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19 - TAS and SAS
Definitions
True Airspeed (TAS)
and
Skydiver’s Airspeed (SAS)
are two methods of calculating the
airspeed of a moving/flying/falling object.
TAS
is a term used in aviation: It is the speed of an object relative to the surrounding air,
regardless of the altitude.
SAS
is a new concept developed by LARSEN & BRUSGAARD:
SAS
is the speed of a skydiver
calculated from measurements of air pressure and temperature and converted to a fixed air
pressure (875.3 mb) and a fixed temperature (+7.080C) which corresponds to 4,000 feet ASL.
TAS
A skydiver’s True Airspeed (TAS) relative to the ground changes as a function of the altitude (air
pressure) and temperature which makes it difficult to compare fall-rates.
Example: A skydiver (in a fixed freefall position) who has a terminal fallrate of 62 meters/sec at
10,000 feet will have a terminal fallrate of 50 meters/sec at 3,000 feet.
It will be seen that the difference in altitude (air pressure) makes it difficult to compare the fall-
rates when measured using TAS.
SAS
The SAS formula calculates airspeed (using the same metrics used with TAS) as though the
complete skydive had been performed at a fixed air pressure and a fixed temperature which
corresponds to 4,000 feet ASL.. 4,000 feet is chosen as the reference altitude by LARSEN &
BRUSGAARD since this is the average altitude at which the working time of a skydive is normally
ended.
Conclusion
Using
SAS
, skydivers in any body position can express their vertical speed by a number (
SAS
).
This number remains virtually constant regardless of altitude with little or no variance due to
temperature differences and can be compared with the airspeeds of other skydivers. This means
that regardless of the elevation of the DZ you are jumping at,
SKYDIVER’S AIRSPEED (SAS)
will be the same for the same body position.
SAS is very useful when doing big formation skydiving. If using TAS, it will seem like the base is
slowing down the fall rate during the entire skydive.
To set TAS / SAS see paragraph 10.10
Summary of Contents for Military ALTITRACK
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