Turbulence and 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.

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"

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.

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

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

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
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