The same way a nautilus swims around in water: jet propulsion. Nautiluses move using a hyponome, which expands to pull in water from the sides of the nautilus, and contracts to expel a jet of water.

The bio-blimps can majestically wheeze across the land using what is essentially an organic bellows, just like the nautilus. Whenever it wants to move, it expands a bladder to pull in air from broad vents on the side of its body, and then compresses the bladder to expel the air through a much smaller vent to propel itself.

Presumably, they already have various valves and sphincters and what not for maintaining their main gasbag, so adding one smaller gasbag for locomotion should be a breeze.

One alternative method of propulsion that could be considered is: Basically having none at all...

Consider a symbiotic system where your 'bio-blimps' are more 'giant floating green houses' - Main creature feeds off light algae/lichen-like secondary lifeforms, which the main creature effectively cultivates as a means to gather energy from the sun by funneling rainwater to them.

Further input could come from birds who nest in/on the bio-blimp, depositing the left overs from meals they collect from elsewhere, which in turns feeds the secondary bio-mass.

Bio-blimp's survival is no longer tied to it being able to navigate toward food, as its food source naturally grows or comes to it, and it is free to drift on the winds while adjusting its lifting bladders to maintain a comfortable altitude.

Don't Fly, Fall with Style!

Ok, so it's going to involve flying too, but...

Your creature is going to have some way to control its buoyancy. Presumably muscles that change the shape slightly, thus changing the amount of air displaced, thus changing the buoyancy. So use this ability to your advantage.

The creature grows small wings or flippers. Not nearly enough to lift the creature, but enough to generate some lift. Now the creature can control it's flight by trading height for speed.

Your creature moves by:

1. Expanding to increase buoyancy and increase altitude


2. Contracting significantly, triggering a "fall"


3. Using its "wings" to control the fall


4. Expanding again before it hits the ground

Effectively, your creature copies the locomotion of birds of prey, only instead of using thermals to gain height, it uses buoyancy. The lift from the wings will have both horizontal and vertical components, so some of the energy will be converted into making the dive longer, and some of it will go into moving your creature towards its objective.

So your sky whales gracefully float upwards before suddenly diving in whatever direction they want to go.

Hydrogen Rockets

Consider that your gasbag is full of hydrogen. It would be entirely possible for it to take some of its excess hydrogen and allow it to escape in a direction of its choosing. It could then generate a spark and move because of the explosion (the valve would have to be stronger than the surrounding skin, of course).

I don't think this is a practical solution, but it seems negligent to not mention it.

Tail Fin

Not sure why you think flippers are impractical. The lower density of air means that thrust for a fin would be lower than in the water, but drag would also be lower. Also, you didn't say how fast the creature needs to move. Almost any fin at all would provide some thrust, so the only question is: what is the maximum thrust you could feasibly get? And that depends on how much mass you can allocate to muscle, the overall shape of the creature, and its surface roughness (to compute friction drag). If the creature isn't trying to compete with birds of prey in the speed department, then the frontal area and surface friction are probably not important, and you could get away with a large, thin, lazily waving tail fin, mostly made of chitin spines and a thin but tough membrane. You could probably get away with a medium-to-small fin also, at a compromise of performance.

Movement

If the creature moves slowly, then spherical is best. If it needs to move quickly, then making it long and skinny (or at least cylindrical) is best but compromises your lifting efficiency. Also, you probably can't put enough muscle and metabolism on it to make it "swim" quickly. Probably have to resign yourself to a very slow whale. It is likely to be unable to overcome most winds aloft, depending on its customary altitude. So, at best, it can maneuver within air currents but has little say in where it actually goes.

Food

A bigger problem, I think, is: what does it eat? Surely it can't catch flying creatures unless it has a clever lure. Surely a floating gasbag simply isn't sturdy enough to survive claws and beaks. I like the idea of the floating greenhouse. This creature can possibly do something that terrestrial plants cannot: move above the treetop/cloud layer to get unobstructed sunlight. That should be the primary motivation for its airborne nature. It can get carbon and oxygen from CO2 in the air, and it could possibly "drink" clouds to get hydrogen/water. But it still needs to fix nitrogen, and get essential trace elements like phosphorus, sulfur, iron, and other minerals. For this, it could either be an herbivore, and munch on treetops to get these vitamins, or it could be a dirt scavenger.

Imagine having long tentacles that can drop down dozens of meters, scooping loose soil from along the ground. Being close to the ground would make it vulnerable to ground-based predators and birds of prey, so it would need to have some kind of defense against those. But it would spend most of its time as high as it could manage because it's too metabolically expensive for most birds to fly very high in the sky. Once it scoops up dirt, it rises aloft, and lets symbiotic mycorrhizal webs in the "buckets" of the tentacles do the hard work of extracting trace minerals from the dirt. Basically, the tentacles would be functionally equivalent to tree roots. Once the symbionts stop giving up nutrients, the skywhale dumps the dirt and grabs some more. Mountains are safer places for gathering dirt because of fewer predators, and less altitude change, but also have lower nutrient content because fewer plant/animal/bacterial species are churning through the soil. So their scavenging would depend on their current needs and the risks involved.

