True Airspeed

[tcco94]

What is an easy way to teach this to PPL students and a more complex way for CPL students? I'm getting lost and confused in my own words at trying to teach why TAS increases with altitude. The density of the air decreases as altitude increases, but the temperature also drops which makes colder air more dense. Propeller efficiency should decrease as altitude increases also, which gave my student the idea we should fly at lower altitudes on cross country flights...for more efficiency.

Any CFI's out there know a way to explain this better?

 

[Jett]

IAS= how fast you put atmosphere into the pitot tube
TAS= how fast you cover a distance in space

As you increase altitude but maintain IAS you are smashing atmosphere into the pitot tube at the same rate, but because it is more spread out, you cover a greater physical distance.

 

[tcco94]

IAS= how fast you put atmosphere into the pitot tube
TAS= how fast you cover a distance in space

As you increase altitude but maintain IAS you are smashing atmosphere into the pitot tube at the same rate, but because it is more spread out, you cover a greater physical distance.

So more along the lines of you're travelling the same speed but you're covering more distance in a given time period...making you "faster" through that "airmass" (the definition of TAS)

 

[Jett]

You are traveling through the same number of molecules of atmosphere for a given time period. You are traveling further through space in order to achieve that.

So, yes, you are moving faster though the airmass. I'd just be careful about defining it as moving through the airmass, that to me sounds like indicated airspeed. I also explained IAS as speed through molecules of air.

 

[Jett]

... and in regards to flying lower for efficiency... A propeller does loose efficiency with altitude, but the big item for GA trainers is loss of engine power. A C-172 for example gets it's highest cruise speed at 8000'.

 

[🦈💜]

... and in regards to flying lower for efficiency... A propeller does loose efficiency with altitude, but the big item for GA trainers is loss of engine power. A C-172 for example gets it's highest cruise speed at 8000'.

That's why they made constant-speed props, so that you can tighten efficiency.

-Fox

 

[CFI A&P]

What I did was draw two airplanes on the white board. Say a C182 @ 3000' and include IAS and temps. Then a C172 at 10,000' and a given IAS and temps. Numbers that were relative to the airframe examples.

Then have the student tell me which was one faster before working it on the E6B. It helps explain who is faster and why, plus include the E6B practice.

Sometimes I would switch it up and go with a Baron, Caravan but I would try to keep the IAS near each other so the student wouldn't always guess the faster IAS.


Sent from my iPhone using Tapatalk

 

[Blackhawk]

http://luizmonteiro.com/Learning_Pitot_Sim.aspx

 

[Jordan93]

IAS= how fast you put atmosphere into the pitot tube
TAS= how fast you cover a distance in space

As you increase altitude but maintain IAS you are smashing atmosphere into the pitot tube at the same rate, but because it is more spread out, you cover a greater physical distance.

This is how I teach it as well. Draw two circles. One circle is at sea level with 10 molecules of air. The other circle is at 10000 feet with 2 molecules of air. Your IAS is staying the same at both altitudes, however at the higher altitude since things are more spread out, you're cutting through the molecules faster

 

[Jett]

This is how I teach it as well. Draw two circles. One circle is at sea level with 10 molecules of air. The other circle is at 10000 feet with 2 molecules of air. Your IAS is staying the same at both altitudes, however at the higher altitude since things are more spread out you cutting through the molecules faster

I like that. I used to line up all my finger tips, touching eachother and say "sea level" then spread them as far apart as I could and say "10000 feer". I feel like your method implies a little more "Science!"

 

[Blackhawk]

While the link above is about the Pitot static system it essentially shows the principle behind TAS as you view the difference between static and dynamic pressure as the altitude changes.

 

[Jordan93]

I like that. I used to line up all my finger tips, touching eachother and say "sea level" then spread them as far apart as I could and say "10000 feer". I feel like your method implies a little more "Science!"

Haha yea. It's just enough science and just simple enough that an idiot like me can understand it! I do the same thing with hot and cold air when I explain how temperature affects performance. Draw two circles, label one hot and the other cold. 10 molecules of air in cold circle and 3 molecules of air in hot circle. You can explain a lot with a few circles.

 

[JustinS]

The way I explained it to my students was that at higher altitude the air is less dense, and for the airplane to "feel" like it's going 100 knots it had to go faster through the less dense air to get the IAS to 100 knots and have the wing have the equivalent of 100 knot airflow (IAS) and as a result the TAS would be faster than IAS as altitude increases.

 

[BCTAv8r]

The density of the air decreases as altitude increases, but the temperature also drops which makes colder air more dense.

That is one of the concepts I'm still trying to understand. Cold air is less dense than warm air, but as you gain altitude, there are less air molecules, making the air less dense. Does the loss in molecules make up for the increased density of cold air?

Edit: I meant to say that cold air is more dense, not less.

 

[AAPalmTree]

None of that crap matters. Find the GS indicator on one of your tubes. That is the ONLY thing that matters in figuring out when your next Starbucks, hotel room or "real" bathroom encounter is gonna happen.

 

[U_of_I_Tweak]

That is one of the concepts I'm still trying to understand. Cold air is less dense than warm air, but as you gain altitude, there are less air molecules, making the air less dense. Does the loss in molecules make up for the increased density of cold air?

In a nutshell, yes. The altitude and therefore absolute air pressure has more of an impact on density than temperature.

