Why does a propeller have three blades?

[Mjwomack]

I've a feeling I might not understand the answer, but I was wondering, why does a propeller usually have three blades?

Instinctively two blades doesn't look very powerful, four looks like it should be easier to balance than three, so why is three the 'default' number of blades on a propeller?

Simple answers appreciated!

[bhouse]

It's a compromise and it's complicated...

But as it was first explained to me, 

• The fewer blades a prop has, the greater its aerodynamic efficiency can be
• As engine power increases, greater diameter or more blades are needed to absorb the power
• Greater diameter needs greater ground clearance, so as power increases, the generally preferred solution is to add more blades and accept the reduced efficiency

There are other factors (eg tip speed, structural strength etc) that must also be considered, but it can be shown with maths (or one can just accept!) that aircraft of moderate performance perform best with three blades.

 

Give me a week or two if you'd like the full explanation!

[Mjwomack]

Thanks, that's my level of understanding of technical things!

' it can be shown with maths (or one can just accept!)'  - I'm a very trusting soul!

[Giorgio N]

In reality there is no default number of blades on a propeller and there have been props with several number of blades, 2, 3, 4, 5 or even more...

As Brian said, the final choice of the characteristics of a propeller for a certain aircraft is a compromise, and yes, it is complicated.

Let's start from the first thing to consider: power. The propelle will have to convert the power from the engine into a thrust given by the aerodynamic forces acting on the propeller blades. If we look at the blades as if they were wings, we can see that if we want a certain lift we need to operate on one or both of two factors: surface and speed. The surface will depend on how many blades I have and how large these are. The speed will depend on the RPM at which the propeller is operating.

Leaving speed out of the equation for a moment and focusing on surface, it's clear that having more blades gives more surface, reason why a 2-blade propeller is rarely seen on powerful types. Of course in theory it is possible to have a large 2-blade propeller, but this means an increase in propeller diameter, that creates other problems and is not generally desired. A 2-blade propeller also generates more vibration than a prop with more blades.

Having more blades however also means introducing more interference between the air flow around the blades, and this is the reason why it is quite rare to see propellers with more than 4 blades and really no traditional propeller has more than 5. Props with more than 5 blades generally use "scimitar" shaped blades that minimize interference.

As a result we have that historically most aircraft above a certain power level have always used 3 or 4-bladed propellers. that are in the end the best compromise between prop diameter and efficiency. One of the question most often asked when talking about WW2 prop designs is if it's best ot have 3 or 4 blades, and the answer is that there's no best, both have pros and cons.

We should also keep in mind that the surface of a propeller can be increased by making the blade wider, and this has been used quite often, just think of the "paddle" blades used on the DH Mosquito or Lancaster or to the very wide blades used by certain late war German types. A wider chord blade has pros and cons (for example higher drag), so again it's a matter of finding the best compromise. German engineers relied heavily on 3-bladed propellers, constantly increasing the chord of the blades. British and US designers standardised on 4 blades (with the exception of a few 5-blade design). In the postwar era really 4 blades have become the norm for the more powerful engines until the advent of the more advanced multi-blade designs

 

I have left speed behind in the discussion... there are two issues regarding speed: one is efficiency as a slower propeller has greater efficiency. Of course efficiency alone does not generate thrust so ideally it's best to have a propeller moving at the speed that gives the highest thrust. Speed however can not be increased over a certain point that is when the tips will enter transonic regime, This is not desirable as induces loss of lift on the tip section and can induce structural problems. Keep in mind that the speed we are considering is the vector sum of the speed of the aircraft and the rotational speed of the tip, reason why propellers have a hard time on fast aircraft. This is also the reason why an aircraft like the Tupolev Tu-95 has propellers turning at very slow speed.

Maximum diameter is also affected by these speed limitations, as a larger propeller at the same RPM will see higher velocity on the tips compared to a smaller one. Of course maximum diameter is affected by geometric limitations, An example of this problem is the F4U Corsair, where the wing had to be designed as it is to allow the propeller to have the required clearance (and a few other reasons).

 

So there's no real best propeller for every aircraft. Each solution is best suited to a certain range of power, with 2-blade propellers today only seen on very low power types and 3-blade on the more powerful ones. The introduction of scimitar blades has led to the use of 6 or more blade designs on the most powerful turboprop types and this has led to a decline in the use of 4-blade propellers, that until the '80s were the norm on these aircraft.