Believe it or not, at almost seven months into my Exec project and I’d never even heard a 162F engine, much less seen an Exec fly, even less flown in one but from day one I was quite surprised by the small dimensions of the Silencer - or Muffler to US builders. There are strict noise level limits for all UK Kit Cars and surprisingly, even stricter limits for Track Cars. I’ve learned from the many cars that we’ve submitted to noise level tests that there’s no real substitute for a large box if you want to maximise gas flow of a powerful engine whilst keeping it as quiet as possible.

The Rotorway ‘silencer’ is anything but a large box. It’s pretty-much a ‘straight through’ 2 1/2 inch exhaust with around ten inches of perforated tubes half way along its length, encased in a stainless casing of only 3 1/2 inches in diameter. There’s some wire wool packing between the perf and the case but it sure ain’t gonna do much in the way of reducing exhaust noise. It’s simple to achieve a massive reduction in exhaust noise level with a heavily baffled silencer but usually at the cost of some engine power. And from what I’ve read, engine power is something the Exec could use more of - not less. But, I was sure I could achieve a few decibels reduction in noise without changing the gas flow characteristics of the original silencer, just by increasing the size of the case around the perf tubes. However, what was not clear to me at this stage is how much of the overall noise output of an Exec can be pinned on the exhaust. I’m sure that the Main and Tail Rotor blades, Engine Induction and Mechanical Engine Noise all contribute greatly. There seems to be more than a little interest from other builders , and indeed Rotorway themselves, in making the Exec more ‘Neighborhood - friendly’ so I decided to sacrifice my factory silencer in the name of research to try and make it quieter. Doesn’t life have a funny way of changing tack? Thirty years ago my 200 Watt Marshall Stack just wasn't loud enough!!

My plasma cutter made light work of cutting away the casing. A cutting disc on a small angle grinder would be an alternative method.

Here’s half the casing removed exposing the wire wool packing inside

Most of the casing removed - just the ends left to grind away

Three lengths of 1 3/8” diameter perf tubes welded to a collector at either end. This is a good design feature. Three small perforated tubes will disturb the sound waves better than one large tube. Ideally, I’d like to see the perf tubes a few inches longer. It seems a waste of silencing potential to make them so short. I suppose it’s possible that Rotorway could have tuned the tube lengths and calculated the back pressure for maximum engine performance but I doubt it. I’ll leave them as they are for now anyhow.

Here are the new case components. I decided on a 5 inch diameter case made from 18 gauge stainless. The end caps are spun 18 gauge stainless. The one on the right has been machined to slide over the 2 1/2 inch tube. The original case is made from 16 gauge stainless. There’s really no reason why it should be so heavy. I expect the new silencer to be a little heavier than the original but It may be a price worth paying.

Ali foil is wrapped around the perfs so that the purging gas stays inside the tube during welding

GROUND TEST

My own engine was far from running so I took the new silencer to Jonathan and David at Southern Helicopters to test it on one of their ships. The particular ship was awaiting its permit renewal and we didn’t have a pilot anyway so the tests were conducted at 100% RPM on the ground. After a slight hiccup setting the ranges on the db meter I managed to get an encouraging set of measurements.

I took noise level measurements at four different positions with both silencers. The meter was held at approximately waist height horizontally at all times. The ambient noise level started at 55 db

1. At six inches distance 45 degrees from the end of the tailpipe.

2. At four feet distance 45 degrees from the end of the tailpipe.

3. Standing by the pilots door pointing the meter towards the front of the ship.

4. Thirty feet away pointing the meter at the ship.

Here are the results:

METER POSITION STANDARD db MODIFIED db 1 122 db 110 db 2 116 db 110 db 3 109 db 104 db 4 98 db 104 db

Immediately upon firing up the engine with the modified silencer we noticed a deeper exhaust note, less rasping and more pleasant to the ear. The close - up results appeared pretty much as I predicted with the biggest change being measured close to the exhaust outlet. Moving the meter away allowed rotor noise and mechanical engine noise to become a contributing factor, reducing the overall change.

The readings from the final test at 30 feet distance from the ship were a little confusing and, I think, an anomaly. The ship (and me) were standing on a concrete roadway with the brick wall of a huge barn behind us 100 feet away so I don’t think we were getting a true reading.

Southern Helicopters took more measurements at a later date from a hovering ship and failed to find much significant overall noise reduction at 20 to 50 feet away.

There are a few things to bear in mind here. The meter I used measures only decibels. I don’t know enough about sound characteristics and measuring equipment but it seems that reducing ‘nuisance’ frequencies would not necessarily result in a decibel reduction. A fly-by test using more sophisticated measuring equipment seems the next step. I’m pretty confident that the 12 db exhaust noise reduction and more pleasing note will be noticeable in the air but the potential is there for much more reduction. Gotta get some bigger guns involved.

Meanwhile, I’ve begun planning a Mk2 version which will have a more scientific baffle design. Experts tell me that it is possible to significantly reduce exhaust noise without sacrificing engine power - and possibly even improve it a little.

Here’s how it looks on my ship. It weighs in at 4.06 Kilograms - 0.5 Kg more than the original. I reckon I could get the Mk2 version down to the same weight as the original, or less.

SILENCER Mk3

Here it is - the Mk3 Exhaust. It actually weighs in at 4.2 Kilo  (0.7 Kg more than the original). Making the tailpipe in lighter wall tube would save loads of weight but it was just too expensive to buy stock for one job at this time. In comparison, the exhaust tailpipe on my Robinson Trainer is wafer thin.

I used a more sophisticated sound level meter this time and took the measurements with ‘C’ Weighting - the setting used for mechanical equipment and engines. The tests were conducted inside my workshop so comparisons with the earlier tests would not be safe.

I took two measurements on each silencer. All taken at 2200 rpm - engine idle speed. Main Rotor Blades not fitted:

1. At six inches distance 45 degrees from the end of the tailpipe.

2. At four feet distance 45 degrees from the end of the tailpipe.

Here are the results:

METER POSITION Mk 2 SILENCER Mk 3 SILENCER 1 117 db 110 db 2 106 db 104 db

It’s evident from previous ground tests that there is so much mechanical and wind noise from the Exec that even relatively large reductions in exhaust noise are almost lost in the overall noise level at 20 to 30 feet away.

I’m sure that the the exhaust noise reduction will be much more evident with the ship in flight. We shall see.

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