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

Adrian spake thusly upon matters weighty: 

Ummm.... fixed max speed on fans.... try this.... probably fixed max 
engine rotational torque to the fan shafts.... pushing denser air 
would mean preforce a slower max rotation speed, ergo possibly the 
same max lift. And even if you had the power to ram the fans around 
faster, you'd cause greater heating in the fans because they'd be 
moving much heavier air (more of it). So perhaps you don't get these 
benefits. 

Just an alternate viewpoint.....

> 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
> 
> 
> 
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Thomas Barclay		     
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