How much strength do you need to pull a levitated object?

[Zanos]

Maybe this is more of a physics question. If I've got some big object I've cast suspension on, which is basically greater levitate, how much strength do you need to actually move it around? Trying to create a discount airship with flying undead.

[Palanan]

Originally Posted by Zanos
Maybe this is more of a physics question.

Well, if you don’t mind the pitter-patter of gently falling catgirls….


Originally Posted by Zanos
If I've got some big object I've cast suspension on, which is basically greater levitate, how much strength do you need to actually move it around?

Is this the Suspension spell from Shining South? According to the spell description, you can move it 45 feet/round by mental command. Are you trying to move it in excess of that distance per round?

[Zanos]

Well, if you don’t mind the pitter-patter of gently falling catgirls….
Oh, I intend to toss them overboard. :smallamused:


Is this the Suspension spell from Shining South? According to the spell description, you can move it 45 feetround by mental command. Are you trying to move it in excess of that distance per round?
Only up or down. I'm trying to move it horizontally as well.

[Palanan]

Originally Posted by Zanos
Only up or down. I'm trying to move it horizontally as well.

EDIT: This is trickier than first glance would suggest. Can you describe the structure you're trying to move?

[Zanos]

This may not be answerable with physics. The text for Levitate indicates the object can’t be moved horizontally, and Suspension doesn’t supercede that.
You can explicitly move horizontally under levitate if you have some kind of external force:

You cannot move the recipient horizontally, but the recipient could clamber along the face of a cliff, for example, or push against a ceiling to move laterally (generally at half its base land speed).

[Palanan]

Yeah, I misread that text the first time around. I mistakenly inferred a force which the spell doesn't exert. :smallredface:

So, it sounds like the spell essentially gives you neutral buoyancy and the ability to ascend or descend. If my understanding is correct, I'd say the force required to move your vessel laterally depends on its mass and air resistance, plus the force required to overcome any winds or other air currents.

.

[el minster]

you could probably move two or three times max drag

[Buufreak]

Maybe this is more of a physics question.


So, it sounds like the spell essentially gives you neutral buoyancy and the ability to ascend or descend. If my understanding is correct, I'd say the force required to move your vessel laterally depends on its mass and air resistance, plus the force required to overcome any winds or other air currents.


Physics (as long as mechanical and not electrical) is something I can do. First off, when dealing with sliding an object along the surface of another object, one needs to know the coefficient of friction. Lucky for us, our objects are floating thing, and literally nothing but air. Thus, the coefficient is effectively 0. We'd also assume that as long as the floating thing is maintaining a perfectly stead, unchanging altitude, it has a continuous opposite force to that of gravity, the force that normally pulls it downward.

As such: since there isn't enough force to pull the object down to the point that it creates friction with the surface of anything earthly or earth-bound-stationary, and it also isn't accelerating upwards (which would ultimately lead to the breach of atmosphere if you did), we assume a zero-sum, and literally any force that would be applied, so long as it is at least partially out of line to the vertical plane, would produce movement.

Now, should the spell simply contradict that, then physics isn't going to help you, and the answer quickly summarizes to "sorry, it's magic, you're SOL."

[Palanan]

Originally Posted by Buufreak
*snip*

To elaborate on this with regard to airborne objects—there are four primary forces acting on anything in the air: lift, thrust, drag and gravity. Lift opposes gravity, and drag opposes thrust.

Levitate and Suspension remove lift and gravity, so the two primary forces in this case will be thrust and drag. The question becomes what thrust is available to move the object, and how the object’s configuration will reduce or increase drag.

So, we can’t continue without more input from the OP on what this craft looks like, and what he’s using to propel it through the air.

[el minster]

Also there is the questioning of steering and I would say at normal speed you if you could lift it as a max load you could steer but at lower speeds you could do more

[Zanos]

So, we can’t continue without more input from the OP on what this craft looks like, and what he’s using to propel it through the air.
I haven't built anything yet, I was hoping to make something with fabricate and wall of iron or wall of stone since I can readily access materials that way without chopping down someone else's forest. Was concerned how much weight would be a factor, since what I can effectively suspend isn't necessarily what I can effectively propel. The end goal is to build something vaguely boat shaped that can comfortably support the whole party as a taxi, which is rather large and includes some bigger than medium minions. Helps if it's relatively fast as well.

