How does a propeller with more blades create more “grip”

I have heard that the more blades a prop has, the more "grip" it has in the air at the expense of speed.

How do more blades create more grip but lose top speed?

Aside from fancy material science which can result in propellers that interact with air more cleanly and is highly proprietary, the primary way a propeller could gain more "grip" on the air is by being more aggressively pitched or having more blades.

In a propeller spec which can be displayed either as:

• [diameter in tenths of an inch][pitch in tenths of an inch], e.g. 5042
• [diameter in inches]x[pitch in inches]x[number of blades per prop], e.g. 5x4.2x3

the pitch of the propeller theoretically represents how far forward the propeller would move over the course of one rotation if it were curring through a solid material. More aggressively (higher) pitched propellers can be said to "grip" the air more than less aggressive pitches because they will go further per rotation.

The same is true for high blade count propellers, which will also go further per rotation due to the fact that there's more surface area for air to exert a reaction force against.

But I think that the idea that propellers need to trade increased pitch and increased blade count for rotation speed is technically wrong. Just like how saying that higher kV motors require lower-voltage batteries isn't technically correct but in practice holds true, I believe the same holds true for propellers.

It is true that aggressively pitched and higher blade count propellers require more powerful motors to drive because they do more work per rotation. This is the reason why in practice these propellers are usually paired with lower-kV motors which spin slower per volt because they require less rotational velocity to do the same amount of work.

It should be noted that some (well-designed) propellers mitigate the increased work required to spin propellers with more than three blades by decreasing the chord length of each blade, thus decreasing the area of the blade and the work required to rotate it. This is where reading the spec sheet of the propellers you want to use and taking a careful look at the aerofoil geometry is key. :)

(cit.)

However, nothing would stop you from pairing a high-voltage battery with a high-kV motor and an aggressively pitched propeller with many blades. The setup would, of course, consume ungodly amounts of power and require some pretty special materials to ensure the propeller doesn't shatter from the intense forces it would feel at those high speeds, but it could technically be done and there isn't anything wrong with doing so.

• One other thing to add is that frequently blade chord decreases as blade count increases. This can help mitigate the effect of blade disturbing the air so there's an optimization which can be done here (although frequently not done by low-end model manufacturers). – Kenn Sebesta Apr 22 '20 at 20:47
• @KennSebesta Good point! I'll add that soon when I get the chance. – ifconfig Apr 22 '20 at 20:56