Turbulence and airspeed

[Jay Honeck]

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 before: 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"

[[email protected]]

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!!!
;<)

[Paul kgyy]

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.

[[email protected]]

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

[Aluckyguess]

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

[Brien K. Meehan]

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.

[Watson]

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 before: 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"
>

[Bob Moore]

"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

[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

[Jay Honeck]

> 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"

[Jim Burns]

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

[Jay Honeck]

> 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"

[B a r r y]

Jay Honeck wrote:

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

Isn't it usually placarded?

[Jim Burns]

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?
>

[Jay Honeck]

>> - 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"

[Jose]

>>
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

[Peter Duniho]

"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

[Doug]

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.

[Jim]

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.

[Icebound]

"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 before: 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.

[Scott Draper]

<<Is the airspeed REALLY increasing in the bumps,
or not?>>

1) I doubt it's a pitot tube issue. Studies have shown that Pitot
tubes are very accurate until beyond the stall AOA.

2) A reduction in static pressure would produce the same phenomenon
and occurs more easily than a Pitot tube problem.

3) Possibly a function of the static longitudinal stability of an
aircraft. In response to an increase in AOA, the natural stability is
to pitch down.

4) While it's true that there is horizontal windshear, the literature
on aircraft structures assume it's minor compared to vertical
windshear. Still might be enough to affect the Pitot tube, but on
average you'd think it'd average to zero.

5) I'm no weather expert, but the air does accelerate and decelerate
as it moves to and from pressure systems. Seems reasonable that
changes in velocity would generate turbulence, rather than vice versa.
Perhaps a different weather system would generate different results.

[Jay Honeck]

> 5) I'm no weather expert, but the air does accelerate and decelerate
> as it moves to and from pressure systems. Seems reasonable that
> changes in velocity would generate turbulence, rather than vice versa.
> Perhaps a different weather system would generate different results.

And we *did* have a big cold front moving into the area.

Thanks for the input.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[[email protected]]

The V-g diagram is usually a good representation of this information.
The best SINGLE diagram I've found, IMO, is in the Jeppesen Instrument
Pilot Manual.

For Va (maneuvering speed), the angle of attack changes with speed and
load. If you are flying slower, your are at a higher angle of attack.
A gust from the front or below will increase the effective angle of
attack, and, before the lift increases enough to do damage, a stall
will occur (at least momentarily).
However, if the gust is strong enough from below, you can damage the
wings even if you are just dangling from a wire. The force on the
wings isn't from too much lift--it's from just blowing the wings off.

AvWeb had a discussion about this a few years ago; the information
might still be in their archives.

Flying Magazine, June 1996, page 106 had another fascinating article on
this as well.
Frequently, we seem to be taught that below Va, we can move the
controls to full extreme without damage. Well, flight 587 in New York
straightened us out on that. You can't go from one extreme to the
other repeatedly.

Another source of info is NTSB Safety Recommendation dated February 8,
2002.

A few years ago, I did a minor Civil Air Patrol seminar on this
topic--not in depth, just about 15 minutes or so. I have a very
thorough Vg diagram I put together from several different sources. If
anyone is really interested, I can try to dig it out of the archives;
it is a powerpoint slide, though quite detailed. I have no idea how to
put it up on the newsgroup, so if anyone IS interested and knows how, I
can email it to them.

[Dylan Smith]

On 2006-02-09, Jay Honeck > wrote:
> 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.

Not anywhere near correct, I'm afraid, as any glider pilot can tell you.
Thermals also qualify as 'updrafts', and I've spent many hours being
kept aloft by these updrafts. Even with our weak lift here, I've got my
glider to 5,300 feet on these, and in Texas I've been at over 8,000 feet
AGL. Some soaring sites get thermal lift up to 12000' AGL. Wave lift
(which can be considered an updraft, as there is a vertical component to
the air) can reach well into airliner altitudes. Gliders at Minden
regularly reach FL300 and higher.

