Gravity Powered Airplane, a blimp with wings

[someone_else]

No, it isn't news, but I want to ask a few questions on the concept.
Here a video of it
here an article about it

Now, to make it short, this thing is a zeppelin with wings, supposed to go up using buoyancy and then deploy wings and start repressurizing helium to become heavier than air again to glide down like a gigantic paper airplane.
The concept then adds a system of turbines that convert some of the air movement during descent into power that is then used to pressurize air into tanks, then used to power all its ancillary systems and be its main ballast.

While the first part sounds reasonable to me, the part about pressurizing air and going fuel-less sounds like bullshit to me.
I also feel a possible violation of thermodynamics (perpetual motion).

So, what I'm asking is:
-What its performance would be or what would you need to modify to get a performance at all (and not a scam)?

[Bottlestein]

While the first part sounds reasonable to me, the part about pressurizing air and going fuel-less sounds like bullshit to me.
I also feel a possible violation of thermodynamics (perpetual motion).

So, what I'm asking is:
-What its performance would be or what would you need to modify to get a performance at all (and not a scam)?

No violation - the power generated is nowhere near enough to power the turbine - you are far from "breaking even", energetically speaking. In thermodynamic terms, a turbine is "inefficient", due to dissipative effects along the blades. In layman's terms, the "usable" portion of the pressurized wave is along the axis of the turbine, but the blade does work on the air in a circumferential direction also. We can make the blades longer, so a greater portion does axial work. But this has its own drawback: The longer the blades, the faster the blade tips move for a given rotational velocity. This brings more of the blade tip into a supersonic flow regime, and shock waves form along the blade. This induces flow separation and kills efficiency since less air is now hitting the blade. This is why, after a certain amount of horsepower, fighters simply do not have pure prop engines - turboprop or turbojet or turbofan have to be used to maintain some efficiency.

[CSJM]

There is no violation here, just a very creative use of the atmosphere. In this case, presuming it takes substantially less energy to pump up the helium bladders and drain air "ballast" than it'd take to simply fly the thing up 10 miles on conventional power, you can have a net increase in energy when repeating the ascend/descend cycle. It really depends on the efficiency of turbines used for power generation and air recompression.

Using atmospheric buyoancy to harvest energy from gravity is a ridiculously appealing concept. If it works, you could eventually just send thousands of the things into some high-speed airstreams to fly up and down 24/7, just stuffing electricity into accumulators that are then swapped out by similar craft.

[Broomstick]

No, it isn't news, but I want to ask a few questions on the concept.
Here a video of it
here an article about it

Now, to make it short, this thing is a zeppelin with wings, supposed to go up using buoyancy and then deploy wings and start repressurizing helium to become heavier than air again to glide down like a gigantic paper airplane.
The concept then adds a system of turbines that convert some of the air movement during descent into power that is then used to pressurize air into tanks, then used to power all its ancillary systems and be its main ballast.

While the first part sounds reasonable to me, the part about pressurizing air and going fuel-less sounds like bullshit to me.
I also feel a possible violation of thermodynamics (perpetual motion).

So, what I'm asking is:
-What its performance would be or what would you need to modify to get a performance at all (and not a scam)?

First, let's just ignore the energy requirements to capture and store useful amounts of helium. That all happens before you load up the zep-plane.

Some of this is not new at all. Use the ram-air of unpowered descent is actually a standard emergency backup technique for all airliners these days, and has even been deployed on occasion. It's called a "RAT" or ram-air turbine and is used to power the big planes in the event of an otherwise total power failure. It provides enough power for a radio, deploying landing gear, and a few other things in the small models used for emergencies. What they're talking about is deploying a larger version. It certainly could work in theory.

As for unpowered descents - no big deal. Once you're up you don't need power to come down. The space shuttle, for example, on return to the atmosphere is a glider - on every space shuttle flight the descent involves no power from engines. In fact, their problem is too much energy and the big trick is dissipating speed/power safely. Airliners have safely landed with no engine power a few times, too. As one of my flight instructors put it, if you're above the ground you're never without power - most of the time you're using engine power but if that quits you still have gravity power. You just have to convert all that potential energy into actual energy in a controlled manner so you have a landing instead of a crash. So the descent phase is entirely possible.

So that just leave the getting-up-there phase. We've been using lighter-than-air and buoyancy to do that since the 1790's.

Yeah, it's possible.

However, it won't be entirely fuelless. Or at least, not in the sense of being without energy input. You'll need some form of energy for steering and working the thing on the trip up, when you won't have ram-air power generation. That might be electric batteries or solar power or... well, a gas-fueled engine somewhere. And that's why it doesn't violate any principles of physics. You'll need a little energy input to start the cycle, after which you're using physics and gravity to extract energy from the environment - which sailboats have been doing for thousands of years. But it will need a lot less fuel input that fixed wing or rotary aircraft.

