IAS vs Altitude

[WannaBePiloto]

Hey folks,

Something I've always been a bit hazy about I'm hoping someone can help me clear up:

If you change nothing else about the plane, as you climb higher in altitude Indicated Airspeed decreases, right? Because less air molecules get into the pitot tube...

So how is that one power setting that gives you say, 100knots at sea level, also manages to keep your Indicated Airspeed at 100knots at 5,000'?

In my mind, I'd think you'd have to gradually increase power as you climb in order to maintain the same IAS you had at sea level. This in turn of course results in you having a higher TAS than IAS (above sea level). But it seems as if that isn't the case in reality as the same power setting used at 500' is the one used at 5,000'?

Sorry for the stupid question. Surely more to follow though! Thanks!

 

[Eric Pauley]

Air gets thinner, less force on elevator, airplane pitches down, TAS increases, IAS stays the same.

At a given elevator setting the airplane is in pitch equilibrium at a specific indicated airspeed, so the pitch will adjust to achieve that.

See page 5-15 (longitudinal stability) in the PHAK for more details.

 

[Pinecone]

What you are missing is the lower air density means less drag. So the same power gives the same IAS. But since there is less air, TAS is higher.

 

[WannaBePiloto]

What you are missing is the lower air density means less drag. So the same power gives the same IAS. But since there is less air, TAS is higher.

I think you are right. That does seem to be the component I was missing and one that doesn't seem to get brought up much in videos/discussions on this topic.

Air gets thinner, less force on elevator, airplane pitches down, TAS increases, IAS stays the same.

At a given elevator setting the airplane is in pitch equilibrium at a specific indicated airspeed, so the pitch will adjust to achieve that.

See page 5-15 (longitudinal stability) in the PHAK for more details.

Interseting. I'll definitely give that section a read (should have probably done that first before coming here).
I'm not sure why thinner air would require less elevator down force... I'd think if anything it would require more as there's less air that the wings can use to produce lift and thus you'd need a higher AoA in turn requiring more elevator lift (in the downward direction) to counter it.
I'll give that section a read and see if it turns the light bulb on for me!

 

[Eric Pauley]

The amount of elevator downforce is a function of IAS, not TAS (makes sense since IAS is the one that effects pressure). So for a given elevator setting/trim the airplane will maintain the same IAS by pitching. It doesn't even matter what the power is set at (well, it does a little because of prop wash). A given trim/elevator setting will maintain roughly the same IAS all the time.

 

[dmspilot]

Air gets thinner, less force on elevator, airplane pitches down, TAS increases, IAS stays the same.

At a given elevator setting the airplane is in pitch equilibrium at a specific indicated airspeed, so the pitch will adjust to achieve that.

See page 5-15 (longitudinal stability) in the PHAK for more details.

This explanation is bizarre. The difference between IAS and TAS has nothing to do with elevator force or pitch stability.

 

[Eric Pauley]

Sure, the TAS part is irrelevant. The point is that the elevator force is a function of IAS.

 

[dmspilot]

Sure, the TAS part is irrelevant. The point is that the elevator force is a function of IAS.

The question is not about elevator force.

 

[Eric Pauley]

Maintaining IAS during the climb is all about elevator force. See the fig from the PHAK.

 

[dmspilot]

Maintaining IAS during the climb is all about elevator force. See the fig from the PHAK.

That has nothing to do with the question, which is about cruise speed, not climb.

 

[Eric Pauley]

That has nothing to do with the question, which is about cruise speed, not climb.

as you climb higher in altitude Indicated Airspeed decreases, right?

I'd think you'd have to gradually increase power as you climb

Anyway all I've got to say is IAS stays the same because of elevator force and longitudinal stability, per the PHAK.

 

[dmspilot]

Anyway all I've got to say is IAS stays the same because of elevator force and longitudinal stability, per the PHAK.

No. The question is about the difference between IAS and TAS, it's not about longitudinal stability.

