TAS and altitude

[scifi]

Hi Dave, Unfortunately 'Air-Speed' is an absolute concept, it is the speed at which air molecules pass a certain point.
It is only because we use a Pitot driven device that the terms TAS, IAS, and CAS have come about, to get over the deficiencies of the ASI.


Surely all we need to know is the Air-Speed, and wind velocity to work out our Ground Speed, to tell us how long our journey will take.
.

[DaveReidUK]

Originally Posted by scifi

Surely all we need to know is the Air-Speed, and wind velocity to work out our Ground Speed, to tell us how long our journey will take.

I'd suggest you try flying at, say, 150 kts TAS at sea level and then attempt the same thing at FL300.

You might change your view about TAS being the only important parameter.

[Derfred]

Originally Posted by disco87

I've been reading about why Jets climb as high as possible, I'm quite comfortable with the changes to SFC and also minimum cruise drag on the airframe. However I'm struggling to get my head around why there is an increase in the TAS.

As far as I'm aware, above a certain altitude the mode of speed management is in relation to the MN. Therefore this would result in a decreasing TAS as you climb at a constant MN, however it seems that the TAS increases from what I've been reading, which has confused me a bit.

I'm just looking for an explanation of why this is the case.

Thanks

Generally correct except for the bit about TAS increasing after climbing at a constant MN. It doesn't. TAS decreases when climbing at a constant MN, due to the fact that the temperature decreases, and therefore the TAS associated with that MN decreases with temperature. Until you reach the tropopause, above which the temperature ceases to decrease and therefore the TAS remains constant with MN no matter how high you climb.

The tropopause will vary considerably due to location, seasons and weather, but is typically around 35000' in a standard atmosphere.

[john_tullamarine]

Not pressure. The air's half as dense at 400, so

Not quite. It is root sigma which is half the sea level value ... but you have the general idea.

[megan]

An instrument that displayed TAS wouldn't be particularly useful

Some light aircraft (think some Pipers were one) had ASI's which showed TAS. Don't really know of what practical use it may be, other than saving reaching for the E-6B.

[Owain Glyndwr]

scifi
Dave Reid is perfectly correct, "Air speed" without an adjective is meaningless.
Technically, a pitot static system measures a quantity known as Equivalent Airspeed which is the airspeed which would be registered at sea level by the number of molecules which pass by per second at the flight altitude and speed. EAS is the quantity you need to calculate the forces on the airframe, or to know how close you are to flight envelope limits. In short, it is the number you need to know to FLY the aeroplane. IAS,CAS are simply corrections to EAS to account for variations arising from the positioning of the pitots and static ports along with compressibility effects.
True airspeed, on the other hand, will differ from EAS by considerable amounts which depend on the altitude and ambient temeperature. It is the number you need to know to NAVIGATE the aeroplane.
I must say I am surprised to find this sort of discussion in a professional pilot's forum!

[wiggy]

...Anyway back to the OP and specifically :

I've been reading about why Jets climb as high as possible,

FWIW ..we certainly don't routinely climb as "high as possible" (i. e. Which in our case would be FMC max)..,,as I recall it years back from CFS days design RPM (on a small turbo jet ) came into the best flight level equation....and suspiciously enough these days (and ignoring winds) our best flight level on the 777 (and I think similar was true on the 744) seems to where the N1 was around 90%.....and usually a thousand feet or more below "as high as possible" ..... in my old age am I misunderstanding best range/design RPM?