16953Re: [ksurf] Re: more helium in sleds...
- Aug 1, 2000Great Ceaser's ghost! Somebody even more pedantic than I am!! I wonder how
long it will be before the flames start for us to take this nitpicking
private. Well, not a peep so far, so I guess I'll have at it some more....
----- Original Message -----
From: "Dave Culp" <dave@...>
Sent: Tuesday, August 01, 2000 6:34 PM
Subject: [ksurf] Re: more helium in sleds...
> Dave Raue:
> >I'm not sure which part isn't true. The point isn't that He can't be
> >used, contained, compressed, etc. Obviously it can. It's that doing
> >so in a practical, cost effective fashion isn't likely. Metalized
> >mylar has pretty poor puncture resistance. And the context of the
> >thread was as an inflation medium for Naish/Wipika designs. These
> >have relatively small inflatable volumes that need to be at
> >relatively high pressure since they provide structural support. It's
> >pretty obvious that somebody could construct a sealed, inflatable
> >foil. Making one that would survive sand fences, thorns, etc would
> >be more challenging and expensive.
> Guess I'm going to have to ask you for some definitions, Dave. Yes,
> the thread is "more helium in sleds." OK, I guess I'll grant you that
> Naish's are "sleds," though I can find no reference work that so
> defines them (including both Wipika's and Naish's web sites) And yes,
> we established, long ago, that trying to get any value from putting
> helium into the tubes of a Naish or Wipika was a *total* waste of
> time. You're better off putting helium in your bike's tires, then
> trying to fly the bike. We've *covered* all this, in this
> interminable thread. Forgive me, I was under the impression we'd
> moved on, and were discussing putting helium into any *other* types
> of kites, where it might be actually, physically possible for it to
> have some effect.
Well, we had, but I got on a tear and resurrected it. Nice analogy with the
bike tire, though. Whoever raised it most recently didn't seem to have read
the archives. In fact this has been covered several years ago in rec.kites
if I remember right.
> Having said that, we (you!) were discussing 1) permeability of
> fabric, and the general expense/impossibility of same, and 2)
> re-compressing helium via mechanical pumps. (Is anyone else on the
> same page? Is anyone else even listening??)
It doesn't matter if WE'RE having fun with this....
> If I were asked to build a helium-filled kite, I wouldn't use a
> single bit of either fabric or film, but a combination, just as
> inflatable boats use (plastic bladder, inside fabric cells). In fact,
> I *have* done this, with good results. No, I'm not going to teach you
> how to do it, nor reveal my suppliers to you. Sorry.
Going the composite route seems like a pretty obvious step for an engineer.
Don't worry, I'm not going to try it.
> Now, you said:
> >... He is going to diffuse out of your bladder like mad. On an
> >atomic scale the baldder is quite porous, like microscopic
> >chickenwire. Chicken wire keeps the chickens in, but it doesn't do
> >much to keep out the mosquitos. Plasma metalization of the bladder
> >perhaps? Otherwise, better get the boys from duPont working on a new
> >bladder material...
> From this, I presume you are talking about bladder material, and
> about helium permeability, of bladder material. Sorry to be pedantic,
> but you keep changing the subject.
YES! The world presents so many interesting challenges! This is what makes
me such a fascinating fellow.
> Real, cheap films already exist,
> they do not need to be developed (I expect DuPont makes some, but I
> haven't checked; I use another manufacturer). They are relatively
> *very* impervious to small molecules, such as helium; much better
> than aluminized mylar, for instance. Weights on the order of half an
> oz/yard can be had. Unsuitable for kitemaking; we're talking about
> *bladder* material (see above; your words, Dave)
Well, in principle we seem to be on the same wavelength. So I now fall back
to the context, not a dissertation on what is possible. The context is sled
kites. Would any of the appropriately impermeable films have the palette of
physical properties needed to serve as a drop-in replacement for the Naish
bladder? I don't know, but I'd be surprised if they could endure the
mechanical abuses as well as HDPE or PVC or whatever they use. But I
haven't given the matter serious study.
> To inflation pressures. I hope we've already determined the issue of
> Naish/Wipika inflation pressures to be moot, with any relation to
> helium, but for what it's worth, I think I answered that one, too. By
> mouth, you are physically unable to exceed about 5 lbs. Certainly 30
> lbs is silly--you put that much in your automobile, then carry 4-5000
> pounds of car around, on 4 little half sq ft patches. It's not
> necessary to argue the point, though.
But I'll argue it anyway since I don't like your analogy :-) I put 120 psi
into my bike tire and load it with 200 lbs. That's 4x the pressure for 1/8
the load. And I'll bet I could do it by mouth if Todd were egging me on.
> You've got the kite? A pressure
> guage's less than $20 at any hardware store. If you unwrap it
> carefully, you could likely return it for a refund after you measure
> the pressure in your Naish.
> > > I've yet to meet the man capable of 5lbs, PSIG,
> > > by mouth inflation. Doesn't mean there aren't several candidates
> > > here!)
> >I hope I'm one of this rarefied group ;-)
> Well, yes, now you mention it... :-)
See above.... fortunately I've got plenty of company.. :-)
> >I think not. The original thread was about inflatable sleds, not
> >foils or flying mattresses. You're not suggesting that the volume of
> >a 15m AR5 is 570 l or even 5700 l are you?
