Question to Mxmanic

Actually, he doesn't.

mike

"rq3" wrote in message
. net...
Anthony, you've got the issue of compressibility precisely backwards. No
surprise.

Rip

Tom L. writes:

The big question is "why does the wake turbulence descend?"

Because it's the downwash from the aircraft's wings. Aircraft stay in the air
by pushing air downward. As the wings pass through still air, they twist the
air downward as they pass. The force required to do this engenders an equal
and opposite force that raises the wings--lift, in other words.

No downwash = no lift.

Turbulence is mostly from wingtip vortices. The vortices exist because air is
twisting over to the top of the wings from the bottom. The vortices are
necessary in order to accommodate the swath of downwash behind the aircraft,
which is descending in relation to the still air on either side of the
aircraft's path.

Is the air volume inside the vortices denser than surrounding air?

Density has nothing to do with it. The air has been pushed downward by the
wings.

Probably not. So the descent is probably not due to gravitational
force.

No, it's not gravity. The air descends because the wings pushed it down.

I am no expert on fluid dynamics and have no access to texts that
answer the question (if there are any), but figure 7-3-5 in AIM is
interesting - it shows a wake sinking at several hundred fpm
immediately after an aircraft, but than stabilizing at several hunderd
feet below the flightpath, i.e. no further sink. This might indicate
that the sink is due to wing downwash.

It is.

If that is the case, than
1. Wake turbulence in steep turns will not move just downward, but
down and out, that is: opposite lift.

Yes.

2. The speed at which it moves will depend on downwash - it's speed,
intensity, strength (?) I don't know which term would be appropriate
here. Whatever it is, it might be much smaller for GA aircraft than
for large aircraft.

The product of air mass times downwash acceleration has to be the same as the
product of aircraft weight times gravity. So a larger and heavier aircraft
produces a larger downwash, albeit not necessarily a faster one.

--
Transpose mxsmanic and gmail to reach me by e-mail.

On Apr 16, 3:59 pm, Tom L. wrote:
The big question is "why does the wake turbulence descend?"
Is the air volume inside the vortices denser than surrounding air?

Whoa, good guess. I just read a reference that said the vortex
descends until it meets air of its own density and then dissipates.
It was surprising to read. Let me see if I can find that site
again...

Kev

On Apr 16, 3:59 pm, Tom L. wrote:
The big question is "why does the wake turbulence descend?"
Is the air volume inside the vortices denser than surrounding air?

Found it, Tom. Ref:

http://www.airpower.maxwell.af.mil/a...ug/carten.html

"Cruise altitude vortices usually level off at about 1000 feet below
the altitude of the aircraft as their density comes into equilibrium
with that of the surrounding air. Decay processes then take over. "

Regards, Kev

"Kev" wrote in message
ups.com...
On Apr 16, 3:59 pm, Tom L. wrote:
The big question is "why does the wake turbulence descend?"
Is the air volume inside the vortices denser than surrounding air?

Found it, Tom. Ref:

http://www.airpower.maxwell.af.mil/a...ug/carten.html

"Cruise altitude vortices usually level off at about 1000 feet below
the altitude of the aircraft as their density comes into equilibrium
with that of the surrounding air. Decay processes then take over. "


Don't underestimate the value of the words "usually" and "about" in that
sentence. You are still trying to absolutely describe something that is very
dynamic.

On Apr 17, 12:24 am, "Maxwell" wrote:
"Kev" wrote in message
http://www.airpower.maxwell.af.mil/a...ew/1971/jul-au...

"Cruise altitude vortices usually level off at about 1000 feet below
the altitude of the aircraft as their density comes into equilibrium
with that of the surrounding air. Decay processes then take over. "

Don't underestimate the value of the words "usually" and "about" in that
sentence. You are still trying to absolutely describe something that is very
dynamic.

True. So I guess we could all agree that where the wake goes, depends
on the surrounding atmosphere and aircraft profile...

Still... if it stayed at the same altitude most of the time (contrary
to NASA reports), or was over 100' tall (as some tried to claim at
first), then EVERY student pilot could hit their own wake all the time
grin.

Kev

"Kev" wrote in message
ups.com...
On Apr 17, 12:24 am, "Maxwell" wrote:
"Kev" wrote in message
http://www.airpower.maxwell.af.mil/a...ew/1971/jul-au...

