Go Back  PPRuNe Forums > Non-Airline Forums > Private Flying
Reload this Page >

CAS on airspeed indicator

Private Flying LAA/BMAA/BGA/BPA The sheer pleasure of flight.

CAS on airspeed indicator

Old 21st Mar 2018, 17:44
  #1 (permalink)  
Thread Starter
 
Join Date: Apr 2008
Location: nowhere
Posts: 1
Likes: 0
Received 0 Likes on 0 Posts
CAS on airspeed indicator

Looking through my Beech V35A POH, I notice that it has a table for important airspeeds with both Calibrated(CAS) and the Indicated(IAS) equivalent speeds which are different than CAS.

But then there is a note that says "The airspeed indocator is marked in CAS values". If so, what is the point of giving IAS values? Doesn't this mean that CAS always equals IAS?
JammedStab is offline  
Old 21st Mar 2018, 20:42
  #2 (permalink)  
Moderator
 
Join Date: Feb 2000
Location: UK
Posts: 14,031
Received 11 Likes on 4 Posts
The ASI is in IAS.

CAS is a necessary step for some calculations, obviously - and it's a requirement to provide a correction in the manual for part 23 aeroplanes, which includes your Beech.

There was a phase in the 50s and 60s when some American manufacturers provided limits and speeds in CAS, not IAS. This was a really stupid idea and was eventually beaten out of them. If your manual does that, make the conversions using the table in the POH performance section, and fly to IAS.

G
Genghis the Engineer is offline  
Old 22nd Mar 2018, 08:30
  #3 (permalink)  
 
Join Date: Feb 2003
Location: Oxford
Posts: 2,043
Likes: 0
Received 0 Likes on 0 Posts
'The ASI is in IAS' is, of course, a tautology. A rather nice one.
tmmorris is offline  
Old 22nd Mar 2018, 08:50
  #4 (permalink)  
 
Join Date: May 2005
Location: France / Qatar
Age: 68
Posts: 1,099
Likes: 0
Received 0 Likes on 0 Posts
Actually the ASI shows ‘ASIR’, or ‘Air Speed Indicator Reading’.

Correct for instrument errors and you will get IAS, in other words what a perfect instrument would read.

Correct for pressure error and you get RAS (or CAS for Americans).

Correct for compressibility and you get EAS.

Correct for density and you get TAS

Correct for wind component and you get GS

But to return to the subject of the thread, I too found it unhelpful back in the day when some US manufacturers marked the ASI dial with colo(u)red arcs, referenced to CAS instead of IAS.
eckhard is offline  
Old 22nd Mar 2018, 11:44
  #5 (permalink)  
 
Join Date: Nov 2005
Location: Zulu Time Zone
Posts: 729
Likes: 0
Received 0 Likes on 0 Posts
eckhard

Actually the ASI shows ‘ASIR’, or ‘Air Speed Indicator Reading’.

Correct for instrument errors and you will get IAS, in other words what a perfect instrument would read.
I'd be interested to see a reference for that (other than wikipedia).

What you read on the dial is by definition the Indicated Airspeed, according to all the many texts where I have encountered a definition for it. It is what you see on the ASI, not what you get after reading the ASI and then applying instrument error correction (which would be some half corrected value between IAS and CAS).
‘Indicated airspeed’ means the speed of an aircraft as shown on its pitot static airspeed indicator.

Certification Standard Definitions - EASA

Indicated airspeed (IAS)—the direct instrument reading obtained from the ASI

FAA Pilot Handbook of Aeronautical Knowledge

The indicated airspeed (IAS) is the actual instrument indication for some given flight condition.

Aerodynamics for Naval Aviators

Indicated airspeed (IAS). The direct instrument reading
obtained from the airspeed indicator, uncorrected for
variations in atmospheric density, installation error, or
instrument error.

FAA Airplane Fllying Handbook

Indicated Airspeed (IAS)
IAS is shown on the dial of the instrument, uncorrected for
instrument or system errors


FAA Instrument Flying Handbook

[The] gauge is calibrated in knots of indicated airspeed (KIAS).