At a high altitude, a warmer airmass will have less density than a colder airmass, but both will have less density than an airmass at sea level.

If you can't do something smart, do something right

 

[tcco94]

This is how I teach it as well. Draw two circles. One circle is at sea level with 10 molecules of air. The other circle is at 10000 feet with 2 molecules of air. Your IAS is staying the same at both altitudes, however at the higher altitude since things are more spread out you cutting through the molecules faster

I like it...but that's the part I'm having trouble explaining to my students. I understand at 10,000 feet you'd have less molecules because the density decreased but how are you cutting through the molecules faster just because there are less molecules going through the pitot-tube?

Here's where my brain went south today when trying to explain this... student failed a stage check for planning too low of an altitude. I started giving him various reasons for why you should fly higher. That's where he brought up the discussion that the decrease in density of air makes it less efficient on his propeller and for that reason he planned an altitude of 4500. I told him that although the efficiency will decrease, his true airspeed will increase with altitude. That's where I think I made him confused, which made me confused at understanding what I am even trying to explain. By definition: TAS = CAS corrected for non standard pressure and temperature. Okay...cool. Now, my airspeed indicator already corrects for the non-standard pressure by the differential of dynamic air pressure (ram air) and static pressure (steady air pressure around my airplane) and then I just put the OAT in the small box and read the true airspeed below....

Now I get the question... "Well why is it higher?"...and then I start getting confused. So the density of the air is decreased, so I have less molecules going through my pitot-tube giving me a lower IAS than I am actually flying.... but why? Are we pretty much saying that my airplane is moving 130 knots but the KIAS is going slower because there are less molecules going through the pitot-tube...or in other simpler words...my airspeed indicator is "lying" to me at how fast I'm moving?

Sorry for the in depth break down but I'm not trying to explain this to a person like you and me...I gotta break this down into some englinese understanding where I can understand it and teach it to my student more effectively than I felt I did today.

 

[WS]

I think you need to separate the temperature and efficiency stuff from this and look solely at the true airspeed vs indicated, and make the other variables equal. Keep it simple. At higher altitude, the pressure is less, so there will be less molecules hitting the airframe and pitot tube therefore the indicated speed will be less than at lower altitude. Meanwhile, the aircraft has to travel a greater distance since there are less molecules to hold it back and keep it flying hence why the difference increases as you go higher.

At very high altitudes, the aircraft may barely be flying due to the "thin" air and have a very low indicated speed, all other things being equal, but may have a much higher true airspeed than at lower altitudes since it is covering a much greater distance to meet up with all these molecules that are keeping it flying.

People much smarter than me get advanced degrees in aerodynamics and study all this in detail. If you stop and think about all the variables like prop angles, efficiency, weight, temp, moisture content etc, it could get extremely confusing for a student, so try to look at one thing at a time.

And yes, the airspeed indicater IS lying to you, but it is an error that can be calculated. If I recall, at lower altitudes like what piston singles use, you lose roughly 2% for every 1000 feet you climb. So at 5000 feet, your IAS is 10% less than at sea level.

 

[tlewis95]

In order to produce the same amount of lift in less dense air, the wing has to move through the air faster. To make 100kts worth of lift at SL up at 7000' the wing (and pitot tube) needs to move through the air at a faster speed.

I like correlating that with a performance lesson as well.

What speed do we approach at? (IAS)
Ok. How about if we are taking off/ landing at a 7000' elevation? (Same IAS)
Why do we need more runway to land at 7000' then at SL then? (Higher TAS)

All the same could be done using a takeoff as an example but then the prop and engine inefficiencies throw more mud at the concept

 

[Jordan93]

I like it...but that's the part I'm having trouble explaining to my students. I understand at 10,000 feet you'd have less molecules because the density decreased but how are you cutting through the molecules faster just because there are less molecules going through the pitot-tube?

Here's where my brain went south today when trying to explain this... student failed a stage check for planning too low of an altitude. I started giving him various reasons for why you should fly higher. That's where he brought up the discussion that the decrease in density of air makes it less efficient on his propeller and for that reason he planned an altitude of 4500. I told him that although the efficiency will decrease, his true airspeed will increase with altitude. That's where I think I made him confused, which made me confused at understanding what I am even trying to explain. By definition: TAS = CAS corrected for non standard pressure and temperature. Okay...cool. Now, my airspeed indicator already corrects for the non-standard pressure by the differential of dynamic air pressure (ram air) and static pressure (steady air pressure around my airplane) and then I just put the OAT in the small box and read the true airspeed below....

Now I get the question... "Well why is it higher?"...and then I start getting confused. So the density of the air is decreased, so I have less molecules going through my pitot-tube giving me a lower IAS than I am actually flying.... but why? Are we pretty much saying that my airplane is moving 130 knots but the KIAS is going slower because there are less molecules going through the pitot-tube...or in other simpler words...my airspeed indicator is "lying" to me at how fast I'm moving?

Sorry for the in depth break down but I'm not trying to explain this to a person like you and me...I gotta break this down into some englinese understanding where I can understand it and teach it to my student more effectively than I felt I did today.

Your IAS at sea level is 100. Your indicated airspeed at 6000 is going to be 100. There are less air molecules up at 6000 ft. Because there are less air molecules up at 6000 feet but you are still getting the same IAS your TAS must be faster because in order for the plane to impact the less molecules at the same "rate."