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, all 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 bottom 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 make it a bit higher.) As someone said: too much info, just fly!!
I found myself volunteering for a role in the Noisy-committee of the club. We have to abide by the rules, so the first question is ”what are the rules?”
Following is just a collection of bits and pieces, just to get started.
Excel sheet for calcs (download to your pc)
All about audio: audio-stuff
Paragraph 9: 60 dBa at the nearest house!! Which is rather an important fact. Whivh would translate into 78dB(A) at our site!
Equipment: anything under 200 Euronen is not worth looking at.. It needs to be a Class 2 certified one.
Laserliner (various prices, from 160-270, so shop around. ) Can be calibrated!! wich is Very Important! Of course those calibrators are not cheap: calibrator. Why you need one (This site has a lot of good ”howto” info.
We also have a very high back ground noise at the field, caused by the Betuwe Lijn. I have measured in excess of 60 dBA when the train passes on a quit afternoon. The houses that we want to ”protect” are much closer to the railway, so they already suffer from much higher noise then whatever we can add. That’s another story!
These guys did a lot of measurements, so use them to our advantage 😉
Worth mentioning is that the allowed noise level at this location is 59dBA during daylight hours. (in the LautstärkeGleichenkarte Elten above.)
So, what is the max level that we are theoretically allowed to produce to reach the 59dBA level?? The distance to the houses is 200 meter (centre of field), therefore a max level is 77dBA is theoretically allowable. This is not to say that we should be lower!
Of course, knowing the level does not help you any further if you want to fix_it. Some kind of octave-filter app is almost a must, but most I-phone/Androids have plenty gadgets to held you find the offending frequencies. (Needless to say, it almost always is the prop/exhaust noise that is the cause of the problem.)
Procedure: from https://www.mfc-hameln-lachem.de/flugsicherheit/laermmessung
Die Messungen sind unter folgenden Bedingungen durchzuführen:
- Das Flugmodell ist so zu positionieren, dass sich der im Abschnitt Lärmmesspunkte definierte Bezugspunkt in einer Höhe von 1 m ± 0,1 m über dem Boden befindet und die Flugzeuglängsachse parallel zum Boden verläuft. Das Luftfahrtbundesamt kann in Sonderfällen eine andere Aufstellungsgenehmigen.
- Zur Vermeidung von Reflexionen dürfen in einem Umkreis von 30 m um das Mikrofon sowie um das Flugmodell keine die Messung beeinflussenden Gegenstände vorhanden sein.
- Die Lärmmessung muss auf einem kurzgemähten Grasboden erfolgen.
- Das Flugmodell ist so zu positionieren, dass sich die Flugzeuglängsachse in einem Winkel von 90° ± 30° zur Windrichtung befindet. Die Lärmmessung hat auf der zum Wind abgewandten Seite des Modells zu erfolgen.
- Die Windgeschwindigkeit darf 5m/sec nicht überschreiten.
- Die Messung muss an jedem Punkt über einen Zeitraum von mindestens 30 s erfolgen; maßgebend ist der höchste in diesem Zeitraum gemessene Pegel.
- Die Umgebungstemperatur muss zwischen 10°C und 30° C liegen; keinNiederschlag.
- Das Umgebungsgeräusch muss mindestens 10 dB(A) unter dem vom Modellerzeugten Geräusch liegen.
- Die Messung muss bei Vollgas erfolgen. Eine Limitierung von Leistung und Drehzahl, die zur Erfüllung der Lärmschutzforderungen vorgenommen wird, ist nicht erlaubt.
- Für die Messung muss ein Präzisionsschallpegelmesser nach DIN EN 60651 oder nach DIN EN 60804, in beiden Fällen mindestens Klasse 2, in der Betriebsart „langsam“ („slow“) und im Anzeigemodus „dB(A)“ verwendet werden. Die Kalibrierung der Messanlage mit einem akustischen Schalldrucknormal zur Überprüfung der Empfindlichkeit der Anlage und zur Ermittlung des Bezugspegels darf nicht länger als zwei Jahre zurückliegen.
Auf unserem Gelände gelten folgende Werte:
- Kolbenmotor 82 dB (A) >> in our situation it is 78
- Turbine 90 dB (A)
Also part of the whole thing a flightlog? (automated/RaspberryPi?)
Find a meter with a usb connection to a laptop and use USB extenders. (For safety mostly)
Make a decent test stand
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!