Protection

To protect against lighting strikes, the skywhales could form a metallic web across their skin, like the shell of an airliner. By offering a low-resistance conductive path for lightning, they should be able to avoid the worst effects of a strike. Iron may be too heavy to use as a conductor, so perhaps they could create carbon nanotubes or graphene as the Faraday cage, which can be constructed out of pure carbon. What would be especially interesting is to create a graphene supercapacitor and have them store some of the charges flowing across their skin to use defensively. This could, of course, also be used offensively. Imagine a region on their top surface which emits predator pheromones (whatever organic volatiles might attract birds, like even blood heme). A bird flies up and comes in for a landing, expecting an easy treat on the gasbag. But when it lands, the supercapacitor, charged up from flying through a nimbus cloud, discharges and electrocutes the unsuspecting bird just as it makes contact with the skin. Then a "mouth" opens up and swallows the victim into the digestive system. Of course, a carnivorous skywhale wouldn't need to eat dirt, since birds would presumably be able to provide all the trace nutrients. However, it might not be able to eat enough birds to meet all its metabolic needs, so symbiotic/intrinsic photosynthesis would still be valuable.

Lift

I think the most difficult problem is actually lift. A fish can change its buoyancy easily because gas has a much lower density than water. So a swim bladder is relatively efficient space-wise. A proportionally sized swim bladder in your skywhale wouldn't give much altitude change. So you probably need to squeeze the entire gasbag to descend, which would presumably cost a lot of energy. The amount of work required to descend all the way to the ground, using just volumetric compression (as opposed to releasing gas) is basically the same as a hiker climbing from the ground to its maximum altitude (the altitude at which the gasbag is fully inflated). Only, the climber weighs half a ton. More problematic than the work (which can be done slowly over a long time) is the amount of force required to squeeze the gasbag because this puts hard lower bounds on the amount of muscle mass needed. The higher the max altitude, the more force required to squeeze it to the ground. Getting above 1000m may be infeasible.

Alternatively, it could simply dump hydrogen to descend quickly, and refill once it gets to the ground or a low cloud layer. At the surface, it would probably need to hover over a decent water source to obtain sufficient hydrogen to ascend. Otherwise, it would need to scavenge water vapor from clouds. And separating hydrogen from oxygen or alkanes or carbohydrates is energetically expensive (although most expensive for oxygen and least for alkanes, I think...methane is probably the easiest H source), so it couldn't do this quickly or easily. It could take days or weeks to refill the gasbag. If it relies heavily on photosynthesis, then dipping below the cloud layer could prove fatal, and possibly strand it near the ground for a long time.

Anyway, those are some things to think about. Have fun!

Up and down is all you need

Why not? Look at Google's Loon. Their balloons navigate solely by going up and down in the atmosphere, catching the winds at various altitudes. This is enormously energy efficient, and I'm guessing your hypothetical creatures are going to be very concerned with conserving energy.

For obvious reasons, wings are by far the best propulsion system as far as animals go.

Since that seems to not be possible due to that whale's anatomy however, and since we've already excluded flippers, I can think of two possible means of locomotion :

• Jet propulsion : Such as the one squids use underwater by quickly ejecting water out of a valve, allowing them to reach great speeds. However due to the low density of air, jet propulsion would likely be very weak in terms of acceleration unless unrealistic amounts of pressure are applied. If you are thinking of a slower type of animal though that would be an idea.




• No locomotion at all! Some animals simply travel by drifting, such as jellyfish. Of course in the air that is not really an option, since food would probably be scarce thus depriving you of the luxury to drift aimlessly just waiting for food to land into your mouth.

In conclusion, the best option then would be a combination of both proposals, that is aerial whales that mainly navigate the wind currents by letting themselves drift along the currents, with the ability to (slightly) steer using jet propulsion. That way no enormous amounts of jet pressure would be needed, thus staying in the realm of plausibility.

I think an animal like that would have the best chance if it mostly relied on:

• Following winds at different altitudes like a hot air balloon


• Having an omnivorous diet & not chasing any quick moving prey. Their diet could consist of nectar & tree top fruits, along with insects & maybe occasionally birds or squirrels


• Instead of chasing prey, relying entirely on passive mechanisms. One possibility would be an organ that charges in the sunlight to glow as an insect lure at night.

The "whale" is somewhat flat and has a "forward balloon" which can be contracted somehow (muscular tissue or whatever).

Contracting it will cause loss of buoyancy and it will start to drop nose first, this, in turn) will cause it to move forward due to its elongated/flattened shape.
Relaxing completely the reverse will happen (rise nose-first), again resulting in forward movement.

Having two forward balloons allows for a certain degree of steering.

Note this "dolphin-like" locomotion won't be enough to overcome wind push, on such a large creature (the same holds true also for all other proposed mechanisms).

The gas in a bag can be heated by directing the blood of the body through the larger, external circulating system that runs through the bag tissue or cooled by holding the blood inside the body. It would ensure up and down movements. this would allow the creature to find the horizontally moving air masses to stay in them and to get some horizontal freedom too. Together with more traditional means of navigating in the air like fins the switch between inner and outer blood circulating system would let it move around but would it be enough to survive I don't know

The question says that fins are going to be too big to be practical. However, actual flying animals have air-adapted fins, and we call them wings. For such a big animal, wings are going to be big, but not as big as they would be for a heavier than air animal or aeroplane, since they don't need to provide lift, just propulsion.