What speed is your actual speed?

[FLS]

So your True air speed can only be slower than your Ground speed if there is a tail wind? 

Correct.

[Wiley]

True air speed is what you need to worry about going fast in a dive close to compressibility.

The gist of one of my favorite things I ever saw Hitech post to someone talking about flight modeling was, 'You are trying to apply broad general logic in an arena where it is not detailed enough to apply.'

With that in mind, I'm curious how this applies.  To me, compression would seem to me to be a similar thing to a stall, just at the other end of the spectrum.  Why would IAS not apply in compression like a stall?  Not challenging the truth of the statement, it just seems counterintuitive to me.

Wiley.

[earl1937]

True speed if your actual speed, but fly by your indicated.

Not quite true, but close! Your ground speed is the speed at which your aircraft is moving over the ground and that is the one which you must use to compute time to your destination! Your true airspeed is just indicated airspeed, corrected for altitude and temperature! Your indicated airspeed is the speed at which the air is flowing over your pitot tube and wings. Your calibrated airspeed is the airspeed corrected for any instrument errors.

[GScholz]

Wiley, stalling is the abrupt loss of lift as the angle of attack increases beyond the wing's ability to keep the airflow from separating from the wing surface. Indicated airspeed is the raw data that comes from your instrument, which is connected to a pressure sensor in the Pitot tube. This device measures the pressure exerted on it by the flow velocity of the air. The higher you fly the thinner the air gets and thus the faster you need to fly to get the same indicated airspeed; there are fewer air molecules hitting the Pitot tube sensor for any given unit of speed and time, so speed must be increased to achieve the same reading. IAS thus also neatly indicates the mass of air flowing past your wing in any given unit of time, which is what you need to generate lift. In thinner air you need more speed to generate the same lift, and in general, without going into extreme detail, the IAS you need to keep flying changes little with altitude, because you need to increase speed with altitude just to get the same IAS reading.

At the other end of the speed spectrum you are facing compressibility, or in other words, speeds where the air molecules are beginning to have trouble getting out of the way quick enough, and start forming shock waves. Compressibility is a Mach effect and thus directly linked to the speed of sound, so the true airspeed is what is important in that end of your aircrafts speed range.

[Wiley]

Compressibility is a Mach effect and thus directly linked to the speed of sound, so the true airspeed is what is important in that end of your aircrafts speed range.

Ah!  Makes perfect sense now.  I didn't quite understand the general idea of compressibility.  Thanks for the answer.

Wiley.

[earl1937]

So your True air speed can only be slower than your Ground speed if there is a tail wind? 

An example of your question is this: You are flying on a 090 degree heading, and your true airspeed at, say, 15,000 feet is 330MPH true! Now, with the wind 270 degrees at 30MPH, then your aircraft would be moving over the ground at 360MPH. If you were heading 270 degrees with your aircraft, with the same 30MPH wind, and 330MPH true air speed, you would now be moving 300MPH over the ground, which is what is referred to as ground speed. Another effect of wind, which a lot of people do not notice is this: your aircraft will climb faster in feet per minute into the wind than it will climbing down wind.

[SkyRock]

a TA152 at 40k   the speed!!!!!!

[Badboy]

So what speed was I actually going? And what is the significance of all three?

Hi Tinkles, this should help.

Airspeed is measured by determining the difference between 2 air pressures about the aircraft. One pressure measurement is taken on the side of the aircraft, or on the side of the pitot-static tube (e.g. a small tube sticking out of the aircraft with its axis parallel to the direction of flight ), and indicates the static air pressure at your current altitude. The other pressure measurement is taken in the front, open end of the pitot tube, and indicates the total air pressure acting on your aircraft as you fly through the air mass. The difference in these 2 measurements is indicated airspeed (IAS). Airspeed gauges are only calibrated for sea-level, standard-day atmospheric conditions. So. unless you are at sea level and the atmospheric conditions match standard day conditions, your airspeed indicator is not indicating true airspeed (TAS), ie your true speed with respect to the air mass. That's why this value is called IAS, it’s what is indicated on the airspeed gauge.

Calibrated airspeed (CAS) is IAS corrected for what is called "position error". This error is due to local airflow effects about the static measuring source. As the aircraft moves through the air, it changes the pressure field around itself. So, you don't get an accurate static pressure reading from the static source. This error is different for every aircraft type, and usually changes for each aircraft with airspeed and configuration (e.g. gear up or down) changes. Through flight test, you can determine what these position errors are, and then determine the necessary corrections to get CAS from IAS. The reason this is called CAS, is because this is what you would read if the airspeed indicator was "calibrated" perfectly, i.e. no position errors.

