From: Adrian Johnson <ajohnson@i...>
Date: Mon, 06 Dec 1999 02:05:04 -0500
Subject: Re: GEVs, ACVs, ground effect, etc etc etc
OK, a bunch of comments about disparate parts of this thread: 1. Down hill thrust vector: > You are making the mistake of thinking of this as a fan blowing air OK, hang on a second. We're actually mixing parts of different threads here. I originally posted the bit about the down-hill component of the thrust vector. At that point in the thread, what we were actually discussing was using the fans on the bottom of the vehicle on "supercharge" to lift the vehicle over hills, obstacles, etc by aerodynamic lift. The conversation turned to the size of rotor blades necessary to do this (they would be BIG by necessity, and unsuitable for the bottom of a vehicle). Then I tossed in the bit about the thrust vector. This didn't really have anything to do with air escaping from the skirt providing some kind of thrust, though that would have SOME effect (I don't think too much, as it would be escaping around the whole skirt at more-or-less the same amount). What I was referring to is the idea that if you have a fan/rotor/propeller fixed in the frame of the vehicle, it produces thrust downwards. When the plenum is closed (ie vehicle is sitting on flat-ish surface) it is acting as a pump, forcing air into a mostly-closed volume, and the "thrust" it provides is by no means holding up the vehicle. But it IS moving a mass of air. When it goes to "supercharge" mode to hop the vehicle over something, the fan is using aerodynamically created "thrust" only (ie the air cushion is no longer relevent). Either way, there is a thrust vector acting away from the force of gravity (ie downwards). If the fan is tilted (say, because the vehicle is travelling UP a hill, and the vehicle tilts backwards), the thrust vector is now not acting directly against the force of gravity. It is "split" as it were, and though part is acting against gravity, part is by necessity acting in the direction perpendicular to the surface of the hill - and thereby part of THAT part is acting to push the vehicle in a direction opposite to that in which it is travelling... I could DRAW this in a vector diagram simply, but my ascii-graphics skill isn't up to it... If the fan/rotor/prop was in a gimbal mounted housing so it always pointed down relative to gravity, this wouldn't be a problem, but as to the ORIGINAL idea, which was to overpower the fans to counteract the losses 'cause of the vehicle tipping, etc my comment still stands - your increased fan power would actually be at least partially working against the vehicle getting up the hill, and you would THEN have to have an even MORE powerful thrust fan/device pushing you up the hill - and all of this seemed really inefficient. As to the point that "if we can posit Grav vehicles then why get hung op over hovercraft going up hills" - well, yes. I buy into the idea that the future-hovercraft has a limited duration VTOL capability using a different type of thrust unit (ducted fan, jets, rockets, etc) but I don't like the idea of grav units at all. If you can use grav units, why not just make a grav vehicle. I know, one could make lots of arguments against that (maybe grav in small units is cheaper, maybe it's easier to make in small units, maybe etc etc). But I see hovercraft as being a "lower tech" alternative in the Tuffleyverse, anyway. It's easy to make, and doesn't require too high an industry base tech level. Hovercraft are things that could be manufactured and supported on distant colony worlds with relatively lower tech. Grav is so revolutionary, if it were common it would change everything - and in my incarnation of the Tuffleyverse is subsequently very rare. We're debating hovercraft 'cause we see them as more plausible on a common basis in the Tuffleyverse. 2. GEV vs. ACV This is kind of a fiddly point here, but there is a BIG difference between "Ground Effect Vehicles" and "Air Cushion Vehicles". Ground effect is an aerodynamic effect encountered when an aerofoil is creating lift (ie flying) close to a surface (ground, water, etc) - usually within a distance roughly equivalent to the length of the aerofoil (well, ish). Roughly speaking, air is compressed between the aerofoil and the surface creating a higher-than-normal pressure zone, and the aerofoil is "artificially" more effective. It creates more lift for a given speed, or to put it another way, you can use a much smaller aerofoil to create a given amount of lift than you would need to fly freely. This makes it really interesting to land gliders, with their low weight but really long wings, 'cause when you are just about to touch down, the wings suddenly become a lot more effective and you have a tendancy to float along for a much greater distance than you would think the glider was able to. Gotta be careful when planning the landing to account for this. Less of a problem with powered aircraft, 'cause their wings are a lot shorter relatively... Anyway, the Russian GEVs that someone else mentioned recently are examples of vehicles designed to exploit this. They are huge great big machines (the biggest version was 707 sized, and could carry several hundred tons of cargo) that can travel at a few hundred miles per hour, within about 10 feet of the surface. Great for ferrying HEAVY cargo over long distances quickly - provided you don't hit rough surfaces. There is a US company building a small civilian vehicle (appx 4 seats) that does the same thing - I dont' know if they ever got it certified. A hover craft is an Air Cushion Vehicle. It does not use the aerodynamic effect called "ground effect" but, as we all know, rides along on a compressed cushion of air contained in a chamber - like an air hockey puck. Similar SORT of idea. Completely different practical effect and technological requirements to use it, though... The Russian GEVs are cool - but rather limited. And you can't go SLOWLY in them - which limits their effectiveness for over-land military use. Air cushion vehicles, on the other hand, are combat proven. I'm sure there could be some kind of VTOL/ACV combination that, using simple (non-grav) tech could lick the "how do I get up a hill" problem, but I only see them operating in the light-armour, transport, scout, type roles - not as front-line MBT's. That "sinking into the lake/swamp/river/soft earth" problem is a big one:) Now, the "put auxilliary tracks on them for climbing hills" idea makes a lot of sense, but they'd be UGLY (if the tracks were on the outside - offends my aesthetic sensibilities...). On the other hand, the tracks could be retractable, and reside in housings up underneath the skirts. When you get to a hill, down come the tracks and they pull you up. Wouldn't be necessarily fast, but would work - and you wouldn't suffer the complete loss of the air cushion - which would happen if you hopped it over the hill. AND would require a LOT less lifting power than the VTOL option. Anyway - there you go... We don't all have to race out to put tracks on the sides of our GZG Hover Jeeps, 'cause they're tucked up underneath...:)