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Almost all propellers used for drones have very thin chord length compared to the propellers used for things like table fans or boats which seem to have blades with much longer chord length. What is the reason that drones most often use propellers with blades that have thinner chord length?

Common drone propeller shape:

drone propeller

Common table fan or boat propeller shape:

Boat propeller

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    $\begingroup$ This similar question explains the issues related to household fans. $\endgroup$
    – rcgldr
    May 8, 2020 at 15:43

2 Answers 2

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Overview

The most efficient propeller is single bladed, has infinitely narrow chord, has infinitely thin airfoil, has an infinitely long blade, spins infinitely slowly, and requires infinite torque. So at the risk of stating the obvious, propeller design is a study of the tradeoffs and compromises required to make something practical. Here's a list of some compromises:

  • length: landing gear height (or water depth) restricts how long a blade can be.
  • material constraints: thicker airfoils resist forces better. Stronger materials allow for thinner airfoils while still meeting required blade strength
  • power-plant constraints: the torque required to spin the propeller should match the motor/engine's efficiency and power curves.
  • number of blades: more blades equal more thrust for a same diameter
  • increased activity factor (aka blade width): wider blades produce more thrust for the same diameter
  • balancing: one prop is very hard to balance without causing lots of drag by the opposing mass.
  • vibration: three props give fewer vibrations than two
  • sound pollution: smaller props spinning more slowly are more pleasing on the ear
  • consumer perception: consumers like to see big blades in their fans, regardless of real-world efficiency and performance.
  • density of the fluid:
    • dense fluids such as water can cause cavitation, which can quickly damage the propeller. One solution is to change tip shape so as to minimize cavitation likelihood.
    • on the opposite scale, with low-density fluids such as air, the blade tip can spin so quickly it stalls.

As it relates to drones

Rotorcraft are a little special, so let's break out by type:

Vertical propulsion

Multirotor drones and helicopters have tons of space compared to boats, and so don't particularly need to restrict their diameter. They choose their rotors based primarily on power-plant performance.

Longitudinal propulsion

Fixed-wing drones with landing gear obey the same rules as full-scale aircraft. If the prop is going to hit the ground on takeoff it's not very useful!

Fixed-wing drones without landing gear, e.g. hand-launch gliders, can use very large folding propellers. These are chosen similarly to multirotors, where it's about efficient power-plant matching.

As pointed out by @RobinBennett, "You don't want it stalled when the aircraft is stationary, so there's a maximum pitch angle - and that sets the maximum speed. If you want to go faster you need to spin the prop faster, and if you've got a fixed amount of power, you need a smaller prop just to spin it faster."

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    $\begingroup$ The other major compromise (and possibly bigger than all the others) is that props need to work across a speed range. You don't want it stalled when the aircraft is stationary, so there's a maximum pitch angle - and that sets the maximum speed. If you want to go faster you need to spin the prop faster, and if you've got a fixed amount of power, you need a smaller prop just to spin it faster. $\endgroup$
    – Robin Bennett
    May 7, 2020 at 14:53
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    $\begingroup$ Good point, updated answer. $\endgroup$
    – Kenn Sebesta
    May 7, 2020 at 16:12
  • $\begingroup$ "You don't want it stalled when the aircraft is stationary". F5B models, are short burst, high powered, high speed glider type models, and the props have a lot of pitch (for the high speed). These props will stall when the glider is stationary, but it's not an issue since the models are thrown when launching, and the excessive amount of power means there is still some thrust even when the prop is stalled. $\endgroup$
    – rcgldr
    May 8, 2020 at 15:10
  • $\begingroup$ That's good to know. I don't think it changes the answer, though, as the stall characteristic still is not desirable. It is instead a result of other compromises. BTW, the tubercles (aka sawtooth) profile for F5D blades is pretty cool! $\endgroup$
    – Kenn Sebesta
    May 8, 2020 at 17:48
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As is explained briefly here on Aviation.SE and here on Physics.SE, the differences in chord length (and other properties of the propellers like diameter and pitch) are mostly derived from the fact that water is ~800x denser than air and results in different optimal properties for highly efficient propellers.

Boat propellers generally are constrained in diameter to limit the draft (maximum depth) of the vessel and thus would have to spin faster to generate the required thrust. This results in large pressure gradients across the surface of the propeller which cause cavitation losses. These pressure gradients are reduced by increased blade count and chord length, which provides more surface area for the water react against and allows the prop to be spun slower.

cavitating boat prop (cit.)

Image of cavitation on a nautical propeller, where large pressure gradients across the propeller surface cause bubbles of low-pressure water which drain efficiency and can result in damage to the propeller upon collapse.

Similarly with desk fans, which are limited in diameter by the available space, and speed (to reduce noise) so blade area is increased to move more air (efficiency isn't as much of a concern).

Multirotors don't have the same diameter limits (having the tips not meet or exceed Mach 1 is the concern) removing the need for high chord lengths.

Relevant Reddit discussion

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  • $\begingroup$ A better way to look at this is that lots of fat blades are just what you have to do get enough thrust when you're limited by diameter. Also, another reason for limiting diameter on small boats is keeping the blades strong enough to handle hitting things. $\endgroup$
    – Robin Bennett
    May 7, 2020 at 14:46
  • $\begingroup$ @RobinBennett That's what I meant to say, did I not? $\endgroup$
    – ifconfig ♦
    May 7, 2020 at 16:18
  • $\begingroup$ While this is a good answer regarding boat propellers, it doesn't answer the question also asked by the OP about why table fans also use propellers with a high chord length, even though they're pushing air, not water. $\endgroup$
    – Ilmari Karonen
    May 8, 2020 at 12:55
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    $\begingroup$ @IlmariKaronen - fans are also limited in diameter and speed (for noise). They don't really care about efficiency but they do want to move lots of air. Big, fat blades move more air but have large tip losses because their tips are so big. $\endgroup$
    – Robin Bennett
    May 9, 2020 at 17:24
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    $\begingroup$ @ifconfig - I've had a try - what do you think? $\endgroup$
    – Robin Bennett
    May 11, 2020 at 10:28

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