TAS

[gallwood]

Hi,

I'm just going through Air Navigation studies at the moment and I have a question on True Air Speed.

TAS is defined as the movement of the aircraft relative to the air mass. Does wind play a part in this, or does the wind velocity cancel itself out with drag?

For example. In a small PA28, travelling with 2300RPM set, it is said to give roughly 100Kts TAS. If flying in to a head wind, will the same power setting still give 100ks TAS, despite the slower speed (presume it is a slower Ground Speed).

I will ask my instructor when I see him, I was just wondering.

Thanks for your help.

Graham

[BackPacker]

TAS is measured in respect to the surrounding air mass. Whether that air mass is moving (with the airplane in it) or not doesn't matter.

To get from TAS to ground speed (GS) you have to factor in the wind. But that's the earliest place where the wind plays a role in your calculations.

[The Dead Side]

Hi Graham,

If you imagine (albeit a classic cliché) the aircraft in the sky, as a boat in a river.

If your engine is not running, regardless of the flow of the river, you will be going at 0 m/s in relation to the water. You may well be travelling at say 10 m/s in relation to the riverbank (or ground) if there is a fast flow.

If you relate this directly to the air, at a constant power setting you will travel (assuming weight, altitude and temperature e.t.c. are all constant) at the same airspeed (CAS/TAS), regardless of the windspeed.

When you bring wind into the equation, you are affecting your speed over the ground, not your airspeed - so you may end up at your destination faster or slower, but the speed through the air will remain steady.

The airspeed you read in your cockpit, is the speed of the aircraft through the air - your 2300rpm will always (again, assuming same W.A.T) give you the same airspeed (and therefore an expected performance) which can be used as a ballpark figure to work with over and over again.

What you don't have on a conventional cockpit layout is a ground speed reading, for this you need to then factor in the wind. But again, unless your bothered about what time you get somewhere, this isn't really an issue - your performance is reliant, amongst other things, on your airspeed.

Hope this helps,
Dead Side

[wrecker]

Don't forget that IAS does not equal CAS which does not equal TAS

[The Dead Side]

Unless at you're at sea level! (Arguably CAS is not the same as IAS, but I suppose errors are small enough to be negated when explaining principles of performance)

[Pilotage]

Ground speed

- corrected for wind

Gives True Air Speed

- Corrected for Density

Gives Equivalent Air Speed

- Corrected for compressibility

Gives Calibrated (or Rectified) Air Speed

- Corrected for system errors

Gives Indicated Air Speed.


Fly at 100kn TAS into a 10kn wind, you're doing 90kn G/S. Turn 180deg onto the reciprocal heading, at the same speed, and you'll now be flying at 110kn G/S.

P

[phiggsbroadband]

If you have a GPS, that will tell you your GS. If it looks different to your IAS then you have some indication of the wind speed (but not its direction.)

If wind conditions have changed, you can decide to take a different Flight Level, to make the most of a good tailwind.... It's nice to gain an extra 60 knots, for no increase in fuel consumption.

[AdamFrisch]

At altitude, TAS normally goes up when IAS stays the same or decreases. TAS however is not a speed that has any relevance to the performance of the aircraft (except for GS). You'r airplane will stall at its IAS speed, not the much higher TAS speed.

This is also the reason why your landing ground speed will always be much higher at high altitudes. It can be quite a surprise to come in to land at 7000ft and see the ground racing by even though your flying the approach at the exact same IAS you do at sea level. This and the performance of aspirated engines are the reason your takeoff roll at high altitudes are longer - but density alone is the reason your landing roll will be longer (because your GS is higher). People seem to have a good grasp of the former when they plan, but tend to forget the latter one.

[peterh337]

TAS however is not a speed that has any relevance to the performance of the aircraft (except for GS).

and flutter...

[24Carrot]

... and coffin corner, since mach 1 is a TAS, not a CAS...

Coffin corner (aviation) - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Coffin_corner_(aviation))

OK, probably of zero interest for the average spam can driver, but the OP has an unspecified exam to pass:)

[Pace]

TAS however is not a speed that has any relevance to the performance of the aircraft (except for GS).

It will do at high altitude airfields where on landing you are flying an IAS but your TAS is higher.

Pace

[Pilotage]

Mach 1 is, strangely enough, a Mach number.

At any given set of conditions, it'll have TAS, CAS, EAS and IAS values - the relationship will keep moving, and is a function of temperature and density.


Mach number is an issue in light and microlight aeroplanes - engine speed and propeller diameter, plus air temperature define tip Mach number. Vibration and loss of efficiency can become problematic above about 0.8M.

P

[24Carrot]

I should have been clearer!

If you specify a temperature, then you have completely specified the TAS which matches Mach 1.

Basically temperature measures the randomly directed kinetic energy of the air molecules, which is a function of their average speed, which determines how fast they can transmit information such as sound waves.

If there is a wind, the Ground Speed of sound is higher in the wind direction, so TAS really is the right measure.

But of course EAS,CAS,IAS etc do differ from TAS depending on density (and various errors).

[gallwood]

Thanks guys for all the comments very informative.

Since reading more of my Nav book for my PPL exam, most of the comments make sense to me :)

Graham