Equivalent airspeed (EAS) is CAS corrected for what is called "compressibility effects". As you go higher and/or faster, individual air molecules can be compressed as they come to rest inside the pitot tube. This "compressing" has the effect of causing the pressure sensor inside the pitot tube to indicate a total pressure higher than the actual value. These compressibility corrections are independent of aircraft type, and depend only on CAS and pressure altitude. They only come into play if you exceed 0.6 Mach number and/or 30,000 ft pressure altitude. The reason this is called EAS, is because this is your TAS equivalent at sea level. That means, take whatever value this is at your current altitude, Star Trek transport your aircraft to sea level, and this will be your TAS. The term is important because for a given angle of attack (AOA), an aircraft behaves the same aerodynamically (ie. it generates the same amount of lift, drag, etc.) at a given EAS regardless of altitude, discounting Mach number effects.

TAS is EAS corrected for air density at your current altitude. Air density is a function of pressure and temperature. Ground speed is TAS corrected for wind velocity.

At sea-level, standard-day atmospheric conditions all of these airspeed measurements will be equal, well except for IAS which is still dependant on those position errors, which are different for each aircraft type. An easy way to remember how their magnitudes relate to one another at higher altitudes is by using the square root symbol as shown in the diagram below:



IAS and CAS are usually very close to one another. For most aircraft, usually within 10 to 20 knots or less. EAS is always less than CAS. For airspeeds of Mach 1.0 or less, the maximum difference will be 30 knots. TAS is always greater than all the other airspeeds, at altitudes above sea level.

An important note, IAS/CAS tells the pilot how the aircraft will behave regardless of what altitude he's at. The same aircraft at 200 KCAS at sea level behaves just like it does at 200 KCAS at 30,000 feet. Remember what I said about a given AOA and EAS above? IAS/CAS is very close to EAS, much closer than TAS even at moderate to low altitudes, much less high altitudes. Therefore, since the aircraft behaves the same for the same airspeed even at vastly different altitudes, this makes flying one a lot easier when referencing IAS/CAS which is why when pilots talk speed, they talk IAS/CAS and in WWII that would have just been IAS. On the other hand if you want to compare different aircraft or know which aircraft is faster you need to know their TAS and so aircraft data and performance reports are normally provided in TAS.

Hope that helps…

Badboy

[Tinkles]

Hi Tinkles, this should help.

Airspeed is measured by determining the difference between 2 air pressures about the aircraft. One pressure measurement is taken on the side of the aircraft, or on the side of the pitot-static tube (e.g. a small tube sticking out of the aircraft with its axis parallel to the direction of flight ), and indicates the static air pressure at your current altitude. The other pressure measurement is taken in the front, open end of the pitot tube, and indicates the total air pressure acting on your aircraft as you fly through the air mass. The difference in these 2 measurements is indicated airspeed (IAS). Airspeed gauges are only calibrated for sea-level, standard-day atmospheric conditions. So. unless you are at sea level and the atmospheric conditions match standard day conditions, your airspeed indicator is not indicating true airspeed (TAS), ie your true speed with respect to the air mass. That's why this value is called IAS, it’s what is indicated on the airspeed gauge.

Calibrated airspeed (CAS) is IAS corrected for what is called "position error". This error is due to local airflow effects about the static measuring source. As the aircraft moves through the air, it changes the pressure field around itself. So, you don't get an accurate static pressure reading from the static source. This error is different for every aircraft type, and usually changes for each aircraft with airspeed and configuration (e.g. gear up or down) changes. Through flight test, you can determine what these position errors are, and then determine the necessary corrections to get CAS from IAS. The reason this is called CAS, is because this is what you would read if the airspeed indicator was "calibrated" perfectly, i.e. no position errors.

Equivalent airspeed (EAS) is CAS corrected for what is called "compressibility effects". As you go higher and/or faster, individual air molecules can be compressed as they come to rest inside the pitot tube. This "compressing" has the effect of causing the pressure sensor inside the pitot tube to indicate a total pressure higher than the actual value. These compressibility corrections are independent of aircraft type, and depend only on CAS and pressure altitude. They only come into play if you exceed 0.6 Mach number and/or 30,000 ft pressure altitude. The reason this is called EAS, is because this is your TAS equivalent at sea level. That means, take whatever value this is at your current altitude, Star Trek transport your aircraft to sea level, and this will be your TAS. The term is important because for a given angle of attack (AOA), an aircraft behaves the same aerodynamically (ie. it generates the same amount of lift, drag, etc.) at a given EAS regardless of altitude, discounting Mach number effects.

