Question to Mxmanic

Tom L. wrote:


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


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. The end result is that the downwash stays at a constant altitude,
or sinks MUCH more slowly than theory would indicate. Not a good
analogy, but you've got me thinking!

Rip

"Mxsmanic" wrote in message
...
Maxwell writes:

That's right Luke, add the power of the trim!!!!!

No, power instead of trim. You need more lift.

Right Luke!!! Right!!!!! The power to add trim.

On Apr 16, 11:15 am, "Maxwell" wrote:
"Kev" wrote in message
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).

I am certainly no expert on the subject, but I think most of the data on
wake turbulence comes from studies held at or very near the ground.

My searches on the web show the opposite... or at least that there's
studies both at altitude and near the ground. For examples:

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

"NASA research has shown that as large aircraft move through the air,
trailing vortices tend to remain spaced less than a wingspan apart
while sinking at a rate of several hundred feet per minute. Over time,
the sink rate will slow and their strength will taper off. Research
has shown, however, that vortices can also rise during conditions of
ambient thermal lifting."

"Aircraft Accident Reconstruction and Litigation" By M. P. Papadakis,
Barnes Warnock MacCormick, states that vortices descend 5-10 fps
(30-600 fpm).

Based on the numbers I recall, they did indeed teach that the wake from a
landing heavy would NORMALLY travel both down and away from the aircraft a 5
kts or so. But they were also quick to mention that a simple 5 kt or so
crosswind componet could leave the vortex in the middle of the runway for
quite some time.

Yes, we were all taught that part.

The problem with trying to use this information at altitude is that you
don't have the ground to help stablize the vertical movement of the vortex.

Here is information taken at altitude: Vortices are 14-36 feet in
diameter, approx the wingspan apart, and sink 160 - 1100 fpm.

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

I just think it's an interesting question, because we've all had it
happen, but no one here can give a definitive reason for it (beyond
"yo stupid of course it does" which is pretty lame even for the usual
Mx bashers ;-)

I think I've convinced myself that since I don't always hit my wake on
a perfect steep turn, and because it mostly seems to happen over areas
of rising air, that the explanation is simple. Unless someone can
post better research.

Thanks, Kev

"Kev" wrote in message
oups.com...

I think I've convinced myself that since I don't always hit my wake on
a perfect steep turn, and because it mostly seems to happen over areas
of rising air, that the explanation is simple. Unless someone can
post better research.


Great, now what do you intend to do with the imformation?

"Rip" wrote ...

We all know it happens. I'm just one of those weirdos that wants to know
WHY it happens. As a result of this thread, it appears that nobody knows.
It's an unstudied regime of flight. I find THAT interesting!

Me too ;-)

I actually tried yesterday... with poor results for the connect ;(

But the GPS track provided an explanation. It showed my 360s were not proper
full circles, i.e. at the exit I crossed the previous flight path at an
angle (more than 45 degrees in fact) instead of actually flying in the same
circle track as the entry of the 360. Not so easy to explain, but the result
was that the airplane was only in the potential wake area for a fraction of
a second. I guess you need to fly so that the flightpath is well aligned
with the original circle, in order to catch the wake.

Back to the theory:
I read some interesting basic aerodynamics of drag. According to the book,
at low speeds the induced drag (which is a side effect of the lift force) is
larger than the parasite drag (caused by frontal area, landing gear etc).
But at higher speeds (above 70 mph in the example case, a light plane)
parasite drag becomes the dominant drag component. Now, the induced drag is
creating the tip vortices, which presumably descend, but parasite drag has
no vertical component, so in theory it should stay in place. So according to
this, the higher the airplane's relative speed, the slower the wake will
descend (if at all).

I look forward to the results of the group's experiments ;-)

"Snowbird" wrote:
I actually tried yesterday... with poor results for the connect ;(

But the GPS track provided an explanation. It showed my 360s were not
proper full circles, i.e. at the exit I crossed the previous flight path at
an angle (more than 45 degrees in fact) instead of actually flying in the
same circle track as the entry of the 360.

That's probably because there was wind aloft. GPS shows your track over the
ground, not your track WRT the moving air mass.

In perfectly calm conditions, GPS track would show a circle if you flew one
properly.

--
Dan
C-172RG at BFM

In rec.aviation.piloting Kev wrote:
On Apr 16, 11:15 am, "Maxwell" wrote:
"Kev" wrote in message
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).

