Airspeed Indication and Relative Wind

"jer" wrote in message
.net...
Scott, you got 100 percent bad advice in the previous responders.

And you are 100% idiot.

The bottom line is that the indicated airspeed DOES have errors depending on
the angle of attack.

You may well be correct that the error comes mostly from the static port and
not the pitot tube, but a) this is not relevant to the question asked (only
to part of some of the answers), b) hyperbole just makes you look as stupid
as you claim other people are, and c) much of the response the original
poster got was correct (in other words, the advice was NOT "100 percent
bad").

Welcome to the newgroup. You made quite an entrance.

Pete

Read my post again. I did not claim that AOA does not affect IAS.
I said that AOA does not affect the pitot to any appreciable degree.
The flow around the static source is another story.


In article , "Peter Duniho"
wrote:
"jer" wrote in message
k.net...
Scott, you got 100 percent bad advice in the previous responders.

And you are 100% idiot.

The bottom line is that the indicated airspeed DOES have errors depending on
the angle of attack.

You may well be correct that the error comes mostly from the static port and
not the pitot tube, but a) this is not relevant to the question asked (only
to part of some of the answers), b) hyperbole just makes you look as stupid
as you claim other people are, and c) much of the response the original
poster got was correct (in other words, the advice was NOT "100 percent
bad").

Welcome to the newgroup. You made quite an entrance.

Pete

jer wrote:
Read my post again. I did not claim that AOA does not affect IAS.
I said that AOA does not affect the pitot to any appreciable degree.
The flow around the static source is another story.



Easy now.

I'd forgotten about the static port and it's position with regard to air
flow. Although my original question focused on the pitot tube, it's
undoubtedly better to consider the entire pitot-static system when
considering the effect of different angles of attack on IAS.

So, if I slip to the right during the aforementioned steep approach and
the static port is on the forward left side of the fuselage, the static
pressure goes up a little bit, I suppose? Then, combining a slightly
lowered pressure at the pitot (due to AOA) plus a slightly higher
pressure at the static port, I get a lower IAS, right?

I realize the total error is still probably negligible. Just armchair
flying.

-Scott

"jer" wrote in message
nk.net...
Read my post again. I did not claim that AOA does not affect IAS.

Sure you did. When you wrote "you got 100 percent bad advice".

The only way that the responses to Scott's posts could have been "100
percent bad advice" would have been if there was no effect on the IAS due to
AOA.

Do you actually know what "100 percent" means?

Pete

If I'm descending slowly with a relatively nose-high attitude - say,
in preparation for a short field landing - does the high angle of the
pitot tube have any effect on indicated airspeed? (I suppose all
designs are different - say this is a 172.)

First, descending or climbing is irrelevant. Only your airspeed
affects the RW.

Second, according to the most authoritative sources, the static source
is the primary culprit behind the difference between IAS and CAS. The
pitot is very accurate until AOA's much greater than our little
aircraft can achieve. I know that popular literature suggests
otherwise.

Seems like the air would be passing slightly "over" the pitot tube
opening rather than "into" it, thus reducing the measure air pressure.
Is this correct? Is indicated airspeed affected by high AOA?

Early in my training, I was warned not to rely on the airspeed
indicator while the plane was in a slip. So I don't even look at it. I
slip pretty hard, too. ("High and hot and slipping like crazy," as
they used to say of Tom Buck.)

Is that dangerous?

all the best -- Dan Ford
email: (put Cubdriver in subject line)

The Warbird's Forum www.warbirdforum.com
The Piper Cub Forum www.pipercubforum.com
Viva Bush! blog www.vivabush.org

"Cub Driver" wrote in message
...
Early in my training, I was warned not to rely on the airspeed
indicator while the plane was in a slip. So I don't even look at it. I
slip pretty hard, too. ("High and hot and slipping like crazy," as
they used to say of Tom Buck.)

Is that dangerous?

I don't believe it's dangerous. When I'm doing a hard slip (C-172/C-152),
the AI is jumping all over the place anyway, so I choose not to look at it.

Incidentally, my instructor has pretty much broken me of using the AI much
anyhow (I'm VFR only). I fly mostly by feel now in C-172's, using the AI as
validation for what I am feeling. I think Rod Machado did an article about
flying not so much by the numbers, but by the feel of the plane. AI's can
lie and if they lie the wrong way (i.e. indicating too fast) and you really
rely on them, they can bite you.

Don't for a minute believe that I am saying that students shouldn't fly by
the numbers, but I believe their instructors should sometimes slap the ol'
rubber sticky over the AI towards the end of their training.

-Trent
PP-ASEL

I don't believe it's dangerous. When I'm doing a hard slip (C-172/C-152),
the AI is jumping all over the place anyway, so I choose not to look at it.

Yesterday I slipped into a private field that normally is one-way from
the west. There was a 12 mph wind from the west, so I approached from
the west. There's a road with 60-foot trees next to the end of the
runway, which is on a fairly steep hill. Slip how you may, it seems
that you never can land shorter than midway on the flat part of the
runway, which in any event is not flat but gently sloping to the west.
This reduces the runway to effectively 1000 feet. It is *very*
exciting, though with a headwind straight down the runway I stopped in
a couple hundred feet or less.

Of course what was in my mind when I went past those trees with my
starboard wing pointing down was this same question: is this
dangerous?

Whether it is dangerous or not, it is certainly exciting, and a whole
lot of fun.

Incidentally, my instructor has pretty much broken me of using the AI much
anyhow (I'm VFR only). I fly mostly by feel now in C-172's, using the AI as
validation for what I am feeling.

I also noticed yesterday that I found myself retarding the throttle
when I hadn't looked at the tach. Evidently I fly now as much by the
sound of the engine as by looking at the engine speed.

(Nor do I fly by the airspeed indicator. I peg the tach at 2150, or
2200 if speed is more important than fuel.)

It was a grand day for flying, if a bit breezy for the Cub.


all the best -- Dan Ford
email: (put Cubdriver in subject line)

The Warbird's Forum www.warbirdforum.com
The Piper Cub Forum www.pipercubforum.com
Viva Bush! blog www.vivabush.org

Scott Lowrey wrote:

If I'm descending slowly with a relatively nose-high attitude - say, in
preparation for a short field landing - does the high angle of the pitot
tube have any effect on indicated airspeed? (I suppose all designs are
different - say this is a 172.)

If your plane has a POH, there should be an airspeed calibration table or
chart in Section 5 "Performance."

My Warrior II's ASI will underread by about 7 kt at a high angle of attack,
and overread by about 7 kt at a very low angle of attack. The ASI in a
Cessna 172P will underread by 6 kt at a low angle of attack (7 kt with full
flaps), or overread by 6 kt at a high angle of attack.

In theory, I think, the ASI could be recalibrated to be more accurate at
high and low settings, but why would the manufacturers bother? The
calibration errors make the plane look like it has a much slower stall speed
and a slightly higher cruise speed than it really does.


All the best,


David

During the very nose high, full elevator mush, I describe in the BRS an
Descent thread, my Airspeed needle was pegged at the bottom, just like I was
sitting on the ramp. I know damn well I was moving forward but the low
speed in combination with the extreme angle of the pitot tube was preventing
the ASI from registering anything.

With a lot of power and heavy right foot, you can maneuver a 172 around
gingerly in level flight with nothing showing on the airspeed and the stall
horn shrieking like a demented banshee. Very good practice.
--
Roger Long