Talk:Equivalent airspeed

How to find CAS at an altitude of FL310, indicated airspeed 250 KIAS and Total Temperature of -30 degrees C?

The given formulas cut the issue a bit short. I will expand as soon as possible to give a useful rule of thumb. —Preceding unsigned comment added by Schorsch VAK191 (talk • contribs) 10:49, 30 November 2007 (UTC)

Intro unclear

Quote: "EAS is the airspeed at sea level in the International Standard Atmosphere that would produce the same dynamic pressure as the true airspeed (TAS) at the altitude at which the aircraft is flying" -- Despite the long complicated sentence, I'd say TAS doesn't produce dynamic pressure, TAS produces no pressure at all, TAS is a math value in navigation (ground- and windspeed calculations) and has nothing to with gas physics. I support R. Bowyer in "Aerodynamics for the professional pilot" (ISBN 1 85310 326 8) who writes EAS is "IAS corrected for position error and compressibility error". --Suaheli (talk) 16:36, 16 August 2011 (UTC)

P.S.: The Bowyer quote would also agree with the formulas shown in the article. The problem is just the intro. --Suaheli (talk) 16:42, 16 August 2011 (UTC)

I guess it just shows that different explanations appeal to different readers. I find the current sentence (EAS is the airspeed at SL in the ISA ... at which the aircraft is flying) is ideal. It explains perfectly the relationship between the aircraft's velocity (TAS), its density altitude and its EAS. Firstly we develop an understanding of air density, then true airspeed, and then EAS follows logically from these two foundation ideas. In contrast, the explanation by Bowyer suggests that first there is IAS, then the correction for position error, then the correction for compressibility error, and finally we arrive at EAS! How many pilots truly believe that to understand equivalent airspeed you must first understand compressibility, then position error and compressibility error, then the need for correction of position and compressibility errors? EAS is actually one of the most fundamental concepts in navigating an aircraft. An understanding of EAS must come long before an understanding of position error and compressibility error is needed, not the other way around.

I see no problem with the existing intro. It is an explanation that appears in many sources on aviation and aerodynamics. Dolphin (t) 22:41, 16 August 2011 (UTC)

"It explains perfectly the relationship between the aircraft's velocity (TAS), its density altitude and its EAS. Firstly we develop an understanding of air density, then true airspeed, ..." -- I agree, I think the idea is fine. But that sentence I'm criticizing doesn't even mention the density. And I still don't understand the logic or grammar: Where's the "if" after the "would"? -- --Suaheli (talk) 16:34, 17 August 2011 (UTC)

Is the following an improvement?

• The equivalent airspeed (EAS) of an aircraft is the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same as it is at the true airspeed (TAS) and the altitude at which the aircraft is flying.

I agree that when trying to comprehend EAS for the first time, a narrative expression such as the one presently in the article (or my suggested alternative above) is not easy to absorb so it is insufficient. I think the math equation is also essential. (I'm talking about the equation showing that EAS is equal to TAS times the square root of the density ratio.)Dolphin (t) 22:57, 17 August 2011 (UTC)

Grammer wise, it's better, I think. But why do you hide such a simple compact word like air density behind this long and complicated abstract term "... sea level in the International Standard Atmosphere at which the dynamic pressure ..."? I don't critisize the sequence of the explanation, be it IAS to EAS via position and compressibility error -- or TAS to EAS via density error. Just name it. --Suaheli (talk) 23:56, 18 August 2011 (UTC)

P.S.: I think I completely understood our language problem only now. You consider the structural integrity of the aircraft, while I consider instrument flying and nav calcs only. You think of pressures affecting the aircraft construction, I think of error corrections affecting the pitot-static system. You just want to say that EAS is all you need to know if you want to check the dynamic pressure against the aircraft. OK, got it. But while this info is good for aircraft designers, it lacks all the info a pilot needs. I think both sides are relevant. Designer and pilot. --Suaheli (talk) 01:38, 19 August 2011 (UTC) -- P.P.S.: Quote: "EAS is actually one of the most fundamental concepts in navigating an aircraft." (Dolphin). I've never seen EAS on the flight deck or in flight planning. Only TAS, IAS, Mach (and sometimes CAS). Fundamentally: IAS and Mach for aerodynamics, TAS for navigation. At least from a pilot's point of view. Can't speak for designers. --Suaheli (talk) 01:48, 19 August 2011 (UTC)