If they can get it to work.

[Feil]

The pressurization pump that moves the helium around has to pay for the dissipative losses of the turbines and all drag forces on the airframe. The airplane therefore must pay for every cent of its straight-line energy. The energy required to take it to altitude is "free" because it was already paid for by the process that generated the helium.

The airplane's method of propulsion may be more efficient than a conventional jetliner. That's engineering, not physics. However, there is no fundamental difference in how it works.

Jetliner: spend V+Waste1 energy on fuel to get to cruising altitude, where it will have V potential energy.
Gravity Plane: spend V+Waste2 energy to make the helium at such a pressure that the airplane will float to cruising altitude, where it will have V potential energy

Jetliner: spend D+Waste3 energy on fuel to overcome dissipative forces associated with translational motion along its flightpath
Gravity Plane: spend D+Waste4 energy on fuel to re-pressurize the helium such that the airplane will float to cruising altitude again

No free lunch. If the Gravity Plane is more efficient, it's because Waste2+Waste4 is less than Waste1+Waste3, not (directly) because it's using an alternative energy source.

[CSJM]

Not only because of that. The gravity-plane expends energy on the various things it performs, but the principal difference is that a jetliner carries its energy source with it - the fuel that the engines use to overcome gravity and air resistance, in addition to powering the plane. The gravity-plane (can we call it a graviplane?) uses air pressure, air resistance, and gravity - it doesn't spend energy directly against them, but rather generates energy by tapping into them. It spends energy on altering its physical properties - buyoancy and drag - and the physics do the rest. Even better, it only spends energy on reducing its buyoancy, not on increasing it - helium is pressurized on descent, and its release from containers isn't an energy-intensive process. Archimedes law (or whatever) pushes the blimp-mode graviplane up, simultaneously propelling it forward due to air resistance acting on wings, and then gravity pulls it down, generating even more power (from the description I understood the descent rate is faster than the ascent) to repressurize the helium until it reaches the bottom of its trajectory, where ballast is purged, helium sacs are inflated, and the process begins anew.

If the thing is ever made to run on hydrogen instead of helium, any extra energy generated on descent may be used to pressurize additional hydrogen tanks by extracting hydrogen from the atmosphere. Or for emergency/additional flight power, with the aid of a hydrogen-using ICE on board.

[Old Peculier]

Starting from launch:

1. Plane ascends due to buoyancy. Forward motion is gained by gliding upwards.
2. Plane compresses gas, using energy. Plane descends due to gravity, gliding downwards.
3. Plane decompresses gas. Plane ascends due to buoyancy, gliding upwards.
4. Cycle 2 and 3

Where does the energy come from in stage 2?

Solar power already done without the need to carry gas compressors, or fly with a large front cross sectional area

Wind power? If there is no wind blowing you are relying entirely on your buoyancy and gravity to drive the turbines. Therefore, because of losses, you will not be able to descend (without releasing gas) to ground level, or to the depth of your previous dive. Eventually you will be stuck in the air with no forward motion, nor ability to dive without releasing gas.

With wind, things will essentially be the same, except you will drift, in the direction of the wind.

Basically this plane isn't a 'gravity plane' it's a buoyancy engine with compressed gas as the fuel. As with all fuels, it is expended in doing work.

[Old Peculier]

By the way, the advantage of a lighter than air aircraft is that it can become more energy efficient by flying slowly, like driving a car.

A heavier than air aircraft cannot do this because flying slowly decreases lift, therefore energy must be spent in some other way to keep the plane up, the extreme case of which is a VTOL/STOVL like this, this, this, or this

[CSJM]

Energy in stage 2 is partially pre-stored during previous descent, partially gathered during ascent. In both ascent and descent the aircraft is moving forward due to having wings - where submarines can rise or dive using forward thrust and wings, this aircraft can achieve forward thrust using rising, diving, and large wings. Energy is generated on both ascent and descent, though more on descent due to a (likely) slow climb rate.

Like mentioned multiple times, it all boils down to the efficiency of turbines and optimizations to the recompression mechanism. If it can acquire more energy during a descent/ascent cycle than it expends on steering and gas compression, it will work. If it can't, it won't. Solar power is always an option of course, and it can help the craft achieve optimal performance (requiring less dive/climb manuevers), but yeah, it'll defeat the purpose of a gravity plane - might as well go with a giant solar-power dirigible.

