On 14.12.18 01:50, Charlie Gibbs wrote:> On 2018-12-13, NY
 wrote:
 >
 >> "Dennis Lee Bieber"  wrote in message
 >> news:g1751edssbc3h2lpgoh3qag0cl9gmjm97h@4ax.com...
 >>
 >> Is there an electronic means of measuring airspeed, which doesn't
involve
 >> measuring air pressure as the plane flies forwards and subtracting the
 >> current static air pressure, corrected for altitude?

There are some, e.g. there are wind sensors using travel
time for ultrasound signals (see Vaisala WXT 532). The real
airspeed is of little interest to the pilot, he is more
interested in the dynamic air pressure, as the airfoils
react to it. The current pitot/static system is head-on
to that purpose.

 > The formula models what's called a "standard atmosphere": at sea level
 > this assumes a pressure of 29.92 inches and a temperature of 15 degrees
 > Celsius.  Pressure decreases exponentially, but below 10,000 feet it's
 > about one inch of mercury per 1000 feet.  Temperature decreases by 2
 > degrees Celsius per 1000 feet up to the tropopause (typically at about
 > 35,000 feet, where the pressure is a quarter what it is at sea level
 > and the temperature is about -55C).  These are theoretical figures,
 > though - actual values will vary depending on barometric pressure,
 > temperature, and moisture content of the local air mass.

The relation between pressure and altitude is not a plain
exponential, as the air temperature decreases 6.5 degrees
(C or K) per kilometer rise, and this changes the density
from the straight exponential model.

The barometric formula relates the relative absolute temperature
(in Kelvin) to the relative pressure at altitude. The altitude
enters the formula via the temperature relation.

    p / p0 = (T / T0) to power 5.255883

p = pressure at altitude
p0 = standard pressure, 1013.2 millibar
T =  temperature at altitude, T0 - 6.5 K / km
T0 = standard temperature at surface, 288 K (15 C).

 >> Are such errors in altitude readings smaller than you'd get with
 >> consumer or professional GPS?
 >
 > Yes.

It depends on the weather. GPS measures the geometric altitude,
but the aircraft instruments are using the pressure formula,
and it will show less than the real airspeed as soon as the
density of the air is less than that in standard conditions
(1.29 kg/m3). The predictions from the barometric formula will
be off as soon as the vertical temperature profile does not
follow the -6.5 degrees / km ratio. An example is a quite
common phenomenon with weather changes called an inversion,
where the temperature temporarily rises with altitude.

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-TV

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