Question to Mxmanic

On Apr 17, 11:06 am, Mxsmanic wrote:
In a turn, a portion of the lift produced by the wings must be used to
accelerate the aircraft laterally, and this portion of the lift is no longer
available to maintain the aircraft's altitude. Thus, without any adjustment
of pitch or power to compensate, any turn will result in a loss of altitude.

All pilots know this and assume that a "turn" includes compensation,
unless they specifically say "descending" or "climbing".

So when you keep saying "a turn will always descend", you just confuse
your readers... especially the ones who don't follow threads closely.
You're in a pilot newsgroup, which means the prevailing terminology is
that of pilots, not necessarily engineers, civilians, or whomever.

Kev

Just spoke to a friend with 26,000 hours. He confirmed that DC-8 and 707
heavies certainly do get a bump as they fly through their own wake
during a 360 degree constant altitude turn. He also said that some
Category D simulators include the effect in their motion repertoire.

Rip


Kev wrote:
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

In rec.aviation.piloting Mxsmanic wrote:
Thomas Borchert writes:

Can't follow you there. That's as useful a statement as "airplanes tend to
be stationary objects..."

In a turn, a portion of the lift produced by the wings must be used to
accelerate the aircraft laterally, and this portion of the lift is no longer
available to maintain the aircraft's altitude. Thus, without any adjustment
of pitch or power to compensate, any turn will result in a loss of altitude.

Yet another true but worthless statement.

One of the first things real pilots are taught in real training in
real airplanes is how to maintain a constant altitude in a turn.

Ergo any real turns by real airplanes will be constant altitude
unless the PILOT has a reason to do otherwise.

Since most 360 turns are done as practice to establish and maintain
the skill, most 360 turns will be at a constant altitude +/- 100 feet.

--
Jim Pennino

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In rec.aviation.piloting Mxsmanic wrote:
writes:

If air truely behaved like an incompressilble fluid at low air speeds,
it would be difficult, if not impossible, to breathe while jogging.

It's possible to breathe water, which is indeed incompressible for all
practical purposes.

And yet another factoid that is only applicable under a very
contrained set of circumstances you are trying to arm wave into
a generallity to prove yourself correct.

And no, it is not possible to breathe water, you can only breathe
gases, if you want to be pendantically, semantically correct.


--
Jim Pennino

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Yes, they do. I just asked a friend with 26,000 hours. He confirmed that
DC-8's and 707's do get a bump as they cross their own wake in a 360
degree constant altitude turn. He also said that some Category D
simulators include this effect in their motion repertoire.

Rip

Tom L. wrote:
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

Anthony, you've got the issue of compressibility precisely backwards. No
surprise.

Rip

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.

Anthony, my boy, your interpretation is incorrect. At Mach, the air has
compressed as much as it can, which is why it takes so much energy to
force a solid object through Mach. You have the concepts reversed in
your head.

Rip

Mxsmanic wrote:
writes:

Did you use Microsoft Air Simulator to do this?

You haven't answered my question.

OK, now wave your hand through a real fluid, I'd suggest water.

Did it feel the same as waving your hand through air?

If you look in books on aerodynamics, you'll find that air is effectively an
incompressible fluid at low speeds, such as those encountered in small
aircraft. It isn't until you get to the transonic range that compression
starts to be an issue, and the rules change substantially at and beyond the
speed of sound.

And it's darned difficult, just like the man said.
Hey, here's an idea, why don't you go get us some empirical data from
personal experimentation, and then get back to us with some actual
information!

Rip

Mxsmanic wrote:
writes:

If air truely behaved like an incompressilble fluid at low air speeds,
it would be difficult, if not impossible, to breathe while jogging.

It's possible to breathe water, which is indeed incompressible for all
practical purposes.

In rec.aviation.piloting Mxsmanic wrote:
writes:

Air does not behave very much like an imcompressible fluid at low air
speeds. Not even close.

That's not what the engineers say.

I am an engineer and have the degree to prove it and I totally agree
with him, so stuff it.

Under some conditions, low air speeds is one of them, air can be
treated like it is an imcompressible fluid.

The difference between TAS and EAS is only about 13% even at Mach 1. Since
small aircraft come nowhere near to Mach 1, for all practical purposes air is
incompressible for most calculations.

Word salad that shows you don't get the concept.

At low airspeed, the equations for incompressible fluid flow are close
enough to what actually happens that they can be used for practical
calculations.

This does not mean that air "acts like an incompressible fluid" in any
way, shape, or form.

Obviously air, being a gas, can be compressed, but taking that into account at
low speeds greatly complicates the calculations, and the final result isn't
significantly different.

Finally, the light comes on...

Well, gee-whiz, you backed into a place where you are finally correct.

Aren't you great?

--
Jim Pennino

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rq3 writes:

Anthony, my boy, your interpretation is incorrect. At Mach, the air has
compressed as much as it can, which is why it takes so much energy to
force a solid object through Mach.

If it were compressed as much as it could be, it would be a liquid.

--
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