A long time ago I started to silence the Carden, and today I managed to obtain some very good numbers. The weather was nice, the wind was calm, and I was off for the day. So, all ingredients were present to make it all happen. At the club we have built a heavy support on which to mount the plane to be tested. In the past I had tested this setup and had found that the power produced was enough to move and tip over this contraption. So, the largest part of the measurement session consisted of securing everything with spikes in the ground, etc. The plane itself is firmly strapped in place, I have an additional (temporary) anchoring point on the u/c, tied to the frame. I do not fancy the idea of something going flying at full power. Anyways, alle went well. At 25 meters/ 90 degrees downwind, according to the prescribed setup I measured 77dB, which is NotBad! The 45/135 degrees measuring stations also showed 77dB (tad less) and the one to the back of the plane only 56 dB. In all instances the exhausts are plainly visible to the dB meter. I had noticed in the past that the sound at the back and front of the plane is a lot less than you would expect. Which tells me I guess that most noise is produced by the prop-tips. The only thing I still need to worry about at some time is that in this test situation, the cylinder temps go a lot higher than when flying, even though nothing is blocked. Just goes to prove that just propeller wind is not the same as the airflow generated by flying. This is also the reason I had to wait for a cooler day, because initially I saw temps up to 180C! Now, with a modified hole at the botom of the cowl, it’s only 150C max. (In flight I see 130C max today. And yes, it is the left cylinder that is always warmer. Seams to be a feature of twins. Maybe one day I’ll try to add more ‘cool’ to that side. (or squeeze the other side, to get make it a bit higher.) As someone said: too much info, just fly!!
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?)
Our neighbouring club held it´s annual fly-in the other day. It´s a really laid back day where every one does what he likes to do, which is mostly fly..
I took the opportunity to bring along my Carden, just to get it used to ´foreign lands´. The club is really only a few miles away, and they do have a nice field, although it is lined with trees on one side, and a nice big tree on another corner.
For reason only know to my inner self I did not fly there, but enjoyed myself anyway. The next weekend it did become clear why I had this nagging feeling something was NotRight.
This is what was left of the throttle servo, I made a stupid mistake, I left the original choke/throttle contraption on the carb, which resulted in the wrong bits blocking free movement of the carb arm at the wrong time. At least it explains the ´low voltage´ warning I got! Lucky enough the batteries had enough oomph to perform a quick kill! The engine was at idle at the time, so other than a fried servo, no harm was done. Since then I´ve had a few fligths without problems. I also did some sound measurements: Getting 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, and you can hear that for sure. So, waiting for something with 3 blades now, and of course, keep the finger on the throttle!