F3A--Noise.

[Troy Newman]

The sound meter doesn't add up all the noises...it measures the loudest one....If you want to reduce your noise level the first place to look today in our current pattern setup is the prop. 

Before the NATS and Teams this year I played with a sound meter and lots of props....The biggest change was prop and rpm...If I could reduce the diameter of the prop and or reduce the RPMS being turned the noise went down....Didn't matter what exhaust system we used..other than different exhausts were more or less restrictive and meant changes in RPM....Regardless....IF you break 400mph tip speed you are going to break the 94db(A) FAI rules for sound.....By the way a 17-12 APC at 7800-7900 is right on the border...a little wind or any wind and it tips the meter over the 94db(A) limit.

Now it is very possible that you can reduce the prop noise to the point where the airframe and or engine sounds are the loudest thing...but my testing showed that it was the prop that was the make or break noise factor...go to an old KF with the Prop tip speed chart. It will give you some numbers.....

Also there were certain prop designs that would break noise limits below the 400mph....The props with thinner blade tips like many of the 3 blades although more quiet in the air...really busted the sound test on the ground. And by the way some of these noisy props had some serious serious pull at lower RPMS compared tot he standard APC setups 16-12 to 17-13 APC on the 140DZ......We basically repeated the results on the OS 140's and the same props.....

There are no points for in the air noise...only a zero for the flight if you don't make noise on the ground. Ask Sean about his first flight at the TEAMS.

TN
  ----- Original Message ----- 
  From: Wayne Powell 
  To: discussion at nsrca.org 
  Sent: Friday, August 08, 2003 11:57 PM
  Subject: RE: F3A--Electric.


  Just thinking out loud here.  Isn't there more to the electric drivetrain than just the motor and prop?  I'd suggest the gearbox as a significant source of sound beyond the prop.  If the tip speed between the two is virtually identical, assuming similar design, there should be negligible difference in prop noise (within reason).  Taking it further, that would mean the exhaust and carb noise are equivalent to the noise created by the gearbox?  It would be neat to compare a direct drive set-up running a prop at the 395 mph tip speed vs. the geared system on a dB meter...

  Wayne Powell
  Bolly Props North America
  http://www.BollyProps.com
    -----Original Message-----
    From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org]On Behalf Of Troy Newman
    Sent: August 8, 2003 7:09 PM
    To: discussion at nsrca.org
    Subject: Re:F3A--Electric.


    Also some have claimed its silent flight....Tony said the sound meter was tipping 93db(A)...That means prop noise is our greatest noise issue....Who cares about in flight noise...the Sound meter is 3M away from the model....

    Just some FYI.....YS 140DZ turning a 17-12 at 7800rpm is 92-93db(A)...OS 140RX same 17-12 at 7800rpm is the same 92-93db(A) this is roughly 394mph tip speed....

    Jason is using a 22" prop at around 6000rpm its hit 93db(A) and if you calculate the tip speed its 392mph.

    So guess what prop noise is the kicker....its not silent flight when you are tested at 3meters.
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