Turbulence and airspeed

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.

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"

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

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"

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.

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"

"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

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?