good test of this would be to see if GROUND speed increased, too,
but the GPS doesn't update quickly enough to tell for sure. And the fact
that it would sometimes stay in the yellow arc for more than just a few
seconds seems to negate this possibility.

So what's happening here? Is the airspeed REALLY increasing in the bumps,
or not?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

OK, I'll take the first shot...
During an updraft (when you feel heavy), the rising blast of air hits
two major surfaces: your main wing, which is near the center of lift,
and the horizontal tail surface, which is way aft. The rising air will
therefore push the tail up, and that means nose down, to some degree.
The nose-down attitude will gain you some speed, since you say you are
holding the yoke steady (as opposed to holding the airplane level).

'Course, this can't work for long, since eventually you will have a
down-gust, which will reverse all that--you'' be nose high and slowing
down.

If you only run into the updraft gusts, consider getting a glider!!!
;)

Assuming that the turbulence consists of a series of up- and
down-drafts, each bump creates a change in the angle of attack, which
would change the airspeed reading. However, I'd expect it to go both
up and down, not just up unless the bumps were generally in the same
direction. They could be unidirectional if you were flying just above
or below a boundary layer, which often occurs in the midwest.

Paul kgyy wrote:
Assuming that the turbulence consists of a series of up- and
down-drafts, each bump creates a change in the angle of attack, which
would change the airspeed reading. However, I'd expect it to go both
up and down, not just up unless the bumps were generally in the same
direction. They could be unidirectional if you were flying just above
or below a boundary layer, which often occurs in the midwest.

Is a 'bump' an updraft, or is it an increase in headwind? The headwind
would make the plane go up and also appear faster, while a drop in wind
makes it lower and slower. The plane may be stable in such a way that
the drops cause the nose to go down and recover speed, but the lifts
do not (as much) cause the nose to rise. The net result would be more
airspeed.

John Halpenny

The air could be cooler or more dense giving a higher speed reading.
Just a guess.

Hmm. Interesting. I'll take a wild-ass guess too.

Turbulence is essentially inertia. That is, when an object's medium
changes, the object experiences acceleration or deceleration until the
medium and object are in balance again.

When the air mass changes, the airplane accelerates in some direction
(up, backwards, sideways, whatever) to match it. If the air mass is
constantly changing (unstable air), the airplane is constantly
adjusting (bumpy ride).

I wonder if the airspeed was reading higher because it actually was
higher, i.e. you entered an air mass which had an average higher speed
relative to the direction of the airplane.

I think this would apply to what would feel like an updraft. The
second most important thing that an airplane does is provide lift when
you give it airspeed. A sudden supply of airspeed would give a sudden
burst of lift, and feel like an updraft. In fact, I think that would
be a lot more efficient way to lift an airplane than blowing air up
from underneath it at any speed.

I'll throw this bit of empirical observation in too: If you take an
airspeed indicator out of an airplane, and hold it in your hands and
shake it, it will indicate changes in airspeed.

I think this would apply to what would feel like an updraft. The
second most important thing that an airplane does is provide lift when
you give it airspeed. A sudden supply of airspeed would give a sudden
burst of lift, and feel like an updraft. In fact, I think that would
be a lot more efficient way to lift an airplane than blowing air up
from underneath it at any speed.

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 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.

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.

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.
--
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.