what do we need so many different airspeed?

[dolpinsky]

mach
tas
ias
cas
eas
gs

why cant we just use one???

[italia458]

You've only listed two speeds - TAS and GS. TAS is your speed through the air and GS is your speed over the ground.

Mach is a ratio, and the rest of those are measures of pressure.

[dolpinsky]

is that mean there are more than two airspeed?
and may i ask why is ias so important on takeoff and landing?
sorry for the stupid questions, i'm reading the bak and got a bit confused

[italia458]

IAS is the pilot's direct indication of EAS - which is essentially a direct measure of dynamic pressure. The wings react to dynamic pressure. At takeoff and landing speeds there is almost no difference between IAS and EAS. Any difference will be accounted for and the speeds published in IAS for a pilot's reference will reflect those differences.

From an engineer's POV, the airspeed indicator should be calibrated in units of pressure such as millibars. But from a pilot's POV, pressure doesn't make sense. They've calibrated the pressure indications for a standard day at SL, meaning that under those conditions, IAS will equal TAS. As altitude increases, IAS will decrease compared to TAS.

[dolpinsky]

ok i got it, thank you bro! ;)

[Uplinker]

Yes, IAS is the indicated airspeed - which the wings are flying at. Obviously this is most important for lift, drag, (stall) and control response.

As italia suggests; airspeed is measured as air molecules causing a pressure rise in the pitot probe due to the forward motion of the aircraft. This pressure is then corrected for many factors such as probe position error, instrument reading error, atmospheric density, etc. etc. to give an IAS reading in the cockpit. I invented a long mnemonic to remember all the CAS, EAS etc. to pass my ATPL exams, but have forgotten it now! The higher you fly, the less dense the air gets, so there are fewer air molecules causing less pressure in the pitot tube for a given speed. Therefore although an observer might see that you were flying at say 280knots, your cockpit gauge might only read 220knots. The former is the TAS, (true airspeed), the latter is the IAS.

Mach is the speed of sound for a given atmospheric density and temperature and is the speed of a pressure wave - which is what sound is - and is around 720Knots, depending on atmospheric density. Aircraft have a Mach indication because at high altitude and speeds, the speed of the pressure wave becomes important for the aerodynamics of the wing and fuselage; If the aircraft is flying at or faster than Mach 1, the pressure 'message' to the air molecules ahead of the aircraft to get out of the way cannot reach them before the aircraft actually does, and this causes a huge increase in drag. Speeds approaching Mach 1 can be seen over the tops of the wings even when the aircraft is flying below Mach 1, so aircraft are designed to have a maximum economic Mach number, in the region of say 0.73 to 0.84, although speeds greater than Mach 1 can be designed for, e.g. Concorde.

Ground speed is only really important for predictions of estimated time of arrival, and how much fuel you will use, which the wings don't care about, although Airbus for one also use groundspeed as a function in approach airspeed calculations.

[flybywire380]

As a follow-up to this thread .... all other things being equal, does stall speed change with altitude?

Uplinker - you mentioned that "pressure" is corrected for many factors to give IAS in the flight deck; not sure if this is the case ....

- CAS is IAS corrected for instrument & position error. CAS is shown in the flight deck
- EAS is CAS correct for "compressibility" effect
- TAS is EAS corrected for density
- GS is TAS corrected for wind

[italia458]

FBW,

It depends what airplane you're looking at to determine whether IAS or CAS is displayed to the pilots. If the airplane has a 'compensator' built in to take the readings from the pitot probe and correct them, then you will get CAS in the cockpit. A lot of airplanes don't have this function so IAS is displayed and there is a chart in the AFM where you can see what the CAS is for any particular IAS within the speed envelope.

