Talk:Airspeed

Merge

Ground speed should be merged into this article, since that article is not much more than a simple definition. Phoenix2 18:58, 31 December 2005 (UTC)

But they are entirely opposite definitions; one is speed through the air, the other is over the ground. What do you propose the title of the merged article should be? ericg ✈ 19:21, 31 December 2005 (UTC)

I agree with Ericg. What do you propose to call the new article? --TheLimbicOne(talk) 20:19, 8 January 2006 (UTC)

Likewise. Improving it to something more than a simple definition is the answer. Meggar 03:46, 11 January 2006 (UTC)

sounds ok

Compressibility

The talk about compressibility (and Bernoulli) in the "Indicated airspeed" section is totally off-base as far as I know. Equivalent airspeed is calibrated airspeed corrected for compressibility. Unless you're in an airliner or jet that enters the high-subsonic range of flight, most airspeed indicators are not corrected for compressibility. Once I'm back at school I'll try and haul out the references to get this sorted out; in the meantime, I'd appreciate confirmation on this if anyone else is knowledgeable. ericg ✈ 08:09, 24 December 2006 (UTC)

No! It's not off-base. Air is definitely compressible. Airspeed is a function of $Q_{c}$, which is the difference between the total pressure (or impact pressure) and the static pressure. Don't confuse impact pressure with dynamic pressure $(Q)$. Dynamic pressure is incompressible and is always defined as one half the density times the velocity squared:

$Q\ =\ {\frac {1}{2}}\rho V^{2}$

Where:

$Q$ = Dynamic pressure (Pa) (pounds per square foot)

$\rho$ = Density (kg per cubic meter) (slugs per cubic foot)

$V$ = Velocity (meter per second)(feet per second)

The total pressure (or impact pressure) sensed by an aircraft pitot tube in forward flight is related to dynamic pressure in a more complex way:

${\frac {Q_{c}}{P_{s}}}={\frac {1}{2}}\rho V^{2}\left({1+{\frac {M^{2}}{4}}+{\frac {M^{4}}{40}}+{\frac {M^{6}}{1600}}}...\right)$

Where:

$Q_{c}$ = Differential pressure (pounds per square foot)

$M$ = Mach number

$\rho$ = Density (slugs per cubic foot)

$V$ = Velocity (feet per second)

Differential pressure $(Q_{c})$ is more easily computed using knots instead of feet per second using the following equation:

$Q_{c}\ =\ P_{o}\left[\left(1+{\frac {\gamma -1}{2}}\cdot \left({\frac {V_{c}}{A_{o}}}\right)^{2}\right)^{\frac {\gamma }{\gamma -1}}-1\right]$

Where:

$Q_{c}$ = Differential pressure (inHG)

$V_{c}$ = Calibrated airspeed (knots)

$A_{o}$ = Speed of sound at standard sea level (knots)

$P_{o}$ = Static pressure at standard sea level (inHg)

$\gamma$ = Ratio of specific heats of a gas at constant pressure/constant volume (1.4 for air)

Please note that the above equation is only applicable when $V_{c}\leq A_{o}$.

When $V_{c}\geq A_{o}$, use the following equation:

$Q_{c}\ =\ {\frac {\left(1+\gamma \right)}{2}}\left({\frac {V_{c}}{A_{o}}}\right)^{2}P_{o}\left[{\frac {\left(\gamma +1\right)^{2}}{4\gamma -2\left(\gamma -1\right)\left({\frac {A_{o}}{V_{c}}}\right)^{2}}}\right]^{\frac {1}{\gamma -1}}-P_{o}$

Now plug in some numbers and do the math. You will notice that at low speeds (below 100 knots) there is little dicrepancy between dynamic and impact pressure while at higher speeds the difference is much more pronounced. Also, notice the formula in the article uses $Q_{c}$ (note the subscript c -- it means "compressible").

