True airspeed
mattcroghan
Hello -
For a given indicated airspeed or mach, does true airspeed decrease with an
INCREASE in altitude? I hope that's clear enough, because as a slow learner
the above still confuses the heck out of me.
Thanks!
Matt
Manuel Portillo Pérez
No. It is just the other way round. True airspeed increases with an increase
of altitude for the same indicated airspeed.
Manuel
Greg Esres
<<For a given indicated airspeed or mach, does true airspeed decrease
with an INCREASE in altitude? >>
Your airspeed indicator measures something called "dynamic pressure",
or "q", which is 1/2*rho*V^2. Rho is air density, and the V is TRUE
airspeed. You can see for a given true airspeed, a lower density,
which occurs at altitude, will produce a lower pressure, according to
the formula, and thus will provide a lower indicated airspeed.
Now, if you take indicated airspeed to be a constant, as your scenario
suggested, then in a low density environment (low Rho), you will
require a higher V to achieve the required pressure.
So, the final answer is that TRUE airspeed increases with altitude,
for a given indicated airspeed.
Mach is different. It's a ratio of TRUE airspeed to the speed of
sound. The speed of sound decreases with altitude (due to decreasing
temperature). So for a given Mach, TRUE airspeed must decrease along
with the speed of sound at altitude.
Stealth Pilot
>For a given indicated airspeed or mach, does true airspeed decrease with an
>the above still confuses the heck out of me.
As you move to higher altitudes the number of molecules per unit of
volume (density) in the air decreases.
At, what is it 10,000 or 18,000 ft, the air density is half that of
sea level.
To maintain the same apparent airspeed as you get higher you will be
flying faster so that the same number of molecules per unit of time
hit the pitot.
For a constant indicated airspeed you will have an increasing actual
speed with increasing altitude.
For a constant indicated airspeed you will have a decreasing actual
speed with decreasing altitude.
Stealth Pilot
Australia
GLPILOTSRV
True airspeed increases approximately 2% in reference to calibrated airspeed
per 1000 feet increase in altitude up to the tropopause. Deviations from
standard temperatures can impact this generalization..
EX: 250 KCAS = about 300 KTAS at 10,000 feet
G. Lee
Carter Nelson
I'm not sure how universal the various flavors of airspeed are. The way I
learned it was based on the following acronym:
ICE-T ("ice tea", insert rapper reference here)
which stands for Indicated, Calibrated, Equivalent, and True, and indicates
the order in which the various airspeeds are derived at.
Indicated Air Speed (VIAS) is simply the raw reading from the instrument,
for example, the delta pressure from a pitot-static probe plugged into
incompressible Bernoulli
Calibrated Air Speed (VCAS) is VIAS corrected for instrument error, position
error, etc., this is airplane configuration dependent and determined through
flight testing or some other process
Equivalent Air Speed (VEAS) is VCAS corrected for compressibility, like
VIAS, but use compressible Bernoulli instead
True Air Speed (VTAS) is VCAS corrected for variation in density due to
altitude, this requires that the ambient density be determined somehow
(typically done by also measuring temperature along with the pressures)
Based on the above definitons, and to answer your questions, let's assume
zero instrumentation error and no compressibility, such that:
VIAS = VCAS = VEAS
then the only correction needed is for changes in density with altitude to
get to VTAS, which is done as follows:
VTAS = VIAS / sqrt(G)
where G is the ratio of density at the given altitude over standard day sea
level density. G will be unity at sea level and then decrease (become less
than one) with increasing altitude. This means that for a given indicated
air speed, the true airspeed will increase with an increase in altitude.
This trend will generally continue as altitude increases.
However, in terms of Mach, the trend is not as simple. VTAS can be computed
from Mach as simply:
VTAS = (Mach) * (Speed of Sound)
The speed of sound variation with altitude is sometimes decreasing,
sometimes constant, sometimes increasing, depending on what altitude you are
at. However, speed of sound generally decreases with altitude up to about
36,000 feet. In this case, for a given Mach number the true airspeed will
decrease with an increase in altitude. This is the opposite of the trend
with VIAS described above.
Hope that helps,
Carter
dbrown
Good explanation except for one thing. Mach depends on temperature, not
altitude. Usually the temperature gets colder as you go up which gets
people to thinking that Mach is related to altitude or pressure. It is not.
dbrown
And I said that wrong. Not Mach. The speed of sound.
Don Stauffer
There ARE airspeed indicators that make the correction themselves, based
on measured pressure and temperature, but these are pretty fancy systems
and not found on smaller, cheaper aircraft. Normal ASI reads as G. Lee
indicates.
--
Don Stauffer in Minnesota
stau...@usfamily.net
webpage- http://www.usfamily.net/web/stauffer
Carter Nelson
> Good explanation except for one thing. Mach depends on temperature, not
> altitude. Usually the temperature gets colder as you go up which gets
> people to thinking that Mach is related to altitude or pressure. It is
not.
This is a deviation from the original posters subject, but this comment
seems a little confusing. Mach number can depend on more than just
temperature, but ultimately, Mach number has a very specific and simple
definition:
M == V / a
where V is velocity and a is the speed of sound. Speed of sound is directly
related to temperature through the following relation (for thermally perfect
gases):
a = sqrt( g * R * T )
where g is the ratio of specific heats for air (1.4), R is the ideal gas
constant for air (1716 ft*lb/slug*R), and T is absolute temperature (use
Rankine for the given R). While temperature does vary with altitude, and
therefore also the speed of sound (and if V is constant, so will Mach
number), this variation is not monotonic. Check out the graphs on this site:
http://www.digitaldutch.com/atmoscalc/index.htm
Note that at some altitudes temperature and speed of sound increase with
altitude.
While the relation given above for Mach number is the common DEFINITION, in
practice Mach number can be computed in various ways. Now, if you have an
instrument that somehow reads V (velocity) and another that somehow reads a
(speed of sound), then just divide the two to get Mach number. However, for
isentropic flows, Mach number can also be computed from measurements of
total pressure and static pressure. See equation 44 in NACA 1135:
http://naca.larc.nasa.gov/reports/1953/naca-report-1135/naca-report-1135.pdf
So in some sense, Mach number does depend on pressure, but be careful using
the word "pressure" without specifying total pressure or static pressure.
dbrown
I just ran your program against my old Pratt & Whitney book.
With the same temperature at 10k, 20k, 30k, 36k up to 65K, the
speed of sound does not change.
To repeat what I meant is that the speed of sound does not change appreciably
with altitude (out to about the 5th significant digit) unless the temperature
changes. Of course the temperature does change, due to the atmosphere.
My point is that if the outside air temperature is the same at whatever
flight level your flying at, the speed of sound is as close to the same as
you can measure.
Mark Drela
>but ultimately, Mach number has a very specific and simple
>definition:
>M == V / a
For airplanes, an equivalent and I think a more useful definition of Mach
number is:
0.5 gamma M^2 = q / p
i.e. M^2 is a measure of the ratio
q / p = dynamic pressure / ambient static pressure
with the constant 0.5*gamma = 0.7 thrown in.
Using this relation, we can write:
0.5 gamma M^2 = (1/CL) * (mg/S) / p
For a given aircraft flying at some wing loading mg/S and some CL,
the flight Mach number depends only on ambient static pressure.
Air temperature and density do not matter.