IAS vs Altitude

[SebIp]

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.



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!

I had kinda point out a few replies back that IAS is the difference between the pitot and the static port.

What happens to your static pressure as you climb?

There is more to it. As others have said your TAS goes up. However the basic thought you had that there’s less pressure on the pitot so there’s less pressure and less airspeed read don’t consider that static port.

Btw you do know all the symptoms for blocked pitot and blocked static ports?

As for the power decreasing at altitude. That’s somewhat a different question in my mind. If you have insufficient power to maintain altitude you will either descend and kept your airspeed as that’s the balance between your CG and the trim and the now infamous elevator pressure. Otherwise you’ll pitch up to maintain altitude but at a lower airspeed.

 

[Eric Pauley]

@tsts4 looks like I was wrong.

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?

This explanation (if you read the whole comment) was about longitudinal stability in climb (which admittedly we now know wasn't what OP was asking about). It's straight out of Chapter 5 of the PHAK: https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/07_phak_ch5.pdf

 

[nauga]

This explanation (if you read the whole comment) was about longitudinal stability in climb (which admittedly we now know wasn't what OP was asking about). It's straight out of Chapter 5 of the PHAK: https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/07_phak_ch5.pdf

Not exactly. As dynamic pressure changes *with no change in trim* there is less force produced by the horizontal stabilizer but the basic pitching moment produced by the remainder of the airplane is reduced by the same amount since both moments are equally proportional to dynamic pressure (assuming no change in propwash or other secondary effects). There is no pitching down and accelerating, equilibrium is maintained so long as dynamic pressure is the only thing changing. This is not a characteristic of stability per se, it is a characteristic (and the definition) of trim, which is somewhat related but not the same.

Nauga,
and the sum of the moments

 

[Eric Pauley]

There is no pitching down and accelerating

Isn't it angle of attack that would not change, rather than aircraft pitch? As altitude increases climb rate decreases (assuming NA) and TAS increases (trimmed for fixed IAS), so the geometric angle (w.r.t. the horizon) of the relative wind gets shallower. So even at constant AOA the geometric aircraft pitch should get shallower.

As a more extreme example, if we cut power completely the AOA would remain constant but the direction of the relative wind (and therefore aircraft pitch) would also change.

I'll admit I may be missing something here, but personal experience says that during a NA climb pitch noticeably levels off and IAS stays constant.

 

[WannaBePiloto]

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?

This is exactly where my mind is at now. It makes sense. It doesn't do a deep dive into unecessary topics.
Only thing I think I might disagree on is that a faster True Airspeed somehow maintains a higher level of lift to keep an airplane airborne in thinner air. I'm of the impression that lift is created from the airplane moving through air molecules (measured by IAS) not through an air mass (i.e. True Airspeed)

Unless perhaps I read that the wrong way. Otherwise, I think you and I are on the same page haha.

I had kinda point out a few replies back that IAS is the difference between the pitot and the static port.

What happens to your static pressure as you climb?

There is more to it.

Not sure what you mean? There's more pressure as you climb? I'll admit there's a bit more pressure inside the cockpit than outside.... but still not sure how you'd be able to say static pressure increases as you climb. Maybe my understanding of pressure is wildly backwards haha. But figured I'd point this out and ask ya to explain since I've inadvertantly created a never ending thread of questions

 

[Eric Pauley]

I'm of the impression that lift is created from the airplane moving through air molecules (measured by IAS) not through an air mass (i.e. True Airspeed)

Precisely. Previous answer is mixing TAS and IAS.

Nauga's answer makes the most sense to me for cruise: https://www.pilotsofamerica.com/community/threads/ias-vs-altitude.142891/#post-3400781

 

[Dan Thomas]

This is exactly where my mind is at now. It makes sense. It doesn't do a deep dive into unecessary topics.
Only thing I think I might disagree on is that a faster True Airspeed somehow maintains a higher level of lift to keep an airplane airborne in thinner air. I'm of the impression that lift is created from the airplane moving through air molecules (measured by IAS) not through an air mass (i.e. True Airspeed)

Unless perhaps I read that the wrong way. Otherwise, I think you and I are on the same page haha.

True airspeed is the ACTUAL speed through the air. The indicated is lower due to the lessened impact of the air against the pitot due to its lower density. The higher true airspeed creates more lift than if we were doing the same true airspeed as the indicated shows. The true is higher because the drag is less. It's one reason airliners fly as high as they can most of the time.