Naval Aviation Schools Command Fundamentals of Aerodynamics
oggers is offline  
Old 22nd Mar 2018, 17:59
  #6 (permalink)  
 
Join Date: May 2005
Location: France / Qatar
Age: 68
Posts: 1,099
Likes: 0
Received 0 Likes on 0 Posts
Yes, I’d be interested to remember where I read that too! I promise that I didn’t make it up, nor look it up on Wikipedia.

Sometimes one recalls a ‘fact’ without knowing from where (or even why) one knows it. It’s also true that sometimes these ‘facts’ turn out to be wrong, and that the memory was faulty.

I want to say ‘Mechanics of Flight’ but someone nicked my early copy and I only have a rather inferior later edition, which I’ve mislaid.

So, if anyone can come up with a reference (come on, ETPS guys!) I’d be very grateful.
eckhard is offline  
Old 22nd Mar 2018, 18:16
  #7 (permalink)  
Moderator
 
Join Date: Feb 2000
Location: UK
Posts: 14,031
Received 11 Likes on 4 Posts
Originally Posted by eckhard View Post
Actually the ASI shows ‘ASIR’, or ‘Air Speed Indicator Reading’.

Correct for instrument errors and you will get IAS, in other words what a perfect instrument would read.
Incorrect, as already stated. ASIR = IAS.

Correct for pressure error and you get RAS (or CAS for Americans).
Negative.

Correct for all system errors - including pressure, and you get CAS (Calibrated Airspeed), known by some Americans as RAS (Rectified Airspeed).


Correct for compressibility and you get EAS.
Correct, but seldom needed below 10,000ft or below 0.6 Mach.

Correct for density and you get TAS

Correct for wind component and you get GS
Correct

But to return to the subject of the thread, I too found it unhelpful back in the day when some US manufacturers marked the ASI dial with colo(u)red arcs, referenced to CAS instead of IAS.
Agreed - Cessna were the biggest offender, but other offenders exist.

G
Genghis the Engineer is offline  
Old 22nd Mar 2018, 18:20
  #8 (permalink)  
Moderator
 
Join Date: Feb 2000
Location: UK
Posts: 14,031
Received 11 Likes on 4 Posts
Originally Posted by eckhard View Post
Yes, I’d be interested to remember where I read that too! I promise that I didn’t make it up, nor look it up on Wikipedia.

Sometimes one recalls a ‘fact’ without knowing from where (or even why) one knows it. It’s also true that sometimes these ‘facts’ turn out to be wrong, and that the memory was faulty.

I want to say ‘Mechanics of Flight’ but someone nicked my early copy and I only have a rather inferior later edition, which I’ve mislaid.

So, if anyone can come up with a reference (come on, ETPS guys!) I’d be very grateful.
PJ Swatton, and he's still wrong.

https://books.google.co.uk/books?id=...%20IAS&f=false

The definition of the Newton just above his incorrect airspeed definitions on that page, is also wrong incidentally.

G
Genghis the Engineer is offline  
Old 23rd Mar 2018, 16:26
  #9 (permalink)  
 
Join Date: May 2005
Location: France / Qatar
Age: 68
Posts: 1,099
Likes: 0
Received 0 Likes on 0 Posts
Although I haven’t read the book by PJ Swatton, the wording of those definitions are very similar to those that I remember. So; maybe Mr Swatton is quoting from the book that I read, or the book that I read used Mr Swatton’s work as a reference, or both Mr Swatton’s book and the one that I read are quoting from the same third source. Is this subject mentioned in ‘Handling the Big Jets’? Maybe I read it there.

Anyway, Ghengis, could you please elaborate on what is wrong with Mr Swatton’s exposition? I have no axe to grind, just looking for accuracy. Also, his definition of the Newton seems ok to me? What’s wrong there?

A quick thought experiment:

Three aircraft of the same type are flying in formation, sufficiently spaced so as not to interfere with each other’s pressure sensing.

They are at the same altitude, in the same air mass and are keeping perfect station.
Therefore, they must be flying at the same TAS.