I suspect that some people may read that "Equivalent Airspeed (EAS) is corrected for compressibility" and then assume that the compressibility factor is then added to calibrated airspeed (CAS) to equal the EAS. Airspeed is by default compressible so to "correct" for compressiblity, you are actually removing the compressibility factor. Think of it like reverse correction. What I mean is, it is removing the compressiblity factor from CAS so that the airspeed corresponds to the dynamic pressure instead of impact pressure. Equivalent airspeed has limited applicability in real life (mostly used in engineering models). Most aircraft use calibrated airspeed in all their charted performance values.

I hope this sort of clears it up. If not, please add to this thread or e-mail me at gacman@hotmail.com. I love talking about this stuff. Sincerely, Matt Gould

EAS and other issues

I was busy writing an air data calculator a few weeks ago, and it occurred to me to look up what Wikipedia had on calibrated airspeed, equivalent airspeed, etc. There are a few issues on these pages that need attention. For example, it is not true to say that at sea level, in no wind, calibrated airspeed is the same as ground speed. That only applies in straight and level flight. Flying vertically up or down, it is possible to have a high airspeed with no ground speed. Also, in the CAS page, the statement

At higher altitudes CAS can be corrected for compressibility error to give equivalent airspeed (EAS).

is a bit misleading. CAS itself is corrected for compressibility at sea level in ISA conditions. EAS is a way of relating the speed at altitude to the speed at sea level on the basis of root sigma alone (lower case Greek letter sigma is the normal symbol used for density ratio), and does not take into account compressibility. In the page on EAS, the text

At sea level EAS is the same as calibrated airspeed (CAS). At high altitude, EAS may be obtained from CAS by correcting for compressibility error.

is likewise misleading. I think it would be better to say that at sea level, in ISA conditions, EAS is the same as TAS. (and you could say at sea level in ISA conditions, CAS is the same as TAS. These are only minor points, though, the bulk of the items on CAS and EAS is fine for subsonic flight. If I knew enough Latex to write the equations, I could launch into a major edit and update them for supersonic flight (which requires iterative solution of the pressure balance because there is no exact solution when pressure recovery across a normal shock is included). However this might detract from the simplicity of the entry. Is there anyone here who contributed to the original entries? There is a possibility that we are hitting a problem over usage of terms here. My earliest source material was internal notes on air data calculation by Trevor Jordan, writtin in the 1960's at Hawker Aircraft Limited. The current reference used in the UK aircraft industry is ESDU data sheet 69026 (which is an outstandingly clear account of the subject). It states "Equivalent Airspeed (EAS), V_e, is the (true) airspeed at sea-level in the International Standard Atmosphere that will give the same kinetic pressure, q, as exists at the aircraft flight conditions." (the kinetic pressure, q, is half.rho.V², where rho is density and V is true airspeed) What reference do our friends in the US use? WhatIsTrue 19:39, 23 February 2007 (UTC)

As it stands, you're right. The article is generally wrong about equivalent airspeed. It isn't generally used as a stepping stone between calibrated and true airspeeds. It's mostly used as an engineering convenience to do performance and structural calculations when compressibility isn't much of a concern. I'm going to set about cleaning it upPaulgush (talk) 20:59, 16 November 2023 (UTC)

Paulgush On Nov 27 you changed a sentence in the lead to say: “As the aircraft climbs into thinner, cooler air, its true airspeed grows ever greater than the airspeed indicated on the ASI.” See your diff.

This is unsatisfactory for two reasons. Firstly, as an aircraft flies into cooler air (with no change in air pressure) TAS gets closer to IAS, not further apart. As you know, this is because reducing air temperature causes the air density to increase. As an aircraft climbs the reduction in air pressure causes EAS and TAS to be different; but this effect is moderated by the reduction in air temperature, tending to bring the two speeds closer together.

Secondly, air is neither thick nor thin. The relevant properties of air, for encyclopaedic purposes, are pressure, temperature and density.

I will change the text in the lead back to the previous text. Dolphin (t) 01:32, 29 November 2023 (UTC)

Indicated???

