Please Reassure Me: Perpetual Motion Machines are Still Bunk, Right? (Gravity Plane)

Factual Questions
1

For reasons that are beyond the scope of the question I have I was skimming through You Tube, and taking a look at odd videos that caught my eye. All was well, even if I were starting to think I was going to move to a bunker for safety from things falling from the sky, until I got to this video.

It purports to describe how something called a Gravity Plane might work, claiming that it can operate indefinitely without fuel, based on stored compressed air and a lifting gas. If you don’t care to go to the video, I don’t blame you. Basically it claims that the craft begins it’s take off phase by inflating gas bags within the booms of the fuselage, to make the craft lighter than air. Then when the craft has reached equilibrium it will transit to heavier than air flight by using compressed air to reduce the volume of the gas bags, whereupon the Gravity Plane will begin to glide. While in glide mode, turbofans will power compressors to regenerate the Gravity Plane’s supply of compressed air. Letting it store up all the energy it needs to begin the lift and glide cycle all over again.

The video references patents by a man named Robert D Hunt, and his company Hunt Aviation. A quick look at their website (here ) mentions that one of the keys to their system is “a proprietary low-boiling-point-liquid.” My gut reaction here is that while there might be reasons to keep the proper chemical name of the liquid secret, without any details I’m going to assume that it would be better called by its proper name: “snake oil.” Then there are the claims about the revolutionary low-drag wind turbine that’s also required to make this thing work. One technological revolution I would withhold judgment, but when the same project requires at least two of them?

For that matter, I have great difficulty believing that enough energy can be taken out of the air-flow around a moving glider to store sufficient mechanical energy in compressed air tanks without severely impairing it’s ability to fly. The emergency air turbines on commercial airliners aren’t near so ambitious, and the Gimli Glider still had major problems with theirs when they were stuck flying without engines.

I also am something of a fan of airships. So… claims that a commercial quality airship can reach 10 miles height sound a bit too good to be true. I know that things like high altitude balloons can go that high, or higher (IIRC the highest free fall performed by a person was done from a balloon.) but those are not rigid hulled craft. ISTR that the relatively low ceiling of dirigibles was one of the factors that killed them as military aircraft during WWI - i.e. even WWI aircraft, as primitive as they were, could fly higher than a dirigible could go. I’ll grant that with modern design and material science the ceiling for such craft may be higher, now. But I don’t buy it being that much higher. And even there, they’re talking about a maximum range on a single rise and glide cycle of approximately 400-600 miles.

I’ll admit, too, the brief look I’ve done has shown a small flurry of articles and releases dating to about 2004, and nothing since then. None of the references I’ve found online to Robert D Hunt predate this scheme, another bad sign.

So, basically, I’m asking - have other Dopers heard of this scam? And is there any reason for me to refrain from mocking this idea, and those people who bought into it? I did a quick search of the archives and found nothing.

2

It’s not an Orbo (a deliberate, outright scam concerning perpetual motion) but it’s definitely baloney.

3

Setting aside issues of perpetual motion…it’s basically just a blimp, innit? Once they get the magical proprietary gas inflating the gasbags? And don’t blimps have trouble flying in Weather with a capital W? So this puppy wouldn’t be able to “hover in place weightless at any time”–only when it was calm.

Which lets out most of North America, at any given moment. :smiley:

4

If they are carrying the gas with them the whole time, and if the volume of the plane’s structure does not change (that is: no gas-bag inflates above / behind / beside it) then the buoyancy can’t change either.

5

What I gathered from what I read was that the bouyant gas was going to be in gas bags, similar to how a dirigible was set up. So, while there may well be a constant mass of the “snake oil” in the craft, it won’t be in a constant volume. But that gets into other problems. A volume for a gas bag that produces a useble lift at sea level may rupture, if you don’t bleed off pressure from inside it, as you rise to several miles altitude.

And the moment that you open up the cycle, like that, you kill any chance for the perpetual motion cycles they were talking about.

6

This sounds like the aeronautical equivalent of the ocean-going ‘gliders’ that have extreme ranges - basically, they’re submarines with wings and inflatable internal airbags. blow the air out, and the things descend a little nose-heavy so they travel forwards. When they get to a certain depth, compressed air inflates the air bags, and they start to rise nose up, so that they travel forwards again. So basically, with just the energy required to compress the air and release it, they can travel immense distances. If they are almost perfectly neutrally bouyant, it doesn’t take much gas exchange to make them work. Little enough that they can be powed by solar cells on the surface. Very little energy can take these things around the world or keep them loitering soundlessly in the ocean for months or even years at a time.

But it’s not perpetual motion. The compressors are not 100% efficient, and neither are the batteries. Every cycle loses some energy, which has to be replenished.

And so it is with your airplane. Expend the power to compress the air in your balloon so you descend, and by the time you get back to your original altitude you’ll find that your impellers didn’t generate anywhere near as much energy as you used to start the cycle. The impellers add drag, which inhibits the aircraft’s ability to glide. That, plus other drag on the airframe consumes energy. That energy must be replaced. Without replacement, the craft will eventually use up all its stored energy, and that’s that.

However, it may be a fairly efficient way to travel long distances by air, if you don’t mind being at the whim of the elements while you ascend by balloon, and it certainly wouldn’t be a fast way to travel. It might or might not be more efficient than a garden-variety blimp or dirigible.

