Essentially a rotor is a rotating wing and the design of that wing generates lift. My question is this, why not increase the surface area of the rotor and slowdown the rotor speed. Rotor strength will also be increased. Slower rotor speeds will also reduce the turbulence created by the larger area of the rotor. I believe that this will also increase efficiency as the reduced speed can result in smaller engines being used to do the same work.

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    $\begingroup$ If you reduce the rotor speed you limit the forward speed of the aircraft because the retreating blade will stall earlier. Aviation is about the art of compromise. A slower rotor might be more efficient, but that's of no value if the helicopter is then too slow to be useful. $\endgroup$
    – user27769
    Jan 1 '18 at 10:13
  • $\begingroup$ It's just hard to make long and strong blades. And rotor speed has its own limit, too, which leaves the only option to be adding more blades. No one want to make a 20 blade rotor if they could get away with 2, yet you still see a lot of 5 blade helicopters. $\endgroup$ Jan 1 '18 at 21:59

Indeed a relatively large rotor has advantages:

  • Low induced velocities: less sand and bushes that are swept up by the rotor airstream.
  • Low autorotational rate of descent.
  • Low power required in the hover.

However, there are some disadvantages as well:

  • The size of the helicopter is less practical at landing, take-off, clearance of buildings etc.
  • The empty weight is higher, a very important factor in VTOL aircraft.
  • The hub drag is higher.

A smaller rotor will have a smaller and lighter hub and a lower overall parasitic drag: more efficient for cruising flight. There is a practical limit to blade radius of about 12 m due to static rotor droop when not spinning. From J. Gordon Leishman:

The static droop can increase quickly for larger rotor diameters and may cause problems when starting and stopping the rotor, especially in gusty wind conditions where the low centrifugal forces on the blades may lead to "blade sailing,"causing the rotor blades to flap and flex and perhaps impact the airframe.

The rotational speed of the rotor is determined by the tip speed, and is set such that at cruise speed the advancing blade tip stays below the drag divergence Mach number. Reducing the tip speed permits a higher flight speed to be achieved.

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There is a design optimum for rotor disk size, determined by weight. The weight over disk area ratio is the disk loading and the analogy with wing loading of fixed wing aircraft is very strong: a large wing area creates more drag and construction weight. The relationship between disk loading and operating weight is partly governed by the square-cube law, and a clear trend can be observed when the two parameters are plotted against each other: there is an optimum governed by weight, and the optimum is not the largest rotor diameter.


Rogers says,

Theoretically the most efficient propeller is a large diameter, slowly turning single blade propeller.

Generally, all else being equal, if you can get a larger-diameter propeller or fan on your aircraft, you will have greater efficiency. We see turbofans getting bigger and bigger and bigger partially for this reason. It also underscores why the AeroVelo Atlas has such large rotors.

So why not just stick giant props on everything? There are many factors, including

  • The practicality of manufacture. Can you build blades long enough and stiff enough without negating your extra efficiency with weight?
  • The arrangement of the aircraft. Can you fit the prop under the wing, for example? Or for a helicopter, will the blades have enough ground clearance at their tips when the rotor isn't spinning?
  • The intended airspeed of the aircraft. As Airsick mentions in a comment, on a helicopter you might get very little forward speed before stalling part of the blade. On a fixed-wing airplane, you'll more quickly run into supersonic flow at the blade tips (which causes unacceptable drag and noise) as you try to spin the propellor faster.

Any aircraft design is the result of rigorous and extensive trade-offs and optimizations. The points you make are valid, but they are usually constrained by other factors.


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