As for propulsion I was thinking either using a haunt shift to make some animated propellers, or just pulling the thing around with a big zombie attached by chains. For the propellers, I could make up to a 20 hd haunting presence haunt the area, so it could animate up to 20 propellers. Plus, it's literally a ghost ship. For the other option, the best flying undead I have access to is a 37 str juju zombie abyssal drake, which is huge and has a fly speed of 150 feet and can take a full set of actions. I can use more than one if necessary.

The last idea I had for propulsion was an energy transformation field keyed to some spell that provides thrust and then enslaving some outsiders with SLAs to make super evil engine room. Not sure what spell to use here, though.

Thoughts on any of those options is appreciated.

[Palanan]

Originally Posted by Zanos
Was concerned how much weight would be a factor, since what I can effectively suspend isn't necessarily what I can effectively propel.

I think this is the key issue. With the masses involved, drag will be negligible compared with the inertia, and how much of a factor that is will probably come down to a DM call.

[DeTess]

So, to expand a bit more on the answer to your question, assuming the spells basically mean that lift and gravity always perfectly cancel out, any decent amount of force will allow you to move the thing, because air drag is a function of the object's speed. That having been said, I assume you want your airship to move at a decent clip.

The force of drag is equal to 0.5*the density of air*the speed of the object squared*the drag coefficient*the crossectional surface area. For the drag coefficient you can assume about 1 for an aggressively unaerodynamic shape (a cube will go a little over 1, but your craft will probably be more like a long cylinder, which is a little over 0.8). You should be able to fill in everything else based on how big you think your craft will be, how fast you want it to go and the density of air at your preferred altitude (but you can fill in 1 here as well, and you won'be too far off if you aren't planning on flying all that high up).

Then all you have left to do is translate the resulting force in newton to a strength score and you're good to go.

edit: though as pointed out, if you want to accelerate quickly as well, inertia will be a bigger problem than drag. Havinga decent top speed is nice, but if it takes a week to get there...

[Quertus]

As i see it,

Step 1 - determine craft (size, shape, composition)

Step 2 - determine craft mass

Step 3 - add passenger / cargo mass

Step 4 - determine thrust

Step 5 - from mass and thrust, determine maximum *acceleration*

Step 6 - hope that there's some online guide to allow you to determine wind drag at different speeds for your shape, to compare to your thrust, to give you maximum velocity.

(Do I qualify for chosen enemy: cat girl yet?)

[TheStranger]

Okay, the physics of this have been answered, but unless you want to calculate aerodynamic properties of your levitated object, what you really need is a reasonable-sounding houserule. So let’s reason by analogy.

What you have here is not unlike a dirigible suspended by magic instead of gas, or a boat floating on air instead of water. Both boats and dirigibles can be powered by propellers, with generally poor maneuverability but decent speed. So an array of propellers seems like a reasonable solution.

A zombie abyssal drake could also probably pull your craft without much trouble given it’s high strength (quite possibly enough to just carry the whole party, though I’m not going to do the math). As has been noted, you’re just dealing with inertia, drag is probably minimal. A good mental comparison would be pulling a boat through the water, which experience tells me isn’t too hard even for a fairly large boat. I would model inertia by dropping the maneuverability of the dragon by 1-3 classes depending on the size of the craft (one for huge, plus one for each additional size category). However, I would also note that tow ropes can be cut, so I would probably lean towards propellers.

[el minster]

I believe RotW has rules for flying creatures using kites and such to lift things

[Quertus]

"Give me a lever long enough and a fulcrum on which to place it, and I shall move the world."

How much strength do you need to pull a levitated object? Anyone can pull it. Even a small child (or skeletal cat girl) can do so. It's not that hard to understand - it's just rocket science. :smallwink: :smallbiggrin:

The only question is, "how fast?".

To determine that, the limiting factors are the force being applied (their strength, or the "thrust") and the force working against it; namely, air resistance. (The top speed of the pulling being, as well as the structural integrity of the craft, can also be limiting factors.)

The thrust is compared with the mass of the craft (including crew and cargo) to determine the acceleration, via the simple formula "force = mass x acceleration".

The top speed… has no good formula(e) that I'm aware of; however, "treat it as a dirigible" seems fair, and Google says that they have a top speed of 115 MPH.

So, assuming that your construction, materials, and passengers can handle such speed (and, if towed, the towing beast(s) go that fast), it sounds like you have a reasonable answer.

