Talk:Airspeed

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[edit] 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

[edit] 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 (pounds per square foot)

ρ

= Density (slugs per cubic foot)

V

= Velocity (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:

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

Where:

Q c

= Impact pressure (pounds per square foot)

M

= Mach number

ρ

= Density (slugs per cubic foot)

V

= Velocity (feet per second)

Impact 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

= Impact 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)

γ

= 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 $V_c \le A_o$ .

When $V_c \ge A_o$ , 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 on slow, low flying aircraft with reciprocating engines). 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

[edit] 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)

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Talk:Airspeed

From Wikipedia, the free encyclopedia

[edit] Merge

[edit] Compressibility

[edit] EAS and other issues

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