Magnus 2009-08-20 21:01:48
We had a discussion about this and the popular explanation as to why TAS
increases with altitude while IAS stays constant was basically:
“Well, since air density is lower at altitude, and the indicated
airspeed stays the same, the airplane must be flying faster to
compensate for the lower density.”
This is true I suppose but to me it’s a really akward way of explaining
it and it’s a kind of backward deduction rather than an actual
explanation as to why flying from A to B is quicker at 10,000′ than at
2000′ (Disregarding wind).
My way of putting this would be something like:
“For the airplane to accelerate thrust has to be greater than drag. As
the airplane climbs the air gets thinner, ie there’s less resistance
holding the airplane back. At the same time, because of the decreasing
air density the engines produce less power, and the propellers become
less effective at converting that power into thrust. So while thrust
does decrease slightly, drag decreases more and the net effect is that
as the airplane climbs, thrust is greater than drag which results in an
At the same time, IAS does not change for the same reason, a decreased
air density. There’s less air, but it’s entering the pitot tube at a
faster rate which keeps the indication constant.”
What do you think of this? I’m trying to keep some sort of balance
between simplicity and comprehensiveness here, and I’m “talking” at the
Chris brooks 2009-08-20 21:01:54
The amount of drag doesn’t change as you go higher. The airspeed to achieve
this drag does.
Magnus 2009-08-20 21:01:59
Sure but how is this higher airspeed attained then? The fact remains
that the decreasing density upsets the equilibrium between thrust and
drag in the favor of thrust, allowing the airplane to accelerate to a
new state of equilibrium, this time found at a higher airspeed.
Mackfly 2009-08-20 21:02:07
KISS works well here!
Sailplanes have no prop / engine but the TAS vs IAS is still there.
Which makes me wonder why I’ve never gone into orbit while climbing?
Go with KISS—-when you find yourself running out of ideas and altitude you
won’t have time to dig into strange thoughts. Mac—
Andrew saranga 2009-08-20 21:02:10
You can make it even simpler by pointing out that the IAS is a direct
measure of the drag. Since drag will always be equal to thrust, when the
air is thinner the airplane will accelerate until the drag become equal
to thrust. Since thrust drops will increasing altitude, so does drag and
Mrcole100 2009-08-20 21:02:24
I think I have a simpler explanation for you. The reason that TAS and
IAS differ with altitude is the air density. While you’re probably
saying I already know that, it’s probably a little more subtle than
you expect. Think of what the airspeed indicator does, it’s a measure
of the dynamic pressure of the air entering the pitot tube. But how
does it compute airspeed? Well, the formula for dynamic pressure is
1/2*rho*V^2. Rho is the density of air and V is simply the airspeed.
So if the dynamic pressure can be measured and the density of air is a
given then V can be computed. If you rearrange the equation to solve
for V you would find that V varies with rho inversely. As rho
increases, V decreases for a given dynamic pressure and vice versa.
The airspeed indicator therefore must be calibrated for a certain air
density in order to provide the correct airspeed. The value of rho
used is the standard sea level air density.
You will notice that unlike the altimeter, which has a k*** allowing
you to recalibrate the pressure setting, the airspeed indicator
doesn’t have a k*** that will allow you to adjust the density setting,
so you always use the same value which is sea level density. As you
climb the true value of density decreases, but the value that’s used
to compute IAS, the sea level density, is always higher. Because the
higher sea level density is in the denominator and is higher than the
true value at altitude, the IAS will always be lower than the TAS.
This is the reason that takeoff and stall speeds are constant
indicated airspeeds. The IAS is a measure of the amount of air
molecules that the wings experience, and that’s all they care about
when it comes to issues of performance such as takeoff or stall.
However, the TAS that is necessary for the wings to see that IAS will
depend on density. So while the stall speed may always be 45 knots
IAS, the TAS that corresponds to that given IAS will vary with
density, whether caused my altitude or temperature increases.
Journeyman 2009-08-20 21:03:19
This is basically correct. Calibrated airspeed is just the pressure
differential, which depends on airspeed and density. They (standards
people) arbitrarily fix the density when they calibrate the instrument,
so as density decreases, the dynamic pressure will drop for the same
true airspeed. Thus, You get a lower instrument reading for the same
It may seem unnecessarily complicated to call this thing airspeed
when it’s not really airspeed but something closely related, but there
are a couple of good reasons. Firstly, the early instruments were
(and mostly still are) mechanical, so there’s no real good way to
mechanically compute TAS even if you do have temperature and presure.
Secondly, aircraft performance depends on the same speed and density
factor that the airspeed meter reads, so you don’t have to change the
airpeed value as altitude changes.
Of the two, I prefer the first explanation. Of the three, I prefer
mine, even if it’s a little more long-winded.
Andrew gideon 2009-08-22 14:20:23
Put the fellow through an MBA program, and he still speaks like an engineer.