All references I found stated the Indicated airspeed was uncorrected for instrument errors. It is what the airspeed indicator indicates. —Preceding unsigned comment added by 12.152.124.38 (talk • contribs)

- and that is what I was taught in flight school (in the US).

I agree with the post above that some of this needs work. All references I found stated the Indicated airspeed was uncorrected for instrument errors. It is what the airspeed indicator indicates.

I think that there is confusion here as to the difference between indicated and calibrated airspeed. Indicated is what the instrument shows - calibrated is corrected for various errors. Or, perhaps, there is a difference between British/Engineering definitions and US/Pilot definitions.—Preceding unsigned comment added by 68.184.132.44 (talk • contribs)

The source given (BCAR section D) was replaced by JARs in 1979, later by EASA. Changing it to line from EASA CS-definitions should fix it. Meggar 23:46, 9 July 2007 (UTC)

Added. Meggar 16:51, 12 July 2007 (UTC)

Airspeed is a vector quantity

It says in the article 'True airspeed is a vector quantity'.. is that really correct? I don't know about aviation terminology, but every physics student knows that speed is a scalar and velocity is a vector.. isn't it air velocity which is a vector quantity..? Zargulon (talk) 22:02, 15 August 2011 (UTC)

You are correct. Any scientist will confirm that speed is a scalar quantity, and the equivalent vector quantity is velocity. "True airspeed" is aviation terminology that has evolved over the past century for use by pilots and others in the aviation industry and elsewhere. Industry terminology is not always the same as scientific terminology. Scientists would understand, and refer to, the velocity of an aircraft relative to the atmosphere. Aviation terminology for the same thing is true airspeed.

Navigators and pilots often combine the true airspeed, wind speed and ground speed when determining the heading they should fly in order to reach their destination. These three speeds are treated as vectors when navigators and pilots draw the triangle of velocities Dolphin (t) 05:37, 16 August 2011 (UTC)

A vector is a speed AND a direction. Even aviators must know both airspeed and heading to navigate properly. The wind triangle you refer to is a way of calculating both HEADING and speed to fly from other known vectors such as METAR. I've removed that sentence, it's completely wrong no matter what background you're from.110.175.228.227 (talk) —Preceding undated comment added 11:33, 25 October 2011 (UTC).

You removed the sentence The true airspeed is a vector quantity. You say it's completely wrong but you don't explain why you think it's completely wrong. Will you explain why you think it's wrong? I have inserted some information about true airspeed and heading constituting a vector quantity. Dolphin (t) 11:43, 25 October 2011 (UTC)

And completely alienated anyone who ever wanted to read this article. It's not a complicated thing to understand, and doesn't require maths beyond trigonometry. Well done. 13:26, 11 March 2013 (UTC)~~ — Preceding unsigned comment added by 110.175.228.227 (talk)

Excellent Work!

I would like to compliment those who have built this page. It is comprehensive, accurate, and useful - and I don't have to go figure out where my old Boeing Airspeed manual got to in my library.

Congratulations! And thank you all.Cronkurleigh (talk) 20:29, 25 March 2014 (UTC)

"Needle alive" listed at Redirects for discussion

The redirect Needle alive has been listed at redirects for discussion to determine whether its use and function meets the redirect guidelines. Readers of this page are welcome to comment on this redirect at Wikipedia:Redirects for discussion/Log/2023 December 15 § Needle alive until a consensus is reached. Utopes _{(talk / cont)} 01:12, 15 December 2023 (UTC)

"Airspeed alive" listed at Redirects for discussion

The redirect Airspeed alive has been listed at redirects for discussion to determine whether its use and function meets the redirect guidelines. Readers of this page are welcome to comment on this redirect at Wikipedia:Redirects for discussion/Log/2023 December 15 § Airspeed alive until a consensus is reached. Utopes _{(talk / cont)} 01:12, 15 December 2023 (UTC)