Airspeed indicator markings

[Tim Helton]

The POH for my 1971 Cessna 150 lists the various v-speeds in calibrated
terms... For example, the minimum flap operating range is listed as 49 mph
CAS, which is 40 mph IAS.

However, on my airspeed indicator the lower end of the white arc only
extends to 49 mph, not to 40 mph as I would expect.

Here's a picture of the airspeed indicator:

http://www.blairsville.com/n6921gx/MVC-866F.JPG

What am I missing here?

tim

[Chuck Forsberg WA7KGX]

The definition of the white arc was changed sometime in the 70's.
It's rather obscure, so please refer to the discussion in the
"training experiences" on my web page.

--
Chuck Forsberg WA7KGX N2469R c...@omen.COM www.omen.com
Omen Technology Inc "The High Reliability Software"
Author of YMODEM & ZMODEM ZMODEM Consulting: $200/hr
TeleGodzilla BBS: 503-617-1698 FTP: ftp.cs.pdx.edu pub/zmodem
POB 4681 Portland OR 97208 503-614-0430 FAX:503-629-0665

[Kirk Stinson]

Tim,
The lower end of the white arc does extend to 49 mph IAS (outer circle)
and 40 knots IAS (inner circle). So the information published in the POH
matches the information presented on the airspeed indicator. Am I
misunderstanding the question?

Kirk Stinson
DWH/Houston, TX

Tim Helton <t...@blairsville.com> wrote in message
news:23hh5.239$C41....@eagle.america.net...

[Tim Helton]

> The lower end of the white arc does extend to 49 mph IAS (outer
circle)
> and 40 knots IAS (inner circle). So the information published in the POH
> matches the information presented on the airspeed indicator. Am I
> misunderstanding the question?

All the speeds, including those in the POH, are in mph... The inner scale
(knots) is there just for reference.

My confusion relates to why the CASs in the POH match the IASs that are
painted on the airspeed indicator.

tim

[Tim Helton]

Here's a picture of the airspeed indicator:

http://www.blairsville.com/n6921gx/MVC-866F.JPG

And a picture of the POH listing the CASs:

http://www.blairsville.com/cas.jpg

See how the IASs match the CASs?

tim

[J. C. Kamienski]

I seem to recall reading something at some point in my training that some
old airspeed indicators actually "indicated" CAS instead of IAS.

I wish I could remember where, now...

Am I imagining I read that, or did I really see it somewhere?

Anyone?

Jill

Tim Helton (t...@blairsville.com) wrote:
: Here's a picture of the airspeed indicator:

[J. C. Kamienski]

Ok, I have to follow up my own post..

http://avstop.com/AC/landing/landing2.htm

"A fine point, but a very important one - airplanes manufactured before
the mid-1970s had their airspeed indicator color-coded speed range arcs
marked in calibrated airspeeds, and shown in miles per hour."

Jill

J. C. Kamienski (kami...@ucsu.Colorado.EDU) wrote:
: I seem to recall reading something at some point in my training that some

[Kirk Stinson]

Sorry...
I see what you are saying....so "CAS" and "IAS" are the same on these
older models. Isn't that odd? What about air density, temperature, etc?

Kirk Stinson
DWH/Houston, TX

[J C]

Well, time for a li'l history ... and let me see if my memory doesn't play too
many tricks ... unfortunately, I don't have a reference for this, just my
memory (not a good thing):

IIRC, sometime in the mid-to-late-'70s -- like around 1976 or 77 -- the rules
for how airspeed indicators were to be marked changed (some other things did,
too, like having a standardized POH among the different airframe manufacturers
-- a real one -- for small, SE aircraft; how the aircraft empty weight was
calculated changed, too). The "industry" didn't all change at the same time,
either ... different manufacturers and different models changed at different
times.

Prior to this date, airspeed indicators and the information in the PIM/POH
were all CAS and a table was provided to show the errors between CAS and IAS
...

After this date, instrument markings -- the POH/PIM -- everything -- changed
to IAS ... made sense to me at the time (but no one asked ... darn!). After
all, we fly IAS, not (necessarily) CAS.

Also IIRC, in the normal cruising speed range for an airplane (not the green
arc, just where the airplane "normally" cruises, like on a cross-country
flight), CAS and IAS are virtually identical. At the low end, near the bottom
of the green and white arcs, and where one normally flies the pattern, there
will be some difference between CAS and IAS, primarily because the pitot tube
isn't pointed directly into the airflow any more. There will be some errors at
the high end, too, but for most of us that won't matter too much.

So this means that your 1971 C-172 has instrument markings and speeds shown as
CAS; C-172s manufactured after 1976 (I think!!) were all in IAS.

JC Boylls

If anyone recalls something different, I'd like to know ...

[J. C. Kamienski]

CAS is just IAS corrected for instrumentation errors.
Correction for air density, temperature, etc will give you TAS.

Jill

Kirk Stinson (sti...@pdq.net) wrote:
: Sorry...

[Kirk Stinson]

Well...my post was not clear at all. In fact it was dead wrong.

Certainly, at standard pressure and temperature calibrated airspeed and true
airspeed are equal by definition. True airspeed is higher at density
altitudes in proportion to calibrated airspeed and indicated airspeed in
this case.

What I still don't understand is why they publish the speeds in CAS. Most of
all, I don't understand why Cessna (in addition to instrumentation errors)
does not take into account and publish how certain angles-of-attack will
affect the indicated airspeeds in relation to CAS. Are they negligible
during normal maneuvers and operations in the C-150?

[Chuck Forsberg WA7KGX]

CAS is also corrected for angle of attack. There is a correction
table for my 1964 Skylane with entries for different configurations.
The important thing to keep in mind is, the rules changed sometime
in the 70's.