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Old 02-18-2018, 08:49 PM   #661
MRaza
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why does speed decrease as altitude increases?
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Old 02-18-2018, 09:46 PM   #662
IvanK
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Good question this is explained by the "Custard Layer theory" Which is the empirical proof that the Air gets thicker as altitude increases ... slightly controversial concept but pretty simple. (Note this theory predates Global Warming theory)

Take your oblate spheroid (Planet earth) and wrap it with an atmosphere (The custard layer)
then rotate the oblate spheroid around its axis ... what happens to the custard layer ... it gets thrown off ...as does the atmosphere.

Nonsense you say .... but lets look at the evidence.
Why is it that (if you accept the theory) that slow aeroplanes that fly in the lower reaches of the custard layer have propellers ? simple the air is so thin they actually need fans to induce air flow. Helicopters who indulge in super thin air "nap of the earth" type flying have the biggest propellers of all.

As you get higher the air gets thicker so thick in fact that propellers can no longer turn so then we bring in jet propulsion which allows thick air to be accelerated through a tube to produce thrust. then of course you get into space were its so thick you cant even get the air down a tube so must resort to non air breathing engines like rockets.

Take also the pilots those who fly in the lower thin portions of the custard layer can breath quite easily unassisted. however as you get higher the air is so thick then masks and ultimately full pressure suits are required ... least the high altitude aviator drowns in the thicker higher level air

Further convincing proof of the theory is the colour of the sky. in the lower thin levels is a lovely cyan sky blue ... however as you climb the sky gets darker until ultimately in space where its really thick its black ! ... pretty convincing so far.

Now of course there many other empirical proofs but lets get back to your question.

"Why does speed decrease with altitude" ... simple in terms of Custard layer theory . Given airspeed is measured by Pitot static systems in the lower or thin portions of the atmosphere the air molecules are free to move and their concentration is obviously less so they can rapidly move through the pitot system. However as altitude increases the air becomes thicker and poor air molecules become congested and constrained by the pitot tubes and can no longer move through it as quickly as at the lower levels and so a lower speed is registered.... all quite simple really. It just takes a little outside the box type thought see the answer.

BTW "Custard layer theory" also explains why Chem trails can linger so long after they have been dispensed .... but much of that subject is of course classified.

Anyway I hope that brief discussion on "custard layer theory" helps answer your question

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Old 02-18-2018, 10:20 PM   #663
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However as altitude increases the air becomes thicker and poor air molecules become congested and constrained by the pitot tubes and can no longer move through it as quickly as at the lower levels and so a lower speed is registered.... all quite simple really. It just takes a little outside the box type thought see the answer.
Not saying your wrong but that goes against everything I've ever learned. I've always been taught that air gets denser the LOWER you go because of other molecules above pushing them down. And that air gets thinner/less dense as you increase altitude so that's why mountain climbers have trouble breathing?!
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Old 02-18-2018, 10:27 PM   #664
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why does speed decrease as altitude increases?
Your IAS will drop because the air pressure in the pitot tube drops. TAS will stay the same. You can actually go a lot faster at high altitude because of less air resistance, e.g. in the F-15 you'll struggle to go above Mach 1.1 at see level but can be above Mach 1.4 at 40.000ft in no time.
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Old 02-18-2018, 10:29 PM   #665
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Your IAS will drop because the air pressure in the pitot tube drops. TAS will stay the same. You can actually go a lot faster at high altitude because of less air resistance, e.g. in the F-15 you'll struggle to go above Mach 1.1 at see level but can be above Mach 1.4 at 40.000ft in no time.
Sorry but what's the exact difference between IAS and TAS?
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Old 02-19-2018, 01:01 AM   #666
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Sorry but what's the exact difference between IAS and TAS?
TAS or True Air Speed is how fast the air is actually moving over your aircraft's control surfaces (or how fast your are actually moving through the air). As such, you need to know it when trying to calculate closure speeds, for instance. IAS or Indicated Air Speed OTOH is, in a sense, is a measure of how much air is actually moving over those surfaces. IAS is an indication of how your aircraft will respond to that air. At high altitude, you can be moving fast (TAS) through the air but your aircraft can be just above stall speed (in terms of IAS) because there is so little air to provide lift, etc. You fly the aircraft in reference to IAS. How the aircraft feels with an IAS of 250 at sea level is more or less how it will respond with an IAS of 250 at 32,000 feet, even though at 32,000 feet, your true speed through the air is 800.
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Old 02-19-2018, 01:21 AM   #667
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" How the aircraft feels with an IAS of 250 at sea level is more or less how it will respond with an IAS of 250 at 32,000 feet, even though at 32,000 feet, your true speed through the air is 800"

