ASI under-read or over read?

[antonybradford]

Hi there,

Can someone explain what happens to the ASI and the indicated airspeed, as temperature increases / decreases?

My thoughts are if the temperature at x thousand feet is ISA + 5, then the density will be less, so the ISA will over read? is this true? Im getting mixed up with how pressure will affect IAS and density...???

If someone could de-confuse me, i would be most grateful!

Regards,

Antony

[Farmer 1]

First of all, Antony, you must realise that a; the ASI is an extremely complex bit of kit. Simple is not the word you would use to describe it. b; like other barometric devices, it is designed to operate in the International Standard Atmosphere, which is an item which does not exist anywhere in the world. For this reason, the further away from the ISA, the greater will be the error in the indications.

I fly helicopters, i.e. relatively low and slow, and an under-reading of 10 knots or less is the norm for me, with the error varying directly with altitude. In my experience it is always an under-reading error.

An error of such magnitude does not normally have any real effect on us - we normally calculate ETA and fuel requirement etc. directly from the ground speed. If we feel so inclined, we calculate TAS with a Dalton computer.

[bookworm]

My thoughts are if the temperature at x thousand feet is ISA + 5, then the density will be less, so the ISA will over read? is this true?

The density is less, but that makes the ASI underread. Lower density means lower dynamic pressure, which is what the ASI measures. Lower dynamic pressure at sea level density (which is what the ASI assumes) would be a lower speed. So the ASI shows a lower speed.

[Farmer 1]

It's a long time since I've done this stuff, and I don't have the books to hand, but what I do manage to remember is: the ASI uses pitot pressure from the pitot tube, and static pressure from the static vents. The former comprises dynamic and static pressure, so the latter is cancelled out to leave dynamic pressure, which in effect is translated into IAS.

I stand to be corrected, but I believe the sea-level air density has only minimal variations, and so has a correspondingly minor effect on the IAS. Pressure and temperature, and their differences from the ISA, are the principal factors.

[bookworm]

I think it all depends on what antonybradford means by "over read".

I was interpreting it as "does the ASI read more or less than the true airspeed?" In which case it "underreads" in conditions of lower than sea level density.

You may be interpreting it as "does the ASI read more or less than the IAS when the temperature in not ISA?"

The latter is a bit circular, because the IAS is what the ASI reads. ;)

[High Wing Drifter]

Antony,

Just think of the ASI as a moleculemeter; that it simply counts air molecules (an oversimplification but effective), then if the air density is less, there are few molecules to be counted, so the ASI would underread when compared to the actual speed through the air, the True Air Speed (TAS).

[FlyingForFun]

I agree 100% with Bookworm's second answer - there are (at least) two ways of interpreting the question.

If you are talking about the effect on IAS vs TAS, the easy answer is to have a play with your CRP1, which will convert IAS to TAS and will take the outside air temperature into account. Try it for a given IAS at a given altitude, and see how the TAS varies as you adjust the temperature.

If you are talking purely about the IAS, then remember that the airspeed indicator, assuming you ignore errors in the instrument, measure the number of molecules of air passing over the wing. Also remember that it is the number of molecules passing over the wing that determine how the wing behaves (the amount of lift and drag it generates for a given angle of attack), and this is why we use the ASI the way we do during flight. If you change the outside air temperature, the number of molecules of air passing over the wing may well change, but it will still be correctly indicated by the ASI. So, from an aviation point of view, no correction is required, but from a navigation point of view it will be.

FFF
----------------

[Old Smokey]

For a given dynamic pressure, at the same altitude, temperature change will have no effect upon IAS/CAS/RAS/EAS. For a given altitude, 100 knots of IAS will be at exactly the same dynamic pressure on a hot or a cold day. TAS will vary, lower on a cold day, higher on a hot day.

Similarly, for a given static pressure, temperature change will have no effect upon Indicated Altitude or Pressure Height. A static pressure of 696.81 hPa will indicate 10,000 feet of Pressure Height regardless of temperature. True Altitude will vary, lower on a cold day, higher on a hot day.

