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Mainly because of what pilots learn about density altitude and aircraft performance, any idea that an increase in temperature could improve performance seems counterintuitive. And indeed it is. Here's what got me thinking about it.

In studying for a written test I ran across a question phrased something like this:

An aircraft flying at a constant power setting flies from a colder temperature to a warmer temperature. What happens to true airspeed and true altitude?

The correct answer turned out to be that true airspeed and true altitude both increased. I knew immediately on the altitude because I remembered reading how very cold temperatures can cause dangerous errors in altimeters, but the airspeed didn't make sense.

I still don't understand why the increased temperature lead to an increase in true airspeed. Obviously a reciprocating engine doesn't perform better in warmer air. We don't install "interwarmers." Does it really mean that warmer air can actually improve performance? Or does this need a deeper explanation?

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  • $\begingroup$ Maybe "improves aircraft efficiency", rather than "performance"? $\endgroup$ – rbp Jul 22 '15 at 15:24
  • $\begingroup$ "I remembered reading how very cold temperatures can cause dangerous errors in altimeters" - Be careful with that; your altimeter, by definition, shows indicated altitude, which is affected by temperature. Your altimeter will read higher than your true altitude in cold air due to the increased density (which is what's dangerous), and thus the indicated reading on the altimeter will decrease as the outside air warms. The question must assume you're also maintaining a constant indicated altitude, in which case as the air warms and your altimeter reading decreases, you'd climb to a higher TA. $\endgroup$ – KeithS Jul 22 '15 at 15:39
  • $\begingroup$ The Instrument Flying Handbook outlines a scenario where the temperature is -50C and the difference between charted altitudes and corrected altitudes had to be calculated in order to conduct a safe approach. I've never been in that situation but it's good to have it in mind. $\endgroup$ – ryan1618 Jul 23 '15 at 4:21
  • $\begingroup$ I'm open to improving my understanding of both facets of this question in future, but best I can tell the reduced air pressure makes the wing more efficient in terms of ratio of drag to true airspeed. The engine may become more efficient, but may render less total horsepower because of the reduced availability of air. $\endgroup$ – ryan1618 Jul 26 '15 at 18:19
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Drag (and lift) increases with density. Density decreased and so did drag. So at the same power, you can fly faster.

Now I don't know whether reduction of power of a normally aspirated spark-ignition reciprocating engine at constant throttle setting would be higher or lower than the reduction of drag. But the question says power setting.

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True airspeed is equivalent airspeed corrected for non-standard pressure and temperature.

With a increase in temperature, TAS has no choice but to increase. Notice that IAS did not increase. It has nothing to do with engine performance.

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  • $\begingroup$ @Federico How does this not answer the question? The OP wanted to know why TAS increases and I answered it. It is more simple than the other answers indicate. $\endgroup$ – wbeard52 Jul 23 '15 at 17:43
  • $\begingroup$ my bad, sorry. You addressed the engine performance, but the question is about the overall aircraft one, that includes lift and drag. So, yes, you are right that this answer the question, but only partially. $\endgroup$ – Federico Jul 23 '15 at 17:49
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A higher temperature means the molecules are moving faster. Assuming a constant atmospheric pressure, that would translate to less number of air molecules in the same amount of space.

Now, drag is caused by the airframe hitting the air molecules. There is less drag, but the power setting stays the same - i.e. the force of forward thrust is constant, but the force of drag is smaller. Airspeed increases.

Of course, with most engines, if the throttle setting stays constant, engine power is less in warm air. So the question assumes that the loss of power is compensated by an advancing the throttle.

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"Obviously a reciprocating engine doesn't perform better in warmer air."

Why not? Horse Power increases with inlet temperature. Have a look at your engine handbook and look for something like "Sea Level and Altitude Performance". There should be a chart or table that makes that clear. I'm looking at the chart in my Lycoming engine manual for my Mooney, and the chart clearly shows an increase of horse power with inlet temperature.

"We don't install 'interwarmers.'"

No, we don't, but your reasoning is wrong. The premise of the question is that the air temperature changes by itself. By the way, the air pressure increase in a turbocharged/supercharged engine has an increase in air temperature as one component (although the smaller one) to provide for higher engine power. We close the cowl flaps in cruise flight to keep the engine warm, so we get higher engine output when an increase in altitude provides for cooler air to begin with!

"Does it really mean that warmer air can actually improve performance?"

yes, but not much. Air density changes (from altitude change) are much more pronounced.

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  • $\begingroup$ When you say "inlet" are you referring to the temperature of the air entering the intake or the turbine inlet? If you mean turbine inlet that's a measure of the temperature of exhaust gas exiting the cylinder, not intake air, so I don't see how it's directly related. $\endgroup$ – ryan1618 Jul 23 '15 at 4:18
  • $\begingroup$ you are referring to reciprocating engines in your post. There are no turbines in reciprocating engines. But then you also talk about cylinders ... there are no cylinders in a turbine engine ... sorry, I don't see how you're making sense here. $\endgroup$ – Andreas Lauschke Jul 23 '15 at 12:54
  • $\begingroup$ Turbocharged reciprocating engines use a turbine inline with the exhaust. When you refer to "inlet temperature" it sounds like you might be referring to TIT or turbine inlet temperature. My point was was the temperature of the air inducted and I think you were referring to the temperature of the exhaust as it exits the cylinder. $\endgroup$ – ryan1618 Jul 23 '15 at 14:23
  • $\begingroup$ Nope. I'm not referring to turbocharged engines, and I'm not referring to exhaust. Inlet is never exhaust. Have a look at your engine handbook. My Lycoming engine manual has a chart for Sea Level and Altitude Performance and lets you compute horse power with inlet temperature as one of the inputs. Don't get hung up on the word "inlet". For most practical considerations, the inlet temp is ambient temp. That's what is the premise of your question: the plane flies into an area of higher temp. That would be ambient temp. You can pretty much assume that's the same temp that your engine sucks in. $\endgroup$ – Andreas Lauschke Jul 23 '15 at 14:28
  • $\begingroup$ Thanks for clarifying that you mean "air inlet" and not "turbine inlet." $\endgroup$ – ryan1618 Jul 23 '15 at 19:18

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