"Cruise altitude vortices usually level off at about 1000 feet below
the altitude of the aircraft as their density comes into equilibrium
with that of the surrounding air. Decay processes then take over. "

Don't underestimate the value of the words "usually" and "about" in that
sentence. You are still trying to absolutely describe something that is
very
dynamic.

True. So I guess we could all agree that where the wake goes, depends
on the surrounding atmosphere and aircraft profile...

Still... if it stayed at the same altitude most of the time (contrary
to NASA reports), or was over 100' tall (as some tried to claim at
first), then EVERY student pilot could hit their own wake all the time
grin.



But if "ifs" and "buts" were candy and nuts, it would be Christimas every
day.

If you can hit your own wake doing 60/360s and holding altitude, keep
practacing. You are more than likely doing something wrong.

"Maxwell" wrote in message
...

If you can hit your own wake doing 60/360s and holding altitude, keep
practacing. You are more than likely doing something wrong.


Correction, if you CAN'T hit your own wake

On 16 Apr 2007 19:26:17 -0700, Kev wrote:

On Apr 16, 3:59 pm, Tom L. wrote:
The big question is "why does the wake turbulence descend?"
Is the air volume inside the vortices denser than surrounding air?

Found it, Tom. Ref:

http://www.airpower.maxwell.af.mil/a...ug/carten.html

"Cruise altitude vortices usually level off at about 1000 feet below
the altitude of the aircraft as their density comes into equilibrium
with that of the surrounding air. Decay processes then take over. "

Regards, Kev


Great! Thanks for the effort.

Now I have a new question -- where is this extra air coming from, and
how?
The vortices grab some additional air molecules and then take them
down. Theere is now a volume of air with missing molecules (if I'm
allowed to speak in K-grade language). These molecules have to be
replaced, and the only source is lower -- in the more dense air that
is in addition getting the extra particles. So there must exist an
additional upward moving air flow outside the vortices.

The pictures showing jets right on top of clouds do seem to indicate
this. The vortices seem to suck in clouds from below and then spin
them.
So there is this secondary air movement starting at 1000' below an
aircraft, moving upwards on both sides of the aircraft and filling the
low density areas left by the vortices and wing downwash.
Interesting.

- Tom

Yes, they do. I just asked a friend with 26,000 hours. He confirmed that
DC-8's and 707's do get a bump as they cross their own wake in a 360
degree constant altitude turn. He also said that some Category D
simulators include this effect in their motion repertoire.

Rip

Tom L. wrote:
On 16 Apr 2007 06:37:13 -0700, "Kev" wrote:

On Apr 14, 4:27 pm, "george" wrote:
I always maintained altitude and rate of turn in steep turns with the
end result being hitting my own slipstream.
As have we all on nice days, and students like to brag about it. Yet
Mx is correct, in theory we should not be able to do this.

I seem to recall recent magazine (web?) articles where the idea that
you can hit your own wake while actually holding altitude, should be
downplayed nowadays. You _have_ to descend a little bit to do so,
which means that, while you might be within the +/- 100' test
scenario, you are NOT holding the same exact altitude.

Hmm. Or else it means that the wake doesn't necessarily descend as
we're taught. On a warm clear day (which is when I've hit my own
wake), I betcha that the wake is being held upward a tiny bit by the
heat from the ground.

Cheers, Kev



The big question is "why does the wake turbulence descend?"
Is the air volume inside the vortices denser than surrounding air?
Probably not. So the descent is probably not due to gravitational
force.

I am no expert on fluid dynamics and have no access to texts that
answer the question (if there are any), but figure 7-3-5 in AIM is
interesting - it shows a wake sinking at several hundred fpm
immediately after an aircraft, but than stabilizing at several hunderd
feet below the flightpath, i.e. no further sink. This might indicate
that the sink is due to wing downwash.

If that is the case, than
1. Wake turbulence in steep turns will not move just downward, but
down and out, that is: opposite lift.
2. The speed at which it moves will depend on downwash - it's speed,
intensity, strength (?) I don't know which term would be appropriate
here. Whatever it is, it might be much smaller for GA aircraft than
for large aircraft.

It would be interesting to do the following flight test:
On a nice day (meaning: perfectly still air) fly turns at different
bank angles and speeds and note when you do and don't experience the
bump at the end of the turn. Do this in different aircraft - low/high
wing, small/large/...

Does anyone know whether big aircraft experience the bump at the
conclusion of their steep 360s?

- Tom