On 2018-12-13, NY  wrote:

> "Dennis Lee Bieber"  wrote in message
> news:g1751edssbc3h2lpgoh3qag0cl9gmjm97h@4ax.com...
>
>> For ground speed, not air speed... Having a 200kt ground speed with a
>> 100kt tail wind means an air speed of only 100kt.
>
> 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?

I don't know of any, but they probably exist.  Still, for sheer simplicity
(i.e. reliabilitly) it's hard to beat a pitot/static system (heated to
resist icing over) driving a pressure sensor.

> What is the typical +/- for air-pressure airspeed and altitude readings?

Airspeed indicators are good to within a few knots, and altimeters to
100 feet or less.

> How much does ground-level air pressure vary from time to time or place to
> place,

Sea-level air pressure typically varies between 29.5 and 30.5 inches of
mercury (with occasional excursions beyond that).  Part of preparation
for flight is to set your altimeter for the current air pressure -
otherwise it could be out by 500 feet or more.

> and does it drop as altitude increases by a consistent formula?

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.

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

Yes.

> I presume both airspeed and altitude gauges include a fairly long
> time-constant in the needle damping to avoid gusts of wind causing
> apparent fluctuations in speed and altitude.

Not that long.  In mechanical instruments the inertia of the parts
provides some damping, but rapid fluctuations will show up.

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