TAS is EAS corrected for air density at your current altitude. Air density is a function of pressure and temperature. Ground speed is TAS corrected for wind velocity.

At sea-level, standard-day atmospheric conditions all of these airspeed measurements will be equal, well except for IAS which is still dependant on those position errors, which are different for each aircraft type. An easy way to remember how their magnitudes relate to one another at higher altitudes is by using the square root symbol as shown in the diagram below:

(Image removed from quote.)

IAS and CAS are usually very close to one another. For most aircraft, usually within 10 to 20 knots or less. EAS is always less than CAS. For airspeeds of Mach 1.0 or less, the maximum difference will be 30 knots. TAS is always greater than all the other airspeeds, at altitudes above sea level.

An important note, IAS/CAS tells the pilot how the aircraft will behave regardless of what altitude he's at. The same aircraft at 200 KCAS at sea level behaves just like it does at 200 KCAS at 30,000 feet. Remember what I said about a given AOA and EAS above? IAS/CAS is very close to EAS, much closer than TAS even at moderate to low altitudes, much less high altitudes. Therefore, since the aircraft behaves the same for the same airspeed even at vastly different altitudes, this makes flying one a lot easier when referencing IAS/CAS which is why when pilots talk speed, they talk IAS/CAS and in WWII that would have just been IAS. On the other hand if you want to compare different aircraft or know which aircraft is faster you need to know their TAS and so aircraft data and performance reports are normally provided in TAS.

Hope that helps…

Badboy

Lots.. of... words..

Thank you guys for all the info.

Tinkles

<<S>>

[FLS]

Another effect of wind, which a lot of people do not notice is this: your aircraft will climb faster in feet per minute into the wind than it will climbing down wind.

Are you sure this is what you meant to write?

[Oldman731]

Are you sure this is what you meant to write?

In reference to the ground.

- oldman

[doright]

Compressibility is a Mach effect and thus directly linked to the speed of sound, so the true airspeed is what is important in that end of your aircrafts speed range.

For Mach effects the mach number is what is important (duh). It is displayed on the E6B. For each airplane mach buffet always occurs at the same mach number no matter the altitude. Same with control lockup. Mach changes with temperature and therefore altitude. Where as TAS is, well true airspeed, independent of altitude and temperature changes.

So say your cruising along at the edge of Mach buffet at .79 mach in a ME163. You increase throttle and now your controls lock up at .83 mach.
At 24,000 feet these occur at a TAS of 549 (375 ias) and 577 knts (394 ias), and throttle is 33%.
At   1,000 feet these occur at a TAS of 600 (593 ias) and 630 knts (620 ias), and throttle is 77%.

[FLS]

In reference to the ground.

- oldman

I would hope so but that's not what he wrote. It reminds me of the old question if an aircraft turns faster with a headwind or a tailwind.

[kvuo75]

Your indicated airspeed is the speed at which the air is flowing over your pitot tube

 

obviously incorrect.

[earl1937]

I would hope so but that's not what he wrote. It reminds me of the old question if an aircraft turns faster with a headwind or a tailwind.

Regardless of what you may think or have heard, just try this in a real aircraft: takeoff into a 20MPH headwind and time your self to 500 feet. Then land, turn around, takeoff downwind and time yourself to 500 feet. You will see that there is considerable difference in the amount of time it took to climb into the wind and the time it took to climb downwind.
Maybe I need to state it in a different way: We want to climb to 10,000 feet, and we are climbing at a 1,000 feet per minute rate of climb, that would be 10 minutes, say into a constant 20MPH headwind, correct?
Now, reverse the climb into a downwind, same 1,000 feet per minute, 20MPH tailwind, climbing to 10,000 feet, should take 10 minutes, right? Sorry Charlie, in the real world, it won't arrive at 10,000 feet in 10 minutes, like you think it would. A lot of the books and so called experts say that you will, but try it and you will find that what I just told you is correct. The bottom line to the statement is just this: An aircraft will climb more efficiently into the wind than downwind. The aircraft will move further away from point of departure when climbing downwind, when it arrives at 10,000 feet, than it will climbing into the wind, because of the difference in ground speed.
The old argument of turns into and downwind is really very simple: turns into or downwind, when started directly into or downwind, will be the same, as the wind effect is canceled because of the same amount of time into and downwind. Now, if you really want to discuss turns into and downwind, consider this: you are flying on a heading of 360 degrees and the wind is from 270 degrees at 30 knots, and you want to make a 180 degree turn to the left, which 90 degree segment of the turn will have the greatest radius of turn?