The flagellar motor of bacteria seems to be a perfect way to have a biological motor but since no complex multicellular creatures have used that method for flight or other uses, it must be harder to repeat at a larger scale.

I am not sure how big of a creature is realistic to have this rotary motor but their body design can as light as possible. I am imaging some tentacle like appendages being spun around like a propeller to hover and achieve flight if possible.

What changes to the flagellar motor design would be needed to scale it up for a larger creatures anatomy?


4 Answers 4


As told here:

Bacterial flagella are helical filaments, each with a rotary motor at its base.

Archaeal flagella (archaella) are superficially similar to bacterial flagella.

Eukaryotic flagella lash back and forth.

Only wings can give UPLIFT

Flagella give motion like oar or propellar for boats. They don't give uplift like wings of an aeroplane. Even the propellar driven planes get uplift from wings and not from propellar.

@Daron is right. Flagella can be used for swimming only and not for flying.

  • $\begingroup$ Like a helicopter or drone, not a plane propeller. $\endgroup$
    – user96146
    Commented May 24, 2022 at 10:22
  • $\begingroup$ @RayHammond Same problem $\endgroup$
    – Daron
    Commented May 24, 2022 at 10:36

Replace the air with Water

enter image description here

Bacteria use their flagellae to swim around in liquid. If you want a creature to fly around with flagellae the first step is to make it fly through water. This is called swimming.


Limits on efficiency make it difficult to believe

As a general rule, planes are considerably more efficient than helicopters - around 5:1 in terms of fuel consumption per passenger mile - this becomes even more of a handicap compared to glider aircraft that take advantage of thermals to stay aloft with no fuel consumption.

The biological requirements for flying already demand a significant challenge for birds requiring differences in bone and lung design (as well as other features) to support practical flight.

The advantage of helicopters over planes is primarily to conditions where the maneuverability, hovering or low-speed flight is an advantage or the benefit of not requiring an airstrip for takeoff/landing. Bird and insect winged flight do not suffer from the same kind of limitation as our fixed wing planes, giving less possible advantage to possible rotary lift systems.

There is just not enough benefit compared to the significant downside in energy requirements to make such an animal viable in the real world. Biology does not permit such an extravagance to survive the competition.

Could you in theory bio-engineer such an organism? Probably - it would be unable to compete in the real world, and would doubtless be limited to short hops because overheating, lung/heart capacity, muscle fatigue, etc. would be definite problems. But humans have a history of breeding animals that could not survive in the wild because they serve our purpose. Domesticated chickens can't really fly, but can use they wings in short hops, though I doubt some of the heavier breeds can even manage a short hop.

I would expect such an artificial design would necessarily be a small animal.

Now, if you open up the possibility of low-gravity worlds, this changes things - the power requirements for all forms of flight drop. Dense atmosphere would also help.

This is all based on the assumption that biologic rotary motors could be scaled to large sizes - It is not certain that this is true. Certainly you could not simply scale up the design used in bacterial motors, but it seems possible that with a proper redesign a large rotary motor would be possible, though far from our current abilities of genetic engineering.

Now, given that the actual question is what changes in the motor design would be needed, I suspect that you are asking for something that is very complex, essentially impossible to answer. We do not design motors using biology, but engineered materials that do not have the same trade-offs and limits.

But even worse, we don't know how to design an organism at all - we can't engineer any de novo organism at all currently. Gene-splicing is simply picking and choosing from existing natural designs in the hope of producing a result that is suitable. We don't have anything close to use as a basis at this time.


As another answer mentions, this sort of appendage is primarily used for moving in liquids. While some microorganisms could theoretically use it to fly, physics stops this sort of appendage from working.

For smaller creatures, the tension between molecules makes the world seem like a different place. For example, water's surface tension is enough to lift or drown an insect, and the fairy fly (a kind of wasp) can almost grab its way through air.

These structures work for microorganisms, including in the air, because at their scale, they are suspended in the fluid (fluid = gas or liquid), so very little force is necessary to keep them lifted.

Now for the rotary part - at a small scale, the forces of things like cell membranes can hold them together - individual molecules can slide past one another while remaining connected, just like surface tension. However, at a large scale, this isn't possible. Entire sections would have to be completely detached, making it impossible for nutrients to travel from one side of the rotor to the other. Not enough blood, lymph, nerve signals, etc. would be able to get through, as basically the entire area would be cleaved for the rotation. Helicopters and Bacteria both don't use any of these parts, so they have no problems.

Additionally, muscles do not work for this kind of motion - a muscle only functions when both ends are attached to something. However, as we just learned, the rotor must be detached from the rest of the creature. Thus, muscles wouldn't work unless the creature was grabbing the rotor and constantly rotating it and re-grabbing it. And then, how is it going to evolve this? It has to find reliable access to very strong rotors, learn to grab it, learn to rotate it without letting go, and then have enough strength to generate the force to use the rotor for lift, which isn't possible due to the square-cube law! And then, it would also have to have a counter-rotor to keep the rest of its body from swinging around the opposite way (ever noticed the little one on the back of helicopters? Newton's laws strike again - rotate one thing one way, you rotate the other way).

Why would this not evolve? Well, for the same reason it hasn't so far - the resources and energy required are too much, the rewards too little, and the connectivity impossible - any species that even tried this method would definitely be out-competed by regular flying species.

Thus, it is practically impossible for any naturally occurring large organism to use this method of flight. The best you're getting is the helicopter seed, which still only falls.


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