Noise characteristics are difficult to pin down. With a recip engine there are two different types of noise. Listening from the rear you hear mostly the exhaust and through this the sounds of combustion. From the front however what you hear is mostly mechanical, valves opening and closing (especially through the carburettor air inlet), the prop gearbox and other assorted noises. Now add a prop to this and you also get aerodynamic phenomena, such as airflow over the prop and shockwave effects at tips sometimes. With a contra-prop you will have interference of these aerodynamic bits so the sound will change somewhat. The third variable is airframe installation, as cowlings can dampen some sounds but also add a vibration spectrum to the whole game. For example you will hear more valve noise at the front with a short air inlet as on a radial, compared to a longer air scoop as on the Seafire. As mentioned above the exhaust is also a big factor as different lengths and shapes can have a large effect, think of the horns section in your local orchestra and how all those pipes make different noises.
Just a few thoughts.
Sign In
