Is 1 propeller always more efficient than 2?

Question

Based on other answers I've read here (1) (2) (3), a single large blade propeller is the most efficient type of thrust you can get.

There are 2 reasons for this (as I understand it). Firstly, because each blade on the propeller causes wing-tip vortices when low pressure air meets high pressure air.

Secondly, each blade on the propeller adds surface area to the aircraft increasing parasitic drag.

So my question is what if both propellers are surrounded by a duct? Assuming it's a perfect fit, the duct will neutralize wing-tip vortices, so now all that's left is parasitic drag.

Now couldn't you just use 2 propellers that equal the surface area of a single large propeller and get the same efficiency with no added parasitic drag?

On a similar note, could you use just 1 small propeller surrounded by a duct and reach similar efficiency (if not better) than a large propeller surrounded by the same duct?

I say "if not better" because the smaller propeller would have less surface area, thus less parasitic drag.

Answer 1

Ducts are only used for highly loaded propellers of a very small radius, namely the fan stage of turbofan engines. For lower disk loadings a shroud or duct would create more friction drag than it saves in efficiency.

If the tip Mach number of the propeller is low enough (say below Mach 0.9), then the single propeller of maximum diameter will be the most efficient way to create thrust. This condition is met when both flight speed and thrust per propeller disk area (disk loading) are low.

As the aircraft gets faster, the tip Mach number can best be limited by reducing the propeller diameter. Now more blades will be needed in order to create the desired thrust, and interference between them will reduce efficiency. Also, the higher disk loading will result in more swirl losses, and now it becomes attractive to run a second, counter-rotating propeller behind the first. The two can be either close together or - even better - on opposite ends of the aircraft.

Answer 2

Looking at a static turning propeller blade, as running on a test bench, it is easy to imagine how a one bladed propeller would be most efficient, and how multiblades would not have "clean" airflow due to blade turbulence. This is indeed true and very noticable when one compares fans with props and is popular theory.

However, it is important to realize a prop moving through air follows a helical path in 3 dimensions. If it is travelling fast enough, than 2 blades will not interfere. This would be an excellent wind tunnel project. Also, pitch would play a role, and studying pitch vs "minimum interference speed for x number of blades at various pitch conditions" would be my dissertation du jour, or for many months.

Secondly, single blade props are very tough on bearings and have been known to suffer vibration issues, which are to be avoided as plague in aircraft.

The 2 blade prop can be perfectly balanced and is very efficient. It is no surprise that it is the blade of choice for light aircraft, with 3 blade also fairly common when ground clearance becomes an issue with more powerful engines.

The 1 blade, although actually applied with low power engines in the past (Piper Cub) yielding some improvement in efficiency, must be balanced with counter weight.