 

[dmspilot]

Air gets thinner, less force on elevator, airplane pitches down, TAS increases

Sure, the TAS part is irrelevant.

It wasn't irrelevant the first time you tried to explain it, it was the entire basis for your answer.

 

[WannaBePiloto]

Sorry guys. Didn't mean to cause a disruption by asking the question I'll figure it out from here I swear

 

[Eric Pauley]

Sorry guys. Didn't mean to cause a disruption by asking the question I'll figure it out from here I swear

You're fine. Read the PHAK, fly, and be enjoy!

 

[Pinecone]

Air gets thinner, less force on elevator, airplane pitches down, TAS increases, IAS stays the same.

Nope. Same pitch and power, same IAS. TAS goes up as you climb due to the reduced ram air pressure on the pitot from the lower air density.

 

[Eric Pauley]

Nope. Same pitch and power, same IAS. TAS goes up as you climb due to the reduced ram air pressure on the pitot from the lower air density.

That initial line was virtually made in jest, but people ignore the second part of the post which is basically directly out of the PHAK. The point is that elevator force varying with IAS provides longitudinal stability. Power is only minimally relevant. Read the PHAK.

 

[texasclouds]

Flying a Citation 550 at FL350, you may see IAS 210 KIAS and TAS 365 KTS. Big difference.

 

[2-Bit Speed]

Sorry guys. Didn't mean to cause a disruption by asking the question

Why the long face? Half the threads here were attempts to start the kind of argument you got here (by accident). That's an accomplishment!!

 

[Doc Holliday]

Flying a Citation 550 at FL350, you may see IAS 210 KIAS and TAS 365 KTS. Big difference.

Painful.

 

[Pinecone]

That initial line was virtually made in jest, but people ignore the second part of the post which is basically directly out of the PHAK. The point is that elevator force varying with IAS provides longitudinal stability. Power is only minimally relevant. Read the PHAK.

Which has nothing to do with the question asked.

 

[Pinecone]

Flying a Citation 550 at FL350, you may see IAS 210 KIAS and TAS 365 KTS. Big difference.

Biggest one I have seen was CAS of about 280 knots, Mach 0.9 = a bit over 500 KTAS.

T-38A at FL390

 

[SebIp]

Remember IAS is a a measurement of difference between the static and pitot.

 

[Eric Pauley]

Which has nothing to do with the question asked.

Debating the intent of OP is pointless. If you want to interpret the question as referring to cruise then sure longitudinal stability isn't as relevant. If the goal is to explain why a given trim setting maintains IAS throughout the climb then it is very relevant.

 

[tsts4]

Debating the intent of OP is pointless. If you want to interpret the question as referring to cruise then sure longitudinal stability isn't as relevant. If the goal is to explain why a given trim setting maintains IAS throughout the climb then it is very relevant.

Actually, it is the point. The OP asked A, you answered B and got called out for it. Own it and move on.

 

[Eric Pauley]

Actually, it is the point. The OP asked A, you answered B and got called out for it. Own it and move on.

I still disagree that OP's question in unambiguous one way or the other. But since you want me to "own it" let's just ask. Then we can move on.

@WannaBePiloto can you please clarify which of these you were asking about:

(A) Why, when setting level-flight cruise power, a given cruise power setting achieves the same IAS at different altitudes
(B) Why, as you climb, the IAS stays constant at a given trim setting without needing to touch the throttle

Thank you!

 

[tsts4]

I still disagree that OP's question in unambiguous one way or the other. But since you want me to "own it" let's just ask. Then we can move on.

Disagree all you want, but IMO it wasn't ambiguous, at least not to most of us. How a simple question regarding IAS turns into a discussion about pitch stability and elevator force is beyond me.

 

[Eric Pauley]

Disagree all you want, but IMO it wasn't ambiguous, at least not to most of us. How a simple question regarding IAS turns into a discussion about pitch stability and elevator force is beyond me.

