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Regardless of the overall design configuration of a helicopter, it appears that helicopters have main rotor blades that all fall into the class of ultrahigh aspect-ratio wing design: that is, you never see a modern helo swinging blades around that look like the wings of a piper cub, for example. What are the design principles that result in thin, long-span, narrow chord main rotor blades rotating relatively quickly for every helicopter I can remember seeing?

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  • $\begingroup$ Check out the Boeing X50 Dragonfly. $\endgroup$
    – Jim
    Commented Dec 6, 2021 at 15:13

2 Answers 2

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Unlike with fixed-wing aircraft, there are very few structural limitations on the planform of helicopter blades.

enter image description here

Pic above was used in this answer and shows the centrifugal force acting on the blades. The blade is hinged to the rotor hub, and from the angle relative to the rotor hub we can see that the lift force on the blade in vertical direction is much lower than $F_C$. That allows the helicopter to use blades with very high aspect ratio, which is aerodynamically the most efficient for a given lift area.

A rotor like in the pic has zero bending moment at the root: it has a hinge. Fixed hub blades do experience a root bending moment but this is also comfortable evened out by the centrifugal forces.

In general, the design principles for a helicopter rotor are:

  1. For a given design gross weight & range, establish a practical rotor disk loading. This results in the rotor disk area => blade length.
  2. Rotor rotational speed is as high as possible, without entering critical Mach on the forward sweeping blade in cruise.
  3. Number of blades is the minimum we can get away with, Rotors with the lowest solidity have the highest efficiency, increasing the number of blades does have some advantages like lower rotor induced vibration and a less distinctive noise signature.

Regarding the last point: the Wop Wop of the UH-1 was a well known distinctive feature. The Huey had two blades with a larger chord than usual: due to the teetering hub design, number of blades was limited to 2, and in order to accommodate the required disk loading the rotor blades have a wider chord than on similarly sized fully articulated rotors.

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  • $\begingroup$ What is the relationship between teetering limits and blade length that limits blade length? $\endgroup$
    – John K
    Commented Dec 6, 2021 at 4:57
  • $\begingroup$ @JohnK Was distracted, have amended, thx. $\endgroup$
    – Koyovis
    Commented Dec 6, 2021 at 5:12
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    $\begingroup$ A shame that you didn't get an upvote when the answer was accepted. +1. $\endgroup$ Commented Dec 6, 2021 at 12:10
  • $\begingroup$ I would remove the statement "Unlike with fixed-wing aircraft, there are very few structural limitations on the planform of helicopter blades." Both have structural requirements, but most of those can be addressed thanks to the wide availability of a variety of materials. The limitations on fixed wing and helicopter blade planforms are governed more by design requirements, rather than structural integrity. There are many examples of high AR aircraft available that show that structural limitations can be overcome for fixed wing high AR aircraft. $\endgroup$
    – MishaP
    Commented Dec 8, 2021 at 10:25
  • $\begingroup$ @MishaP The statement is exactly the reason why. Wing /blade load and structural integrity iare the primary design criteria. $\endgroup$
    – Koyovis
    Commented Dec 8, 2021 at 22:15
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comment under question: "check out the ... X 50 Dragonfly"

Well, both crashed. Reason: rotor created too much turbulence around the fuselage, creating an uncontrollable pitch up tendency.

In addition to @Koyovis answer, a very high aspect ratio (allowed by the "centrifugal force" on the blades as they turn) reduces turbulence, thereby reduces interference with the following blade (and anything else that happens to be around).

The reduction of turbulence per unit rotor lift produced is by the same principle of reducing drag in high aspect wings: smaller, weaker trailing edge and tip vortices. This is also why high aspect propellers, as few as possible, are most efficient.

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