As for stall speed changing with altitude - yes it does. Usually very small changes though. The 'truest' indication of performance deficit or surplus is measured based on EAS - even though your stall speed will be published in IAS, it is the EAS that matters. When you climb the air becomes more easily compressible and, therefore, there will be a bigger difference between IAS and EAS at altitude versus SL. The IAS stall speed will increase with increase in altitude so as to keep the EAS the same. That takes care of the compressibility problem. There is also the effect of Reynolds number. There are other people on here better qualified to talk about the effect of Reynolds number on the aerodynamic forces created by an airplane but the general principle is that Reynolds number decreases with increases in altitude and decreases with decreases in speed. You can find out more about it here: Reynolds Number (http://www.grc.nasa.gov/WWW/BGH/reynolds.html)

All in all, the changes are very small mostly because the stall speed is a relatively small number (the smallest in the normal flight envelope). If an airplane stalls at 120 KEAS at SL, it stalls at 120 KCAS. If it's at 35,000' the KCAS becomes 121.6 KCAS - a difference of only 1.6 knots!

[HazelNuts39]

CAS is IAS corrected for instrument & position error. CAS is shown in the flight deckTo be precise, IAS is the value read on the dial of the airspeed indicator, corrected for instrument error by reference to the calibration of the instrument against a known pressure source.

CAS is IAS corrected for errors in the pitot and static pressures. The static pressure error is called 'position error' because it attributable to the position of the static pressure source. The pitot pressure error is mainly due to the water drains in the pitot tube, but can become greater at extreme angles of attack.

Large modern airplanes use an air data computer (ADC) or Air Data/Inertial Reference Unit (ADIRU) to correct the measured pitot/static pressures for the known errors, and to calculate the CAS that is shown on the flight deck. On those airplanes IAS equals CAS, provided the airplane stays within the envelope for which the pitot and static pressure errors are known.

[HazelNuts39]

The 'truest' indication of performance deficit or surplus is measured based on EAS - even though your stall speed will be published in IAS, it is the EAS that matters. Well, no. EAS is relevant only as long as the air can be considered to be incompressible, i.e. at low Mach numbers.

CAS is greater than EAS because the pressure in the pitot is higher than it would be in incompressible flow. The compressibility that causes CAS to be higher than EAS affects all aerodynamic pressures that keep the airplane in the air.

[Groundloop]

To be truly accurate Groundspeed is NOT an airspeed.:=

[Airmann]

Ice-T Pretty Cool Drink

I C E T
P C D

(I)AS corrected for (P)osition or instrument error gives you
(C)AS corrected for (C)ompresibility effects gives you
(E)AS corrected for (D)ensitiy gives you
(T)AS

[flybywire380]

italia458/Hazelnuts39 - thanks for your replies - very helpful.

Cheers,
fbw380

[SuperflyTNT]

Is EAS directly proportional to drag?

Came across a book where it says, air frame cruise drag decreases with an increase in higher altitudes. This is mainly because we fly at a constant mach number above FL260. With a constant mach number EAS decreases with an increase in altitude. Hence drag decreases, as EAS is directly proportional to drag.

I get how EAS decreases with increase in altitude with constant MN. Don't understand how EAS is directly proportional to drag.

[HazelNuts39]

Came across a book where it says, air frame cruise drag decreases with an increase in higher altitudes. Not true in general, e.g. when climbing at constant CAS. However, when climbing at constant Mach close to Mmo it is true because your speed is getting closer to minimum drag (green dot) speed. If you continue at constant Mach below 'green dot', then your drag increases again (assuming you don't run out of thrust or hit buffet).

[SuperflyTNT]

OK thats understandable as drag increases with any increase of decrease below green dot speed/VIMD.

Also, quick doubt - Does your Mcrit speed increase with increase in altitude?

[Microburst2002]

It is a very honest and good question!!!

We have IAS
We would like to have EAS, which happens not to be a speed, in the velocity sense.

But there are errors. CAS is IAS corrected for position and instrument errors.
If we corrected also the compressibility error (which stems from the pitot tube compressing the air, and this compression is sensed as increased speed) then we would have EAS.

We would like to have TAS, too, because we need it for navigation and other stuff. TAS is the speed relative to the air mass in which we are flying. This one is truly a velocity. If we add the wind effect we will have GS, which we obviously need for navigation purposes. And of course it is a velocity.

EAS, and its children (CAS and IAS) is a way to read pressure in knots. At sea level in the ISA atmosphere, EAS and TAS are the same. EAS is the speed that you would need in the ISA at sea level to get the same dynamic pressure as you get at your TAS in your actual air mass. It is not an actual velocity. You need to know it because dynamic pressure is what makes the airplane fly.

