True Air Speed Vs Indicated Air Speed

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Laurelix

Airman 1st Class
293
197
Jun 13, 2016
I wanted to showcase the difference between TAS (True Air Speed) compared to IAS (Indicated Air Speed)

P-51D


Stall Speed at 9611lb (4360kg) loaded weight - 110.7mph (178km/h) at Sea Level
(No flaps, level flight)

That's Wing Lift Coefficient of 1.31 CL_Max which is standard for a laminar flow wing.

With this info I can calculate the stall speed at different altitudes by changing the air density to match different altitudes.

This is the TAS it needs so it doesn't stall at altitudes with different air density.
IAS = TAS at Sea Level, however TAS has to increase to maintain same IAS stall speed due to decreasing amount of air as altitude increases.

ALTITUDE / SPEED / AIR DENSITY
Sea Level - 178km/h - 1.225kg/m3
1000m - 187km/h - 1.112kg/m3
2000m - 196km/h - 1.007kg/m3
3000m - 207km/h - 0.9093kg/m3
4000m - 218km/h - 0.8194kg/m3
5000m - 230km/h - 0.7364kg/m3
6000m - 242km/h - 0.6601kg/m3
7000m - 256km/h - 0.5900kg/m3
8000m - 272km/h - 0.5258kg/m3
9000m - 288km/h - 0.4671kg/m3
10,000m - 306km/h - 0.4135kg/m3

This is why stall speed is measured in IAS.
P-51D has 178km/h IAS stall speed but it needs to at least travel at 306km/h at 10,000m to maintain 178km/h IAS and avoid stalling. As a result of higher stall speed in TAS for all planes, as you get higher the more sluggish planes turn. Their radius of turn increases drastically and also their power output is also reduced at higher altitudes making the power to weight ratio worse and worsening the sustained turn capability.

FUN FACT:
To get into Space and leave gravitational pull of earth you need to go beyond 100,000m altitude.
If P-51D was to reach 80,000m altitude, it would have to travel at 45,800km/h TAS (Mach 37.1) to maintain 178km/h IAS in order to not stall.
This is why even jets like SR-71 Blackbird have service ceiling of 26,000m altitude.

Conclusion:
TAS = How fast you travel from point A to point B. Basically how much distance you move in set amount of time.

IAS = The airflow over your wings. You need to go faster to maintain same airflow due to decreasing amount of air as you go higher in altitude. All the TAS speeds at different altitudes = 178km/h IAS

———

Another Fact:
Every plane has a structural limit and there is a maximum limit at every altitude where the plane will fall apart at certain TAS due to the extremely high IAS.
The newer next generation jets which are suppose to reach Mach 6 will still be limited to same speed at low altitudes to earlier jets, the difference is the capability of going even higher which allows the jet to achieve greater TAS before reaching structural IAS limit of an airframe. Jet technology today is at its Apex level right now that increasing max speed is the same as saying - Increasing Service Ceiling. It's a matter of creating engines which can still operate at altitudes with ridiculously low air density which would increase service ceiling.
 
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Airspeed can be very complex or simple, depending a lot on your aircraft and what you are doing. There are quite a few airspeeds that engineers and pilot use, here is a list. This is remembered by the acronym ICE-T and to this I will add GS and Mach.

I is for indicated or basically what your cheap airspeed indicator reads

C is for calibrated, or your expensive indicator that corrects for errors of installation e.g. losses in line and errors in pitot tube location. This is what your wings see and react to.

E is is for equivalent which is calibrated airspeed corrected for air compressability. This is used by engineers in their analysis of performance and for calculating true airspeed.

T is for true airspeed or the speed at which your aircraft is actually flying through a block of air. It is equivalent airspeed corrected for density.

GS is for ground speed or your true airspeed corrected for movement of that block of air over the ground, i.e., wind. This is a definite need for navigation.

Mach is the ratio of you aircraft true airspeed (I believe) compared to the speed of sound at the local conditions. Most aircraft have Mach limitations and that is the point where supersonic airflow occurs over some portion of the aircraft which causes certain control issues.

If you are flying your little putt-putt around the neighborhood or flying off to grandma's house 50 miles away at 8000 ft., all you need is indicated airspeed indicator and a calibrated thumb. However, in a higher performance aircraft with serious nav needs you need either indicated or calibrated airspeed indicators and a means of calculating GS which in turn means you need to calculate true airspeed, which means you need basically some sort of computer or computed data. And you also need to know weather conditions. Flying in high performance modern aircraft you are back to the simple indicated/calibrated indicator because all that other stuff is calculated for you. Oh, for higher performance aircraft you may need a Mach meter.

Simple stuff for a complex subject.

Oh, for supersonic airflow you have to go back to the drawing board since supersonic airflow does not act like subsonic airflow.
 
This must be wrong. TAS is how fast a plane or other is actually travelling through the air it is in. A civil airliner just set a speed record across the Atlantic going supersonic in terms of ground speed (A to B) because it was in a very fast high altitude air current, actually a downgraded hurricane. The plane was just going at its normal speed in air, but the air it was in was going at more than 150MPH relative to the ground. BTW if you were actually "in the know" about what leading edge military aircraft can and cant do you wouldnt be posting it here.
 