At altitudes nearing the airplane's service ceiling you'll find that the airplane's attitude is more nose-up in level flight. That's because more angle of attack is needed to provide the lift in such thin air. Service ceiling is defined as that altitude at which the airplane's rate of climb falls below 100 fpm; it can climb higher but it will do so slowly, and at some point you'll be in the climb attitude but not climbing anymore.

Too many pilots never caught the truth of the relationship between airspeed and angle of attack. And that's even when they're being trained in slow flight, where the airplane has to have a high AoA at low speeds to generate the same amount of lift. This lack of understanding leads them to wonder why their landings are so lousy. It's because they approach too fast and carry that airspeed all the way into the flare, where any up-elevator results in ballooning or a bounced landing, or they end up landing flat (low AoA necessary at that high airspeed) in order to get it onto the ground, and now they try to fix the speed with braking, and they skid the tires. In extreme cases they get wheelbarrowing that can wreck the airplane, or they run off the end of the runway. Or they bust the nosegear.

There are plenty of resources on successful landings, and YouTube is not one of them. These folks need to go back to the textbooks and get hold of the theory they missed.

 

[Eric Pauley]

The higher true airspeed creates more lift than if we were doing the same true airspeed as the indicated shows.

Could you explain this?

Are you arguing that, at the same IAS/AOA, an airplane at a higher TAS will produce more lift?

If you crack open the PHAK (which I believe counts as a textbook), you'll see the following equation for lift:



See ρV^2? Once you ignore compressibility and other things that have no effect on GA, that is (proportional to) IAS^2. TAS is irrelevant.

The cruise discussion here has really been played out. Nauga explained it perfectly: https://www.pilotsofamerica.com/community/threads/ias-vs-altitude.142891/#post-3400781

 

[Tantalum]

Hell.

To Richard Feynman's point, it is evident that there are very few people who actually understand this well enough to articulate and teach it

OP asked a straightforward question

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'?

The air is thinner, less drag, the plane "speeds up" to maintain the same "equilibrium"; TAS goes up, IAS stays same

I'm sure someone will eviscerate this post but it's the answer I've given all my DPEs without issue

 

[SebIp]

This is exactly where my mind is at now. It makes sense. It doesn't do a deep dive into unecessary topics.
Only thing I think I might disagree on is that a faster True Airspeed somehow maintains a higher level of lift to keep an airplane airborne in thinner air. I'm of the impression that lift is created from the airplane moving through air molecules (measured by IAS) not through an air mass (i.e. True Airspeed)

Unless perhaps I read that the wrong way. Otherwise, I think you and I are on the same page haha.



Not sure what you mean? There's more pressure as you climb? I'll admit there's a bit more pressure inside the cockpit than outside.... but still not sure how you'd be able to say static pressure increases as you climb. Maybe my understanding of pressure is wildly backwards haha. But figured I'd point this out and ask ya to explain since I've inadvertantly created a never ending thread of questions

Your static port measure ambient (for the most part) pressure. The IAS is the difference between the pitot ram pressure and the static. So as your climb the static pressure decrease. So if you consider what that means to your original assumption of higher = less pitot ram pressure, what does that mean?

 

[WannaBePiloto]

Your static port measure ambient (for the most part) pressure. The IAS is the difference between the pitot ram pressure and the static. So as your climb the static pressure decrease. So if you consider what that means to your original assumption of higher = less pitot ram pressure, what does that mean?

I'm not even sure what we're discussing at this point haha. You had literally just said "What happens to your static pressure as you climb? There is more to it."
Now you are going back to saying as you climb the static pressure decreases (true). So ima just leave that one alone I think.

Hell.

To Richard Feynman's point, it is evident that there are very few people who actually understand this well enough to articulate and teach it

OP asked a straightforward question


The air is thinner, less drag, the plane "speeds up" to maintain the same "equilibrium"; TAS goes up, IAS stays same

I'm sure someone will eviscerate this post but it's the answer I've given all my DPEs without issue

Thank you. This is originally what I came here to find out haha. 100 rabbit holes later and we've touched on pressure differentiels and elevators and TAS somehow generating lift and everything inbetween.

If I stop replying it's because I have a busy week coming up. Thanks all for the comments and participation.

 

[Pinecone]

Only thing I think I might disagree on is that a faster True Airspeed somehow maintains a higher level of lift to keep an airplane airborne in thinner air. I'm of the impression that lift is created from the airplane moving through air molecules (measured by IAS) not through an air mass (i.e. True Airspeed)

Lift is a function of IAS.

Vne is a function of TAS.

Some aircraft, like the U-2, fly high enough that the IAS at Vne is nearly the same as the stall speed in level flight. Called the coffin corner. Can''t go faster, can't go slower.

 

[WannaBePiloto]

Lift is a function of IAS.