The relation between TAS and EAS was arbitrarily stablished by some scientific. It is a convenience. There is no such thing as the "density error". Yes, you will read about it but we need EAS. TAS is not good for flying purposes.


All in all, the changes are very small mostly because the stall speed is a relatively small number (the smallest in the normal flight envelope). If an airplane stalls at 120 KEAS at SL, it stalls at 120 KCAS. If it's at 35,000' the KCAS becomes 121.6 KCAS - a difference of only 1.6 knots!


CAS is a very erroneous speed at 35,000 ft. The difference between IAS and CAS can be small, but the difference between EAS and IAS is very big, in the range of 15 kt. That is why stall speed increases with altitude. equivalent stall speed remains relatively constant, but IAS and CAS will increase with mach number.

Also, compressibility effects also have a part in stalling and even stall EAS will vary with mach number.

[HazelNuts39]

EAS is the speed that you would need in the ISA at sea level to get the same dynamic pressure as you get at your TAS in your actual air mass. It is not an actual velocity.Correct.

You need to know it because dynamic pressure is what makes the airplane fly. Wrong. Dynamic pressure is a theoretical concept, a convenient reference pressure for calculating aerodynamic coefficients. The real pressures generated in compressible air make the airplane fly.

[aterpster]

The three practical speeds are:

TAS

IAS

and

GS.

The methods of the finer points of aerodynamics as to how they are derived is done for you quite nicely by the software and hardware on modern aircraft. Let the aerodynamic engineers concern themselves with other then the foregoing three.

At altitude Mach becomes important but it is a functional form of IAS that accounts for compression.

All three (four) speeds are very important and can be significantly different.

To properly control the airplane below the altitude where Mach supercedes IAS, then IAS is paramount.

Without TAS you don't know how fast you are going through the air. That makes a big different for takeoff and landing performance at higher elevation airports (as well as high density altitudes).

Without GS you don't know when you're going to get to wherever you are going. :)

[Microburst2002]

Wrong. Dynamic pressure is a theoretical concept, a convenient reference pressure for calculating aerodynamic coefficients. The real pressures generated in compressible air make the airplane fly.


Dynamic pressure is also an empirical fact. It is the kinetic energy per volume unit of air. The more dynamic pressure, the more Lift, the more Drag, etc... And that is why you need to know it, because the aerodynamic forces depend on it. Real air has dynamic pressure.

When compressibility effects become noticeable then you need to know your mach number, too, because it will affect aerodynamic forces and set your limits before dynamic pressure does.

[HazelNuts39]

From Boeing Jet Transport Performance Methods, Measurement of Airspeed

impact pressure and dynamic pressure

The difference between the total pressure and static pressures sensed on the airplane, ( pt - ps), is called impact pressure. In the chapter entitled “Physics of Air”, however, we defined ( pt - ps) as dynamic pressure, ½ρV². What’s the difference?

Impact pressure and dynamic pressure are the same at low speeds, below 200 or 250 knots, in which speed range the density of the air is considered to be unaffected by the airplane’s speed. At higher speeds, the air density around the airplane is affected by compressibility and the term ( pt - ps ) is no longer equal to ½ρV². Just remember that impact pressure is defined as ( pt - ps) whereas dynamic pressure q is defined as the kinetic energy of the airflow, ½ρV².

When computing aerodynamic forces, you’ll use dynamic pressure frequently. For airspeed measurement, however, which must cover both the lower speeds where there’s no compressibility effect and the faster airspeeds where there IS some compression, we’ll use the term “impact pressure” to denote ( pt - ps ). Dynamic pressure is also an empirical fact. It is the kinetic energy per volume unit of air. The more dynamic pressure, the more Lift, the more Drag, etc... And that is why you need to know it, because the aerodynamic forces depend on it. Real air has dynamic pressure.Real air has impact pressure. The more impact pressure, the more Lift, the more Drag, etc.

When compressibility effects become noticeable then you need to know your mach number, too, because it will affect aerodynamic forces and set your limits before dynamic pressure does. The point is that compressibility affects aerodynamic forces at all speeds. The following graph shows the change of lift coefficient due to compressibility at given angles of attack (alpha) versus Mach number. The data points are from the aerodynamic data base of a large transport airplane. The ratio of impact pressure qc to dynamic pressure q0 is shown by the red line.

