Question to Mxmanic

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

"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

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.

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

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

As you correctly point out, we all know that it happens because we have all
done it; and when we flew eights around pilons, we hit our own wake quite
decisively each time we crossed the center point.

Thus, clearly, it doesn't matter whether we might have found a more
impressive bump lower down; the salient point is that a portion of the wake
was above the flight path when we returned to that place in the atmosphere.

Actually, most of the writings about wakes and sinking air, insofar as I can
tell, only discuss the motion of the central portion of the wake.
Additional writings, regarding the (very reall) potential for upset discuss
the central area of the vorticies--which settle at a lesser rate and expand
as they settle. Our actual experience strongly implies that the vortices
expand at least as rapidly as they settle.

I see that Snowbird has already posted links to my favorite illustration of
this, plus quite a few more, so I'll stop.

Peter

"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

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.

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

Mxsmanic wrote in
news
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.


No, it doesn't, fjukkwit. Only most of it.

Send me fifty bucks and I'll explain why to you


bertie

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