Question to Mxmanic

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

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,

How tall is the wake?

Jose
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On Apr 16, 9:41 am, Jose wrote:
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,

How tall is the wake?

Good point. Still, using the calculator at:

http://www.csgnetwork.com/aircraftturninfocalc.html

It's going to take about 30 seconds to fly a 360 steep turn at
100kts. My wake _should_ descend about 150' during that time (300
fpm). I can't imagine a C172 wake being tall enough to stay in my
way unless something else is ocurring (me descending, or the wake
staying up).

Would love to hear a decent explanation. Kev

My wake _should_ descend about 150' during that time (300
fpm). I can't imagine a C172 wake being tall enough to stay in my
way...

I can. 150 feet is not tall at all for a wake. Remember, the air
around the wake is also being dragged by the wake vortex.

You're going to bump into something.

Jose
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Jose writes:

I can. 150 feet is not tall at all for a wake.

It's extremely tall for a small aircraft. The wake would probably be about 50
feet high.

Remember, the air around the wake is also being dragged by the wake vortex.

But at very slow speed. Even the downwash itself is moving slowly, only a few
knots. Any of the winds that one often encounters at altitude would be enough
to rapidly disperse it.

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In rec.aviation.piloting Mxsmanic wrote:
Jose writes:

I can. 150 feet is not tall at all for a wake.

It's extremely tall for a small aircraft. The wake would probably be about 50
feet high.

Remember, the air around the wake is also being dragged by the wake vortex.

But at very slow speed. Even the downwash itself is moving slowly, only a few
knots. Any of the winds that one often encounters at altitude would be enough
to rapidly disperse it.

You have no idea of the altitudes or winds aloft at which most people
practice maneuvers in real airplanes.

Wind by itself will not "disperse" anything, it will just move it.

It takes turbulant air to disperse things in the air.

--
Jim Pennino

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writes:

You have no idea of the altitudes or winds aloft at which most people
practice maneuvers in real airplanes.

Neither does anyone else. You don't know until you get there.

Wind by itself will not "disperse" anything, it will just move it.

Winds interacting with each other will disperse lots of things.

It takes turbulant air to disperse things in the air.

See above.

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On Apr 16, 10:22 am, Jose wrote:
My wake _should_ descend about 150' during that time (300
fpm). I can't imagine a C172 wake being tall enough to stay in my
way...

I can. 150 feet is not tall at all for a wake. Remember, the air
around the wake is also being dragged by the wake vortex.

Hmm. We're going to have to define a wake, methinks. I can't find
anything about body wakes, for example. Do they give much of a
bump? Glider pilots, are you listening?

On the other hand, wingtip vortices are a well-researched topic, and
if a Boeing 727's is only 9' in radius, it would be hard to imagine a
vortex being more than 5 feet in radius for a C172, if that much.
Even if larger, and sinking very slowly, it should still be 50-150'
below the aircraft if the other parameters (altitude, wind) are
static.

Regards, Kev

On the other hand, wingtip vortices are a well-researched topic, and
if a Boeing 727's is only 9' in radius,

cite? I remember seeing pictures of wingtip vortices (of fair sized
aircraft) and they looked like they were more than 18 feet across.

Jose
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On Apr 17, 12:02 am, Jose wrote:
On the other hand, wingtip vortices are a well-researched topic, and
if a Boeing 727's is only 9' in radius,

cite? I remember seeing pictures of wingtip vortices (of fair sized
aircraft) and they looked like they were more than 18 feet across.

Sorry was not cited here, but in other posts. To repeat:

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

Yes, I would've thought much bigger too, but then they wouldn't be as
much a threat so far behind an aircraft if they expanded quickly in
diameter. Apparently if flaps or spoilers aren't used, the danger
area behind a 747, for example, extends many more miles than we're
usually taught:

http://www.nasa.gov/centers/dryden/a...4-14-DFRC.html

Still haven't found much on really light aircraft ( 26,000 lbs), but
the concept should be the same, albeit at a much smaller amplitude.
There are equations for calculating the vortex, but they seem hard to
get at on the web.

Of interest: wingtip vortices were first formally written about in
1907 (!), and the use of vertical fins to cut down the drag on wings,
dates over a decade before that.

Regards, Kev