True Airspeed

tcco94 said:
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?
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You aren't cutting through the molecules faster. You are moving through the molecules at exactly the same rate. But now the molecules are further apart. Ask your student if he could swim faster in syrup or water...

BCTAv8r said:
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.
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Temperature is a measure of molecular energy. Your classic Mercury thermometer is being "squeezed" by the air around it. When the air is hot, there is a lot of molecular energy, the atoms that make up the air are basically vibrating and bouncing off the thermometer. The force they impart into the Mercury causes it to rise in the tube. At high altitude there are far fewer atoms to bounce off the thermometer, the temperature decreases.

When you think of colder air being more dense think of it at sea level. Yes, cold air at sea level is always denser than warm air at sea level. When you start going up in altitude the pressure will decrease because there is less atmosphere above it pushing down, wether it is cold or warm to begin with won't change this.
 
Why not just explain/instruct the whole ICE-T concept of airspeeds from the beginning (instead of trying to leap from I directly to T)?
 
The airspeed indicator functions by comparing the ratio of ram air pressure from the pitot tube to the static air pressure. At sealevel where the air is more dense, this ratio is obtained at a slower TAS. At a higher altitude, the aircraft must travel at a higher TAS to move through a grater volume of air just to obtain the same ram air/static air ratio and display the same IAS.

Knowing that we will takeoff using the same indicated airspeed at sealevel as we would at higher a higher altitude airport, say 6,000 ft, calculate the takeoff distances for sealevel and 6,000 ft elevation. Look at the distance required and the IAS used and then calculate the groundspeed for both. You'll come up with a greater liftoff speed at the higher altitude. If you frame this as a "no wind" condition, you should be able to relate groundspeed and TAS for your students. We need a higher TAS at a higher altitude in order to develop the same the same amount of lift at the same IAS.

As someone stated earlier, the discussion of temperature on the density of an air mass should be done with air masses at similar altitudes. Typically, we're concerned about the effect air temperature on density altitude for our performance calculations. The performance calculations are made for a given field elevation and the density variables will be atmospheric pressure and air temperature. Again, it may be helpful to frame the discussion of the effects of temperature on air density to air masses at the same altitude.
 
tcco94 said:
Any CFI's out there know a way to explain this better?
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The wings only care about the mass of the air flowing around them. When the air is less dense, you need more of it moving over the wings to get the same lift... So you move faster. The engine only cares about the mass of air going into the cylinders. Less dense air, less air goes in. Possibly less fuel too (if you lean the mixture), so you get less power.

At least, that's how I explain it.
 
drunkenbeagle said:
The wings only care about the mass of the air flowing around them. When the air is less dense, you need more of it moving over the wings to get the same lift... So you move faster. The engine only cares about the mass of air going into the cylinders. Less dense air, less air goes in. Possibly less fuel too (if you lean the mixture), so you get less power.

At least, that's how I explain it.
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Okay, this is all starting to click in my head now. I think my head was not in the game yesterday and I was just making circles at my own explanations. Your IAS is lower because of less molecules but we still need molecules to flow over the wing to produce lift...so we move faster through the air to get that same amount of lift we were getting at 2000 feet...making our TAS faster.

If I draw the circles like @Jordan93 said, I would just explain that I need more molecules at 10,000 feet and to achieve that we go faster.

Does that all make enough sense?

SurCharles said:
The airspeed indicator functions by comparing the ratio of ram air pressure from the pitot tube to the static air pressure. At sealevel where the air is more dense, this ratio is obtained at a slower TAS. At a higher altitude, the aircraft must travel at a higher TAS to move through a grater volume of air just to obtain the same ram air/static air ratio and display the same IAS.
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That just sounds confusing to me because at sea level under standard atmospheric pressure, your IAS or CAS would equal TAS. Every increase of altitude from that point would be an increase in TAS from your IAS.
 
Keep it simple and make it relatable. With these concepts, I always make my students explain it back to me to make sure they really understand it.
 