You and I are in agreement (except with what you said the Vne is a function of TAS...). I was trying to say that I disagreed with the idea presented by the person I was replying to.

 

[Eric Pauley]

Vne is a function of TAS.

In light GA isn't Vne (almost?) always an IAS? Here's mine, at least:



In the case of the U-2, my understanding is that Vne is set by mach number, which is not a fixed TAS.

 

[WannaBePiloto]

In light GA isn't Vne (almost?) always an IAS? Here's mine, at least:

View attachment 117486

In the case of the U-2, my understanding is that Vne is set by mach number, which is not a fixed TAS.

Yeah I've never heard heard of that one either (that Vne is a funciton of TAS). I would say I'd be surprised if that was correct... but then again... I'm learning new things every day hahah

 

[Rgbeard]

Lift is a function of IAS.

Vne is a function of TAS.

Some aircraft, like the U-2, fly high enough that the IAS at Vne is nearly the same as the stall speed in level flight. Called the coffin corner. Can''t go faster, can't go slower.

Is there a dislike button somewhere?

 

[Eric Pauley]

Is there a dislike button somewhere?

I’ve been wondering that a lot recently.

 

[Velocity173]

Is there a dislike button somewhere?

What @Pinecone indicated is true.

 

[Eric Pauley]

What @Pinecone indicated is true.

Got a link?

Part 23 would seem to disagree: https://www.govinfo.gov/content/pkg/CFR-2017-title14-vol1/pdf/CFR-2017-title14-vol1-sec23-335.pdf

Recall: EAS is basically equivalent to IAS for spam cans.

 

[Rgbeard]

FYI - Here's a limitations page for a rather ordinary hershey-bar special:

I don't see any mention of TAS. It's all about IAS, and the arcs/lines on the airspeed indicator.

 

[Rgbeard]

Lift is a function of IAS.

Vne is a function of TAS.

Some aircraft, like the U-2, fly high enough that the IAS at Vne is nearly the same as the stall speed in level flight. Called the coffin corner. Can''t go faster, can't go slower.

You might want to Google "Coffin Corner" before talking about it. Not sayin'. Just sayin'.

 

[Eric Pauley]

FYI - Here's a limitations page for a rather ordinary hershey-bar special:

I don't see any mention of TAS. It's all about IAS, and the arcs/lines on the airspeed indicator.

View attachment 117488

Why waste time providing data when you can just appeal to "authority"?

 

[Rgbeard]

 

[Velocity173]

Got a link?

Sure.

https://pilotinstitute.com/airspeed-indicator/

http://flyingdonald.blogspot.com/2011/03/what-flutter.html

https://www.boldmethod.com/learn-to-fly/aerodynamics/coffin-corner-where-vne-and-mmo-meet/


While your airspeed indicator has a redline for Vne, it’s most likely (typical GA FW aircraft) established for flutter. Flutter is directly affected by TAS and not IAS. Vne could also be from critical Mach (TAS) as indicated by the U2 “coffin corner” example.

Helicopters have Vne’s as well but not due to flutter reasons. The primary reason for Vne is at high speed the aircraft can get into retreating blade stall. But, there’s also a Vne based on the TAS of the blade itself and that’s critical Mach of the advancing blade also at high speeds (very cold temps). Vne (as an IAS) is reduced as altitude increases. Vne adjustment can be due to external loads as well but that’s a whole other topic.

 

[Eric Pauley]

Sure.

https://pilotinstitute.com/airspeed-indicator/

http://flyingdonald.blogspot.com/2011/03/what-flutter.html

https://www.boldmethod.com/learn-to-fly/aerodynamics/coffin-corner-where-vne-and-mmo-meet/


While your airspeed indicator has a redline for Vne, it’s most likely (typical GA FW aircraft) established for flutter. Flutter is directly affected by TAS and not IAS. Vne could also be from critical Mach (TAS) as indicated by the U2 “coffin corner” example.

Helicopters have Vne’s as well but not due to flutter reasons. The primary reason for Vne is at high speed the aircraft can get into retreating blade stall. But, there’s also a Vne based on the TAS of the blade itself and that’s critical Mach of the advancing blade also at high speeds (very cold temps). Vne (as an IAS) is reduced as altitude increases. Vne adjustment can be due to external loads as well but that’s a whole other topic.

Aviation Safety later publised what basically reads as a retraction of that article: https://www.aviationsafetymagazine.com/features/vne-revisited/

We shouldnt have stated VNE is an indicated airspeed.

Also regardless of what airspeed is safe, Vne is a regulatory speed, which by regulation is EAS/IAS for Part 23 aircraft (nit: at spam can altitudes).