P.S.
The graph clearly shows that CAS (representing impact pressure) is actually a 'truer' indicator of aerodynamic performance than EAS (representing dynamic pressure).

http://i.imgur.com/shbcEhR.gif?1

[aterpster]

Good stuff for performance engineers.

[Microburst2002]

what is the legend in those graphs, exactly?

Real air has impact pressure. The more impact pressure, the more Lift, the more Drag, etc.

Then why don't we use an indication of that magnitude to fly an airplane?

Why do we use two instruments (altimeter and ASI) instead of one?

[HazelNuts39]

what is the legend in those graphs, exactly?The curves labeled alpha=3 through 7 (degrees) show cL/cL0, the ratio between the lift coefficient at a Mach to that at M=0 for the particular AoA or, if you like, the lift coefficient in real air divided by that in incompressible flow.

The red line is qc/q0, the ratio between impact pressure (pt - ps) and dynamic pressure ½ρV².

Perhaps you should ignore the thin blue (highest) line, it is the pressure correction pc/p0 according to Prandl-Glauert for small perturbations, e.g. thin airfoils at small angles of attack.
Then why don't we use an indication of that magnitude to fly an airplane?Well, the ASI provides just that, it indicates CAS, which is aerodynamically more relevant than EAS, as illustrated in the graph.

Why do we use two instruments (altimeter and ASI) instead of one? You're not interested in altitude?

[italia458]

HazelNuts,

You say that CAS takes into account compression of the air which affects aerodynamic forces at certain speeds and is therefore a better indication of aerodynamic performance. Is that a correct interpretation? I see your point, and it makes sense, but I do have other questions.

When computing aerodynamic forces, you’ll use dynamic pressure frequently. For airspeed measurement, however, which must cover both the lower speeds where there’s no compressibility effect and the faster airspeeds where there IS some compression, we’ll use the term “impact pressure” to denote ( pt - ps ).

That quote you included states that when computing aerodynamic forces you use dynamic pressure. That seems to contradict what you're writing, doesn't it?

I agree that compressibility does affect the aerodynamic forces on the airplane, which is why we need to take it into consideration when flying at high speeds and/or altitudes. However, when measuring airspeed, the pitot tube measures total pressure which means that it slows the air down to a speed of zero, relative to the airplane. The more you slow the air down, the more the air is compressed. The difference between the pitot tube and the airplane is that the air flow around the airplane doesn't get slowed to zero, excluding the boundary layer. It makes sense that there would be compressibility, and expansion, of the air depending on which location it's at on the airplane but I don't think the effects would be nearly as drastic as the difference between CAS and EAS.

[HazelNuts39]

That quote you included states that when computing aerodynamic forces you use dynamic pressure. That seems to contradict what you're writing, doesn't it?No, it doesn’t. Tradition and convenience dictates that aerodynamic coefficients are referenced to dynamic pressure, in expressions like: Lift=cL*½ρV²*S. Because of that arbitrary definition of (lift) coefficient, ½ρV² must be used to calculate (lift) force from that coefficient. What many simplistic introductions don’t explain, is the variation of that coefficient with Mach number (and sometimes with Reynolds number).

The difference between the pitot tube and the airplane is that the air flow around the airplane doesn't get slowed to zero, excluding the boundary layer. It makes sense that there would be compressibility, and expansion, of the air depending on which location it's at on the airplane but …Actually, the airflow is slowed to zero at the stagnation points, such as the nose of the radome or the leading edge of a straight wing. But that is really unimportant, because the forces on the airplane are the result of pressures all around the airplane, and in real air all changes of pressure from the undisturbed static pressure are subject to compressibility.

I don't think the effects would be nearly as drastic as the difference between CAS and EAS. Have another look at my graph and think about it. For the data shown in this example, the effect is greater than the difference between CAS and EAS when Mach is greater than about 0.6.

EDIT: Below M.6 the effect is only 50% of that predicted by the Prandtl-Glauert model.
EDIT 2: To borrow from another thread: If your theory doesn't fit the experiment (flight test evidence) - it's WRONG!