Regards,

Old Smokey

[bookworm]

For a given dynamic pressure, at the same altitude, temperature change will have no effect upon IAS/CAS/RAS/EAS

I think I agree with all except EAS. If you change the static pressure, there's a variation in EAS vs CAS. I guess if you said "at the same pressure altitude" then that would ensure the same static pressure.

[High Wing Drifter]

Gawd! Judging by the question, Antony is just starting out. With that in mind, the advanced concept of EAS has no relevance to a PPL student and must surely confusing. Instead of CAS I think it better to use the term RAS as this specifically means manual rectification of the IAS reading and usually by using the PPL standard issue CRP-1 whizz wheel. Whereas CAS means correction of the dynamic pressure automatically by an Air Data Computer, only found on advanced machinary...again no relevance to a PPL student (or certainly not at this stage!)

[Final 3 Greens]

Anthony

HWD's molecule counter concept is helpful.

If you ever take off from a high altitude airport (e.g. Big Bear in California, nearly 7,000 above sea level), you will notice that your ground speed (at the airspeed indicated) appears a lot higher than it would do at sea leve.

This is because the ASI is counting less molecules due to the reduced density of the air.

But you need to achieve the same airspeed to lift off, due to the laws of physics!

Now if the temperature rises, the air will get a little less dense and there will be less molecules to count, so you will need even more groundspeed to achieve the same indicated airspeed.

Seeing this with your own eyes is truly a learning experience, as the theory becomes tangible!

Without the benefit of a tangible reference, you need to visualise HWDs molecule counter and as the air gets less dense (because of increased altitude or warmer temperature), there will be less molecules to count and therefore the ASI will under read compared to true airspeed.

Good luck with the rest of your PPL.

[bookworm]

Gawd! Judging by the question, Antony is just starting out. With that in mind, the advanced concept of EAS has no relevance to a PPL student and must surely confusing.

He's either just starting out (in which case I think I answered his question in my first post) or he's asking something very subtle. We're not sure which. :) Granted, the EAS debate with Old Smokey would be better taken to Tech Log!

Instead of CAS I think it better to use the term RAS as this specifically means manual rectification of the IAS reading and usually by using the PPL standard issue CRP-1 whizz wheel. Whereas CAS means correction of the dynamic pressure automatically by an Air Data Computer, only found on advanced machinary...again no relevance to a PPL student (or certainly not at this stage!)

Can't find much support for that. The Oxford view (http://www.oxfordaviation.net/forum/topic.asp?TOPIC_ID=24) seems to be that the terms are equivalent, and that RAS is just older and more British.

I certainly don't follow how the correction can be done with a CRP-1. IAS to CAS involves correction for measurement errors, which is instrument- and aircraft-specific, and needs a look-up table. Correction of CAS to TAS is algorithmic.

http://www.mathpages.com/home/kmath282/kmath282.htm

looks good for the technically-minded.

[High Wing Drifter]

Bookworm,
I certainly don't follow how the correction can be done with a CRP-1. IAS to CAS involves correction for measurement errors, which is instrument- and aircraft-specific, and needs a look-up table. Correction of CAS to TAS is algorithmic.
Yes I agree. My statement was misleading. All I meant was that the RAS/TAS calcs are on the CRP-1. However, having just taken a look at my CRP-1, to my suprise is says "RAS CAS". So I am forced to agree, RAS and CAS are the same thing :O However, I'm sure it is terminologically incorrect to say a 737 EFIS displays RAS...a bit weak? I'll get me coat.

[bookworm]

Well HWD I'm no scientific lexicographer, but I think it doesn't matter mu... oh hang on, no, I am a scientific lexicographer, that's right. ;) Good discussion.

[Old Smokey]

IAS = Indicated Airspeed, Dynamic pressure plus all of the errors.

CAS = Calibrated Airspeed, Dynamic pressure minus all of the errors except for compressibility).

RAS = Rectified Airspeed, the same as CAS.

EAS = Equivalent Airspeed, CAS/RAS minus compressibility error (which is related to Mach Number and TAS)

For a given CAS/RAS at a fixed Pressure Height, EAS will be the same, unaffected by temperature variation. The fact that compressibility error is dependant upon Mach Number or TAS, would seem to be contradictory due to temperature involvement. In the conversion from CAS/RAS to Mach Number OR EAS, the absolute temperature is 'self cancelling' within the formulae.

and, to finish...........