I don't know how you get between two same-power cruise settings at different altitudes if "you change nothing else about the plane, as you climb". This part of the text pretty clearly refers to a single period in time where the airplane configuration remains constant and altitude changes. I.e., a climb. OP may not have intended that or have been unclear (which is fine), but I don't see how a literal interpretation of the question leads to comparing two cruise regimes. During this climb the constant IAS is maintained because of pitch stability and elevator force.

 

[SebIp]

Let’s make this more fun. Air density changes with temperatures. Your actual AGL changes with temperature.

1,2,3 Discuss!

 

[Eric Pauley]

Let’s make this more fun. Air density changes with temperatures. Your actual AGL changes with temperature.

1,2,3 Discuss!

If you fill your tanks volumetrically your weight changes with temperature too.

I make sure to open the storm window and clean the bugs off from the climb so I get a little cruise speed boost.

 

[Pinecone]

I make sure to open the storm window and clean the bugs off from the climb so I get a little cruise speed boost.

Serious competition gliders can actually do that in flight.

 

[Eric Pauley]

Serious competition gliders can actually do that in flight.

Amazing. TIL.

 

[Hang 4]

To stir the pot a bit more, if you maintain the same power setting (throttle, not %power) in a climb in a normally aspirated, piston airplane, IAS will drop as the air gets thinner and the power output of the engine drops.

 

[Eric Pauley]

To stir the pot a bit more, if you maintain the same power setting (throttle, not %power) in a climb in a normally aspirated, piston airplane, IAS will drop as the air gets thinner and the power output of the engine drops.

Have you actually tried that? The whole point I am trying to make is that the elevator maintains constant IAS in the climb even if power decreases. The reduced excess power manifests as reduced climb, not reduced IAS. If anything speed would go up slightly as the prop wash would be reduced somewhat (though I don’t think this is a noticeable effect). See the PHAK.

Don’t take my word for it. Set max rate of climb trim and climb to 10k. In my Arrow this requires basically no trim adjustment to maintain 90kts.

 

[Daleandee]

Serious competition gliders can actually do that in flight.

Yes sir ...

 

[WannaBePiloto]

I still disagree that OP's question in unambiguous one way or the other. But since you want me to "own it" let's just ask. Then we can move on.

@WannaBePiloto can you please clarify which of these you were asking about:

(A) Why, when setting level-flight cruise power, a given cruise power setting achieves the same IAS at different altitudes
(B) Why, as you climb, the IAS stays constant at a given trim setting without needing to touch the throttle

Thank you!

I would edit the the initial post I could haha. I've apparently caused lots of confusion.
Rewording attempt here:

Take an airplane flying 100kias at sea level..... Now put that same airplane at 8,000' and same power setting (throttle untouched) ....how is it still flying 100kias?

Expectation would be that as you climb, IAS would start to decrease (less air molecules going into pitot tube), and thus I'd need to increase power to keep the same IAS; which of course would increase TAS above IAS. But... I'm starting to get the idea that the decrease in air density leads to a decrease in drag (parasitic) and that's why the plane is able to fly faster at the same power setting. That's probably over simplified... maybe not even correct... but it's what I'm going to go with until further notice haha.

Nope. Same pitch and power, same IAS. TAS goes up as you climb due to the reduced ram air pressure on the pitot from the lower air density.

See I think that's the potential falacy that lead to this question to begin with. I think it is misleading to simply say: "TAS increases simply because reduced ram air pressure in the pitot." It actually just increases because you're going faster. The question becomes... (as I tried to reword above)... why/how are you going faster than you were before at lower altitude if you didn't increase power?


I've enjoyed reading through the banter and the explanations above! Hopefully my second attempt at wording this initial question clears some confusion. I apologize if I lead anyone down the wrong rabbit hole haha. Thanks!

 

[Eric Pauley]

I would edit the the initial post I could haha. I've apparently caused lots of confusion.
Rewording attempt here:

If I'm flying 100kias at sea level..... and I climb up to 8,000' ....how am I still flying 100kias if I didn't increase my power setting?

