Re: [GZG][FH] Planet types - atmosphere effects on VTOL's

Ok - this is getting a bit esoteric, but there's a point at the end...

>Hi all!  I'm new to the list, and new to GZG, just picked up FT MT and 
>DS2, great systems.  Anyway, regarding the comments on VTOLs, speed and

>carrying capacity.
>

Hello! Welcome to the list.  Glad to have you with us.

>The fans on a VTOL have the same power output in the dense atmo as a 
>standard, so, even though more mass will get moved through it, same
mass 
>will be ejected at a lower velocity (conservation of momentum), so, no 
>increase in speed. Also, drag is a non linear function, generally it is

>a function of the square of the velocity of the vehicle, so drag is a 
>major deal, in fact, the VTOL may even LOSE some speed.
>
>Carrying capacity will not go up, again since the greater mass is 
>ejected at a lower velocity.  The denser atmosphere gives no
advantages, 
>and creates disadvantages at low altitude.
>

I suggested that carrying capacity could go up, to some degree.  Here's
my
reasoning - again based on pilot experience, not by any stretch on
expertise in physics...  Aerodynamic lifting surfaces (say, wings on a
plane, propellers, rotors on a helicopter) all work less efficiently at
lower atmospheric pressures.  Both fixed wing aircraft and helicopters
can
take off at higher elevations (up to a point), but with reduced maximum
load - for example the helicopters the Indian Army uses in the Himalayan
mountains (operating at 15 - 20,000 feet or higher) can carry troops,
but
at less than half their rated max capacity (or lower).	To get a fixed
wing
aircraft off a runway at a given weight will require a higher takeoff
speed
and much longer takeoff run at higher airport elevations.  The reverse
is
true - you can take off in shorter distances at slower speeds at lower
elevations, with higher atmospheric pressure.  This stuff I know by
experience.

Here's where I start hypothesizing:  VTOL aircraft using lift fans would
generate lift in two ways - the aerodynamic lift of the fan blades (same
as
a helicopter rotor) and the displacement of air by the fans.  A lift fan
in
a denser atmosphere would have a higher total potential lift.  The fan
blades will have a fixed maximum speed (you don't want the blades to
exceed
the speed of sound, or you get nasty shockwaves happening, etc etc -
that's
why helicopters have a slow maximum speed) - but at that maximum speed
they
will have a higher potential aerodynamic lift and will displace a
greater
volume of air molecules, therefore generating more total lift compared
to a
lower atmospheric pressure - up to a point...  The point being that as
you
pointed out, drag increases non-linearly - so there will be a point at
which the greater potential lift is surpassed by drag, and your net
result
is either that the lift and drag balance out, or the drag exceeds lift
and
you lose out.  The other limiting factor is the power of the engine
driving
the lift fan.  While there is a fixed maximum potential speed for the
fan
blades, it will require considerably more power to get them to that
speed
(because of the extra mass of air you are moving and extra drag) - VTOLs
now are designed with engines that produce more power than the rotors
can
transfer to the air, to provide emergency reserves, etc.  In the future,
when operating in denser atmospheres is a possible requirement, maybe
they
design VTOL engines with even more power reserves?  One further wrinkle
to
this that I would kind of like to ignore 'cause I don't really know what
the effect would be is that the maximum potential speed of the lift fans
will actually decrease in higher atmospheric pressures - because the
speed
of sound will decrease, and therefore the point at which you hit the
Mach
shockwaves comes sooner...  So what does that do to the potential output
of
the lift fan?

GAME EFFECTS:

In the end, what I suggest is this:  for game purposes, up to some
arbitrary point with higher atmospheric pressures you will see a limited
increase in the carrying capacity of VTOL's, but a lowering of their
maximum cross-country travel speed.  Beyond this arbitrary point, the
capacity of VTOL's drops off sharply, 'cause you've reached the point
where
drag is surpassing the lift benefits of denser air and/or the max. power
output of the VTOL engines to compensate...  

I say arbitrary, 'cause I don't know what the points should be - so I'd
make it up.  Say, you get a performance increase of 25% carrying
capacity
at 1.5x earth normal pressure.	But the cutoff for benefits is 2x earth
normal - at which point the VTOL's decrease in efficiency.  At some
point,
you'll need extremely specially designed vehicles (like a modern glider
-
with high lift, low drag surfaces) to operate in the high pressure
atmospheres.  Maybe powered balloons (ok, that sounds kind of silly), or
vehicles that do not rely on aerodynamic lift at all, like grav
vehicles.

So, there you go.

Hope I didn't violate too many laws of physics.

Adrian