I would really like to be able to write EAS is the speed that appears on the airspeed indicator of an aircraft with a perfect airspeed indicating system. Unfortunately I have never seen that sort of statement in any reliable published source so I can't use it on Wikipedia. (For reasons of WP:RS)

By perfect airspeed indicating system I mean a system in which:

1. the static port has no position error - always senses the air pressure at the altitude at which the aircraft is flying
2. the pitot tube has no alignment error - always points into the oncoming flow and senses total pressure
3. the airspeed indicator has no mechanical errors or calibration errors
4. low-speed aircraft so that compressibility effects are so low they can be ignored

In any rigorous explanation of aircraft speeds it is necessary to begin with ground speed & wind speed, then TAS, then EAS, then PEC and IAS, then compressibility and CAS and Mach No. This is the order it is taught in courses on aerodynamics and aerospace engineering. EAS can be determined from either of two directions, depending on what initial information is provided. If you know TAS then the appropriate explanation for finding EAS is the one presently given in the intro to Equivalent airspeed, but if you know IAS then the appropriate explanation is the one about IAS corrected for PE and compressibility etc.

At lift coefficient's used in the cruise the PEC has to be very small, the difference between EAS and IAS is also small and it is reasonable to talk about IAS when you really mean EAS. In my experience pilots first learn about TAS and IAS. It is much later that they learn about position error and PE correction. Before they know about PEC, whenever they talk about IAS they are really talking about EAS. Many years ago I had quite a bit to do with the Embraer Bandeirante. In those days its flight manual quoted airspeed limitations in EAS. (It is much more common for flight manuals to quote airspeed limitations in CAS but the Bandeirante's flight manual seemed to be unique which is why I remember it.) Cheers! Dolphin (t) 02:31, 19 August 2011 (UTC)

Thanks. "EAS is the speed that appears on the airspeed indicator of an aircraft with a perfect airspeed indicating system" -- You're writing "perfect airspeed indicating system". This is a good example of a confusion potential. Define "perfect" :-) For stall control, IAS is perfect. For navigational timing, TAS is perfect. For structural control, EAS is perfect. I think different readers will approach the article from different backgrounds. And if only one certain background is assumed, e.g "EAS=perfect", it may confuse other readers who use another definition of "perfect". Cheers! --Suaheli (talk) 05:29, 19 August 2011 (UTC)

I just reread that intro and again had to shake my head. The whole picture that this language creates is just wrong, in my opinion. The statement implies that different types of airspeeds (IAS, CAS, EAS, TAS) produce different dynamic pressures! Of course, that's not true, they aren't pressures, they are speeds, taking various factors into account (e.g. TAS takes density into account). Funnily, all these speed types take the same pressure into account, because they are all derived from the same pitot-static system. It's the aircraft moving through the airmass which produces dynamic pressure, not the airspeed value. This pressure doesn't change if you switch from IAS type to TAS type or whatever. The windshield glass or flaps or pitot tubes encounter a dynamic pressure that is obviously indpendent of what kind of airspeed formula is used in the cockpit or in the designer bureau. Airspeed types are mathematical expressions, not pressure producers. --Suaheli (talk) 16:53, 21 August 2011 (UTC)

P.S.: Perhaps the intro should include something like this: "If an aircraft moves through the airmass [under ABC conditions] or [producing ABC dynamic pressure], then EAS is equivalent to [XYZ ...] ..." -- This would also explain the meaning of "equivalent". --Suaheli (talk) 17:06, 21 August 2011 (UTC)

P.P.S.: Well, you could say with 100 knots on the EAS we will encounter a different actual dynamic pressure than with 100 knots on the TAS. So, I guess the core problem in the current intro is the missing reference of "dynamic pressure". It's unclear what it refers to. Does it refer to the actual value? Or just to the theoretical value which the EAS incorporates? Or to the theoretical value which the uncorrected IAS incorporates? Etc. The intro mentions multiple comparisons, but none of them have a reference and that is highly confusing. At least for me. --Suaheli (talk) 17:15, 21 August 2011 (UTC)

I have refined the wording to avoid the impression that it is the True Airspeed that is producing the dynamic pressure. Dolphin (t) 00:19, 22 August 2011 (UTC)

Question

- Hello, Someone can give me the right reference for the last equation giving CAS/EAS link? thank you very much!—Preceding unsigned comment added by 79.93.145.86 (talk) 22:13, 23 August 2011