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Keith Oakley - n/a
15-May-07, 08:33 AM
I've now collated the dba flyby measurements taken by ELS as each craft crossed the finish line at the recent Rother Valley National. The line was on a full power part of the course so the figures are a good guide, however the line was wide and on occasion craft took the corner wide and closer to the meter so some individual readings may be high. The average is probably the most reliable guide.



The limit is 96dba so obviously those with averages over that need to be working on the problem, particularly the top 3. From noise analysis so far silencers are the key problem. As well as ensuring any stuffing is up to scratch, in some cases bigger and better silencers are needed. You may also want to consider turning the exhaust into the plenum, although that brings it's own problems. Running the fan above 150m/sec tip speed also seems to generate excess noise.



no average loudest

39 99.5 105

5 99.2 106

33 98.3 103

316 97.7 102

2 97.4 102

16 97.4 103

50 97.3 100

34 96.7 103

32 96.6 102

1 96.3 101

37 94.9 101

42 94.5 98

110 93.4 95

144 92.8 98

76 92.6 94

83 92.6 98

100 92.6 95

160 92.5 93

55 92.2 104

52 92.1 94

140 92.1 98

70 92 98

54 91.8 97

150 91.7 99

88 91.6 94

36 91 96

169 91 98

53 90.9 97

78 90.8 95

194 90.7 98

J20 90.4 94

81 90.3 92

46 90.2 96

18 89.8 92

40 89.6 94

87 89.6 95

J19 89.6 92

145 89.4 98

118 89.3 96

72 88.9 95

117 88.3 93

133 88 97

191 87.1 92

201 83.5 96

J11 82.5 89

jon_curtis - n/a
15-May-07, 05:59 PM
are the louder craft generally the faster ones?

Keith Oakley - n/a
15-May-07, 06:31 PM
Yes - generally noise levels go up with bhp and tip speeds but there are some interesting questions there - eg nos 16 and 1, unquestionably the fastest craft (around 8secs/lap faster than 39 & 5) but are ranked 6th and 10th in the noise league.

jon_curtis - n/a
15-May-07, 08:38 PM
me thinks their setup is more efficent then ?

French Lake - n/a
15-May-07, 09:23 PM
We ought to be able to do some theoretical analysis of this.

By taking the craft's number, we can see from info available which class the craft is in, identify the engine type, fan size & speed blah blah

As Keith says, tip speed will make a difference, but so will tip clearance - any particularly out-of-round ducts will increase the effective noise levels, as will any other inefficiencies.

There are some useful rules of thumb for sizing of silencers - I need to dig them out, but they are along the lines of X times the engine capacity or Y times the hp (to give pretty much the same result) - a good big can could take 3 or 4 dB out of the readings without upsetting the back pressures.

Get these bits right and the difference is dramatic.

I'll try and pull some of this together and post it here - probably next week sometime as I've a skirt to finish and a pick-up to fix first!!

tonybroad - n/a
16-May-07, 07:13 AM
good work Keith - i think my results are somewhat skewed with laura using the same craft in juniors (less revs) but the peak is as expected



when you say noise is from silencers is that the end cans or the body of the exhaust itself or is that still under investigation



Tony

Keith Oakley - n/a
16-May-07, 07:46 AM
Good point Tony - looking quickly at the data Lauras FJ figs are mostly in the 80's whilst your F3 figs are 90's. Your NoiseCam analysis is in the 'difficult to decode' box at present, could you pm me with details of your expected rpm and gear ratio?



On the more general point yes it is too early to call exhaust outlet v exhaust body - I can see examples of both. But I think the general trend is that the exhaust outlet is noisier. I think the simple truth is we need more efficient silencers on many craft.