"C.Fleming" wrote:

"root" wrote in message
...

And also that the same glider on Mars has to fly 10 times faster in
order to obtain the same lift, balancing the same weight. But due to
the lower gravity on Mars, its weight would be lower, so the normal
(best L/D) speed on Mars would be less than 10 times this speed on the
earth. This speed can further be reduced by reducing the wing loading,
which provides some benefits on the earth that are no more valuable on
Mars, like speed, which is rather to high, and penetration, which makes
little sense. So it is not unbelievable that soaring may happen on Mars
at speeds between mach 0.5 and 0.7.

Most of that sounds ok -- except your guess at mach speeds. Where do you
come up with 0.5 to 0.7 mach? Remember that as air density decreases, so
does the indicated airspeed at which we reach 1.0 mach. From memory (of
many hours staring at airspeed/mach indicators), on Earth: 340 knots ias =
0.8 mach at approx. 28,000 ft., while at 39,000 ft., the ias drops to 260
knots while maintaining 0.8 mach. On Mars, with an air density of less than
1% of Earth's, it appears clear to me that because mach 1.0 will be reached
at a very low indicated airspeed, a conventional glider (the original post
referenced a PW-5) wouldn't have a chance.


Sorry, your calculations with weird units don't have an obvious meaning to
my metric educated mind. I never thought of indicated airspeed, only tried to
evaluate the ratio of true airspeeds on Mars and the earth to ensure similar
(e.g. best L/D) flight conditions. My idea was that the speed of sound, while
affected by the change in conditions, should not be affected by a very important
factor. The factors involved are absolute temperature, molecular weight of the
gas(es) and gamma (Cp/Cv). Gamma depends only on the atomicity. While I don't
know exactly what are the components of the martian atmosphere, I guess it is
not methane or CO2, but rather diatomic gases with molecular weigth near O2 and
N2 as found on the earth. As all this is under a square root, changes must be
huge to become significative, same thing for temperature. Halving the temperature
on earth only decreases the speed of sound by a factor 0.7, and this is pretty
cold.

So 10 times the gliding speed on the earth is about the speed of sound on the earth,
if the reduction of gravity and wing loading gives a factor that overrides
the change in the speed of sound, subsonic soaring may be possible on Mars.