True Airspeed and Temperature

[oktex88]

Can anyone explain to me why true airspeed increases with a increase in temperature?

Looking at the Piper Arrow charts it clearly shows that a increase in temp there is a increase in TAS.

I always thought that performance is better with lower temps, therefore you can get a faster TAS.

Can anyone fill me in with what I'm missing here?

Thanks!

 

[Alchemy]

True airspeed increases as air density decreases. Warm air is less dense than cold air, all else being equal.

Hotter air decreases your takeoff and landing performance for the same reasons that a higher altitude airfield does. The air is less dense in both scenarios.

Your wing needs X number of air molecules flowing over it in order to generate enough lift to takeoff.

At sea level and 15 degrees, a liter of air has more molecules than it does at sea level and 35 degrees.

Simplistically, a pitot tube measures air molecules, so to get enough air molecules to get to, say, a 50 knot IAS rotation speed, you'll need to really be going perhaps 55 knots true airspeed before your airspeed indicator says "50".

So at 35 degrees you have to go faster to get "15 degrees" worth of air molecules to flow over the wing and into the pitot tube. This means your takeoff and landing rolls will be longer. This is because you're going faster in terms of groundspeed when you takeoff and land.

Does this make sense?

To sum it up: Thin air (hot, high, low pressure) allows the airplane to go faster, assuming the engine's perofmance or maximum mach number doesn't preclude it from doing so, because the air molecules are farther apart and the airplane moves faster through the air to achieve the same amount of lift and IAS. Thin air also makes the airplane fly at a higher speed to takeoff and land, necessitating a longer runway. Thick air (cold, low, high pressure) makes the airplane go slower, but it also allows it takeoff and land land at lower speeds, allowing a shorter runway.

 

[Xiph]

What is causing your confusion, at least from what I can see, is the way the chart is written. You are correct that performance is better with lower temps, but the TAS charts in the Arrow POH I have are all based upon a certain power setting - 75%, 65%, etc. That takes the performance/temperature equation out of the picture, because 130BHP (65% power according to my POH) is 130BHP, whether it's ISA+20 or ISA-20. You may not even be able to get 130BHP out of the engine at ISA+20 at certain altitudes, but the TAS charts don't factor this in (look at the note on the power setting tables, on the other hand, and you'll see that it is addressed - you'll have to increase MP as temperature goes up in order to maintain a certain percentage of power - obviously this cannot be done indefinitely).

So if you're getting 130BHP out of the engine, you'll benefit from the lower drag that warmer (and thus less dense) air gives you, which is why the TAS is higher. This is similar to the increase in TAS that you get from flying at a higher altitude, and for the same reason.

Some POHs (like those for most Cessnas, if you have access to one of those) do their charts differently, and there you can see the effects of temperature more easily. Whereas the Arrow chart uses percent power as the basis for finding TAS, the 172 chart uses RPM. If you look at a particular RPM at a particular altitude, you'll see that percent power drops off with an increase in temperature, and you'll see that TAS generally does as well (though not linearly - in some cases it might even go up, which I believe is due to the decrease in drag being more than the reduction in power, though I'm not absolutely certain on that).

 

[juxtapilot]

Can anyone explain to me why true airspeed increases with a increase in temperature?

Looking at the Piper Arrow charts it clearly shows that a increase in temp there is a increase in TAS.

I always thought that performance is better with lower temps, therefore you can get a faster TAS.

Can anyone fill me in with what I'm missing here?

Thanks!

I think you may be over-thinking what your chart is telling you. Like everyone else has said, when air temperature increases, the air density decreases. In less dense air the pitot tube won't have quite as many air molecules going into it. The IAS will be lower so you'll need to go faster (increase TAS) to compensate for the less dense, higher temperature air.

For example, the IAS at a low temp can be 100 kts, while at a high temp you may have an IAS of 90 kts while at the same true airspeed as your lower temp 100kts. Increase your IAS from 90kts to 100kts, and as you can see your true airspeed will increase as well. In that same situation if you fly from a high temperature to a low temperature, and maintain the same IAS your true airspeed will decrease.