Aircraft A has been blessed with an ASI that has been constructed ‘perfectly’ and has no instrument errors.
Aircraft B and C have been fitted with ASIs that were made on a Friday afternoon and unfortunately suffer from some instrument error.

The ASI readings are as follows:

Aircraft A: 200kts
Aircraft B: 199kts
Aircraft C: 201kts

If one assumes that the ASI reading is actually IAS, then entering the three values into a computer to calculate the TAS would yield three different results. This cannot be correct, as the three aircraft are flying at the same TAS.

Therefore, the three IAS values must actually be identical, even though the ASI readings differ.

Should we care? No, of course not. In practice, we can assume that ASIR is equal to IAS, which is what is assumed by the handbooks and publications quoted above. But a test pilot might care, if the error was significant enough.
eckhard is offline  
Old 23rd Mar 2018, 16:53
  #10 (permalink)  
 
Join Date: Nov 2005
Location: Zulu Time Zone
Posts: 729
Likes: 0
Received 0 Likes on 0 Posts
Hi eckhard

If one assumes that the ASI reading is actually IAS, then entering the three values into a computer to calculate the TAS would yield three different results. This cannot be correct, as the three aircraft are flying at the same TAS.
Indeed. But the TAS derived would not be correct in each case because the instrument error is different in each case. Nonetheless the reading on the dial is still by defintion the IAS.

If you wanted the precise TAS you would need to know the actual instrument and position error of your particular installation but you don't have that. Instead you have the AFM IAS/CAS correction (if you even bother) and you assume it is good enough because it has been certified within a tolerance.

You could equally have all 3 gauges reading the same IAS and all 3 aircraft maintaining formation. But after checking the TAT in each aircraft and "entering the three values into a computer to calculate the TAS", you would again get 3 different values because all the TAT gauges are not quite reading the same. And again it cannot be correct because the aircraft are actually "flying at the same TAS". Or for TAT substitue altimeter reading and same problem again. Welcome to the real world!
oggers is offline  
Old 23rd Mar 2018, 17:31
  #11 (permalink)  
Moderator
 
Join Date: Feb 2000
Location: UK
Posts: 14,031
Received 11 Likes on 4 Posts
Re: Swatton.

1 Newton is the force to accelerate a mass of 1kg at 1m/s^-2, not 1kgx1m/s^-2

Kg are mass and Newtons are force. So 1kg causes about 10 N force downwards at 1g. It is untrue to say that 1kg is about 10N.

ASIR = IAS, they are simply different words for the same thing.

Whilst you can technically separate out gauge errors from pressure/position errors - it isn't actually done that way. Gauge errors are assumed to be trivially small and randomly distributed, so we only consider PEC in total as the total difference between IAS and CAS.

G
Genghis the Engineer is offline  
Old 23rd Mar 2018, 17:46
  #12 (permalink)  
 
Join Date: May 2005
Location: France / Qatar
Age: 68
Posts: 1,099
Likes: 0
Received 0 Likes on 0 Posts
Thanks oggers and Ghengis.

I have no argument with either of you. I am very happy to inhabit the real world and do indeed feel welcome. Sometimes it’s interesting to inhabit the theoretical world for a while and then return, blinking and confused, into reality!

I found the reference on page 7 of ‘Handling the Big Jets’ in google books.
That must be where I read it. DP Davies thought it was worth mentioning. Perhaps he had read Swatton’s book or maybe Swatton cribbed it from him? Maybe it was taught on the ETPS course which DPD attended?

I shall ponder and wonder further on Sunday when I watch my ASI reading 300kts with a TAS of 490 and a Mach of 0.85, the TAT showing 35C warmer than the SAT!

Isn’t physics interesting? And isn’t an aircraft an ideal place from which to observe some of the ideas?

