When you are running a flight school, training future commercial pilots, you are making an important contribution to the national transportation system by alleviating the current pilot shortage.
That’s great. Nevertheless, sooner or later you will be the target of noise complaints. Usually fueled primarily by residents near your airport. Here is a typical example: https://flaglerlive.com/148245/flagler-aiport-noise/
While reduction of emissions is our number one priority, we are also working on the reduction of airplane noise. Interestingly, both goals can very well be pursued at the same time. And the results of our noise reduction efforts are as impressive as those for the carbon footprint minimization.
Let’s first take a look at what causes the noise of light aircraft powered by a piston engine and propeller. Basically, there a three sources of noise:
- The piston engine
- The propeller
- Aerodynamic noise
No. 3 is not so important. Aerodynamic noises are not what causes people to regard airplanes as annoying. When you watch a glider, there is not much to hear from the ground. Even powered airplanes are not that noisy anymore once they are on final approach with the engine idling.
The question is which of the other 2 items generates the noise. Most people would say it comes from the engine and the lack of efficient mufflers. Big surprise here, in most installations the propeller noise is perceived as more annoying than the engine noise.
The reason is that our propellers typically run at around 2,700 RPM on takeoff. This takes the propeller tips close to the speed of sound and creates in most cases more noise than the engine itself – even if at full power.
Why do our propellers run at such high speed? They could run much quieter if they ran at – let’s say – around 2,200 RPM.
Turns out that the RPM that our typical aircraft engines are rated for is a compromise. The engine could actually turn out more power at a higher RPM. But as we had not so good experience with reduction gears in the 1940s and 50s, it became common practice to bolt the propeller directly to the crankshaft of an aircraft engine.
An example is the IGO-480 series. Lycoming developed this engine in the early 50s based on the O-320. The O-320 is designed for a propeller bolted directly to the crankshaft. It is rated at 2,700 RPM maximum.
The IGO-480 is a geared engine (IGO = injection, geared, opposed). Although it uses many of the same components, it is now rated at 3,400 RPM at takeoff. The difference is the reduction gear. It makes the propeller run at only around 2,180 RPM at full power.
Unfortunately, because of shortcomings in metallurgy and design, geared engines had many problems in the 50s, 60s, and 70s. No manufacturer wanted to touch them anymore. Also because they acquired a bad reputation among pilots over the years.
In the early 80’s Rotax introduced their – at the time – new 912 series of light aircraft engines. They were geared from day one. Since 1982 they have built more than 40,000 of these. Rotax produced more aircraft engines than any other manufacturer in the last 40 years.
Today they have a good reputation in aviation circles. And they are a big step forward when it comes to noise reduction. Their propeller speed is only around 2,200 RPM on takeoff.
The Rotax 915 iS, the engine which will drive our new MySkyECO MS-1L, is even quieter. It has a new muffler to reduce the combustion noise even more and we are going to equip it with a newly developed 3-blade-propeller which is electronically kept a best performance and efficiency all the time (FADEC).
Our new aircraft will not only be a giant step forward when it comes to reducing the carbon footprint. It will be as big a step forward when it comes to noise reduction.
This holds true for our future projects as well. They will all have reduction gears. Even the electric MS-1e. The propeller will run probably even slower making it almost impossible to hear it while flying overhead.