TAS and Altitude

uwochris

Flightinfo's sexiest user
Joined
Dec 21, 2001
Posts
381
Total Time
2500+
Hey guys,

Just want to get a confirmation check on the relationship between TAS and altitude, for a constant power setting.

I was under the impression that IAS is constant at varying altitudes, but TAS is not. The reason being that in the higher levels, the less dense air implies less drag for the airplane, and thus a faster TAS. So, I assumed a higher altitude gives a higher TAS for a given power setting.

However, my performance chart in my POH shows that as the pressure altitude increases, TAS decreases. Why the discrepancy?

I was told that it is because my C-172 is not turbocharged, and that TAS only increases with higher altitudes for turbocharged aircraft. This seems weird because in my POH, on the Endurance chart, it shows that TAS is higher as altitude increases for constant power settings. Again, another discrepancy.

Am I missing something??
 

414Flyer

Down with Chemtrails!
Joined
Jan 20, 2002
Posts
4,948
Total Time
4200
TAS and IAS do not know the aspiration of the engine you have.

The reason it will come more into play in a turbo charged engine, is that you can hold a normal cruise power setting up into the flight levels typically.

A normally aspirated engine will hold 75 percent power up to somewhere around 7-8 thousand feet I believe, and after that you power drops off with altitude, and I dont think you will have any increase in TAS at that point, because your IAS you can get, will be less.

TAS will increase about 2 percent per 1000 ft in altitude, at a given IAS, so as you can see, the higher you can climb, the bigger the difference will be between the two.
 

bobbysamd

Well-known member
Joined
Nov 26, 2001
Posts
5,710
Total Time
4565
IAS v. TAS v. Pressure Altitude

Remember ICET?

Indicated = what you read off the ASI
Calibrated = indicated corrected for installation error
Equivalent = calibrated corrected for compressibility (a non-issue in your 172)
True = equivalent corrected for pressure and temperature

What happens is the higher you go the less dense is the air. Remember that also from flight phys and hypoxia? Fewer molecules are available to pack the pitot and to be referenced against the air in the static port. So, for a given power setting, indicated airspeed decreases. True airspeed increases. Try it. Try setting up the OAT v. pressure altitude ring on your 172's ASI and read off TAS directly from that ring.

Hope that helps. Continued good luck with your training.
 

banned username 2

Banned
Banned User
Joined
Nov 25, 2001
Posts
3,254
uwochris said:
This seems weird because in my POH, on the Endurance chart, it shows that TAS is higher as altitude increases for constant power settings. Again, another discrepancy.
Your Endurance chart shows the TAS you should maintain for maximum endurance at a given altitude, not necessarily the Max TAS you can maintain.... Your endurance chart probably has you flying a constant IAS for endurance but when you get to higher altitude that given IAS equates to a higher TAS...
 
Top