IAS, or more accurately CAS, is an imprecise and indirect measurement of the amount of force the air imparts on the wing as the wing moves through the air. TAS is the speed the wing must be moving to attain that force.
 
I like to use the Space Shuttle concept. Whats the speed of the space shuttle in space? 18,000 MPH. That is fast! Why is it so fast? No drag to slow it down! Wonder what its indicated airspeed is. 0. No air going into the pitot tube. Time for the shuttle to land. As it descends the airspeed indicator will start to move, but the drag starts to slow the craft down. When will indicated match the true airspeed? When it lands at sea level.
 
Everyone saying that at higher altitudes the airplane needs to move through the air faster to hit the same number of molecules needs to be careful what you're trying to say. Moving through the air faster....higher up with the same IAS you're going at the exact same rate through the air.
Instead say you have to cover a greater physical distance in the same amount of time to get the same number of molecules.
 
Stinger said:
Everyone saying that at higher altitudes the airplane needs to move through the air faster to hit the same number of molecules needs to be careful what you're trying to say. Moving through the air faster....higher up with the same IAS you're going at the exact same rate through the air.
Instead say you have to cover a greater physical distance in the same amount of time to get the same number of molecules.
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Greater physical (zero wind) distance + same amount of time = moving through the air faster
 
Reading this thread has confused me a bit. In order to solidify my understanding; in a no wind condition, TAS will match groundspeed, correct?
 
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I think we have 2 lessons here:

1) Always check your student's planning prior to a check ride.

2) Don't use that examiner anymore.
 
@tcco94 What exactly did your student fail for? Surely there had to be more involved than just choosing a less than optimum altitude. To my knowledge there isn't any one particular altitude for flying (per POH). Were there other considerations like terrain, weather, airspace, direction of flight etc that were not planned for?
 
Boskru said:
@tcco94 What exactly did your student fail for? Surely there had to be more involved than just choosing a less than optimum altitude. To my knowledge there isn't any one particular altitude for flying (per POH). Were there other considerations like terrain, weather, airspace, direction of flight etc that were not planned for?
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No his altitude planned was fine. His route had no obstacles of terrain or anything. Just want to clarify I said it was a stage check...so an internal check from my school pre solo. Didn't give the student any specific reasons, just said that there were many reasons to plan higher. Student even tried to explain that all VORs were usable (cited the AFD) and that airspace was all E and clear of the B. I didn't disagree that higher was better and they didn't talk about true airspeed...but I brought it up..stating he could go faster with more altitude and have more fuel efficiency as my two biggest reasons he needs to plan higher.

That begun a confusing conversation that I was having trouble teaching in a different way. After some rest of the day of this post, almost all of these different ideas you guys helped me with clicked. I just got home that day and wasn't satisfied with my teaching method on true airspeed and needed help on a better method to teach this.

Which I appreciate the numerous replies and answers. Clearly this is a tough topic so it was good to have some better ideas and understandings to use in the future.

There were a few items he failed on that I did not agree with but it's only internally so I don't have an option to pick my own coworkers lol. That's not the point of this post...I just wanted some different teaching methods on this area rather than just accept the fact my method wasn't getting the point across.
 
okay, failing a stage check makes me feel better for the student than a checkride failure over that issue. Sounds like your student did their due diligence when planning that route.

It may be worth talking to the stage check examiner about that issue though. I know as a student it can be extremely frustrating when you can defend your planning and end up failing anyways. Plus, it gives the student comfort to know that you have his/her back.
 
Acrofox said:
When a student fails without error, the system has failed the student.
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As a Check Airman for a 141 flight school, there is usually more to the story. My bet is he failed a PTS task by not meeting the published standard. But during the debrief, the check-airman gave a whole list of tasks to help the student (they usually do have a passion to teach). From my experiance the student/instructor will go telling everyone about the nitpicky items and forget what that the real reason is that the student nearly put it in the ditch.. or whatever the case may be...

Just my suspicion, but who knows.
 
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