NAMBA April 2020 Web.pub

PROPWASH April 2020 5 How can you design a tuned pipe to use the finite amount of energy in the exhaust? The basic Wisniewski pipe article has the first factor. You need to space the convergent cone the right distance from the exhaust port. This depends on the speed of sound which depends on the average exhaust gas tempera- ture in the pipe. This is around 200 to 300 degrees centigrade for nitro engines, much cooler than gasoline engine exhaust. That tends to make nitro pipes long for the same rpm. The convergent cone’s reflected wave amplitude is directly proportional to the reflecting angle - steeper equals higher amplitude. Wave duration is directly proportional to cone length. A longer cone equals a longer duration. Fig. 4 - MACS Aero Pipe Designing the diffuser and the pipe maximum diameter is a different problem. The larger the center diameter, the lower the pressure the pipe can generate. There are several empirical formulas that have been developed from years of experience. An ideal diffuser acts like a horn. How quickly it flairs is described by the horn coefficient. This is approximated in most pipes by a series of diverging cones, each steeper than the previous. In the case of a larger belly diameter for the reflecting cone we get the best of both worlds, identical duration (same length) with higher amplitude (steeper cone angle). Normally to get steeper angles you would need to shorten the cone length, thereby automatically reducing the duration, and to some extent this relates to powerband width as well. Having a parallel mid-section means that you have no bias in the use of the finite wave energy available. However, going bigger means the diffuser is affected, thus reducing the available energy for the reflection intensity. The reflecting cone then by definition becomes steeper. However, that may not be what the engines dynamic system needs, and as less energy is available, the steeper rear cone will not in fact have any greater amplitude. Fig. 5 shows subtle changes in the shape of the converging cone area as well as the diffuser that can be used to custom tailor the pressure waves. (Continued on page 6)