Hercules Engines

(B2431) wrote:



OK, I see where you are coming from. The pitot pressure DOES vary by altitude.
To compensate for that when measuring airspeed you use static pressure to give
you altitude. Some aircraft detect static pressure from holes on the side of
the pitot tube and others have the static ports located elswhere.



But you need BOTH Dan, you cannot measure airspeed AND altitude
unless you have BOTH pitot pressure AND static pressure...
--

-Gord.

From: "Gord Beaman" )
Date: 1/18/2004 12:48 AM Central Standard Time
Message-id:

(B2431) wrote:



OK, I see where you are coming from. The pitot pressure DOES vary by
altitude.
To compensate for that when measuring airspeed you use static pressure to
give
you altitude. Some aircraft detect static pressure from holes on the side of
the pitot tube and others have the static ports located elswhere.



But you need BOTH Dan, you cannot measure airspeed AND altitude
unless you have BOTH pitot pressure AND static pressure...
--

-Gord.

That's what I said.

Dan, U. S. Air Force, retired

On 18 Jan 2004 08:08:50 GMT, (B2431) wrote:

From: "Gord Beaman" )
Date: 1/18/2004 12:48 AM Central Standard Time
Message-id:

(B2431) wrote:



OK, I see where you are coming from. The pitot pressure DOES vary by
altitude.
To compensate for that when measuring airspeed you use static pressure to
give
you altitude. Some aircraft detect static pressure from holes on the side of
the pitot tube and others have the static ports located elswhere.

But you need BOTH Dan, you cannot measure airspeed AND altitude
unless you have BOTH pitot pressure AND static pressure...

That's what I said.

Did you also say that you can't correct them without OAT, outside air
temperature? That's the third element of a pitot-static air data
system.

Mary

--
Mary Shafer Retired aerospace research engineer

"Mary Shafer" wrote...

Did you also say that you can't correct them without OAT, outside air
temperature? That's the third element of a pitot-static air data
system.

What is required -- static air temperature (SAT) or total air temperature (TAT)
for the OAT correction?

If TAT, how is it measured, and how many thermocouples are required, in what
configuration?

From: "John R Weiss"
Date: 1/19/2004 2:05 PM Central Standard Time
Message-id: _LWOb.85390$sv6.201965@attbi_s52

"Mary Shafer" wrote...

Did you also say that you can't correct them without OAT, outside air
temperature? That's the third element of a pitot-static air data
system.

What is required -- static air temperature (SAT) or total air temperature
(TAT)
for the OAT correction?

If TAT, how is it measured, and how many thermocouples are required, in what
configuration?


It's OAT. Air speed measured with just pitot and static pressure is referred to
as indicated airspeed, IAS. If you temperature compensate it becomes true air
speed, TAS.

Basic mechanical analog TAS indicators use one of two methods of temperature
compensation: one uses a capillary tube from a bulb mounted on the exterior of
the of the aircraft , the other uses a knob where you can manually dial in OAT.
The C-130s I worked on had the capillary type.

Thermocouples are not used for electrically detected OAT. What is used is a
temperature bulb usually mounted flush to the skin of the aircraft. If you look
at a C-130 you will see a 2"(?) disk on the skin on either side ofthe nose
wheel well. Those are temperature bulbs. They vary in resistance due to
temperature change.

For the life of me I can't remember if the resistance increases or decreases
with temperature drop.

Dan, U.S. Air Force, retired

(B2431) wrote:



It's OAT. Air speed measured with just pitot and static pressure is referred to
as indicated airspeed, IAS. If you temperature compensate it becomes true air
speed, TAS.


Well, it's been many years but ISTR that there's another step
here. Isn't there such a thing as RAS?, maybe Rectified Air
Speed?...IAS corrected for temp? (or maybe CAS?). I thought TAS
was IAS corrected for altitude?

Man!...it was something like 27 years since I last stepped down
that six foot ladder under the tail of an Argus...and about 50
years since the F/E course where I learned this stuff (so cut me
some slack guys!).
--

-Gord.

"Gord Beaman" wrote in message
...
(B2431) wrote:



It's OAT. Air speed measured with just pitot and static pressure is
referred to
as indicated airspeed, IAS. If you temperature compensate it becomes true
air
speed, TAS.


Well, it's been many years but ISTR that there's another step
here.

Yep, Dan is incompetent again.

Isn't there such a thing as RAS?

CAS, or computed airspeed.

, maybe Rectified Air
Speed?...IAS corrected for temp? (or maybe CAS?). I thought TAS
was IAS corrected for altitude?

You got it Gord, I knew you would remember.

Man!...it was something like 27 years since I last stepped down
that six foot ladder under the tail of an Argus...and about 50
years since the F/E course where I learned this stuff (so cut me
some slack guys!).

Dan and you can work out the pressure and temperature compensation.

"Mary Shafer" wrote in message
...
On 18 Jan 2004 08:08:50 GMT, (B2431) wrote:

From: "Gord Beaman" )
Date: 1/18/2004 12:48 AM Central Standard Time
Message-id:

(B2431) wrote:



OK, I see where you are coming from. The pitot pressure DOES vary by
altitude.
To compensate for that when measuring airspeed you use static pressure
to
give
you altitude. Some aircraft detect static pressure from holes on the
side of
the pitot tube and others have the static ports located elswhere.

But you need BOTH Dan, you cannot measure airspeed AND altitude
unless you have BOTH pitot pressure AND static pressure...

That's what I said.

Did you also say that you can't correct them without OAT, outside air
temperature? That's the third element of a pitot-static air data
system.


Yes, but would you believe that this whole squabble began by trying
to correct Tarver's misconception that the "Total" fitting of the two
fittings on an air data computer (labeled Static and Total) referred to
two or more pitot gizmos (Tarver hates to call them tubes) instead
of altitude pressure plus impact pressure?

JK