# of Propeller Blades

[vikingBerserker]

What really determines the number of blades to be installed on an aircraft?

If you had a plane that normally had only 2 blades, could you stick 4 blades on and get a higher speed and better fuel economy?

 

[FLYBOYJ]

It depends on engine torque, RPM, available HP and even the number of engines. Engineers will determine the ultimate efficiency based on some of these factors.

 

[Stitch]

As FLYBOY said, it depends on a number of factors, the biggest of which is available HP; too many blades will make a "weak" engine inefficient. You can roughly judge the HP of any given reciprocating engine by how many blades it's propeller has; at the beginning of the War, the "average" HP of a fighter plane's engine was around 750 HP, and they typically only had two or three blades. By the end of the War, 2,000+ was not uncommon, so you saw a lot more 4 and 5-bladed propellers, particularly if they had a smaller diameter.

 

[Colin1]

So why did the Luftwaffe almost universally stick with 3-bladed screws?
Would a 4-bladed screw have made the Fw190D, Bf109K or Ta152H any faster?

 

[FLYBOYJ]

So why did the Luftwaffe almost universally stick with 3-bladed screws?
Would a 4-bladed screw have made the Fw190D, Bf109K or Ta152H any faster?

Hard to say - maybe faster but at what price? You never get something for nothing. A 4 bladed prop might result in a higher airspeed but at what fuel consumption? Although you have an extra blade you also have additional weight. Again something engineers would have to think long and hard about.

 

[Capt. Vick]

As far as efficiency is concerned, believe it or not a single blade prop is best. Unfortunately test with a counter weighted single blade prop were less then successful.

 

[vikingBerserker]

Thanks for the feedback, it was one of the 2am questions that popped into my head.

 

[vanir]

Germany tested some 4-bladed props for the Me-109K and Ta-152 aircraft (the Ta-153 was intended to have one and much of the Ta-152 was based on it but used Fw-190 airframes).

On high torque and/or high altitude engines they used wider paddles for the 3-blades. The Fw190A9 with its more powerful BMW801 variant was supposed to get wider blades (in reality most of these went to 190F9 attack models due to strategic priorities and the A9 got the A8 prop and so probably lost a little performance potential). All Ta152 preproduction series and prototypes can also be quickly distinguished from Fw190D variants by their much wider paddle blades, and of course their climb rates and altitude performance are fantastic (about 2m/s faster initial climb at sea level than a D9 for the 152C and even better for the H).
The British simply added another blade. Americans used a larger diameter prop. Any of these measures helps with overspeeding, climb and altitude performance.
As mentioned already and also to my understanding torque, engine operating speed and horsepower ratios, also aircraft operational altitude mostly define which type of prop is best.

 

[Mark Evan Salutin]

Hey Guys,

I have some experience to share with you taken from Hartzell Propellers, I believe it could help a lot:

If two-blade propellers are the most efficient, then why don't all propellers have two blades?

The short answer is because efficiency doesn't propel the airplane, thrust does. Efficiency is the ratio of the power coming out of the propeller to the power going into it. A two-blade propeller is capable of achieving a higher efficiency than a three-blade propeller and so on, but at the same time it uses less power and produces less thrust.
If you were to operate a propeller at a lower power setting than that for which the efficiency is at its peak, you would have a lower thrust and also a lower efficiency. Likewise if you operate at a higher power setting, the thrust will be higher but the efficiency is lower there also. There is therefore an optimum power setting for each propeller where its efficiency will be highest. If conditions require more thrust than is available from this optimum power setting, then the power must be increased and prop efficiency begins to fall off from its peak value. There reaches a point where a propeller operating at a power higher than that which results in peak efficiency has the same efficiency as a prop with more blades operating at less-than-optimum power. Further increases in power favor the performance of the propeller with more blades. This is because the propeller with fewer blades is no longer operating at its peak efficiency.

 

[wells]

If there were any guns that had to be synchronized to fire through the propeller arc, less blades = higher RoF

Structural limits largely determine how many blades. You can add blades, but then they become increasingly narrow, less rigid and may fail under loads. For a given diameter, more blades = more efficient. This has been known for a LONG time ( the 40s ). The only reason less blades might give you more thrust is because you can increase the diameter for a given blade area. In cases where the diameter is fixed, you want the highest number of blades, within structural limits.

 

[BillF]

Lets not forget too, that the more blades a prop has, the smaller the diameter the prop can be with the same level of thrust...

Whoops, I see Stich already kinda said this...