[Old Peculier]

Like mentioned multiple times, it all boils down to the efficiency of turbines and optimizations to the recompression mechanism.

No it doesn't.

If it can acquire more energy during a descent/ascent cycle than it expends on steering and gas compression, it will work. If it can't, it won't.

It can't. It won't.

you will not be able to descend (without releasing gas) to ground level, or to the depth of your previous dive. Eventually you will be stuck in the air with no forward motion, nor ability to dive without releasing gas.

If you want to know why, look up the physical definition of work and read about it.

[CSJM]

Mm-hmm. Work. How's this - the aircraft itself does no work. It does not exert "a force over a distance". The only part of the craft that performs work is the high-pressure pump that makes air replace helium inside the craft's "balloons" (I don't remember the proper word for those things). All of the craft's motion, upward and downward, is the result of two forces - air pressure and gravity - acting upon the craft. What you're implying is that it is impossible for the craft to gather enough energy on its ascent/descent cycle to fully repeat the gas recompression process. That may well be the case, which is why the project is still on paper. However.

Consider the amount of energy it'd take to move the massive hulk of a submarine by any distance, up or down. Then consider that much less energy is required to fill and empty the ballast tank, with nothing "expended" because the water is taken from and released to, the outside of the vehicle. It'll be the same here. At the peak of the climb, the aircraft will force air amidst the helium balloons, reducing their lifting force and making the aircraft descend. As it starts the descent, turbines catch wind and provide power to refill the air tanks that act as ballast. At the bottom of the trajectory, air ballast is released from everywhere (including from jet nozzles to push the whole thing up), leaving some for the peak descent point, and the lighter-than-air-again craft starts climbing. I think it's all fairly sound, physics-wise. All that's left is to build it and see if it works.

[Old Peculier]

Mm-hmm. Work. How's this - the aircraft itself does no work. It does not exert "a force over a distance".

Buoyancy and gravity are forces applied on the aircraft.

The only part of the craft that performs work is the high-pressure pump that makes air replace helium inside the craft's "balloons" (I don't remember the proper word for those things). All of the craft's motion, upward and downward, is the result of two forces - air pressure and gravity - acting upon the craft.

Yes, 'air pressure' (buoyancy), and gravity. Ask yourself how much energy the pump uses when pumping gas. It will be as least as much (actually much more) as could be gained by turning gravitational potential energy and what kinetic energy it has into useful energy by gliding downwards.

What you're implying is that it is impossible for the craft to gather enough energy on its ascent/descent cycle to fully repeat the gas recompression process. That may well be the case, which is why the project is still on paper.

As I said, this is an aircraft with an internal fuel source (stored as buoyancy), like most others. It could easily work, but not as you imagine: drawing energy from the environment. Instead it is just a lighter than air aircraft, with all the advantages that sort of thing has, but with a novel engine.

If you look at a plane like the harrier, you will see that what it did that made it successful was to use the same engine for vertical thrust as for horizontal thrust, by means of nozzles (this was quite tricky to achieve). This is essentially the same, in that it uses the pump to both descend and deliver forward motion.

Consider the amount of energy it'd take to move the massive hulk of a submarine by any distance, up or down. Then consider that much less energy is required to fill and empty the ballast tank, with nothing "expended" because the water is taken from and released to, the outside of the vehicle.

Nearly a Gigajoule, minimum, to move say a nuclear powered attack submarine like HMS Astute 100 meters up or down. If it is going down, the energy already existed in potential, and if it is ascending using buoyancy that is how much energy the water pumps will use, minimum. Why do you think less energy than that will be used? Do you know, or does it just sound right to you?

By your use of the word "expended" I suspect you are confused between fuels and ballast. In the aircraft the internal 'fuel' is the lighter than air gas, with the energy stored as buoyancy, rather than chemically. Using the word fuel may not be technically accurate, but it is providing the same role in that is carried with the aircraft, and drives its motion for a time at least.

It'll be the same here. At the peak of the climb, the aircraft will force air amidst the helium balloons, reducing their lifting force and making the aircraft descend. As it starts the descent, turbines catch wind and provide power to refill the air tanks that act as ballast. At the bottom of the trajectory, air ballast is released from everywhere (including from jet nozzles to push the whole thing up), leaving some for the peak descent point, and the lighter-than-air-again craft starts climbing. I think it's all fairly sound, physics-wise. All that's left is to build it and see if it works.

"As it starts the descent, turbines catch wind and provide power to refill the air tanks that act as ballast"

Catching wind (ie. air that is moving, as opposed to still air that the craft it moving through) is fine, wind farms obviously do that, as do sailing ships. However, with no wind it will not fly far, and using wind it will be at the mercy of the wind strength and direction in a way that other aircraft will not be.

