
In a previous post entitled, “The Problem with Skinny Propellers,” we talked about how the vast majority of drone manufacturers have made the most unhappy trade-off with their propellers and electric motors: (i) propellers can only spin so fast before there is “boundary layer separation” (i.e., the propeller stops producing lift) but (ii) electric motors need to spin fast to achieve peak efficiency.
The “skinny propeller” is the unhappy result of that unhappy trade-off. It needs to be skinny enough to spin fast enough without losing lift (thousands of RPMs) and to stay somewhat close to the electric motor’s efficiency band (typically > 10,000 RPMs). So both the skinny propeller and the electric motor are compromised designs – and unhappily inefficient. Sadness all around.
What is needed is a (happy) way so that the propeller can spin slowly (like hundreds of RPMs – thereby optimizing the propeller’s ability to produce lift) and so that the electric motor can spin fast (thereby optimizing the electric motor’s efficiency by spinning at RPMs where it is most…efficient).
And, yes, that is a shameless endorsement of the Modovolo Lift because that is exactly what we have done – and it is one of the (many) reasons the Lift is able to achieve long flight times.
You may have noticed that the Modovolo Lift’s propeller is NOT skinny. It’s beefy. It has a wide chord – especially at the hub base. The Lift’s beefy propeller can spin at a few hundred RPMs yet still produce way more lift and thrust than a skinny propeller spinning at many thousands of RPMs, i.e., it’s far more efficient.
Then you may have noticed that the Lift’s propeller has this odd twist and narrower tip. And you may have asked yourself: “what the heck is that?”
It’s a good question. And this is where this post gets geeky because the answer is: advanced genetic-algorithm software program.
Genetic algorithms are super cool. They’re designed to mimic natural selection. Think: Darwinism and the survival of the fittest.
So how does it work? This is a vast oversimplification but the algorithm spawns millions of variants, tests them for fitness, and then starts “evolving” the variants by doing biologically-inspired things such as mutations and crossovers to the find the most “fit” out of that initial set of millions. It then takes that “fit” group and “evolves” another population of mutants and crossovers that is even more fit. Then repeat. Then repeat again. And again.
Based on “fitness” criteria of high efficiency and maximum thrust, we’ve used that advanced genetic-algorithm software program to evaluate millions of possible propeller designs. And that sucker cranked this for weeks before finding a relatively narrow set of possible candidates. Our Nvidia GPU was glowing hot.
And that is how the Lift’s beefy propeller was born – out of the primordial soup of millions of genetically unfit variants.