18

Thrust is proportional to the change in momentum of the air passing through the prop - i.e. how much the prop speeds it up. The power required to do this is proportional to the kinetic energy of the air, which is proportional to speed squared. That 'squared' is the problem. A smaller prop acts on less air, so it has to speed it up more to generate the ...


17

An explanation for why this is can be found in this Aviation.SE answer: Why aren't large, low-speed propellers widely used?, which I'll paraphrase here. The thrust a propeller generates is a function of its velocity and geometry. It makes sense that a propeller spinning faster will also generate more thrust. For a smaller propeller to generate the same ...


6

Comment number 7 at this link is an excellent source (and fairly trustworthy as it was written by Joshua Bardwell). The main reason specified for the efficiency is that for a given amount of thrust, a larger propeller (on a motor with an appropriate KV) will draw less current than a smaller propeller on a higher KV motor, so there is less sag. Another ...


6

Rounded leading edges are better for subsonic aerofoils. Props and wings have to work at a range of angles of attack. The airflow has to split to pass either side of the aerofoil, and a rounded leading edge allows the separation point to move smoothly. Sharp leading edges cause a lot of drag at extreme angles of attack when air has to flow across the sharp ...


5

It's really a challenging question to answer because flexible and stiff propellers both have their advantages and disadvantages in terms of efficiency. Flexible propellers are typically lighter than stiff propellers, so they spin-up more efficiently than stiff propellers. On the other hand, when a propeller is spinning at higher speed, a flexible ...


3

Lower kV motors will spin more slowly but with more torque. Spinning more slowly (with the same prop) will give you less thrust and less power (and thus lower current) at full throttle. However, a well setup quad doesn't spend much time at full power, especially when you're considering your total flight time. The amount of thrust required to fly is still the ...


3

Multirotor propellers with too much flex can actually negatively affect thrust generation efficiency, but not in the way you might suspect. During high throttle punches, the propellers experience significant amounts of stress. If the propeller isn't stiff enough, the propeller can begin to flatten out across the chord, deforming and changing the shape of ...


3

The easiest way to think about this is to remember that drag increases with the velocity squared. A smaller propeller will have to spin faster to achieve the same thrust as a larger propeller, and spinning faster creates more more drag.


Only top voted, non community-wiki answers of a minimum length are eligible