From the Ray Prouty Archives: Considerations of Blade/Hub Geometry

By Ray Prouty | March 4, 2018
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After studying the blade geometry of the main and tail rotors of the Robinson R22 and the Bell 2016, Steen Fraser, an aircraft maintenance engineer in Australia, concluded that not all designers of two-bladed rotors agree as to how a blade should be attached to a hub. His conclusion generated a few questions.

Main-rotor offsets

Submitting drawings of the geometry of the two main-rotor blade attachments (Figure 51-1), Fraser asks:

  • Why are the feathering axis and the center of gravity (CG) offset on the Robinson blades?
  • Why does the Bell 206 have the bade-retention bolt in front of the feathering axis?
  • Why does the Bell 206 manual caution “Sweep blades aft, never forward”?

I was unable to come up with good answers so I sent the letter to the designers.

About the R22, Frank Robinson wrote:

“The 0.28-inch offset allows the blade centrifugal force to compensate for part of the rotor torque, thus reducing the moment carried by the pitch-change bearings.

“The 0.7-inch forward offset of the blade root places the blade root’s shear center closer to the center of the pitch-change bearings for structural reasons.”

Figure 51-2 shows how the centrifugal force acting at the forward shifted CT has a forward component that opposes the rear-pointing drag force.

On the same subject, C. E. Covington, Bell Helicopter’s chief of rotor system design, wrote:

“On the Robinson helicopter, the location of the blade CG ahead of the center of rotation is primarily to balance the steady drive torque with the blade centrifugal force.

“This ‘torque offset,’ as we call it, is incorporated in some Bell rotors, including the Model 412 four-bladed rotor, and the V-22 and XV-15 proprotors, and some tail rotors.

“The two-bladed main rotors — 206, 212, UH-1, 540 and 214 – have the feathering axis passing through the center of rotation and no torque offset is employed.”

A long time ago

Answering Fraser’s question about the location of the 206’s blade-retention bolt was a it harder, as the designers of the 206 rotor are no longer around.

“I believe,” Covington explained, “it was to allow the main bolt to pass directly through the spar extrusion. This would have no been the case had the bolt been located on the pitch-change axis.

“On the hubs with drag braces, this forward location causes a sharing of the centrifugal force between the main and drag-brace bolts. This keeps the drag-brace bolt loaded in tension, thus reducing fretting in the joints.

“I don’t believe there is any aeroelastic reason for the bolt location on the 206 [which does not have drag braces], since the blade is rigidly aligned in flight by the blade bolt and the root end latches.”

Blade sweep

Covington’s answer to Fraser’s question concerning sweeping the Bell 206 main rotor blades is short and to the point:

“The caution to sweep aft, never forward, is to avoid the possibility of a pitchup instability of a forward swept blade.”

The Bell 206 blade can be swept fore of aft using the sweep adjustment nuts. But on a swept-forward blade (or wing), the lift is acting in such a way as to twist the blade noseup, thus increasing its lift even more.

If the blade were not stiff enough, it could become unstable and twist off. This “aeroelastic instability” is what Bell is trying to prevent by cautioning not to sweep the blade forward.

Tail rotors, too

Also questioning the design of tail rotors, Fraser pointed out that the blades on the R22 and Schweizer Model 300 are swept forward in the direction of rotation about 3 deg. He asks if this is designed to reduce pedal forces and, if so, how?

Robinson responds:

“The tail-rotor blade’s quarter chord is forward of the pitch axis to offset part of the blade’s ‘tennis racket’ moment and the negative pitching moment due to camber.”

The tennis-racket effect (named for the feeling in the wrist when the racket is swung at an angle) is produced by centrifugal forces acting on the blade’s leading and trailing edges. It causes them to want to fly at flat pitch.

Without special provisions, such as a bungee or helper spring the pilot must exert steady pedal forces to hold the pitch required for generating the antitorque thrust.

Another factor, the R22 and the 300 use combered airfoils on the tail-rotor blades. These airfoils produce nosedown pitching moments, which also would require compensation with pedal forces. By sweeping the blades forward slightly, the lift acting ahead of the feathering axis can overcome these effects and help the pilot hold the pitch.

But isn’t forward sweep dangerous?

Why, you might ask, is forward sweep dangerous on main-rotor blades, but is used on tail blades? It is because, in contrast to main-rotor blades, tail-rotor blades are usually stubby and have sufficient stiffness to avoid instability for moderate amounts of forward sweep.

However, Bell has used a somewhat different scheme on its recent tail rotors. The OH-58D, for example, has a different alignment than previous Bell tail rotors and actually has some aft sweep.

“The OH-58D pitch-change axis,” Covington explains, “is at the blade’s one-third chord, so that the effective aerodynamic center (at the one-quarter chord position) helps the pilot pitch the blade up against the tennis-racket moment, acting in the same manner as the forward sweep in the R22 and Model 300 tail rotors.”

The discussion above illustrates some of the subtle considerations that a rotor engineer must consider to produce an effective and safe design.

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