Gyroplane noise

[Penguin]

When the SCCA started monitoring race car sound emissions, we quickly learned to point the exhaust tip away from where the sound monitoring equipment was set up.
I wonder if pointing the exhaust tip up would make it quieter on the ground.

[Heron]

Point it sideways!
No direct wave down, no rebound from rotor . . .
Heron

[WHY]

Steve McGowan used to have the loudest gyro around, now it's one of the quietest

Tony

[Redbaron]

Originally Posted by WHY

Steve McGowan used to have the loudest gyro around, now it's one of the quietest

Tony

Let's be real here tony me and you both want a tractor style gyro that is easy to build and safe and can be built under 100 hours :-) noise levels don't count. :-)

[ckurz7000]

Originally Posted by Hornet Flyer

However, I agree with Heron there is a possible resonance issue with the rotors. At least with 2 strokes, not so much with fours.

I would not expect any significant influence of the rotor on the sound emitted by the engine/exhaust. First, rotor blades are small in width compared to the wavelength of sound. Second, the rotation frequencies are much lower than engine rpm or firing rates of cylinders.

-- Chris.

[Resasi]

From purely personal observation gyro rotor noise is minimal and inconsequential...until loaded up in a tight turn when the rotor slap begins. This to me a delight to hear. The engine noise of a two stroke is more invasive than a four stroke, however muffling matters.

Ron Awads Yaminator at B days when he first flew it was delightfully loud, to those who like the sound of power. The next time he came he had become a 'good neighbo' and his engine sound was very acceptable to those 'picky' elements always present in society.

Peter is quite rightly concerned for we are looking for a new home to fly from and here in the UK, where space is a premium and the 'picky' are many, noise pollution is a pet hobby horse for the masses.

Gyros are not considered as very good neighbors. At our present location in the depths of the Cotswold countryside we still managed to get those who would write to the RAF to complain that their peace was being disturbed.

[bryancobb]

I remember an article on THIS VERY SUBJECT in Kitplanes back in the mid 90's. The INTAKE SILENCER that Rotax produced made the most noticeable difference, followed by their after muffler.

I remember there not being very much you can do about the prop noise because you MUST spin it at the right RPM or its efficiency goes off a cliff.

[Heron]

Pointed up, the sound will be deflected in part and be more noticeable.
The intake silencer is a jewell to me, that rattling is very anoying!
Remember rotors have their noise and deflect some air down!
A good tuned exaust is necessary!
For those around the patch there is not much to do! Noise will be there!
Prop noise is also anoying sometimes, I remember the echo from inside open hangars, like some wailing animal!
Heron

[SandL]

Hi again , just wondering if Steve Mcgowan can comment having gone through the process and ... just a thought , if a sound wave is circular, what happens if we change the shape, I bet someone somewhere has done some reasurch on it. Based on these posts I'm thinking about the posibility of not only directing the exhaust upwards but how about a change in the shape of the pipe end, elyptical or significantly flattened, or inserting some kind of spreading vanes in an expanded tail pipe. so I'm thinking that each "pop" would be disipated more. Also could aerodynamics around the pipe be enhanced, I'm now thinking about big jet high bypass angines are quieter than the older straight jets as were fitted to the likes of the VC10, could a venturi be fitted round a tailpipe, is there a benefit, if the tail pipe were pointed into the prop (assuming no burning) will it disipate the sound or will it produce a reverberating sound ? I believe the jet exhaust exhaust of the F117 being sent out of many small wing boxes much quieter than say a hornet. (dont know how the power/ engines compare and I realise it was done to reduce infra red, but maybe there is something there.
thanks all VERY helpful so far and much food for thought
Peter

[Vance]

In our testing the shape of the exit didn’t make much difference until it gets small enough where the boundary layer almost closes it up.

It is my opinion that is how a super trap works. The disks are close enough together to have the flow be restricted by the boundary layer and I feel it bends the waves. The annual discharge also changes the speed as it crosses from inside to outside which sort of changes the sound.

A two stroke exhaust is relatively uncomplicated but easy to screw up. If I had a two stroke to quiet I would use whatever the manufacturer recommended and save myself a lot of trouble and hard work.

Sometimes on a two stroke an inlet muffler will change the jetting because it captures the fog that stands off from the carburetor mouth and there will be very little loss in performance.

An inlet muffler on a two stroke is a simple thing but also easy to screw up. In my experience poorly designed one can make the tuning challenging.

I feel a properly designed after muffler can get rid of a lot of sound energy without much loss in performance.

I feel even with a very loud prop that a reduction in exhaust has reduction in the annoyance factor value because sound energy is cumulative.

We spent many hours on the dynamometer designing exhaust systems and we still just cut and try because we are always confused.

Testing is only fun for some unknown number of hours and after that dynamometer testing is just hard work.

That is why if it were me I would just take advantage of someone else’s hard work.

In my experience without a dynamometer and skilled operators it is nearly impossible to accurately tell how much power and engine has.

We once tested all the latest trickiest exhausts for a Yamaha RD350, a 21 cubic inch two stroke twin and in spite of everyone’s fantasy the stock mufflers produced close to the same peak power and always had a more progressive power curve. What people would feel in the seat of their pants was the sudden change in power, not the horsepower being produced.

An aircraft operates over a much narrower power band so it is a little easier to get a feel for the power.

The direction of the exit is very important but I feel even this can be confusing with the sound waves reflecting off of things and interacting with the propeller and propeller obstructions in unexpected ways.

Thank you, Vance

[eddie]

My subaru powered RAF has a 5 bladed prop with the exhaust going through the prop in the mid portion,the noise is an unsual sound, and is louder than normal.vance is correct is saying that the noise is louder. best regards,eddie

[WHY]

I'm not sure, but I think Steve McGowan had a Pro design his muffler, very effective too.

Tony

[Vance]

In my opinion rotary engines are hard to muffle and have a very hot exhaust that requires exotic materials to have a reasonable life.

I feel their un-muffled exhaust note is unpleasant.

In my opinion the exhaust on a gyroplane is usually short so there is no time to cool the hot exhaust of a rotary engine before the muffler.

Thank you, Vance