[Feil]

Jesus. If I was going to deny the first law of thermodynamics, I would come up with something a hell of a lot cooler than a blimp with wings.

[CSJM]

Who's denying it though? The thermodynamics laws concern a closed system with no energy input, while here we have a craft with outside forces clearly acting upon it.

As for having no wind to use, that the aircraft never flies straight up or falls straight down - it always moves forward due to its aerodynamic profile, thus creating a motion of air that will spin the turbines. "Wind" in this case is air moving relative to the craft, not relative to earth. It may well be completely still air. There can be a problem if there is actual wind blowing the way the craft is moving, which can decrese the efficiency of the system, but my guess is these things will try to stay in relatively constant air streams to keep their operations stable.

Oh, and yeah, I may be completely wrong on the ballast versus actual motion thing, but really. Presume the aircraft in neutral buyoancy mode. Its ballast tanks are filled just enough to counteract the buyoancy of its helium balloons, which in turn counteract the gravity. When it releases stored air, it receives upward thrust, releasing the energy stored in the compressed air. Then it fills up the air tanks again, storing energy in tanks. If it worked just on that principle, it'd need fuel. But losing and gaining air simultaneously affects buyoancy. Gaining air reduces buyoancy, releasing air restores it. I think this allows the craft to have significantly more force acting upon it than it could achieve with just stored fuel. But really, this has to be actually built before we can see if it can work. It won't work with a submarine, because you need twice the energy to go twice as deep - water pressure increases linearly with depth. But air pressure declines in a much smaller margin, resulting in much larger travel distances per energy expended on air compression. I think it's significant.

[someone_else]

Ok, so this gimmick as-imagined is a violation of thermodynamics even in your book.
But other than that, you seem to agree that the concept isn't so bad, and told me I need to provide some energy to keep it working if I want to avoid creating an (impossible) perpetual motion machine.

How much energy I need to provide to this gimmick to repeat the cycle?
That's = to the wasted energy I know, got any aggressively eyeballed estimate?

Also, any comparison with a winged aircraft?

[Broomstick]

I don't have the ability to run precise figures on energy usage, but it will burn less fuel than a fixed wing aircraft of comparable mass/capacity because fixed wings burn fuel to maintain lift and the gravblimp doesn't. Just remember, there's no free lunch. You will have to input some energy into each cycle, the question is how much. Utilization of bouyancy, gravity, air currents, and so forth can be surprisingly efficient - the world record duration of gliding flight is, if I recall correctly, 9 days as an example. That wouldn't be practical for real transport, as they were utilizing air currents around mountaintops to gain their needed lift, but it shows that you really can extract energy from the environment.

But, lighter-than-air aircraft are so different than fixed wings it is hard to compare in a meaningful way.

[Simon_Jester]

Yes, 'air pressure' (buoyancy), and gravity. Ask yourself how much energy the pump uses when pumping gas. It will be as least as much (actually much more) as could be gained by turning gravitational potential energy and what kinetic energy it has into useful energy by gliding downwards.

I would like to see the math. This sounds interesting.

[Old Peculier]

Yes, 'air pressure' (buoyancy), and gravity. Ask yourself how much energy the pump uses when pumping gas. It will be as least as much (actually much more) as could be gained by turning gravitational potential energy and what kinetic energy it has into useful energy by gliding downwards.

I would like to see the math. This sounds interesting.

I'm a bit rusty so it would take more time than I have to give a rigorous answer. My statement was a prediction based on conservation of energy.

[Malagar]

The real question as far as I see it is not is it is more eficient than a fixed winged aircraft?
But is it more efficient than a blimp?

[Bottlestein]

Yes, 'air pressure' (buoyancy), and gravity. Ask yourself how much energy the pump uses when pumping gas. It will be as least as much (actually much more) as could be gained by turning gravitational potential energy and what kinetic energy it has into useful energy by gliding downwards.

I would like to see the math. This sounds interesting.

The math is far less "amazing" than you'd think:

You simply run stage 1 of the Brayton cycle, connected to a nozzle. The efficiency for the combined system is strictly less than the Brayton efficiency (since the nozzle is not reversible).

As I and Old Peculier pointed out, this does not violate the 1st or 2nd laws at all based on the following reasons:
1) Pumping the helium requires more work than the blimp does while rising.
2) Fabricating the insulating material for the "gasbag" (heat transfer = pressure change, so it has to be insulated) requires far more energy than the expanding gas produces.
3) The turbine - system produces less energy than the work gravity does on the system (see my 1st post).