Propellor noise

We have all heard the siren-like sound of an underpropped airplane : mostly this heart-ripping sound is worst when you are in line with the propellor, and it gets a little better when you are out of the plane of rotation.

Many propellor brands and types are available, some are better than others.

 Some people think that their propellor gets noisy when the tip of the blade approaches the speed of sound ( Mach 1). This is not correct, and luckily, this is not the case. As our props are not designed to go that fast, and neither are the airfoils that are used on them, this would lead to considerable damage. The chart below shows what the Mach number of the prop tip is for a given prop size and rpm.

 The air that moves over curved side of the propellor airfoil, however, goes a lot faster than the propellor tip itself because it has to follow a curved surface. The speed of the air that makes this movement could in some cases approach the speed of sound, leading to formation of shock waves, vibrations, noise and a drop in the efficiency of your propellor.

The airfoils that are used to make our propellors are simple flat-bottomed “Clark-Y”-type airfoils in the 10-12% thickness range,  and these typically have an extremely low critical Mach number ( 5-5,5 Mach).  In addition to that, the chord of a prop is extremely small, so the Reynolds number is low. On airfoil data charts, a non-linear drag rise can be seen around and past the critical Mach number, caused by shock wave formation. This effect is observed at slightly higher Mach number in the case of thinner airfoil sections, or when the leading edge is swept at the tips.

Propellor tip geometry plays a crucial role : swept-back tips with thin a

 Anyway, you should try to stay below Mach 0,5 for a very quiet setup, and certainly below 0,6 for a reasonably quiet setup. The chart below shows what this means for your setup. For in-flight conditions, add roughly 10% to the on-the-ground rpm of your engine.

A more technical explanation can be found in an article in the “Sound Task Force” pages of www.mini-iac.com.

 Keep the rpm down : if we use performance-enhancing measures, we should  try to use this to turn a larger prop, not to make the same prop faster. In many cases, we see that somebody used a 10x6 on a .46 size engine ( which is underpropped to start with), then added a tuned pipe, and now is boasting about another 1500-2000 rpm  rpm gain. Do not use a pipe to crank the rpms up, use it to turn a bigger prop at the same rpm. A bigger prop ( more pitch especially) means better efficiency.

The high static thrust that is obtained on the ground by a low-pitch, high rpm setup, could be worthless in the air because thrust will most drop when the flying speed goes up. A high pitch prop will see much less thrust drop when the flying speed goes up.

Remark : it might take lower nitro-content fuels or head shims to keep your engine from detonating ( and overheating) in these low-rev, high load situations.

 Use small diameter props with a high pitch wherever possible to keep the prop tip speed down. I am the first to admit that this approach does not always work for draggy airplanes, where maximum static thrust is important. 

Word of advice : in case of too high a pitch, the airfoil of the prop will be stalled when the flying speed is zero ( noise test on the ground), which is then leading again to higher noise readings. This kind of setup will also lead to poor vertical performance ( low thrust at low airspeed). This is another illustration of the interconnection between noise and efficiency : a stalled airfoil is inefficient, and gets rid of the supplied energy by generating noise instead of thrust.

 The word is that multi-bladed props are better than two-bladed props when it comes to noise. My experiments with 3 bladed props have been rather disappointing : I switched a 13x8 APC ( 2 blade) for a 12x8 ( 3 blade Graupner) on my Lanier Laser with an ST61K on a home-made pipe. With the same rpm reading ( 9800 rpm on the ground), I got the same noise reading for both setups ( 74 dBA at 7 meters), but clearly poorer performance in the case of the 3 bladed prop. My opinion is that the only advantage of a 3 bladed prop is that it brings the prop diameter down ( and thus prop tip speed down, leading to lower noise readings), but at the cost of a lower efficiency because the blades are working in each others downwash. Maybe a solution for the bigger engines, but definitely not the way I would go again for the smaller ones ( up to .60-.90 size).

 Keep away from the propellor, and not just for your fingers’ safety. The downwash that follows each blade of the propellor, is a pressure wave. The closer to the trailing edge of the propellor, the more powerful this pressure is. The wave hits everything in its way, creating turbulence. This means the carburetor, cowl, etc should be kept as far away as possible from the propellor arc. A spacer ring behind the propellor can bring the propellor as much as 10-12 mm forward, reducing the noise that is generated by the propellor wash hitting the nearby objects. A word of caution is in place : the spacer should run perfectly round, so should not create an unbalanced situation, which could in its turn lead to vibration and damage to the airframe or worse.

A large washer between prop and engine brings the prop further away from the airframe.

 Conclusions :

 Remember, if we hear noise, it is because we are spending energy to generate noise. It is better to use this energy for thrust. This motto is backed by my observations that the quietest propellors also turn out to be the most efficient ones.