The Silent VTOL
May 22, 2019
Brian L. Hinman
Clover Aviation, Inc.
About twenty years ago, I had this crazy idea of commuting to work in a helicopter. I became obsessed with the dream of living on the scenic California coast, spending my work day in the heart of Silicon Valley, and looking out at the ocean for sunset dinners. For a number of reasons, I was ahead of my time, but I learned a lot about helicopters trying to realize my dream. I learned how to pilot them, I learned the physics of how they work, and I pondered a day when the flight controls would be “fly-by-wire,” much like the drones that were soon to appear. But the most important lesson I learned is how much people hate the noise of helicopters, and how noise and safety issues have limited the mass-scale adoption envisioned in the 1960s.
Do flying things have to make noise, whether birds, bees or helicopters? Surprisingly, the answer is no. While a housefly buzzing around the bedroom at night can be quite annoying, I was intrigued to learn that owls, in contrast, are almost completely silent in flight. How can an owl that weighs 50,000 times more than a housefly be quieter in flight? Pulkit Sagar and his colleagues answered this question in a paper they published in 2017. The owl has many unique mechanical attributes, including long velvet down feathers that absorb sound, wings with a serrated leading edge to reduce turbulence, and trailing edge fringe to reduce vortex generation. Combining these evolutionary advancements, silent flight gives the owl the unique ability to capture its prey by stealth.
If nature can achieve silent flight, why can’t we engineer aircraft to be silent as well? It turns out that we probably can. As far back as 1947, NASA engineers made modifications to a Stinson L-5E aircraft, toward the goal of creating an ultra-quiet aircraft suitable for reconnaissance missions. By reducing the propeller rpm (and hence tip speed), increasing the number of propeller blades from two to five, and muffling the engine noise, the modified aircraft sound level was reduced by 20dB (99% reduction in noise power) relative to the standard design. While not completely silent, the difference in sound level between the standard and modified aircraft was dramatic in overflight testing.
Reducing helicopter noise has proven more challenging. For many years, it’s been known that the noise level is generally proportionate to the rotor tip speed raised to the sixth power. While this seems like the obvious “knob” to turn, it’s not so straightforward to reduce the tip speed by an arbitrary amount. The helicopter configuration most commonly seen today was invented by Igor Sikorsky in 1939, consisting of a single main rotor and a single anti-torque tail rotor. In forward flight, the advancing blade of the main rotor increases in lift, while the retreating blade decreases in lift, creating a dissymmetry of lift across the rotor disk. Tricks are played in rotor design to try to minimize this effect, though it’s fundamental to the physics of the Sikorsky configuration. At high forward velocity, the retreating side of the rotor disk is in a stall state, producing no lift, thereby making the aircraft unstable. Hence, single rotor helicopter designs must maintain a tip speed of Mach 0.4 or higher to achieve an acceptable forward cruise velocity.
Given the limitations on tip speed in the Sikorsky architecture, there are interesting things that can be done to reduce noise by re-shaping the rotor blades. Traditional helicopter blades shed a strong vortex in rotation, with the air sticking to the blade and being flung off the end, not unlike what happens on swept-wing airplanes. When the next blade comes into contact with the vortex created by the predecessor, there is an undesirable “Blade Vortex Interaction” (BVI). The shock of the vortex striking the blade produces much of the characteristic helicopter sound we know and hate.
Several companies have developed advanced blades to address the BVI issue. Twenty years ago, Eurocopter introduced the “Blue Edge” blades that minimize the vortex shedding, while at the same time increasing the rotor efficiency. Keeping the rotor tip speed constant relative to the standard design, the real-world results were remarkably good. Measured results showed up to 5dB reduction in noise power, while at the same time increasing rotor efficiency by 7%. More recently, computational fluid dynamics (CFD) analysis has allowed high-fidelity optimization of rotor blades, balancing hover performance and forward flight performance. The recent work of Dr. Gunther Wilkein Germany, for example, brings helicopter blades a lot closer to the perfection of the owl’s wings.
As we move into the new era of electric VTOLs, there is an opportunity to exploit the full range of design optimizations to achieve virtually silent flight:
- Most of the designs in process have multiple rotors on either side of the main fuselage. A key advantage of multiple rotors is the ability to reduce the tip speed without concern for the dissymmetry of lift, since the lift on each side of the aircraft will remain constant in forward flight.
- Low “disk loading” is another key tool to reducing rotor noise. That is, the amount of weight per swept area of the rotors should be minimized, balanced by the need to maintain stability in wind-gust conditions. Early piston helicopters, such as the Bell 47 and Robinson R22, have disk loadings of about 2.6 lbs/ft2, mostly because of piston engine power limitations. With the advent of turbine helicopters, disk loading has tended to increase, causing increased noise. The Bell 407, as an example, has a maximum disk loading of 6.2 lbs/ft2.
- Advanced blade designs that minimize vortex creation are critical for new eVTOLs.
- Finally, the use of an electric motor itself is a huge reduction in noise relative to either piston or turbine engines, no matter how well they’re muffled.
Putting all the ideas together, the proposed industry standard that VTOLs must “blend into the background noise” of an urban environment is very attainable. Designed correctly, the VTOLs in process will reduce noise by more than 20dB relative to commercially available helicopters. Landing near-silent VTOLs in densely populated areas, without disturbing workers or residents, will shift public sentiment and allow an easing of restrictive local ordinances prohibiting helicopter landings. This will be essential to realize the vision of Urban Air Mobility that so many of us imagine for the future.