DAS = Density Airspeed. The 'phoney' TAS that results from computing TAS from CAS and Density Height, without consideration of compressibility. OK at Low speeds and Altitudes, horrendous errors at high speeds and high altitudes.

Not sounding off, a lot of PPLs go on to CPL, ATPL, you know the drill.....

Regards,

Old Smokey

[bookworm]

I find all this stuff about "minus compressibility error" quite bamboozling. I'm not for a moment suggesting that it's wrong, but let me share my way of looking at it. It may help some -- if it doesn't ignore it!

Call the dynamic pressure Pd and the static pressure Ps.

EAS is a measure of Pd/Ps, i.e. EAS is a function that depends only on (Pd/Ps) and quantities that are constants.

CAS is a measure of Pd - Ps, i.e. CAS is a function that depends only on (Pd - Ps) and quantities that are constants.

TAS can be calculated from Pd/Ps and the temperature.

Thus in order to work out your TAS you can:

1) Measure Pd/Ps (i.e. EAS) and make a correction for the known, non-standard temperature. At standard sea level temperature (15 degC) the constants in the EAS function are arranged so that EAS = TAS.

2) Measure Pd-Ps (i.e. CAS) and do some potentially more complex manipulation which involves making a correction for the known, non-standard temperature and the pressure Ps. At standard sea level temperature (15 degC) and pressure (101.325 kPa) the constants in the CAS function are arranged so that CAS = TAS.

The classic ASI attempts to measure Pd-Ps (it's easier mechanically than Pd/Ps). Thus we're saddled with using 2 most of the time.

Finally, we can note that the correction in 2 depends on pressure/temperature i.e. density, plus some higher order terms in Mach number. Thus at low Mach numbers (i.e. before accounting for compressibility), we can approximate TAS by making a correction to CAS based on a single quantity, density.

[Old Smokey]

What you've given there, bookworm, is Density Airspeed (DAS). Entirely acceptable up to about 250 KTAS and 10,000 feet.

Regards,

Old Smokey

[Flik Roll]

Some ASIs you can adjust for OAT. I have never bothered! Does anyone do this?

[bookworm]

What you've given there, bookworm, is Density Airspeed (DAS). Entirely acceptable up to about 250 KTAS and 10,000 feet.

Excellent. Thanks Smokey, that makes perfect sense. I'd not come across that term before.

The page I quoted earlier suggests that the fraction error is of the order M^2/8, i.e. about 3% at M = 0.5, but approaching 10% at typical CAT cruise Mach numbers -- very much in line with your assertion.

[High Wing Drifter]

Some ASIs you can adjust for OAT. I have never bothered! Does anyone do this?
To get an estimated TAS you mean? Neither have I. The squares method is good enough for navigation. You basically take (((height/5000)+2)^2/100)+1 * IAS. Or in a usuable rough working mental rule of thumb form :

For 5000' TAS = IAS + 3^2% (or basically 10%)

For 10000' TAS = IAS + 4^2%

For 15000' TAS = IAS + 5^2%

For 20000' TAS = IAS + 6^2%

...etc...

For 40000' TAS = IAS + 10^2% (or double your IAS!).

[funfly]

I seem to remember that someone bought Concord's ASI as a souvenir and was surprised that the maximum was well below the speed of sound.

[overmars]

if i remember correctly, the mach number is dependent on the ambient temperature and pressure. and if i also remember correctly, it is easier to reach Mach 1 at a higher altitude.

[Old Smokey]

overmars,

Mach Number is totally dependant upon dynamic pressure (CAS or EAS) and the static pressure (Ps). Temperature does not come into play.

For example 300 Kt CAS at 30,000 feet (EAS = 285.0), Mach Number is always 0.7905, regardless of temperature.

Calculation of TAS WILL require Mach Number and Temperature.

It is easier to reach any Mach Number at a higher aktitude. Total drag is the sum of the High speed polars, (wave drag due to mach number above Mcrit) and Low speed drag polars due to EAS (dynamic pressure). As altitude increases, for a given mach number, the low speed drag polar decreases due to decreasing EAS/CAS.

Regards,

Old Smokey