Ask an Engineer:

My second Ask An Engineer column:

Why are the Fenestron (tail rotor) blades unevenly spaced?

The spacing between rotor blades (main or tail) is constant on most helicopters. The fundamental reason for this is that each blade affects the air immediately behind it in its plane of rotation. As a rotor blade moves through the air, it disturbs that air, mainly forcing it downward. The following blade, on moving through this disturbed air, produces less lift than it otherwise would (due to a reduction of what’s called the induced angle of attack, which I’ll discuss more in a future column). Since lift is a force (that pushes upward on the blades), production of lift stresses the blades, and more lift corresponds to higher stresses. The stress each blade feels, therefore, is related to the distance between it and the blade in front of it; the further apart two blades are, the greater lift a following blade produces, and the higher its stress. Any rotor blade that produces lift also produces turbulence in the air behind it, which increases stresses on the blade following it, but this stress increases the closer the blades are. Unfortunately, these effects don’t cancel each other out, which means that the stresses each blade feels vary depending partly on how close it is to the blade in front of it. It therefore makes the most sense to space blades evenly, so the stresses are evenly distributed, as uneven stresses increase wear, increasing maintenance costs.

The SA 340 Gazelle, a light utility helicopter, was the first designed with a Fenestron, and its blades are evenly spaced. (The manufacturer, by the way, was a French company called Sud Aviation, which became part of Aérospatiale, which become part of Eurocopter, which became part of Airbus, which is why so many Airbus helicopters–including our EC135–use Fenestrons instead of conventional tail rotors, and why very few others do.) The noise signature of the Gazelle, though, had a significant high-pitched wail to it, which only worsened when the Fenestron was attempted on later (larger) helicopters.

To learn why, it’s important to understand exactly what sound is. Sound is just waves of pressurized air; high pressure, followed a short time later by low pressure, followed by high, etc. If these waves follow each other in a regular pattern, we hear a constant tone. If the waves are closer together, that’s a higher-pitched tone. If the difference in pressure is high, we perceive that as a louder noise.

In the Gazelle, as each blade moved through the air, it disturbed the air as noted before. One byproduct of the production of lift is that the air above a blade has lower pressure than the air below it (that’s another way to think of lift: higher pressure below a blade, with lower pressure above, pushes the blade upward, as if there were an upward force on the blade itself). Since the air affected by one blade is quickly felt by the blade behind it, this pressure difference, to some extent, is transmitted from a leading blade to a following blade. This oncoming air is split, some going above the blade (where its pressure is reduced), and some going below, where its pressure is increased. In either case, though, air going through the rotor system experiences at least one cycle of pressure rise and drop. Note though, that when the spacing between the blades is the same, the ranges of times between the pressure cycles are approximately the same for each blade. This means that the range of sound frequencies generated by each blade is approximately the same, which in the case of the Gazelle just happened to average out to be uncomfortably high-pitched. And its Fenestron (like all later ones), though small, packed a punch, producing hundreds of pounds of force. This meant that the differences in pressure between the peak and trough of each wave were high, ensuring the wail would be loud.

The solution to this problem was exceedingly clever; the engineers just changed the spacing between the blades, such that some were closer together, and others further apart. That meant that each leading/following blade combination produced a different range of frequencies, such that no single one could dominate to create a wail. And since the blades themselves were small, and since there were so many of them (10 in our EC135, though others have between 8 and 18), the stresses that each created and felt were relatively small, so the difference in stresses between closely-spaced blades and those further apart was small, with the result that the stress-induced maintenance issues detailed earlier were almost nonexistent. This tradeoff of slightly higher maintenance and manufacturing costs for a much quieter noise signature was so favorable that every subsequent Fenestron design incorporated it.

Interestingly enough, uneven blade spacing isn’t only a feature of Fenestrons; the Boeing AH-64 Apache attack helicopter, though it has a conventional (unshrouded) tail rotor, unevenly spaces its four blades: two are 45° apart, and two 135°. In its case, designers were willing to pay the increased costs in maintenance and weight of an uneven design in order to lower the helicopter’s total noise signature, as an attack helicopter needs to be as quiet as possible.

Uneven Fenestron blade spacing is an excellent example of pure engineering genius; it solved a vexing problem without significantly increasing weight, or manufacturing or maintenance costs. Helicopters are extremely complex pieces of machinery, and it’s only due to instances of brilliant engineering such as this that they’re able to do all the amazing tasks we ask of them.