And, assuming that your "20 propellers" provides thrust per unit mass roughly analogous to that of a dirigible, you can probably crib off its entry (in A&E, iirc) for maneuverability, as well.

[Twurps]

edit: though as pointed out, if you want to accelerate quickly as well, inertia will be a bigger problem than drag. Havinga decent top speed is nice, but if it takes a week to get there...

And don't forget: whatever intertia does for your acceleration, also holds true for your deceleration. (and any other redirecting your speed/steering) Which I think we can all agree is actually even more important (at least in real life). Nothing more annoying than seeing a mountain appear out the fog a good couple of minutes away, and still not being able to avoid a collision. Or finally spotting the baddies from the air, only to find them long gone by the time you've come full stop.

[Segev]

Since there's no friction, the normal elements D&D uses to model carrying/pushing/dragging won't work very well. With no friction, you could push a billion-ton weight. Only its momentum would resist you.

The question, therefore, isn't if you can push it, but how fast with how much force.

Force = (mass)x(acceleration). If you want to be able to start and stop it quickly, you need more force.

You can actually translate strength scores to force exertion by looking at how many pounds you can lift.

However, because this will lead to a lot of math and try to model things that D&D doesn't model (specifically: acceleration and deceleration), I would just have levitation multiply the amount you can drag by somewhere between 2 and 10. I'd play around with the numbers until I got something that "felt" right for the effect I wanted, because in practice what you're modeling is the ability to overcome the object's momentum in a short period of time.

In fact, another way to do it would be to model acceleration thusly: strength => amount of speed that can be added with a standard action dedicated to pushing/pulling. You can do this somewhat precisely, even. Acceleration is just the change in velocity, so translate Strength into Force based on how much it can lift (pounds of weight ARE a measure of force), and then use Acceleration = (Force)/(Mass). You'll need to do some unit conversions to work it out properly.

But once you know how much acceleration a given strength can give a given mass, you can get that acceleration into units of (feet/round)/round where a round is 6s, and just use that to determine how much somebody can speed up or slow down the item.

[DeTess]

How much strength do you need to pull a levitated object? Anyone can pull it. Even a small child (or skeletal cat girl) can do so. It's not that hard to understand - it's just rocket science. :smallwink: :smallbiggrin:


Nah, that's merely rocket engineering. It isn't proper rocket science until the assumption that the earth is flat no longer holds* and the therm 'eccentricity' isn't used to refer to someone's weird uncle.

Source: I literally studied rocket science :P

*This is actually a very common assumption when doing math on the flight dynamics of aircraft XD

[Zanos]

But once you know how much acceleration a given strength can give a given mass, you can get that acceleration into units of (feet/round)/round where a round is 6s, and just use that to determine how much somebody can speed up or slow down the item.
So if a creature can (roughly) accelerate it's maximum drag weight to it's top speed in 1 round, could I just divide the levitating objects mass by maximum drag weight to determine how many rounds it takes to reach the creatures top speed? So ex: it takes a creature 10 rounds to accelerate an object that weighs 10 times it's maximum drag weight to top speed. And a similar amount of time to decelerate if I have the creatures pull in the opposite direction from it's movement(which could get interesting).

I guess this doesn't factor in air resistance, but I don't think my DM wants to check my math on the airships coefficient of friction. Although I did quite literally ask for this. :smallredface:

[Segev]

So if a creature can (roughly) accelerate it's maximum drag weight to it's top speed in 1 round, could I just divide the levitating objects mass by maximum drag weight to determine how many rounds it takes to reach the creatures top speed? So ex: it takes a creature 10 rounds to accelerate an object that weighs 10 times it's maximum drag weight to top speed.

I guess this doesn't factor in air resistance, but I don't think my DM wants to check my math on the airships coefficient of friction. Although I did quite literally ask for this. :smallredface:

Sounds workable to me. Remember that it also will need to slow the object back down, and will need to spend this time changing its direction (both slowing it in the old direction and speeding it up in the new one), so it will be like dragging a box on ball bearings over an ice-skating rink.

But the base numbers should work and naturally lend you the complexity, rather than needing complex rules to get simple results.

[KillianHawkeye]

Coming at this from a different perspective:

A character can generally push or drag along the ground as much as five times his or her maximum load. Favorable conditions can double these numbers, and bad circumstances can reduce them to one-half or less.

I would say that being levitated definitely counts as a favorable condition as it completely eliminates surface friction.