> 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.

Again, try some gliding in the summer to understand this better. Quite
often in a glider, you feel one wing rising faster than the other - you
bank into this rising wing because this is where the strongest lift is.
Small, strong thermals can have a very marked boundary and it's quite
easy to have half the plane inside the thermal and half of it outside.

--
Dylan Smith, Port St Mary, Isle of Man
Flying: http://www.dylansmith.net
Oolite-Linux: an Elite tribute: http://oolite-linux.berlios.de
Frontier Elite Universe: http://www.alioth.net

[Jay Honeck]

> A few years ago, I did a minor Civil Air Patrol seminar on this
> topic--not in depth, just about 15 minutes or so. I have a very
> thorough Vg diagram I put together from several different sources. If
> anyone is really interested, I can try to dig it out of the archives;
> it is a powerpoint slide, though quite detailed. I have no idea how to
> put it up on the newsgroup, so if anyone IS interested and knows how, I
> can email it to them.

Email it to me at I'll upload it to the binary
channel (alt.binaries.pictures.aviation) you ya! (Pictures are verboten
here...)
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Jay Honeck]

>> 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.
>
> Not anywhere near correct, I'm afraid, as any glider pilot can tell you.
> Thermals also qualify as 'updrafts', and I've spent many hours being
> kept aloft by these updrafts.

Understood, but I'm making a distinction between "lift" (which is a
consistent area of "updraft") and "turbulence" (which is an inconsistent
area of "updraft" or varying relative wind, i.e.: wind shear).

The line is fine, admittedly, but the sky is complex enough to require it.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Doug]

I'll give you one piece of advice that I have learned by experience. I
fly out of Boulder, CO. West of Boulder is the Continental Divide
rising to 14000+. The prevailing west winds come over the ridge and on
the east side of the ride there is unseen pockets of 'rotor' type
turbulence. Going west you are climbing and are slow, so if you hit
them it's not too bad. But coming east, you are descending. Pilots need
to keep their speed down here. It is easy to point the nose down and
gain speed. Sometimes, except for these turbulence pockets, the route
is smooth, so that doubles the temptation to come down fast. When you
hit the pocket of turbulence it is usually just one or two "thwaps",
like giant hit the top of the wings with a big flyswatter. Then smooth
again. This is one place where keeping an eye on Va is essential.

[[email protected]]

Thanks, Jay . I did so.
I'm on google notes, and apparently it does not carry alt.binary
groups.
Probably for good reason, but in this case it's a bummer.

[gyoung]

I have a problem with most of the answers posted here because I don't
think they reflect the physics of flight.

First, tho', here's the way I picture turbulence.

Turbulence occurs when an aircraft moves from a volume of air moving at
a certain velocity into air moving at a different velocity, and the
transition from one volume of air into the other occurs quickly. It is
important to look at velocity as the combination of speed -and-
direction relative to some reference system. (It is helpful to me to use
a reference system external to the airplane.) So, turbulence may be
caused by updrafts and downdrafts (vertical movement of a body of air
relative to the air surrounding it), or by rapid changes in horizontal
velocity (like the 'swirling air' or 'burbling air' that others have
described).

Another situation in which an airplane may be 'tossed around" is when
one part of the airplane (say the left wing) is in a body of air that is
moving at a different velocity than another part of the aircraft (say
the right wing). Such differentials in velocity aren't likely to exist
for a very long time over such short distances, so they cause a form of
turbulence. So let's say the left wing goes into an updraft, passes
through it quickly, then returns to air that is moving the same velocity
as the right wing; the aircraft will jerk towards the right.

Now to other explanations of the airspeed question: One principle I
believe applies is the First Law of Physics" - "Conservation of Energy"
(see http://en.wikipedia.org/wiki/Conservation_of_energy"). In level
flight, and at a constant power setting, an aircraft whose airspeed is
disturbed -will return- to the airspeed at which it was flying before
the disturbance. The return to that airspeed will be delayed by the
time required to accelerate (or decelerate) until the thrust (power)
matches drag. The amount of time required will be related to the mass
of the airplane and the difference between power produced by the engine
(thrust) and the drag of the aircraft.