actually its behavior is very very different, aerodynamically its at 250 but inertia is there for the very much higher TAS. The aeroplane is a lot more sensitive at the higher altitudes tasks like close formation are a fair degree more difficult at higher altitudes even at the same IAS as aircraft inertial response is TAS based. To the pilot the aeroplane feels a lot more squirrly at the higher altitudes than the lower altitudes. For the same IAS you can manhandle the aeroplane a whole lot more at the lower levels than you can at the higher levels.
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Old 02-19-2018, 12:17 PM   #668
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" How the aircraft feels with an IAS of 250 at sea level is more or less how it will respond with an IAS of 250 at 32,000 feet, even though at 32,000 feet, your true speed through the air is 800"

actually its behavior is very very different, aerodynamically its at 250 but inertia is there for the very much higher TAS. The aeroplane is a lot more sensitive at the higher altitudes tasks like close formation are a fair degree more difficult at higher altitudes even at the same IAS as aircraft inertial response is TAS based. To the pilot the aeroplane feels a lot more squirrly at the higher altitudes than the lower altitudes. For the same IAS you can manhandle the aeroplane a whole lot more at the lower levels than you can at the higher levels.
You're right, of course, which is why I said more or less. I simply wanted to get the idea across that IAS had more to do with how the aircraft handled, while TAS had more to do with speed/time calculations.
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Old 02-20-2018, 06:33 AM   #669
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Originally Posted by MRaza View Post
Not saying your wrong but that goes against everything I've ever learned. I've always been taught that air gets denser the LOWER you go because of other molecules above pushing them down. And that air gets thinner/less dense as you increase altitude so that's why mountain climbers have trouble breathing?!
Custard Layer


To answer your question in it’s simple form, less dense air will result in a lower differential pressure between the pitot tube and the static ports (and hence lower IAS) for the same true airspeed as pressure altitude increases.

Some things on airspeed you might want to know,based on your question. Thanks Ironhand

The definitions I use are:
TAS is the actual speed at which the airplane moves through the airmass that surrounds it.
IAS is the speed observed on the air speed indicator.

Just to be clear.....

The airspeed indicator utilizes the difference between pressure measured at the pitot tube and the static port(s) to measure airspeed. This airspeed indicator is designed around a sea level standard atmosphere.

The airspeed indicator is subject to instrument and position errors. If you could correct for these errors you’d have CAS, which stands for CALIBRATED airspeed.

Another way to define TAS is as CAS corrected for nonstandard temperature and pressure. So as you go higher and the air is less dense, you will actually have a higher TAS than CAS, which IAS tries to display. CAS = TAS at sea level in a standard atmosphere.

Just to complicate things, in high speed airplanes, you also correct CAS for adiabatic compression flow. So when you get high and fast, air is compressed at the pitot tube giving false readings. If you correct CAS for this compressibility then you get EAS, which is called equivalent airspeed.

If you really want to get deeper into this kind of thing get yourself a copy of “Aerodynamics for Naval Aviators”
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Old 02-21-2018, 10:57 AM   #670
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Will be nice you can add your name to F-15 display name, like cap. Alf.Snake
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