altitude up, barberpole down ...

[Chairman]

Someone asked a question on another forum and it sounded so simple until I actually thought about it. Now my brain hurts. Can someone explain it to me in simple words please ?

The question was - why does the barberpole speed of an aircraft come down as the aircraft climbs.

It sounded like a no-brainer - the barberpole comes down because the air is thinner, and the ASI is basically just a well calibrated pitot pressure gauge, so less air molecules = lower IAS and the pole comes down accordingly.

But, why ?????

Surely a given *indicated* airspeed isn't going to do anything at FL300 that it wouldn't do at sea level ? If a plane can handle sufficient air molecules hitting it for it to feel like 240kts, shouldn't it be able to handle that many molecules at FL300 as easily as it can at FL0 ?


Gary

[Ian Warren]

Heres another part to your canundrum .. at FL30 .. the aircraft would have to go faster to get from point to point because at altitude , the dia: of the earth has been increased

[Chairman]

The GROUNDspeed would have to be steadily increasing anyway to maintain a constant AIRspeed as the air thinned out.

Wouldn't it ?

Now I'm really confused !

Gary

[ardypilot]

At a constant speed, IAS will be the same at every altitude. TAS increases with an altitude increase due to the decreasing density of the air- think about the aircraft as pacman, and the molecules as the little blue dots he is meant to gobble up. They get further and further spaced apart the higher you go, so you need to travel a greater distance in the same amount of time to eat them.

Groundspeed is just TAS + or - windspeed.

[Chairman]

You what ??? If TAS remains constant, wouldn't IAS decreases as altitude increases ?

Anyways doesn't answer the question (which I had such high hopes of you being able to explain ) - planes are speed limited by how much air pressure caused by forward movement they can handle, or to continue the analogy, by how fast pacman can scoff the dots without barfing.

As altitude increases pacman takes longer to eat the same amount of dots, or eats less dots in the same amount of time, because they're further apart.

My question is why can't he just move faster, and eat the exact same amount of dots which we know he can do in the original amount of time which we know he can do ? I think that's what you said ("so you need to travel a greater distance in the same amount of time to eat them") but obviously for some reason it doesn't work that way, because a plane that's allowed to do 240kts at sea level isn't allowed to to anything like that at altitude.

Gary

[deaneb]

QUOTE (Chairman @ Jul 3 2009, 01:30 PM) <{POST_SNAPBACK}>

You what ??? If TAS remains constant, wouldn't IAS decreases as altitude increases ?

Anyways doesn't answer the question (which I had such high hopes of you being able to explain ) - planes are speed limited by how much air pressure caused by forward movement they can handle, or to continue the analogy, by how fast pacman can scoff the dots without barfing.

As altitude increases pacman takes longer to eat the same amount of dots, or eats less dots in the same amount of time, because they're further apart.

My question is why can't he just move faster, and eat the exact same amount of dots which we know he can do in the original amount of time which we know he can do ? I think that's what you said ("so you need to travel a greater distance in the same amount of time to eat them") but obviously for some reason it doesn't work that way, because a plane that's allowed to do 240kts at sea level isn't allowed to to anything like that at altitude.

Gary

The truth is the plane is flying faster at a higher altitude, otherwise it would be pointless climbing up there in the firstplace wouldn't it?. So although IAS, which is related to pressure is decreased, the actual speed of the aircraft relative to the ground is higher. Why is pacman not able to go even faster and gobble up the dots - because the dots gobbled up by the engine and being ejected out as thrust are suffering the same problem - they are further apart and less of them. It a case of balance of thrust and drag. Overall jet aircraft are faster and more efficient at higher altitudes, prop driven aircart are more efficient at lower altitudes and speeds.

Deane

[Chairman]

That makes perfect sense, but still doesn't answer the question. Let me try it this way :

My question is why does the maximum permitted indicated speed of a plane reduces with altitude.

It's not about why IAS reduces with altitude, or why planes aren't able to maintain a constant IAS as they get higher, it's why does the max allowable indicated airspeed come down as the air thins out.

I'd have thought that if a plane could hang together at a certain airspeed then that was that, regardless of how fast it was actually going, as long as the indicated airspeed was below the limit it would be ok. 240 kts IAS is 240 kts IAS at any altitude - the higher you go the faster the groundspeed will be, but the amount of air hitting the plane will remain constant. So why does the limit reduce as the air thins out ?

Gary

[deaneb]

QUOTE (Chairman @ Jul 3 2009, 09:12 PM) <{POST_SNAPBACK}>

That makes perfect sense, but still doesn't answer the question. Let me try it this way :

My question is why does the maximum permitted indicated speed of a plane reduces with altitude.

It's not about why IAS reduces with altitude, or why planes aren't able to maintain a constant IAS as they get higher, it's why does the max allowable indicated airspeed come down as the air thins out.

I'd have thought that if a plane could hang together at a certain airspeed then that was that, regardless of how fast it was actually going, as long as the indicated airspeed was below the limit it would be ok. 240 kts IAS is 240 kts IAS at any altitude - the higher you go the faster the groundspeed will be, but the amount of air hitting the plane will remain constant. So why does the limit reduce as the air thins out ?

Gary

I guess because the dynamic pressure is only one factor. As you increase in altitude the speed of sound lowers. Shockwaves can start to play havoc as these will begin to develop in low pressure areas when the aircraft is still below Mach 1, so this can also be a limiting factor for higher cruising aircraft (commercial jets)
Additionally turbulence and manouevring is related to weight, not air pressure and as speed increases, then stress on an airframe (G load) will increase. This is related to the actual speed of the aircraft, in other words if the aircraft is designed for normal manouevres at 240 Kts as a "real" speed then at sea level this equates to 240 knots, but at a higher altitude this IAS drops off due to pressure, although the "real" speed of the plane is still 240 Kts.
Hope that makes more sense. At the end of the day its about what the speed is relative to, be it TAS, IAS, GS

[Chairman]

Yes, that helps a lot. I'll call this one answered, many thanks

Gary

[HardCorePawn]

IAS (Indicated) is not necessarily a "true" reflection of how fast an aircraft is moving through the air... hence TAS (True Airspeed).

The barberpole is really the maximum TAS that you should be flying, but is shown as IAS, as that is all you have to reference on your instruments (talking steam powered here... I am pretty sure them flash computer driven ones can show TAS).

At low altitudes, the IAS is pretty close to TAS (allowing for the various errors inherent in ASI's...). However, at higher altitudes, given the same TAS, the IAS is lower (the thinner air doesn't generate as much dynamic pressure within the Pitot sytem as at lower level)

Therefore, the barberpole speed (shown in IAS) must come down...

eg. barberpole at 2,000' = TAS 240kts ~= IAS 239kts
barberpole at 35,000' = TAS 240kts ~= IAS 200kts

(ps. these are made up numbers to illustrate a point)


To test this out, take an aircraft (like the mooney g1000)... fly along at 2000'... and check the various speeds. Then climb it up to like 18000' and check the various speeds. You should find TAS remains constant-ish... but IAS gets less the higher you climb.