Cruising at 7000 ft MSL with manifold pressure = 22, rpm = 2300, fuel flow = 150 I'm only achieving 115 KIAS.  The User Guide chart shows I should be getting about 175 TIAS.  Why the big difference?

Your comparing KIAS to KTAS ( the cruise chart in the manual is showing KTAS)

My understanding is they are only the same at sea level.

15 hours ago, Slayer68 said:

My understanding is they are only the same at sea level.

They are the same in standard athmosphere (1013.25 hpa or 29.02 inHg at sea level and 15° C). As a rule of thumb: to get your KTAS you have to add 2% per 1.000 to your KIAS. For example, if you KIAS is 100 kts at 10.000 ft, your KTAS will be 120 kts. As you can see, that wouldn't explain the difference in the original post (115 kts KIAS at 7.000 ft is roughly 131 KTAS).

 

 

The higher your altitude, the less dense the air becomes, so there is less force being applied through the pitot tube, that results in a lower airspeed being shown on the airspeed indicator.  Wind direction and speed will also affect the relationship between IAS and TAS.

3 hours ago, stans said:

Wind direction and speed will also affect the relationship between IAS and TAS.

I believe that should only be true in the case of a sudden wind change (i.e. windshear). And even at that the difference would only be very transitory, quickly stabilizing at it's previous indication.

From a practical standpoint, wind speed and direction only affects groundspeed.

Jesse

Edited November 4, 20223 yr by JesC