In the short span of time during which the aircraft moves from air
moving at the first velocity to the second, the velocity of the air
relative to the aircraft (angle of attack and/or yaw) will change
abruptly, the occupants will feel 'turbulence', and the aircraft will
find itself in a different flight condition. But assuming there isn't
another change in the airmass velocity surrounding the aircraft (or a
change in power setting or trim), it will return to the same flight
attitude as before the change. When there are repeated, frequent
changes in airmass velocity, it will be difficult to observe the effect,
but it -is- a Law of Physics.

As for the proposed explanations that address the pitot/static system:
Once the aircraft has returned to the original airspeed, and assuming
there were no changes in power, trim, or significant change in gross
weight, the plane will be going through the air at the same angle as
before the 'perturbation' - it will be flying at the same angles of
attack and of yaw. So the air pressure sources will be seeing the same
air as before which should result in the same readings on the instruments.

But back to Jay's question: What might explain why airspeed increased
in turbulence? Here's another idea - a phenomenon described in the
April/May 2005 issue of Air & Space Smithsonian, an article that
discusses flying sailplanes and an phenomenon they call "dynamic
soaring". I really don't understand it well, but it seems to be that
one can 'gain energy' for the 'aircraft system' by "exposing the
airplane's belly to stronger winds" for brief periods of time, flying
back into winds not so strong, returning to the stronger winds, and
going back and forth. So, I guess the airplane extracts some energy
from the stronger winds (weakening them I assume), and uses that energy
to go faster (or in the case of sailplanes, stay aloft longer). What do
you think?

George Young
T-34, Comanche, C-182/172/152, Mooney, and Arrow pilot

[[email protected]]

Jay put this on his website:
http://makeashorterlink.com/?S3CF25F9C

[Bob Moore]

wrote in news:
> http://makeashorterlink.com/?S3CF25F9C

However the current FAR Part 23 gust requirements have changed
from those (30 fps) shown on the posted Flight Envelope. The current
requirements are as below.

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 at altitudes between sea level and 20,000 feet. The gust
velocity may be reduced linearly from 50 f.p.s. at 20,000 feet to 25 f.p.s.
at 50,000 feet.

Bob Moore

[[email protected]]

Thank you--especially for paying attention to the details. I don't
have a copy of part 23.
I'll update my diagram.

The primary point of the whole exercise was that there was much more to
the Va issue than just "slow down to Va and you can do what you want".
I was very surprised at what I learned when I was challenged to look
into the details.

[Jim Macklin]

Va is pilot input to the controls, Vb is turbulence.

FAR Parts are all available at www.faa.gov



--
James H. Macklin
ATP,CFI,A&P

--
The people think the Constitution protects their rights;
But government sees it as an obstacle to be overcome.
some support
http://www.usdoj.gov/olc/secondamendment2.htm
See http://www.fija.org/ more about your rights and duties.


> wrote in message
ups.com...
| Thank you--especially for paying attention to the details.
I don't
| have a copy of part 23.
| I'll update my diagram.
|
| The primary point of the whole exercise was that there was
much more to
| the Va issue than just "slow down to Va and you can do
what you want".
| I was very surprised at what I learned when I was
challenged to look
| into the details.
|

[Doug]

I hit some bigtime thwaps between Carson City and Lake Tahoe once.

[Jim Macklin]

We can still try. Reminds me of the joke about teaching a
pig algebra. Wastes your time and annoys the pig.


"Bob Moore" > wrote in message
. 121...
| "Jim Macklin" > wrote
|
| > Va is pilot input to the controls, Vb is turbulence.
|
| I've been preaching this for years Jim, to no avail in
this
| group.
|
| Bob Moore

[Doug]

Well, Va is placarded on the instrument panel and is in the POH in most
planes. How do I find out what Vb is for my airplane (a 1996 Aviat
Husky)?