So, for example, a character with 10 Strength (and a maximum heavy load of 100 pounds) should be able to tie a rope to and drag a levitating object weighing up to 1000 pounds by himself. Minus the weight of the rope, I suppose.

[Zanos]

Coming at this from a different perspective:


I would say that being levitated definitely counts as a favorable condition as it completely eliminates surface friction.

So, for example, a character with 10 Strength (and a maximum heavy load of 100 pounds) should be able to tie a rope to and drag a levitating object weighing up to 1000 pounds by himself. Minus the weight of the rope, I suppose.
I always assumed favorable conditions there meant like, wheels. The skate psionic power I suppose is similar in that it mostly eliminates surface friction and it lets you treat an object as 1/10th the weight for the purposes of dragging it. That might be applicable but it's a different effect.

[Asmotherion]

My bet is, you'll be able to navigate it with the wind, and easyer to move than in the water.

I'd argue that a simple mage hand can slowly propel it too, but it's probably a stretch, given it's mass and air resistance. Air elementals or a telekinesis spell (probably from a ring of telekinesis) would do the trick though.

[Gavinfoxx]

https://docs.google.com/document/d/14zilT4WGOyHM0AfpG4-GmD2FkgDg1HZ9HC1cTleQHds/edit

[Clementx]

Coming at this from a different perspective:


I would say that being levitated definitely counts as a favorable condition as it completely eliminates surface friction.

So, for example, a character with 10 Strength (and a maximum heavy load of 100 pounds) should be able to tie a rope to and drag a levitating object weighing up to 1000 pounds by himself. Minus the weight of the rope, I suppose.

This. Everyone else is putting way too much effort into this. In addition, an airship is large enough that it becomes the frame of reference for tactical movement, so all issues of maneuverability and acceleration are moot for combat. OP, the more complexity you put into this reduces the fun anyone is going to have with a flying airship by an equal amount.

[Quertus]

OP, the more complexity you put into this reduces the fun anyone is going to have with a flying airship by an equal amount.

Curiously, the 3e build minigame is highly complex, and a source of fun for some. So I think you're mistaken about fun and complexity being inherently inversely proportional.

I think *where* you place complexity matters greatly for its reception. "Airship tactical movement" is certainly one place where *some* groups might welcome the added meaty crunch.

[Zanos]

This. Everyone else is putting way too much effort into this. In addition, an airship is large enough that it becomes the frame of reference for tactical movement, so all issues of maneuverability and acceleration are moot for combat. OP, the more complexity you put into this reduces the fun anyone is going to have with a flying airship by an equal amount.
Well, it does kind of matter if I can't stop my airship quickly. I won't be the only game in the skies. That said I think most of the other airships are made out of wood rather than hardened iron and stone, so the consequences will be more social than material. :smallcool:

In any case as Quertus mentioned, I wouldn't engage in these thought exercises if I didn't enjoy them. And I have no intention of tormenting my DM if he doesn't want to entertain me in this regard.

[Necroticplague]

To elaborate on this with regard to airborne objects—there are four primary forces acting on anything in the air: lift, thrust, drag and gravity. Lift opposes gravity, and drag opposes thrust.

Levitate and Suspension remove lift and gravity, so the two primary forces in this case will be thrust and drag. The question becomes what thrust is available to move the object, and how the object’s configuration will reduce or increase drag.

So, we can’t continue without more input from the OP on what this craft looks like, and what he’s using to propel it through the air.

Wouldn't an object's sheer inertia/mass also be a factor? So more massive objects, even if gravity isn't at work, would be harder to move around than smaller ones, simply because their larger mass needs more force to move (F=MA).

[Palanan]

Originally Posted by Necroticplague
Wouldn't an object's sheer inertia/mass also be a factor?


Originally Posted by Palanan
With the masses involved, drag will be negligible compared with the inertia, and how much of a factor that is will probably come down to a DM call.

I addressed that in my next post down from the one you quoted.

[Fizban]

Coming at this from a different perspective:


I would say that being levitated definitely counts as a favorable condition as it completely eliminates surface friction.


