Turbulence and airspeed

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.

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"

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.

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

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"

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.

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.

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

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