BaldEagel

I have now repeated these tests in the open air of my flying field and not in the garden as before the results are supprising, I did not expect the reflected sound off the surrounding trees to make that much difference to the sound volume.

MVVS 58 Pro, 22x10 3W prop, 5900rpm J-TEC large pitts silencer 90DB at 7 Meters.
MVVS 58 Pro, 22x10 3W prop, 6300 rpm MTW 75K with 25mm dia 210mm manifold, 83 DB at 7 Meters

The open area took 2Db's off the pitts silencer and 3Db's off the cannister, I suppose that is logicall as the lower sound would have a lower reflective index? therefore an extra Db off. I don't think I have mentiond before this engine is mounted on Isolation mounts to help lower sound atenuation through the airframe.

Mike

pe reivers

If you have reflecting walls, the differences are even larger. Wind also may add some dB, so you should not measure if the wind surpasses a calm condition. Also, measurements over a hard surface like packed dirt or clay may increase readings by 3dB.
so measure over a grass surface on a calm day. Take care that the person holding the plane does not reflect or blanket the noise.
Noise measurement is not always conclusive and leaves room for error and interpretation.

edit:
BTW, your header is shortish. This makes the sound harsher, and throttling a bit more critical. Independent tests by Me and MTW showed 250mm as best header length for the TD75K muffler. The header for the TD75 can be shorter, because the inner mufler header part is longer.

BaldEagel

All very good stuff, so what you are saying is that the real sound level could be even lower, it does seem in the air to be much less noisy than others. My manifold length as you know is dictated by the distance I had to install the cannister in my 85" Yak the wing tube was in the way, but despite that the throttle responce is very good, but then I fly this mostly on less than WOT, its a wonderfull engine and so powerfull, must get a three blade prop for even less noise the trouble is the ground clearance does not lend itself to a prop over 22" may investigate a different UC.

Mike

pe reivers

a 22x10 3-blade prop will be about ideal at ~6000 rpm. In that case you would not have to change the UC
I found that 3W props put quite some extra load on the engine, and the thicker foil makes then slightly noisier than carbon props.

BaldEagel

Thanks for confirming my thoughts Pe, what make of 3 blade do you recommend? I can get Mejzlik, Engle and Biela quite easily? in 22x12 all, 22x14 Engle, 22x10 Mejzlik.

Mike

BaldEagel

Thought it about time to update this thread, I am still flying the QQ Yak with the MVVS 58Pro fitted with a 22x10 3 blade Mejzlik it seems to be an ideal combination, the power from this engine is so over the top for this airframe that even at take off its on part throttle and of course most of the flight also, more than very happy with this engine and airframe combination, at times I have trouble getting the Tx back from my pals at the flying site, I will have to have a word with the flying committee it seems that we are having too much fun.

Mike