From: Adrian Johnson <ajohnson@i...>
Date: Tue, 01 Dec 1998 11:52:12 -0500
Subject: 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 > 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 > a function of the square of the velocity of the vehicle, so drag is a 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.