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RC series Airfoils

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Peter McCloud 03/02/2017 at 05:090 Comments

The previous rotor designs used generic NACA 2412 airfoils. These airfoils were developed over 80 years ago, but they are well documented and good general purpose airfoils. However to get the most thrust, airfoils specifically designed for rotors are required. Searching around led to the RC series of airfoils

RC series airfoils

The RC series of airfoils are specifically designed for rotorcraft applications. The naming scheme is RC(X)-YY where X is the number in the series and YY is the airfoil thickness in percent chord. Two of the airfoils, designed for the inboard section of rotor blades are the RC(4)-10, RC(5)-10. Data for these airfoils is found in:

NACA TP 3009 Aerodynamic Characteristics of Two Rotorcraft Airfoils Designed for Application to the Inboard Region

The other airfoil is the RC(6)-08 airfoil, designed for rotor tips. Data for this airfoil is documented in:

NASA TM 4264 Aerodynamic Characteristics of a Rotorcraft Airfoil Designed for the Tip Region of a Main Rotor Blade

Applying the new airfoils

Previously the rotor.py code was only written to use a single airfoil for the entire rotor. Recent updates allowed using two different airfoils, one a the tip and the second at the root. Changing the tip airfoil from the NACA 2412 to the RC(6)-08 means a 25% reduction in the chord. Since the bending moments are low at the tip, the thinner section doesn't affect the structural strength significantly.

Changing the root airfoil from the NACA 2412 to the RC(5)-10 airfoil means a 17% thinner airfoil. It may not seem like much, but the bending strength scales by the thickness cubed. Keeping the chord constant and changing from a 12% to 10% thick airfoil means a 42% reduction in bending strength.

To keep a similar strength with the thinner airfoil, the root chord will be increased from 4.5 to 5.4 inches to keep the overall thickness the same at the root. This leads to a negligible decrease in aerodynamic efficiency, but a big difference in the structural design.

So how big is the difference?

Rotor hover performance is measured by Figure of Merit (FM) vs. Efficiency. Hovering isn't doing any actual work so efficiency doesn't really apply. The FM is the ratio of the required power vs. the ideal required power. The previous rotors had a FM of about 60%. Analysis suggests the new rotors using the RC airfoils will have a FM of 80%.

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