Last edited by eckhard; 23rd Mar 2018 at 18:10. Reason: Reference to ‘Handling the Big Jets’ by DP Davies
eckhard is offline  
Old 23rd Mar 2018, 22:11
  #13 (permalink)  
 
Join Date: Mar 2002
Location: near an airplane
Posts: 2,361
Likes: 0
Received 1 Like on 1 Post
Originally Posted by eckhard View Post
Therefore, the three IAS values must actually be identical, even though the ASI readings differ.
In addition to what others (more learned than myself) have already mentioned, in your experiment the three IAS values are not identical as a correction from IAS to CAS is theoretically only valid for a specific airframe. For reasons you mentioned in your post, you can have different readings and therefore different IAS values. But fortunately, these differences are small and randomly distributed. I would change the wording to '...the three IAS values should be identical...'
Jhieminga is offline  
Old 23rd Mar 2018, 22:52
  #14 (permalink)  
Moderator
 
Join Date: Feb 2000
Location: UK
Posts: 14,031
Received 11 Likes on 4 Posts
Or in reality "the three IAS values should be close enough that the differences don't matter".

G
Genghis the Engineer is offline  
Old 24th Mar 2018, 00:44
  #15 (permalink)  
 
Join Date: Dec 2005
Location: West Sussex, England
Posts: 487
Likes: 0
Received 0 Likes on 0 Posts
Dear eckhard,

I think you also forgot to factor in the quantum effect !

p.s. I use GPS too, and lastly, re. the ASI - whatever it reads - checked at the stall is the most significant indication to remember.

mike hallam.
mikehallam is offline  
Old 24th Mar 2018, 08:29
  #16 (permalink)  
 
Join Date: Jan 2009
Location: Oxford, UK
Posts: 1,546
Likes: 0
Received 0 Likes on 0 Posts
Hello Mike. You are correct - checked at the stall is the true reading! I was flying a rented Russian glider in the women's European Championships a few years ago. We had a lovely evening before, celebrating with other participants....and I was not as careful as should have been reviewing my ground crew's preparations! On being towed up by a Wilga on pre competition practice, the ASI read initially zero, then wound round the dial to the stop!

The airtow was progressing normally, so I released at usual height (still slightly confused by all information being in meters) and did a series of gentle stalls. Instruments still no help, but the attitude was sufficient to recognise safe speed. All the same my ground crew came rushing up after I landed to say "too fast! too fast!"

Better than too slow, I reckoned.
mary meagher is offline  
Old 24th Mar 2018, 08:46
  #17 (permalink)  
 
Join Date: May 2005
Location: France / Qatar
Age: 68
Posts: 1,099
Likes: 0
Received 0 Likes on 0 Posts
Hello Mike,

Yes, I will have to recalculate; and Mary,

Great story!
eckhard is offline  
Old 24th Mar 2018, 14:07
  #18 (permalink)  
 
Join Date: Jun 2001
Location: Fragrant Harbour
Posts: 4,783
Likes: 0
Received 0 Likes on 0 Posts
The way I learned it and is the 'standard' Central Flying School answer is that Indicated Air Speed corrected for pressure error and instrument error becomes Rectified Air Speed. Correct RAS for compressibility and you get Equivalent Air Speed. Correct EAS for Density error and you get True Air Speed.

This is essentially the explanation given in AP3456, the RAF Manual of Flying.
Dan Winterland is offline  
Old 24th Mar 2018, 16:19
  #19 (permalink)  
 
Join Date: May 2005
Location: France / Qatar
Age: 68
Posts: 1,099
Likes: 0
Received 0 Likes on 0 Posts
Thanks Dan, sums it up well.
eckhard is offline  
Old 24th Mar 2018, 17:14
  #20 (permalink)  
 
Join Date: Oct 2017
Location: UK
Posts: 5
Likes: 0
Received 0 Likes on 0 Posts
Mr Engineer,

I'll give Peter a call. He lives just up the road and is an ex-colleague of mine. The book publisher in the past has injected some typo errors which were not picked up by the preafrooder, of which I was one.

At the request of the publisher all print submissions were sent as Word documents.


Mr DW.

Nowt wrong with CFS in our day.

dook
dook is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell or Share My Personal Information

Copyright © 2023 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.