[[email protected]]

Jim Macklin wrote:
> Va is pilot input to the controls, Vb is turbulence.

To learn something new, all I need to do is try to show I already know
something. :<)

I'd never run across Vb before. I'll figure out how to include that.

Thanks again.

[Jim Macklin]

The list is in FAR Part 1. It is changed every so often, as
new terms are added. From Google
Vg Diagram
Vg Diagram. The flight operating strength of an airplane is
presented on a graph
whose horizontal scale {should be vertical scale -Ed.} is
based on load ...
avstop.com/AC/FlightTraingHandbook/VgDiagram.html - 5k -
Cached - Similar pages

The Zoom Maneuver - FCI Emergency Maneuver Training
Referring to the VG Diagram below, we see a distinct
boundary shown as a ...
On this VG Diagram, we can see the 1-G stall speed is
approximately 63 kts. ...
www.fcitraining.com/vid_clip7_zoom_maneuver.htm - 27k -
Cached - Similar pages

FCI Emergency Maneuver Training: Effectively Recovery from
....
Below is a velocity versus G-loading (VG) diagram or as
the test pilots call it
... VG Diagram. The airplane will stall if you attempt
flight outside of the ...
www.fcitraining.com/article2_fci_training_oct02.htm -
40k - Cached - Similar pages

Gleim FIRC - Easy Does It
VG diagram? Can you say, "ARGH"? Stability, controllability,
and maneuverability?
Can you say, "SNORE"? I attacked Lesson One with the hope
that, perhaps, ...
www.avweb.com/news/reviews/181574-1.html - 39k - Cached -
Similar pages

[PPT] Vg DIAGRAM
File Format: Microsoft Powerpoint - View as HTML
Vg Diagram. Maneuvering Speed (Va), a Significant.
Limitation. 0. 1. 3.8. -1.52.
5.7. -2.8. CESSNA 310. CATASTROPHIC. AIRFRAME. STRUCTURAL.
FAILURE ...
www.faa.gov/about/office_org/
field_offices/fsdo/orl/media/ppt/pres.ppt - Similar pages





> wrote in message
oups.com...
|
| Jim Macklin wrote:
| > Va is pilot input to the controls, Vb is turbulence.
|
| To learn something new, all I need to do is try to show I
already know
| something. :<)
|
| I'd never run across Vb before. I'll figure out how to
include that.
|
| Thanks again.
|

[[email protected]]

Oh Boy! More Reading!
I always did like the Vg diagrams. Honest!
The one on the AvWeb site looked good, but too small to read and it
won't enlarge. Maybe I'll email AvWeb and get a better copy.

Thanks for the links.

[Private]

"gyoung" > wrote in message
news:A8uHf.174246$oG.52951@dukeread02...
snip
> But back to Jay's question: What might explain why airspeed increased in
> turbulence? Here's another idea - a phenomenon described in the April/May
> 2005 issue of Air & Space Smithsonian, an article that discusses flying
> sailplanes and an phenomenon they call "dynamic soaring". I really don't
> understand it well, but it seems to be that one can 'gain energy' for the
> 'aircraft system' by "exposing the airplane's belly to stronger winds" for
> brief periods of time, flying back into winds not so strong, returning to
> the stronger winds, and going back and forth. So, I guess the airplane
> extracts some energy from the stronger winds (weakening them I assume),
> and uses that energy to go faster (or in the case of sailplanes, stay
> aloft longer). What do you think?
>

This seems similar to what John Denker refers to as the "albatross effect"
http://www.av8n.com/how/htm/maneuver.html#sec-albatross
see section 16.17.2

Happy landings,

[gyoung]

The albatross was mentioned in passing in the Air & Space Smithsonian.
But your reference provides an explanation that I can follow better -
thanks.

george