Finally got a 3 blade prop, because of availability I selected an Elster 26*12. Thanks to the nice sunny weather I was able to do some empty field noise tests. The plane was placed at the center of the field. The air temp was 30C, wind between 2 and 4 m/s. Humidity around 80%. I performed 4 measurements at 25 meter distance, placed at 90 degrees around the plae. Of course it would have been nice to have the plane sitting 1 meter high, but that’s impossible without a lot of effort. Rear of the plane: 68-70 dBA, Front, 80-82 dBA, left and right in line with the prop also 79-82 dB. I only ran to engine to max rpm long enough to get a stable dB reading. RPM readings (per Jeti) were 5760). As with all these measurements, it is very inaccurate, it produces nothing more than a guestimate. (Meter +/- 2 dB, wind, geese you name it..) The only way to really measure is in a ”dead” room, and that’s not going to happen. This is all about getting some kind of feel for ”is it loud or not”. So my average would be between 77-79 dBA. at 25 meter. I feel that this is quite acceptable, when I compare it to some of our small electric models. (mine is quieter!) A single flight was performed, and I can honestly say that many times I could not hear the sound of the engine when flying less then full throttle. From the previous post:
..72 dB at 25 meters in front of the plane, but 84 at 90 degrees, in line with the prop. This is with a Mejzlik 28*10 2 blade, running at 6150 RPM (temp =20C or thereabouts). In the air I see values of almost 7000 RPM,
So in short, switching to a 3 blade cuts down the noise by half (-3 dB) and costs me 400 RPM. Performance wise I think I am ok. I still run the engine on the rich side, it needs further tweeking. (I’m forever tweaking, so what’s new?)