Airspeed

[ben123]

Once again I have myself quite confused over the definitions of IAS, CAS, EAS & TAS. So if anyone could give me a hand to answer these two questions, it would take me from "im fairly sure im right" to "at least he thinks the same" ;o) , Cheers ;

Q1) When an aircraft climbs from sea level to altitude at a constant mach number, the true air speed (TAS) :-

a) Will decrease
b) Will increase
c) Remains constant

Q2) Given that an aircraft is in a climb but maintaining a particular indicated airspeed, the true air speed will:-

a) Increase
b) Decrease
c) Remain the same


Thankyou

[Genghis the Engineer]

Question 1
Mach number is TAS divided by the local speed of sound. The local speed of sound is proportional to square root of local temperature. As a general rule, (below the tropopause and ignoriing inversions) as you climb it gets colder, so the local speed of sound decreases.

This means that at a constant Mach No. your TAS will be decreasing also.

Above the Trop, and assuming you're not going to reach the mesopause, temperature is pretty much constant so the TAS should stay constant at a given Mach No.



Question 2
True airspeed = exactly what it says.

Equivalent airspeed = what a perfectly working ASI will read, setup for ISA sea-level conditions. For EAS multiply TAS by the square root of relative density.

Calibrated airspeed = EAS plus compressibility errors. Anywhere below 0.5M you can ignore the difference.

Indicated Airspeed = CAS plus system and position errors (known as PEC, Pressure Error Corrections). This is what you actually see on the dial.


It's a more or less universal rule that as you climb, density goes down. So, at anything above sea-level TAS will be bigger than CAS. The higher you climb, the bigger difference.

That was the long answer, the short answer is C.

G

[qwertyuiop]

You cant get a better explanation than that, can you!

I believe the answer to Q2 is actually A. A typo from Genghis?

[Bellerophon]

ben123

I’ll give a go, I’m sure others will do it much more eloquently!


Q1)...A...Decrease.

Put simply, the TAS of the speed of sound (Mach 1.0) decreases with temperature. Temperature decreases with height, so maintaining a constant Mach number whilst climbing means that you are maintaining a constant Mach number whilst the temperature decreases, therefore the TAS will decrease.


An aircraft flying at Mach 1.0, at Sea Level on an ISA day, will have a TAS of 661 kts.

If it maintains Mach 1.0 whilst climbing to 30,000 ft, in ISA conditions, the TAS will decrease to 589 kts.

If it maintains Mach 1.0 up to 40,000 ft, the TAS will decrease further to 573 kts.


Q2)...A...Increase.

Again, as simply as I can, a basic airspeed indicator is just a pressure sensing instrument. The pressure in the pitot tube we assume is ½pV2, and the instrument is calibrated to read correctly in the dense air found at sea level.

In order to maintain a constant airspeed indication (IAS) as we climb we must maintain a constant ½pV2 in the pitot tube. To do this when the outside air density p is decreasing, then V (TAS) must increase to compensate, and keep the pressure in the pitot the same.

Using the ASI in less dense air than it was calibrated for, by climbing significantly above sea level, means the airspeed indications it gives (IAS) will under-indicate the actual airspeed (TAS) of the aircraft because of this reduction in the air density.


An aircraft flying at 160 kts IAS, at Sea Level on an ISA day, will have a TAS of 160 kts.

If it maintains 160 kts IAS, whilst climbing to 20,000 ft, in ISA conditions, the TAS will increase to 220 kts.

If it continues at 160 kts IAS up to 40,000 ft, the TAS will increase further to 325 kts.


Hope this helps!

Regards

Bellerophon

[Genghis the Engineer]

Definitely a typo - A it is.

As my old school maths teachers used to say, always show your working and you can't lose too many marks for a wrong end result. :O

G

[FlyingForFun]

Bristol have a fantastic graph in their notes for working out the answer to this type of question. Unfortunately, I had a go at describing the graph and how to use it, but it ended up being a garbled mess of words, so I deleted it. Unless anyone with better English-skills than me wants to have a go, see if you can track down a Bristol student to show you the graph.

FFF
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[Gerund]

This is a bit of a cheat but saves working it out:

Climb: -CTM+

Descent: +CTM-

Where C = Indicated or Calibrated Airspeed
T = True Airspeed
M = Mach No

Eg in a climb

Constant C, T and M increase
Constant T, C decreases, M increases
Constant M, C and T decrease

Other way around for a descent.