Expectation would be that as you climb, IAS would start to decrease (less air molecules going into pitot tube), and thus I'd need to increase power to keep the same IAS; which of course would increase TAS above IAS. But... I'm starting to get the idea that the decrease in air density leads to a decrease in drag (parasitic) and that's why the plane is able to fly faster at the same power setting. That's probably over simplified... maybe not even correct... but it's what I'm going to go with until further notice haha.



See I think that's the potential falacy that lead to this question to begin with. I think it is misleading to simply say: "TAS increases simply because reduced ram air pressure in the pitot." It actually just increases because you're going faster. The question becomes... (as I tried to reword above)... why/how are you going faster than you were before at lower altitude if you didn't increase power?


I've enjoyed reading through the banter and the explanations above! Hopefully my second attempt at wording this initial question clears some confusion. I apologize if I lead anyone down the wrong rabbit hole haha. Thanks!

Gotcha, so the question is about what happens during that climb? In that case you can prove to yourself that it’s all about the elevator using the following experiment:

Durinf your climb, at some point pull the power back a bit. If it were just that the power reduction and drag cancel out, you’d expect IAS to decrease. It may temporarily, but what you’ll see is that the elevator then loses authority and the plane pitches forward, causing IAS to get back to equilibrium. A given trim setting maintains (roughly) the same IAS regardless of power or altitude.

 

[WannaBePiloto]

Gotcha, so the question is about what happens during that climb? In that case you can prove to yourself that it’s all about the elevator using the following experiment:

Durinf your climb, at some point pull the power back a bit. If it were just that the power reduction and drag cancel out, you’d expect IAS to decrease. It may temporarily, but what you’ll see is that the elevator then loses authority and the plane pitches forward, causing IAS to get back to equilibrium. A given trim setting maintains (roughly) the same IAS regardless of power or altitude.

I think maybe I should have just taken the word "climb" out of my question because you are very focused on that aspect and that wasn't the intent of my question. Perhaps that's where the confusion is coming from.

I'm not asking about what's happening in the climb itself. I'm just saying take an airplane at sea level and at 60% power, it's flying 100kias. Now, put that same airplane at 8,000' and at 60% power, it's flying 100kias. How. Why.

I promise, cross my heart and hope to die, I will not attempt to re-word the question or continue to resurect this thread after this

 

[nauga]

I'm not asking about what's happening in the climb itself. I'm just saying take an airplane at sea level and at 60% power, it's flying 100kias. Now, put that same airplane at 8,000' and at 60% power, it's flying 100kias. How. Why.

I promise, cross my heart and hope to die, I will not attempt to re-word the question or continue to resurect this thread after this

...and I hope I don't regret wading in. However....

Drag is a function of (among other things) dynamic pressure. Dynamic pressure can be represented as a function of equivalent airspeed, which is calibrated airspeed corrected for compressibility, and calibrated airspeed is indicated airspeed corrected for pitot-static errors. At typical GA airspeeds the compressibility correction is negligible, and at higher GA speeds pitot-static errors are *generally* small and essentially negligible. All that is a long way of saying at the same IAS at different altitudes (all other things being equal) you are generating roughly the same amount of drag.

Thrust = drag in a steady climb or cruise, so if (IFF) power (and therefore work and thrust at the same dynamic pressure) does not change significantly as altitude increases then drag will equal thrust (i.e. airspeed will stabilize will stabilize) at the same equivalent (~indicated) airspeed at the same power setting. *True* airspeed will, of course, be different.

Nauga,
tested

 

[Dan Thomas]

Air gets thinner, less force on elevator, airplane pitches down, TAS increases, IAS stays the same.

No.

As the air's density decreases, the drag decreases and the airplane goes faster. That faster airspeed maintains the lift to keep the airplane airborne in that thinner air, and it also maintains the necessary downforce on the stab and elevator.

Where do you get such stuff?