Question to Mxmanic

Mxsmanic wrote:
mike regish writes:


We have. Rising air. It happens all the time over the plowed cornfields I
fly over. The plane doesn't also rise with the air because we are
maintaining out altitude above the ground.


Therefore you are descending into your wake. QED.

Excellent, Anthony! This is almost a real breakthrough for you! We'll
work on your semantic errors in a moment, but the mere fact that you've
just admitted here, in public, that an aircraft at constant altitude can
run through it's own wake is tremendous progress on your part. Good boy!
See what happens when you take my suggestions to re-examine your
erroneous assumptions?

So let's work on those semantics, shall we? News flash! Earth to
Anthony! An aircraft at a constant altitude is not descending! We'll all
wait while you digest that. No, no, take your time. We all know it will
require several days of intense concentration and pointless posting for
it to sink into your limited neural capacity.

Seriously, I'm deeply gratified that you picked up a new skill today; to
wit, the ability to assimilate information which goes against your
preconceived notions, and actually synthesize a situation that would
allow such a thing to be so. I know it's hard for you, but the majority
of the human race does it almost effortlessly. Don't worry, it gets
easier with practice. Anything else we can help you with?

Rip

On Apr 16, 10:45 am, Mxsmanic wrote:
Kev writes:
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.

Then you must be descending through the rising column of air.

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.

Kev

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.

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

Gary writes:

What part of "maintaining our altitude" sounds like "descending" to
you?

The part that is combined with rising air.

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

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.


My post had nothing to do with descending.
It had to do with why you're treated like
a fool on aviation forums. You should address
that issue rather than getting fixated on
steep turns and descending....

"Rip" wrote ...

Oh well. The entire thread has forced me to ask myself just what the wake
behind an aircraft looks like. Like every other pilot, I know you can
intercept your own wake during a constant altitude turn, but it would be
neat to be able to SEE all of the air masses at work.

Thanks for your smoke insights ;-)
There are some great photos depicting the tip vortices he
http://www.airliners.net/open.file/1091105/M/
http://www.airliners.net/open.file/1008033/M/

Here are some real masterpieces :

http://www.airtoair.net/gallery/gallery-vortices.htm


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:

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.
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.

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).

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?

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.

Beautiful, Snowbird. I've seen the effect in reality, but those are some
magnificent photos!

Rip

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 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

RomeoMike wrote:

I don't know if he "cannot" or will not or just wants to get under
everyone's skin. What you say in this post is correct. But why do people
keep responding and arguing ad nauseum with someone who can't or won't
get it? What's the dynamic? I doubt that there has ever been a pilot who
has not flown into his own wake in a constant altitude 360. So this is
not a topic that one pilot needs to prove to another pilot with a
different opinion.

The only dynamic is between the pilots on the group, certainly not with
MX. But, as I mentioned, the thread forced me to ask myself just what it
was I am "running over" when I hit my own wake turbulence. Does it
matter? Probably not, but this enquiring mind wants to know. I still
don't have the answer. Rising wingtip vortices in warm air? Prop wash?
"Burbles" from the passage of non-lifting surfaces like the fuselage?

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!
Perhaps it could lead to some super-terrific drag reduction technique,
like surfing on your own wake? After all, that's why geese fly in "V"
formation.

Rip