[Flinty]

Question about the 40k physics of how ''hover disc'' units might work.

By ''hover discs'', I mean something akin to the item supporting Kirkegard's Scribe, as posted here: http://www.the-conclave.co.uk/forum/index.php?topic=1220.0; essentially a self-supporting unit that is not warp powered a la a Disc of Tzeentch, but relies on some form of mechanical system.

I'm assuming they are more technically advanced than using some form of downward air propulsion (in the more 'expensive' examples anyway) and instead work on a similar basis to suspensor units - albeit on a larger scale.

If that is correct, and the disc produces a force that negates or possibly repulses gravity (err...I'm sure you see what I'm getting at), does the disc exert a downward force proportional to its weight? Or does the unit merely ride on an ''anti-grav cushion''?

I was wondering (from a rules perspective, natch) in that they might be fine moving about on a solid surface, but imagine if they ventured out onto a rickety gantry, or up a set of shonky stairs? If it rides on a cushion, is it possible/impossible to insert or force anything between the disc and the ground surface? If you could, would they fly off at a comical/dangerous trajectory?

Edit: clarity ?!

[DapperAnarchist]

Another good question is do they work against mass (making the mass "less" in some way) or gravity (countering gravity in some way) - those who have read Consider Pheblas will know why this is important, though Orbitals don't seem to appear very often in 40K.

It is an "anti-grav" unit, though whether that means its pushing down, reducing mass/gravity, or a forcefield generator creating a box that the object sits on is never really explained, though Land Speeders and other anti-grav vessels seem to be able to go over objects without any effect beyond backwash, unlike being flown over by a low-flying helicopter...

[MarcoSkoll]

My interpretation is a suspensor or anti-grav drive somehow alters the gravitational constant, allowing it to increase, lessen, or even reverse the effects of gravity on the drive.
Reverse is the important part, because then it creates an upthrust that can be used to lift other objects.

So, it "pushes down" in the same way as gravity pulls down on me - relative to the local "gravitational well". It doesn't push down on the floor beneath it any more than floor below me is pulling down on me. Yes, the floor and I do have a gravitational interaction, but it is trivial.
Thus, in this case, a rickety floor essentially has no force on it, because the forces are between the planet and the hover plate.

As for how far it decides to be from the local COG, in the case of an Astartes Land Speeder, it will be programmed to respond to the pilot's instructions. With a hoverplate or something, I imagine it has sensors that tell it to maintain an altitude of around three feet off the surface below, and instructions to keep itself level, and avoid excess accelerations up or down.

So, if you inserted something between the plate and the floor, it might raise up slightly as it detected an increase in the average height of the surface below, but it wouldn't fly off at a wild angle. And if you shoved them off a balcony, it would probably detect the fall in the level of the ground below, slowly (and safely) sinking until it reached the appropriate level above the floor.

That said, it might come with override controls to disable the "floor clearance" and just let the user manually control altitude, allowing them to hover straight up, across huge chasms or whatever.

[Flinty]

Ah, there we go, the clarity I was looking for. All seems to make sense, but I think I might introduce an earlier/substandard model which might not be fitted with some of those fail safes - the potential for the unexpected with a model constructed on a Friday afternoon at the local Forgeworld is just too good to pass up...