I always assumed favorable conditions there meant like, wheels. The skate psionic power I suppose is similar in that it mostly eliminates surface friction and it lets you treat an object as 1/10th the weight for the purposes of dragging it. That might be applicable but it's a different effect.
Basically nothing actually uses the favorable x2 line, or possibly even the x5 drag line (which I would presume is supposed to be a 5' speed drag given it's place after the 5' x2 lift and stagger- I'm pretty sure a str 10 person shouldn't pull 1,000lbs at 20' even with wheels). Arms and Equipment Guide tells us what wheels are worth, and it's effectively x4- or rather, that wheeled vehicles use 1/4 the combined weight to find the speed of the drey creature, and sleds use 1/3. Even so, 1/10 is a perfectly fine arbitrary modifier for "frictionless."

Except it doesn't get into the real problem of pulling a levitating barge, which is stopping it. So I'd actually say to go with your previous plan of getting a rough calculation of the maximum speed your drey creatures can accelerate it to against air resistance (and how long it takes), and use those numbers. The main problem with the "I'll just permanently levitate something and push/pull it around" airship is the massive lack of control, so the drawn vehicle rules that are written for surface+friction and all the control that gives, don't make any sense here. And since the point of jury-rigging stuff like that is supposed to be dealing with the non-standard problems that arise from non-standard situations, just letting it snap to an easier set of rules misses that point. The floating iceberg style of "airship" should have all sorts of problems getting moving, not crashing into things, and lining up for docking procedures.

[TheCount]

So, it's probably outside the thread, but can I ask why not use something already printed?
A&EG has some, eberon has 3, frostburn has the floating icebergs and faerun has the bloody expensive halruan ones that use suspension as you plan to do.

As for moving it..... Permanent animated objects?
You could even animate the engine itself, lock it in a lead room and it would be safe.... Unless dead magic zone, probably.

Here is the best guide I could find about this topic.
https://docs.google.com/document/d/14zilT4WGOyHM0AfpG4-GmD2FkgDg1HZ9HC1cTleQHds/mobilebasic

[Efrate]

You can also do something with shrink item if you make it massive first then shrink to preferred size. Use something super lightweight (paper) then shrink item it so its weight drops substantially. Then just wrap the whole thing in walls of force so its indestructible. Maybe riverine? Not sure on riverine weight but force is weightless. Or go eberron something something bound elementals and hand wave it cause magic.

[Zanos]

Except it doesn't get into the real problem of pulling a levitating barge, which is stopping it. So I'd actually say to go with your previous plan of getting a rough calculation of the maximum speed your drey creatures can accelerate it to against air resistance (and how long it takes), and use those numbers. The main problem with the "I'll just permanently levitate something and push/pull it around" airship is the massive lack of control, so the drawn vehicle rules that are written for surface+friction and all the control that gives, don't make any sense here. And since the point of jury-rigging stuff like that is supposed to be dealing with the non-standard problems that arise from non-standard situations, just letting it snap to an easier set of rules misses that point. The floating iceberg style of "airship" should have all sorts of problems getting moving, not crashing into things, and lining up for docking procedures.
Yeah, I was thinking of using multiple creatures and commanding them to fly at varying speeds to control turning, breaking, etc and justifying it with an IC kn:engineering check.

That said I think the less painful approach here is to just stick a haunt shift into the damn thing to make it an animated object then cast some long duration fly spell on it.


You can also do something with shrink item if you make it massive first then shrink to preferred size. Use something super lightweight (paper) then shrink item it so its weight drops substantially. Then just wrap the whole thing in walls of force so its indestructible. Maybe riverine? Not sure on riverine weight but force is weightless. Or go eberron something something bound elementals and hand wave it cause magic.
Shrink item has a very strict size limit.

[el minster]

Races of the wild has these rules for nets that flying creatures can carry things with if you could get a really strong flying servant you could just use it to tow people also they have kites if you need to tow more weight

[Bullet06320]

http://www.spelljammer.org/

just build a spelljammer helm and your all good

[Asmotherion]

So, it's probably outside the thread, but can I ask why not use something already printed?
A&EG has some, eberon has 3, frostburn has the floating icebergs and faerun has the bloody expensive halruan ones that use suspension as you plan to do.

As for moving it..... Permanent animated objects?
You could even animate the engine itself, lock it in a lead room and it would be safe.... Unless dead magic zone, probably.

Here is the best guide I could find about this topic.
https://docs.google.com/document/d/14zilT4WGOyHM0AfpG4-GmD2FkgDg1HZ9HC1cTleQHds/mobilebasic

That, plus apply a Zepelin Principle with a Planar Bound Fire and Air Elemental as a failsafe against Dead Magic Zones.