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.

Hey Jay:

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?

Regards,

Watson


"Jay Honeck" wrote in message
news:ScyGf.763242$_o.277999@attbi_s71...
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"

"Jay Honeck" wrote
We would set the throttle and prop RPM so that we would be clearly out
of the yellow arc,

Damn, Jay, I'm plum proud of you son! You didn't slow to Va. :-)

Bob Moore

"Watson" wrote
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".

The 'Maneuver Envelope' with which the average pilot is familar,
and which contains Va, is not the same as the 'Gust Envelope'.


Section 23.333: Flight envelope
(c) Gust envelope. (1) The airplane is assumed to be subjected
to symmetrical vertical gusts in level flight. The resulting
limit load factors must correspond to the conditions determined
as follows:

(i) Positive (up) and negative (down) gusts of 50 f.p.s. at VC
must be considered.........

(ii) Positive and negative gusts of 25 f.p.s. at VD must be
considered...........

Quoted from Aerodynamics for Naval Aviators:

"As a general requirement, all airplanes must be capable of
withstanding an approximate effective +/- 30 foot per second
gust when at maximum level flight speed for normal rated power.
Such a gust intensity has relatively low frequency of occurrence
in ordinary flying operations. The highest reasonable gust
velocity that may be anticipated is an actual veritical velocity,
U, of 50 feet per second."

Bob Moore
ATP, CFI

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"