# Why does IAS decrease with lower air density if power is constant?

TAS increases with less dense air, which is intuitive. What I don't understand is why IAS decreases.

This is assuming sub-transonic speeds where compressibility effects are negligible.

To phrase this in more practical terms, if you look at the climb table in a POH for a turbocharged airplane you will see full throttle climb rate drops with an increase in altitude/temp, even before the MP drops. EAS and propeller efficiency would not seem sufficient to explain the IAS drop seen.

I would think that given a TAS, pressure altitude, and temperature, one could derive the IAS but I have not seen any math to do this.

This is in reference to a turbocharged propeller driven airplane that is capable of maintaining power at higher altitudes.

• My current guess is that this is related to less aerodynamic efficiency at higher altitudes, as described in the AFH, but that’s basically where it ends. I’m interested in why this occurs and to what extent.
– John
Commented Oct 17, 2019 at 1:50

Because even with constant power, thrust decreases with velocity, also known as TAS.

Drag depends mainly on the dynamic pressure, which is what IAS represents, so if you had the same thrust, you'd have the same IAS too.

However a piston engine produces approximately constant power (if your RPM is in the optimal range), and power is thrust times velocity. So the turbocharger lets you have the same power in the less dense air, but because your true airspeed is higher, the thrust is a bit lower and therefore your (maximum attainable) indicated airspeed is as well.

And then there is the effect of RPM. Engine power (of any engine) is limited by RPM, growing up to some optimal speed and then declining (and then you get to the red line). If you have a fixed propeller, your available power grows with speed first as the RPM increases with decreasing effective pitch, but then you cross the optimum and the power starts reducing and then you hit the red line and have to retard the throttle. And effective pitch again depends on TAS.

• IAS drops significantly in actual airplanes with the same percentage power setting. It could be that lower is constant while thrust drops so I have updated the question.
– John
Commented Oct 17, 2019 at 1:43
• @John, thrust drops even for constant power, but in most aircraft you also don't get the same power with the same throttle setting. To get constant power you need to maintain constant manifold pressure, but that usually means gradually moving the power lever forward. Commented Oct 17, 2019 at 17:16
• My question is why thrust and/or drag changes with changing air density given constant power, rather than if it occurs (it is known it does). Only simplistic explanations seem to be available ("it's just less efficient," etc.).
– John
Commented Oct 17, 2019 at 21:05
• @John, and I have already answered that question as best as I could. Drag (which is force), depends on EAS (IAS), but for the same power, thrust is inversely proportional to TAS, so you have less of it at the same IAS. Commented Oct 18, 2019 at 11:19
• Jan, thank you. Do you have any more information on why thrust decreases with velocity given equal power? This would seem to be the reverse of how some engines (ex: ramjets) operate. Is this related specifically to propellers? Since the airfoil on the wing seems roughly as efficient, or at least the AOA is the same, this seems counterintuitive to me.
– John
Commented Oct 18, 2019 at 18:02