A more efficient way to do it would be to just use a balloon to get you up into the stratosphere and let the wind carry you around the Earth.

7

I found a blurb on another site about the “Gravity Plane” that said that it uses the warmer temperatures near the earth’s surface to vaporize the “snake oil” to achieve lift. Once it reaches its peak, the snake oil cools, condenses, and the craft sinks. The feasibility of this aside, it appears that it does have an identifiable outside source of power as it relies on the sun to maintain a temperature gradient in the atmosphere.

8

I had no real problems with the site claiming that it was an energy efficient means of travel, and one that would use a minimum of external energy for each rise and glide cycle. It’s when they started talking about “flying endlessly” without any fuel use at all that I got annoyed.

When one starts looking at the details, I have other questions, of course - hybrid craft usually have to sacrifice performance in some areas to meet the design needs of having to perform in different modes. It’s true for amphibious vehicles, it’s even true for aircraft that operate both at both subsonic and supersonic speeds, and I have to believe it’s going to be true for a craft that is part dirigible, and part glider.

Part of the way that they get their range numbers are based on being able to match the glide ratio of pure gliders. For those who aren’t familiar with the term, a glide ratio is an expression of how far, on average, a glider will travel while falling a given distance. The number that the Hunt Aviation site gives is that good gliders approach a glide ratio of 60:1. That is 60 feet travel for every foot of lost height. If you look at their numbers for range on their lift and glide cycle, 400-600 miles, and remember that they claim to expect an initial rise of 10 miles, you can see just how high a glide ratio they’re expecting.

Anyone who has been around a GA airport has to recognize that gliders are among the more specialized airframes around. I have grave reservations that a hybrid dirigible and glider would be able to approach, let alone match, the performance of traditional gliders.

To get back where I started, the idea does seem fascinating, and I’d have no trouble if people were talking about it holding potential for highly efficient fuel use for air travel. It’s when the claims moved into the realm of fantasy that I got annoyed.

9

In that case, all we have to do to assess the feasibility of this scheme is determine the efficiency of the compressors so that we know how much solar power we must absorb to compensate for their losses, and something similar for the drag of the plane itself as you never get back as much energy in a descent as you used on the ascent. Being warmed near the Earth’s surface, rising, cooling and falling again works just fine for the circulation of the atmosphere itself - the only question is whether the vehicle can absorb enough energy to run. I’d guess “not”, but it’s not quite a perpetual-motion machine, though you’d probably do better with a regular dirigible with electric motors driven by solar panels.

10

It doesn’t seem to be making any claims that are contrary to the laws of physics - it might be that they’ve been rather generous or enthusiastic in some of their calculations of efficiency, material structural properties, etc, but the idea itself looks like it could work, without needing perpetual motion, rotating magnets and coloured energy vibrations.

11

You are right. This site has a photo that shows a couple of those 60:1 gliders. They have a shape rather different from that of the proposed Gravity Plane - a lot more wing and less fuselage. It should be noted that they achieve 60:1 at rather low speeds (around 70 kts). When you push the speed up to something reasonable for transport - say, 120 kts - the glide ratio would drop significantly.

It’s also worth noting that those “sweepable” wings shown on the GP would probably not make sense. Swept wings start to make sense when a plane flies at a substantial percentage of the speed of sound - where glide ratio would be low. The cost, complexity and weight of wing-sweeping mechanisms would pretty well preclude their use on a lighter-than-air craft. (Indeed, it would be quite a challenge to carry fixed wings of the size depicted.)

12

I think fixing the sweeping wings on with a single bolt like that probably isn’t a good idea either.

I just watched it again, and this bit is just stupid, stupid, stupid:
It’s at about 1:02 in the video, and it says:

Wuh du fuh?

You want to compress air into the rigid shell

Your available power source is stored compressed air

You decide the best way to do this is to use the stored compressed air to drive a pneumatic motor, the output of which is attached to an air compressor.

And… why?

13

Okay, that’s officially :smack:. Duh.

14

Hey, hey hey, Mr. Pessimism! I don’t think the Validation Process is due to be reported for another 26 days or so: *“The validation process began in January 2007 and is expected to conclude by the end of this year.” * Steorn - Informasi Teknologi Terkini dan Terbaru

Sure, they have one little bad demo because it turns out the thing will not work if there is any artificial lighting, but then to go and call them scammers? The Steorn guys just had their picture taken with Ireland’s President McAleese in October, for goodness’ sake. Is that not validation enough? I am still hoping to get my money back from my investment there. C’mon, man.

15

I saw that picture, actually. I have a feeling President McAleese doesn’t know precisely who those guys are, and was just asked to pose for a photo at a conference. :slight_smile:

I’m actually pretty surprised they even popped up at all, given the, uh, lack of explanations for the demonstration fiasco. I figured they’d be in Tahiti by now.

16

That bothered you? That makes much more sense to me than one of the other uses for the compressed air stored in the plane: Vectored thrust to make it rise faster! (Beginning at about 0:25 in the video.) That seems even more extravagantly wasteful than using the compressed air to reduce buoyancy at the top of the rise part of the cycle.

17

Dunno - it just seemed like the sort of stupidity a schoolkid wouldn’t entertain for more than a moment or two.