TAS and Temperature

[uwochris]

Hey guys,

I got a question regarding TAS and temperature effects:

If TAS increases as you ascend into less dense air, why does not TAS increase (for any given power setting) in warmer air? After all, warm air is less dense than cold air.

Perhaps I am just a little confused because I always try to associate TAS with groundspeed. I always have had difficulty trying to differentiate between groundspeed and airspeed, which I think is the source of my confusions here.

For example, it seems odd that an increase in a headwind can increase indicated airspeed, yet decrease groundspeed. It just seems contradictory to me.

I hope someone can clarify this for me.

Thanks in advance!

 

[100LL... Again!]

If TAS increases as you ascend into less dense air, why does not TAS increase (for any given power setting) in warmer air? After all, warm air is less dense than cold air.

Don`t forget: In warmer air, for any given power setting, INDICATED will decrease due to the increase in density altitude, and subsequent power loss. The effects may cancel each other out, or it may change one way or the other. Yes, the air is less dense, allowing for less drag, but the ability to make power is reduced.




Perhaps I am just a little confused because I always try to associate TAS with groundspeed. I always have had difficulty trying to differentiate between groundspeed and airspeed, which I think is the source of my confusions here.

For example, it seems odd that an increase in a headwind can increase indicated airspeed, yet decrease groundspeed. It just seems contradictory to me.

An increase in headwind CAN NOT change your airspeed. Gusts can, over a very short time period, change Indicated Airspeed until the airplane `catches up`with the wind (windshear is a good example). If there is a steady-state wind, wind changes will NOT affect airspeed - groundspeed only.

Hope this helps

 

[bobbysamd]

Airspeed v. groundspeed

An increase in headwind CAN NOT change your airspeed. Gusts can, over a very short time period, change Indicated Airspeed until the airplane `catches up`with the wind (windshear is a good example). If there is a steady-state wind, wind changes will NOT affect airspeed - groundspeed only.

Another way to look at it is once your airplane breaks ground it becomes part of the airmass. Its speed is then measured relative to the ground track. In other words, if you have a direct tailwind your groundspeed will be your true airspeed plus whatever tailwind component. Reverse the situation and your groundspeed will be your TAS less the headwind component.

That's why it's so amusing to fly a very slow airplane, such as a Cub, into a strong headwind. The Cub seems like it's standing still in the air.

Hope that helps a little more.

 

[Singlecoil]

Hey guys,

I got a question regarding TAS and temperature effects:

If TAS increases as you ascend into less dense air, why does not TAS increase (for any given power setting) in warmer air? After all, warm air is less dense than cold air.

Boy, you ask some good questions. Let's assume we are talking about a reciprocating engine propeller aircraft without a turbo. The answer is for a given power setting, that is percentage of Brake Horsepower (70% or whatever), TAS does increase in warmer air. I think where you are getting confused, is that to achieve the same percentage of BHP in warmer air, you will need to set a higher engine RPM. So it appears you are setting a higher power setting, when in reality, if you look it up in your POH, the percentage of BHP will be the same. Notice the fuel flows will be about the same at the two different settings, even though engine RPM is different.

Most comparisons with TAS are with IAS that is why your question requires a little more thinking. I'm sure you remember you start with IAS, correct it for instrument error to get CAS, then correct that for pressure altitude and temperature (density) to get TAS. From there, you apply the wind to get your groundspeed. IAS and CAS are very close in most trainers so you can more or less disregard that error. But don't forget about it, because it is a factor in other airplanes. IAS (CAS) will equal TAS at a pressure altitude of sea level on a standard day (59 degrees F). TAS will equal groundspeed if there is no wind, or if it is a direct crosswind. If you are still at that sea level pressure altitude and at colder than standard temperature, IAS will now be HIGHER than TAS. If it is warmer than standard, IAS will be LOWER than TAS. Of course, when you are on approach, you don't care about that, you just care about IAS as that is the measure of how much air is flowing over your wing.

That'll do for the long-winded answer I guess. Does that help at all?

 

[uwochris]

Thanks for the responses. It is a little more clear now.

It is just difficult for me to visualize why groundspeed and airspeed shoud be different- to me, they should be equal. I can see why groundspeed is affected by various winds, but not how your IAS or TAS are affected by winds and temps. I always thought colder air = more dense, therefore more power can be created, therefore you'll get higher g/s, higher IAS, and higher TAS for any given power setting.

Also, I forgot to mention before... can IAS be affected by warmer/colder than ISA temps or different density altitudes? For instance, if I fly my 172 at 2300 RPM at Sea Level, than at 2300 RPM at 10,000ft, should IAS drop? (ie. less dense air implies less ram air pressure in pitot tube) Or, is the decrease in ram air pressure compensated for by the decrease in static pressure (ie. lower air pressure?) One application here may be flying an approach at a Colorado airport which is 9000' ASL, as compared to my home town (912' ASL)- if my POH says to fly approaches 65-75 KIAS, does this hold for all density altitudes, or should I apply a correction since Colorado's air pressure will be much less, and pitot ram air pressure will drop significantly, yielding a lower than actual IAS.

Thanks again for those who responded.

 

[Cardinal]

should I apply a correction since Colorado's air pressure will be much less, and pitot ram air pressure will drop significantly, yielding a lower than actual IAS.

This is where the beauty of the system is, where it actually makes sense. What the airspeed indicator "feels" - and therefore reads - is exactly what the wing and aircraft feels. If that ASI reads 80 knots the wing will give you 80 knots worth of effectiveness, whether you're at 10 feet or 10,000. Your true airspeed will vary, but what that ASI is reading is dynamic pressure, the same thing that gives life to your wing.

Now how long before somebody will insist on dragging compressibility into this?

 

[Singlecoil]

Whoa dude!

You appear to be a little balled up on this one. Reread my post again. When you are flying an approach, IAS is all you are concerned with. Think of IAS as being a measure of how much air is flowing over the wing.

In your example, it is correct that at the 9000 foot airport the air is less dense.
Remember the lift equation? Lift = Cl x 1/2 x Rho x S x Vsquared. At the 9000 foot airport, Rho (air density) is less therefore you must increase something else to achieve the same amount of lift that you had at the lower elevation (higher density) airport. What you increase is velocity. Velocity in this equation is TAS. Therefore you will be flying approaches at a higher TAS to get the same amount of lift over your wing. However you already know that at the 9000 foot airport, TAS is higher than IAS. That's right, if you fly the approach at the same IAS that you did at the 900 foot airport, your TAS is automatically increased, exactly compensating for the lower air density. You don't need a correction. That is why you need more runway at higher elevation airports to land, you are actually going faster, even though IAS is the same as it was at 900 feet.

 

[bobbysamd]

High altitude TAS

This is where the beauty of the system is, where it actually makes sense. What the airspeed indicator "feels" - and therefore reads - is exactly what the wing and aircraft feels. If that ASI reads 80 knots the wing will give you 80 knots worth of effectiveness, whether you're at 10 feet or 10,000. Your true airspeed will vary, but what that ASI is reading is dynamic pressure, the same thing that gives life to your wing.

That's a good explanation. Consider the ASI as a form of pressure gauge. The pitot receives ram air and compares it to outside ambient air received through the static source. Instead of presenting the pressure in something like pounds-per-square- inch it presents it in knots, or mph, or kph.

That's why you have to apply a correction if you use a source of alternate static air if your regular static air hole freezes over, such as in icing conditions. Your airspeed indicator may freeze. Static air inside the cockpit, if that's your source of alternate static air, will be lower inside the cockpit.

I realize that just went a bit afield. You'll learn all that during your IR course. Anyone for EAS?