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Number off Blades
Once the blade area has been selected, the rotor designer must chose the number of blades. Since, for this choice, the aerodynamic considerations are relatively secondary, he will usually make his choice based on the conflicting recommendations of those specialists concerned with vibration, noise, weight, cost, and operational suitability. Some of their concerns can be organized in terms of the advantages shown below.
Advantages of Low Number of Blades Low rotor wreight Low rotor cost Ease of folding or storing Low vulnerability to combat damage High blade torsional stiffness
If these considerations do not lead to an immediate decision, the aerodynamicist might be asked for some recommendations. Unfortunately, these will also be conflicting; both as applied to hover and to forward flight.
As discussed in Chapter 1, a performance penalty in hover is caused by blade – vortex interaction. For a small number of blades, this should be less because the tip
vortex has a chance to get out of the way before the next blade passes by. This should provide a slight power advantage in hover. This advantage, however, might be lost if the small number of blades results in such a stubby shape that the tip loss region is a significant portion of the blade area. On the other hand, splitting the required blade area into many small chord blades might introduce Reynolds number penalties on maximum lift coefficient and skin friction if the chords are less than about 5 inches.
Some design teams limit the minimum aspect ratio of the blade outboard of the flapping hinge to about 12. This is done to ensure that the natural frequency of the second flapping mode will be below 3 cycles/revolution.
In forward flight, a potential advantage of a low number of blades—such as two—is in lower hub drag, as indicated by the hub drag survey presented in Chapter 4. Against this advantage is the disadvantage that the wake left by a rotor with a small number of blades is pulsating, which generates a higher induced power than the smoother wake left by a multibladed rotor. Some basis for this observation will be found in reference 10.2.