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I used the GPS method to check the ASI gauge on my Ercoupe. I found it read about 5 mph too low when below about 100 mph. But based on looking at the calibration chart in the Ercoupe 451E manual, it seems that reading too low at the low end and reading too high at the high end is "normal" and gives you a extra margin over and above the colored markings on the instrument.

The FAA requires you to placard the ASI if it is off by 5 kts or more.
 
P-39 Expert said:
IAS is speed through the surrounding air.
TAS is the speed of the plane over the ground below.
Click to expand...
Indicated airspeed ("IAS"), what is read on an airspeed gauge connected to a Pitot-static system.

Calibrated airspeed ("CAS"), indicated airspeed adjusted for pitot system position and installation error. Each aircraft has an airspeed correction table in the manual.

Equivalent airspeed ("EAS"), calibrated airspeed adjusted for compressibility effects.

True airspeed ("TAS"), equivalent airspeed adjusted for air density, and is also the actual speed of the aircraft through the air in which it is flying.

None of the airspeeds have ANYTHING to do with speed over the ground. Speed over the ground is called ground speed or groundspeed.

When you are in your car, your IGS (indicated groundspeed) is what is shown on your speedometer. Your speedometer might be off because you changed the differential gearing without changing your speedometer gear (as when you install a 3.73 differential gear in a 2002 Camaro SS that originally came with a 3.42 differential) and could also be off because you are running a non-stock tire diameter (say you installed 285/35-17s on a Ford Mustang GT PP1 that originally came with 275/40-17s originally).

I've done both of the above and I re-programmed my speedometer accordingly in the Camaro and didn't worry about it in the Mustang I still have because my actual speed is slightly lower than the speedometer, and I've never gotten a ticket despite cruising about 5 mph over the posted speed limit as a result
 
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This is 100% correct.

Ground speed is the speed relative to the ground at any given moment. This is effected by winds aloft, as the aircraft moves with the air mass.

The speed from point A to B is called block speed.
 
Somewhere in the deep dark past, i wrote airplane performance programs for a major aerospace company. Indicated airspeed, as others have said is the airspeed shown on the air speed gauge. All airplanes have some position error in the indicated airspeed. The presence of the airplane body and wing affects the pressure at both the mouth of the pitot tube, and the static port. The effect can be significant. One model of plane i worked on initially regularly had more than 200 feet difference between the pilot's altitude indicator and the copilot's system. Not a problem with steam gauges, but oh so noticeable with the digital cockpit. Fixed by using pitot-static tubes that stood further from the body.

Once one has corrected for position error, one has calibrated airspeed (and calibrated altitude). It usually takes flight tests to determine position error, and most companies assume that the position error is the same from one plane to the next of the same model, or close enough for guv'mint work.

And one really only needs True Airspeed during flight test. Stall speed for a given configuration of flaps and gear is the same airspeed, no matter the altitude, until one gets so high that the stall speed is approaching high subsonic speeds, but that is another Aerospace Master's degree Thesis

Somewhere, i believe i have an equation to convert CAS to TAS and its a long fellow. Can't believe i used to do those calcuations with a simple hand calculator.

i'll look for that equation sometime when i'm not trapped by a dog on my lap.
 
Its easiest to first convert KCAS to KEAS and then KEAS to KTAS.
KEAS = 1479 * SQRT(((((1 + 0.2 * (KCAS * 1.688 / 1116.45)^2)^3.5 - 1)/delta + 1)^(1/3.5) - 1) * delta)
where delta = local static pressure / static pressure at sea level

Then, KTAS = KEAS / SQRT(sigma)
where sigma = local air density / air density at sea level
 
I took the info in an FAA Advisory Circular and created an Excel spreadsheet to enable GPS data to calculate indicated airspeed so the accuracy of the ASI can be checked. You fly three different directions at least 60 degrees azimuth apart at a set altitude and a given indicated airspeed, write down the GPS data, and then do it again at a different airspeed. I found mine was reading about 5 MPH too low and placarded it anyway, although I found it too hard to take readings at the upper end of the aircraft's capabilities; I needed a co-pilot to do that.
 
Speed of the airplane over the ground is called, oddly enough, "groundspeed" and not TAS.

And aside from correcting airspeed for temperature and altitude factors, the airplane has a certain amount of error in its airspeed measurement system. According to the pilot's manual, my Ercoupe ASI reads too high at the upper end of the scale and too low at the lower end of the scale, thereby giving you some margin.

A friend of mine recently had a new airspeed indicator installed in his airplane and on his first landing was concentrating it, intent on seeing just how slow he could get the airplane before it touched down . As a result, he forgot to put the landing gear down.....
 
Flying at an IAS of 0 MPH and a height of 0ft is quite an achievement.
 
Looks pretty good save 'TAS = speed over the ground". TAS is that speed which the airframe/wing 'sees'. Otherwise a 90kt headwind is uninteresting.
 
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