 

[nauga]

Lift is a function of IAS.

Vne is a function of TAS.

Lift is a function of dynamic pressure; which is a function of equivalent airspeed, or true airspeed and local atmospheric density.
Vne is a function of TAS if flutter is the limiting condition. If it's, say, your windscreen imploding or some static aeroelastic issue due to dynamic pressure then it's a function of equivalent airspeed.

Nauga,
limited accordingly

 

[Velocity173]

Aviation Safety later publised what basically reads as a retraction of that article: https://www.aviationsafetymagazine.com/features/vne-revisited/



Also regardless of what airspeed is safe, Vne is a regulatory speed, which by regulation is EAS/IAS for Part 23 aircraft (nit: at spam can altitudes).

But they still stated Vne as a function of flutter, is TAS. The reason why it’s confusing is that most ASIs are in indicated. Therefore it’s displayed as IAS but the reason could be a TAS limit that the pilot must account for. It’s the difference between what is referenced for Vne and what is actually the limiting factor (flutter / critical Mach).

 

[Eric Pauley]

But they still stated Vne as a function of flutter, is TAS. The reason why it’s confusing is that most ASIs are in indicated. Therefore it’s displayed as IAS but the reason could be a TAS limit that the pilot must account for. It’s the difference between what is referenced for Vne and what is actually the limiting factor (flutter / critical Mach).

I agree that's true in a limited sense.

The statement "Vne is a function of TAS." is false for two (EDIT: I guess one) reasons:

1. Exactly what Nauga said. Some aircraft are not flutter limited.
2. Vne is a regulatory speed. It is, by regulation, a regulatory speed, set based on flight testing and measured in EAS. (EDIT: I'm going to take this one back since the testing is probably done based on engineering analysis of TAS, so in a sense Vne is still a function of TAS)

FWIW in my 32 thou spamcan I'm betting dynamic pressure breaks something before flutter.

 

[nauga]

2. Vne is a regulatory speed. It is, by regulation, a regulatory speed, set based on flight testing and measured in EAS.

I realize you said you took this back, but will pursue it a little all the same. Vne is not flutter speed. Flutter speed (the lowest speed where divergent flutter may occur) is driven by true airspeed. Airplanes whose limiting condition is set by flutter but have Vne as indicated or calibrated will generally have that Vne set with some predefined margin to ensure that TAS remains reasonably well below flutter speed at (published) Vne. In that instance, Vne in [IAS/CAS/EAS] is still a (maybe nonlinear, discontinuous, or even arbitrary) function of TAS.

FWIW my airplane's Vne is 183 KTAS. I have an estimate of TAS available in the cockpit but also use rules of thumb for IAS to remain below Vne.

Nauga,
and the things he has seen

 

[Eric Pauley]

I realize you said you took this back, but will pursue it a little all the same. Vne is not flutter speed. Flutter speed (the lowest speed where divergent flutter may occur) is driven by true airspeed. Airplanes whose limiting condition is set by flutter but have Vne as indicated or calibrated will generally have that Vne set with some predefined margin to ensure that TAS remains reasonably well below flutter speed at (published) Vne. In that instance, Vne in [IAS/CAS/EAS] is still a (maybe nonlinear, discontinuous, or even arbitrary) function of TAS.

FWIW my airplane's Vne is 183 KTAS. I have an estimate of TAS available in the cockpit but also use rules of thumb for IAS to remain below Vne.

Nauga,
and the things he has seen

Great explanation, thank you!

 

[Velocity173]

I agree that's true in a limited sense.

The statement "Vne is a function of TAS." is false for two (EDIT: I guess one) reasons:

1. Exactly what Nauga said. Some aircraft are not flutter limited.
2. Vne is a regulatory speed. It is, by regulation, a regulatory speed, set based on flight testing and measured in EAS. (EDIT: I'm going to take this one back since the testing is probably done based on engineering analysis of TAS, so in a sense Vne is still a function of TAS)

FWIW in my 32 thou spamcan I'm betting dynamic pressure breaks something before flutter.

Yeah at some point it’ll be a structural failure but not at Vne in smooth air. At .9 of design dive speed, there’s a large margin for error for any structural failure.

I think Pinecone’s statement is mostly correct and definitely doesn’t warrant the dislike comment. He simplified the two (IAS / TAS) to make them easier to understand. I think the specific point he was trying to make about Vne is that it isn’t always IAS. It could be less based on atmospheric conditions and in the case of some aircraft (helicopters) could be far below indicated redline.

 

[Doc Holliday]

 

[Eric Pauley]