Prop Tip Speeds Create Majority of Noise

You Can Help by Flying Friendly
By Syl Heumann
Do you know the diameter of your propeller? If you don?t, read on, because it has a very important effect on noise outside your aircraft. We are continually faced with noise complaints at airports, and some of them are warranted too. As the tip speed of your propeller approaches the transonic range, the noise level outside the aircraft takes a tremendous jump. The transonic range begins about 625 MPH, or Mach .85, and continues until the speed of sound which is 736 MPH (Mach 1) at sea level. This increase in noise is not apparent inside the aircraft (as outside). Beginning at approximately 625 MPH, the tip of the propeller is going fast enough to cause compression of the air and is causing at least two mini sonic booms per revelation.

No doubt you have heard aircrafts take off and noticed the noise level increases greatly just as the aircraft is opposite of your position. The reason is that the tips are at transonic speeds, and the sound is radiated on a path perpendicular to the path of the aircraft. The sound appears to drop off as soon as the aircraft has passed, but that disk of noise is following the aircraft, and will continue until the prop tip speed is reduced. It is important to understand that once the threshold of transonic speed is approached, any increase in RPM causes a dramatic increase in noise outside the aircraft.

To determine if your aircraft has a potential for excessive noise look at the chart on this page. Generally speaking, aircraft with 250 horsepower and above with two bladed props are the biggest offenders. Look closely at the chart to understand how important a 100 RPM reduction can be. A reduction of 100 RPM will usually reduce the excessive noise factor, and a reduction of 200 RPM is even better. Plan on reducing RPM as soon as you feel it is safe, or by 500? above ground level (AGL). The calculations do not include the forward speed of the aircraft because the effect is negligible.

The next time you your aircraft is in annual don?t forget to have the tachometer calibrated. A glance at the chart will show how important 100 RPM will be in reducing noise. If all pilots consider these suggestions, we will have fewer noise complaints which will make airport neighbors and pilots much happier.

Propeller Tip Speed- MPH