> Well, yes, I am going to suggest you've made yet another arithmetic
> error. Sorry. ;-)
Quite possible, thank god I'm not designing bridges!
> >Assume that a AR5
> >bladders are about 20cm diam cylinder 5 meters long (for 15.5m??). I
> >get a volume of about 37 l, even less than my estimate of 57 l.
> A "square" tube (no taper) 20 cm by 5 meters will contain about 0.157
> cubic meters, or 157 liters. I've never measured an AR5, but suspect
> the average tube diameter is a good bit more than 8" (20cm). Perhaps
> you took the number from me? I was referring to an average thickness,
> for a foil.
Yeah, whatever, we can move the decimal around a lot and the AR5 still won't
float. I guess we agree on that.
> >>You cannot pump helium with "squirrel cage" type compressors or
> >>fans. The molecule has too little mass; is too light weight (duh)
> >>for these to work effectively. Nothing to do with molecule size.
> >>Squirrel cage
> >Sorry, it has everything to do with molecule size. Size relates to
> >electron configuration which relates to atomic number (as a glance at
> >the periodic table will indicate) and both are lead to design
> >considerations. A chemist will see the size and mass as interrelated
> >functions of atomic weight and electron configuration. Tell him the
> >size and he knows the mass. Perhaps an engineer can take the more
> >simplified view of atoms/molecules as dimensionless point masses?
> >Anyway, diffusion relates directly to size and influences valve,
> >container, and material design (the chickenwire metaphor). In any
> >case, taking a tire pump or sears air compressor to the beach ain't
> >gonna work (which I guess you concur with?).
> Back to semantics again. We seemed changed the subject (*you* changed
> the subject), away from fabric permeability, to compressibility and
> handling of gasses inside metal devices (pumps). I thought I was
> being pedantic again, but if you'll read your own post, Dave, you
> stopped talking about permeability, and switched to gas compression.
> My comments relate to gas handling, not fabric, which I covered
Obviously you thought you had pedantry all locked up. Sorry, you lose. And
I'm undisiplined too! ;-) Anyway, my object wasn't a technical symposium
on the design challenges of lighter-than-air kites. Seriously though, your
cogent contributions to the group have been authoritative and informative.
But it's a mistake to take my rants too seriously (unless it's about
> Let's not split hairs, though: Sure, you're right, molecule size is
> all important, with regards to fabric permeability. OTOH, your
> chemistry lesson holds little water, and demonstrates why chemists
> don't build kites. Which is smaller, Dave, a molecule of helium or a
> molecule of hydrogen? A molecule of ordinary air, or of water?
> (*answers at the end of the post. Don't cheat--give it a guess before
> you look!)
> Last, FWIW, I *do* take an oversized tire pump to the beach. I use it
> for sucking gas *out* of the kite, and exhausting it onto the
> > > OK, OK. Physics in *small* bites. I get it... :-)
> >Bring it on dude, bring it on!! :-))
> OK. Hydrogen, when it exists as a pure gas, is a diatomic molecule.
> That is, it's two atoms, connected together. Helium, which is inert,
> is a monatomic molecule--a single atom, floating free. Though
> physically, a hydrogen atom is of smaller diameter than a helium atom
> (though nowhere near 1/2 the size, as its atomic weight might suggest
> to Dave's chemist), two hydrogen molecules connected together are
> quite a bit larger than a single helium atom, so the first answer is
> Similarly, both oxygen and nitrogen, which together make up about
> 98-99% of air, are diatomic molecules. Two atoms of oxy or two atoms
> of nitrogen, always connected together. Water, H2O, has three atoms,
> but it's a biggish oxygen atom, connected to two tiny hydrogen atoms.
> Overall, the water molecule is smaller than either oxygen or nitrogen
> molecules. Yes, there are devices which are demonstrably "air-tight"
> and yet will leak water. Usually water vapor, but the stuff gets in
> and wreaks havoc with fine instruments and connections, even so.
> You don't need to take my word on either of these; ask any
> high-school or college chemistry teacher.
You know not with whom you are dealing, Dr Culp. The chemist would never be
taken in by this sophistry. Now it is you who must define your terms. As
you're doubtless aware, atomic radii can only be interpreted in the context
of electron configuration and chemical bonding (if any, none for helium).
The crystal radii of hydrogen and helium are, respectively, 0.25 and 1.53
angstroms. Yet the orbital radii are 0.53 and 0.29 angstroms. So,
presumably you refer to crystal radii. Yet, for diatomic hydrogen, the
Heitler-London treatment gives an internuclear distance of 0.742 angstroms,
smaller than crystal radius of helium. As you correctly point out, atomic
hydrogen doesn't really exist in nature since its electronic configuration
isn't stable. Since the chemist knows all this, and is versed in molecular
orbital theory, only the high school teacher (or professor, depending on
where...) would infer that hydrogen is half the size of helium based on
atomic weight. As for diffusion, molecular size is only one factor (which I
guess you know full well). Polarity, electron configuration, partial
pressures, etc all play a role too, sometimes an even greater one. But all
other things equal, size matters (which I also guess you know full well).
This has been fun, and a nice diversion while on tedious phone calls. But
soon I'll have to start doing real research to carry on. I therefore resign
from this thread and hope it has entertained you as well. (But don't
provoke me further or I'll be forced to invoke my buddies who are REAL
scientists, not posers like me :-))
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