I am certainly no expert on the subject, but I think most of the data on
wake turbulence comes from studies held at or very near the ground.

My searches on the web show the opposite... or at least that there's
studies both at altitude and near the ground. For examples:

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




|||||
"NASA research has shown that as large aircraft move through the air,
|||||
trailing vortices tend to remain spaced less than a wingspan apart
while sinking at a rate of several hundred feet per minute. Over time,
the sink rate will slow and their strength will taper off. Research
has shown, however, that vortices can also rise during conditions of
ambient thermal lifting."

"Aircraft Accident Reconstruction and Litigation" By M. P. Papadakis,
Barnes Warnock MacCormick, states that vortices descend 5-10 fps
(30-600 fpm).

Where's the data for C172 sized aircraft?

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

I see no justification for this.

snip rest

--
Jim Pennino

Remove .spam.sux to reply.

On Apr 16, 1:39 pm, Mxsmanic wrote:
Jim Stewart writes:
*Every* pilot (at least in the US) learns steep turns
in the context of the FAA's practical test standard.
That's a steep turn while holding your altitude +/- 100
feet.

If you meet your wake, you're descending.

--
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I have sent the following question to an aerospace engineer at the
Rand corporation for his response...I'll keep you posted:
There is a thread on one of the aviation usenet groups that got me to
thinking...
In training, it it routinely common for a pilot to practice steeps
turns, and when you reach the roll out of a 360 degree turn with a
bank angle of 45-60 degrees, you will feel a hard bump, which most
instructors say is due to going through your own wake. One of the
commentators is arguing that this can't happen due to the fact that
wake turbulence descends. But it is a clearly easily demonstrated
effect. Is it our own wake? Or are we creating a vertical vortex
with the maneuver of a steep turn? (rather than the wake of wingtip
vortices). Is the data on wake turbulence behavior applicable to a
shrply turning aircraft?

Snowbird writes:

Now, the induced drag is
creating the tip vortices, which presumably descend, but parasite drag has
no vertical component, so in theory it should stay in place. So according to
this, the higher the airplane's relative speed, the slower the wake will
descend (if at all).

The entire air mass behind the aircraft is descending. The downwash descends,
and air from above moves down to replace it. While parasitic drag is not
associated with lift and thus has no vertical component of its own, any
turbulence it creates will still drift downward with the downwash, although
perhaps less quickly than the downwash itself, depending on where the
turbulence leaves the aircraft.

--
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Snowbird writes:

I guess Mxmanic uses the FAA AIM as his main source in his "research".

That is only one of many sources. They all say the same thing.

a) "Flight tests have shown that the vortices from larger (transport
category) aircraft sink at a rate of several hundred feet per minute,
slowing their descent and diminishing in strength with time and distance
behind the generating aircraft."

Note the explicit reference to large aircraft. In fact, it seems all actual
wake turbulence safety studies have involved large aircraft, i.e. B707 and
larger. This is in fact quite natural, as there was no real safety issue
before the large jetliners appeared.

The wakes of smaller aircraft descend as well.

b) "Test data have shown that vortices can rise with the air mass in which
they are embedded."

There you are, official proof to the statements of several of our
contributors.

Including myself.

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

Yes. Although the downwash itself should be strongest when the aircraft is
dirty and slow. The reason clean and slow produces stronger _vortices_ is
that it only produces one pair, whereas flaps and other control surfaces can
produce multiple vortices of smaller size that tend to interfere with each
other and reduce overall turbulence.

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).

Which makes it all the more difficult to understand how a pilot could feel his
own wake in a level 360-degree turn.

The interesting study question here, for the light airplane case, would be
the relation between the tip vortices (which presumably sink, as for large
aircraft) and the propwash (which is basically horizontal). I think glider
pilots can testify that the propwash is the dominant one, at least close
behind the tug airplane - any soarers out there who can comment?

You're neglecting the downwash, which is present in all aircraft. Downwash
tends to pull all turbulence behind the aircraft down with it.

But realistically, as the wake behind a light aircraft is no real safety
hazard, there is no compelling reason to study this case. So unless someone
can produce a reference, let's rely on the observational data from countless
pilots.

And ignore the factual data from countless resources? What makes pilots more
reliable? Most pilots barely understand how lift works to begin with.

--
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