This is one for the helicopter pilots and engineers in my network. I thought I knew why the tail rotor blades pitched when they flapped and what delta 3 hinges were all about on helicopters. Right up to the point I stopped to look carefully at the tail rotor on the Bell 429 I currently fly! The hinge is rigged in the opposite sense to everything I have flown before. When it flaps, the pitch increases too which seems madness as they seem complimentary and liable to snap the blade off!

As an example of what I mean, here is the delta-3 hinge on a Bell 429 – it seems to go the “wrong” way…

The conventional explanation for this phenomenon (called a delta-3 hinge) is that as the tail rotor blade flaps, it reduces its pitch to bring everything back to stability. This is a “positive” delta-3 hinge. But it turns out it’s not just Bell 429 that has a hinge which goes the other way. The UH60 and OH58D are the same and indeed any helicopter with the pitch change rod behind the blade. Let’s get into it.

First it’s time to get digging into the books – including the works of Prouty of course. I also dug out some work by the late Frank Robinson whilst he was Bell Flight in the 1970’s.

Hinges on rotor blades do not necessarily have to be perpendicular to the blade axis and the behaviour of the rotor can be tuned by angling hinges or moving control rods to provide coupling between the drag, feathering and flapping of blades.

Juan De La Cierva experimented with various axes during development of the autogyro in the 1930s and produced a naming convention to describe the hinges. The delta 3 hinge is a hinge where flapping and feathering motions are coupled with the hinge lying in the plane of rotation (delta refers to the feather/flap cross-couple and 3 refers to the plane of rotation).

Without a delta 3 hinge a degree of cross coupling is still likely on the rotor blades as the pitch rods remain fixed while the rotor blade flaps, inducing pitching or dragging motions. These motions can be undamped and oscillatory leading severe vibration. Cierva found that by change the angle of the hinge of the blade he could reduce the maximum flapping angle of his unpowered rotor which has a lot in common with the tail rotors of today.

The delta 3 hinge can be either at a positive angle (increased flapping results in decreased feather angle) or a negative angle (increased flapping results in increased feather angle).  A positive delta 3 angle is shown below. Note the pitch change rod is in front of the blade for this positive delta 3 unlike the Bell 429 shown at the start where the pitch change rod is behind the blade for a negative delta 3 angle.

Each option provides several advantages for the designer, particularly with reference to the tail rotor:

• Simpler layout of control rods in tail rotors with greater than 2 blades.
• Reduced vibration as the point of maximum flapping and maximum feathering can be phase shifted away from each other.
• Reduced flapping angles due to phase shifting the point of maximum flapping from maximum feathering with either positive or negative delta 3 hinges.
• Damping of tail boom oscillations dependent on the structure of the tail boom

Of course, as with everything on helicopters it is not a free ride, so the choice of hinge could also increase vibration. The choice of hinge is also heavily dependent on whether the blade has a hinge offset. A negative delta 3 with a hinge offset is a recipe for high vibration levels according to the research.

The delta 3 hinge can be created by introducing a change in the hinge itself or by locating the pitch change rod such that the motion of the blade as it flaps introduces the appropriate motion. Typically tail rotor pitch control rods are located in front of the blade for positive delta 3 effects or aft of the blade for negative delta 3 effects. The lead angle of positive delta 3 inputs tends to be larger than the lag angle of negative delta inputs due to the potential for further vibration effects at large negative delta 3 angles.

Summary

The really important finding though is that both positive and negative delta-3 hinges can reduce the vibration of the tail rotor and improve damping. Experimentation and the extant body of knowledge are likely to drive a particular design team to a certain solution for their aircraft. I wonder if there is a shared history between the engineers at Bell and Sikorsky leading them to go for negative delta-3?

Have you read my other articles?

• Lights, helipad, action! The problem with new helicopter pad lights
• Helicopter on Fire – Could accident investigators have learned more?
• The Ultimate Medical Helicopter – Selecting the right machine for HEMS
• Aerial teamwork – Practical tips for working together by staying apart in emergency aviation
• HEMS Landing Sites – Reliable places to drop your medics