Question to Mxmanic

Kev writes:

Not necessarily. Visualize that I begin my turn over a field where
the air is rising slightly. The rest of my turn is over another area
(lake perhaps) where the air is static. I am not descending through
the rising column yet I manage to hit my own wake because it was held
in place.

Since these would be very unusual circumstances, they cannot substantiate the
claim that pilots routinely meet their own wakes in 360-degree turns.

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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.

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Rip writes:

I don't know, but I'm going to find out! I can envision an aircraft with
light wing loading, like a Cessna for instance, compressing the air
locally as it creates lift. After passage of the wing, the lift created
downwash would rebound upward, kind of like skipping a stone on the
water.

Virtually no compression occurs at the speeds of a Cessna. Compression is
only an issue at high speeds. At low speeds, air behaves very much like an
imcompressible fluid.

The end result is that the downwash stays at a constant altitude,
or sinks MUCH more slowly than theory would indicate.

The downwash does not stay at a constant altitude. It sinks. It has to,
otherwise the aircraft couldn't stay in the air.

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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

writes:

Where's the data for C172 sized aircraft?

Small aircraft work the same way, since they have wings that work the same
way.

People are assuming numbers for a specific type of aircraft are
applicable to very different aircraft.

These facts are applicable to all fixed-wing aircraft.

I see no justification for this.

You're assuming a difference where no evidence for a difference exists.

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On Apr 16, 3:47 pm, "Snowbird" wrote:
I guess Mxmanic uses the FAA AIM as his main source in his "research".
Section 7.3.1 is about wake turbulence. A couple of interesting quotes from
that section, that Mx has not seen fit to share with us:

Heh. Many of his responders seem to have done even less "research".
Instead they substitute insults for information, hoping they'll look
smarter than him. They don't seem to realize that it just makes them
look dumber.

c) "The greatest vortex strength occurs when the generating aircraft is
HEAVY, CLEAN, and SLOW."

In contrast, a light aircraft doing a 360 is usually LIGHT, CLEAN and
(relatively speaking) FAST. Very different conditions, especially regarding
two major sources of wake: the AoA of the wing (which affects the tip
vortices) and the power setting (which affects the propwash strength).

Of course, LIGHT does not mean "light aircraft". Some 152s are
vortex HEAVY in the case of big instructors and students ;-)

For vortex strength, the term HEAVY is used in a relative manner. A
small plane that is lightly loaded will create less vortex strength
than the same small plane that is heavily loaded, because the actual
AOA is larger in the latter case.

The actual AOA is the key for (HEAVY) more load, (CLEAN) less flaps
and (SLOW) less speed. It's greater in all those cases.

Auugh. Four year old calling me. Later..
Best, Kev

"Mxsmanic" wrote in message
...
writes:

Where's the data for C172 sized aircraft?

Small aircraft work the same way, since they have wings that work the same
way.

People are assuming numbers for a specific type of aircraft are
applicable to very different aircraft.

These facts are applicable to all fixed-wing aircraft.

I see no justification for this.

You're assuming a difference where no evidence for a difference exists.



Priceless!!! You are dead wrong again.

On Mon, 16 Apr 2007 23:33:54 +0200, Mxsmanic
wrote:

Rip writes:

I don't know, but I'm going to find out! I can envision an aircraft with
light wing loading, like a Cessna for instance, compressing the air
locally as it creates lift. After passage of the wing, the lift created
downwash would rebound upward, kind of like skipping a stone on the
water.

Virtually no compression occurs at the speeds of a Cessna. Compression is
only an issue at high speeds. At low speeds, air behaves very much like an
imcompressible fluid.

The end result is that the downwash stays at a constant altitude,
or sinks MUCH more slowly than theory would indicate.

The downwash does not stay at a constant altitude. It sinks. It has to,
otherwise the aircraft couldn't stay in the air.

It doesn't have to continue to sink forever. It can stabilize its
position at some point.

To explain the encounter with one's own wake turbulence we need some
quantification for a particular aircraft/bank/speed:
- radii of the vortices
- "sink" rate ("sink" meaning movement away from the flight path, not
necessarily downward)
- final "sink" distance

E.g. if the vertex radius is 15 feet and sink rate 20 fpm, we hit the
wake after a 30 second turn.
If the radius is 15 feet, sink rate 100 fpm, and final distance 10
feet, we still hit it.
And so on.

So what are the right numbers?
We know from experience that a right combination of those numbers
exists in reality.

- Tom

On Apr 16, 11:00 am, Mxsmanic wrote:
writes:
Just because you have never experienced it and can't understand it
from your many hours of playing the Flight Simulator game doesn't
mean it doesn't exist.

My study of aerodynamics thus far indicates that it is impossible, unless you
descend to catch your descending wake. Wakes _always_ descend. It's a
consequence of the mechanism that produces the lift that sustains the
aircraft, and it's unavoidable. Every source that I have consulted points
this out, without exception. Your mere assertion to the contrary is
completely unpersuasive in comparison.


My only comment on this subject - I'm not going to bother arguing
about it, as 1) according to the aerodynamics, it definitely IS
possible to fly through your own wake turbulence in steep turns, and
2) like most people who have actually done steep turns in a real plane
in calm air, I've done it.

Tip vortices (the major part of wake turbulence) extend outward and a
fair distance UPWARD from the wingtip. As well, since the plane is
banked in a turn in the described situation, the vortices descend at a
much slower rate than they would for S&L flight. A simple search will
reveal several NASA and other studies with graphics and photos
depicting an aircraft flying through smoke and the resulting tip
vortices. One of the photos that is in a few studies shows a small Ag
plane, with the upward spiral of the tip vortices reaching 2-3 times
the height of the aircraft, and outward from the wing by half a
wingspan, near the ground. As ground effect decreases the vortices,
at altitude they can be larger.


It just means you don't know a whole lot about real flying or what
happens in a real airplane.

No, it means that I look at the facts, and I don't depend on the legends.

Then you haven't found all of the facts.

Have you ever done a short field take off in your Flight Simulator
Cessna with the springy gear and had the mains vibrate for a few
seconds shaking the airplane?

I don't fly the Cessna, and I fly only at airports with hard, smooth runways
that won't bounce the aircraft around.

Planes bounce just fine on hard, smooth runways - ask any pre-solo
student.

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In rec.aviation.piloting Mxsmanic wrote:
writes:

Where's the data for C172 sized aircraft?

Small aircraft work the same way, since they have wings that work the same
way.

People are assuming numbers for a specific type of aircraft are
applicable to very different aircraft.

These facts are applicable to all fixed-wing aircraft.

I see no justification for this.

You're assuming a difference where no evidence for a difference exists.

Yes, you are quite correct; there is no differece between a 747 and
a C172 in the Microsoft Flight Simulator game.


--
Jim Pennino

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