Ever taken a good look at the engines of planes like the A340-300s, A330-300s, or some A320 and A321s? You might have been surprised to notice a peculiar resemblance to something quite unexpected – a common household hair dryer. These jet engines, with their distinct shapes, don’t just boast an intriguing design, they serve a very specific purpose. The shape helps to improve thrust and efficiency, a critical requirement for any jet engine. These jet engines use what is known as a Common Nozzle Assembly (CNA), which lends them their unique shape resembling that of a blow dryer.
A CNA is a kind of “exhaust mixer”.
The Role of the Common Nozzle Assembly
So, what exactly does a CNA do? The primary function of a CNA, which functions as an exhaust mixer, is to mix the airflow from the fan and the core. The CNA is shaped like a convergent duct, which increases the velocity of the mixed gas and provides additional thrust. The result? Greater efficiency and enhanced performance. Engines that feature a CNA include the CFM56 of the A340-300, the Rolls Royce Trent 700 of the A330-300, the IAE V2500 of A320 and A321, and the RR RB211 and Pratt & Whitney PW2000 of Boeing 757, among others.
An exhaust mixer’s primary function is to reduce the velocity of the air exiting the engine core, which directly leads to a decrease in the engine’s noise output – a crucial factor in ensuring a smooth and comfortable flight experience. Furthermore, exhaust mixers lowers the temperature of the exhaust.
One distinct advantage of an exhaust mixer is its potential to amplify thrust, a major performance enhancement. In a jet engine, thrust equates to the product of exhaust mass flow and exhaust velocity. This slows the fastest exhaust speeds from the engine’s core while simultaneously speeding up the average exhaust velocity, yielding more thrust with the same quantity of energy.
The Changing Trend
Recently, however, we’ve seen a shift towards large high bypass ratio engines, which means more bypass air is available to transfer heat from the core. Coupled with the large, heavy duct required at the back of the nacelle, this has led to the conclusion that the benefits no longer outweigh the drawbacks.
The question then arises – why isn’t a CNA used in new high-bypass engines if it’s so effective at improving performance and efficiency? It all comes down to the size and weight of the engines. New high bypass turbofan engines like the GE90-115B, Rolls Royce Trent XWB, and GE9X, have cowlings with substantial diameters, meaning that a CNA of the same diameter would have to be extended until the end of the turbine. The additional weight that this would bring could negatively impact the overall performance of the aircraft.
A common nozzle assembly (CNA) functions as a unique type of exhaust mixer, enhancing the power and efficiency of engines, which consequently leads to some jet engines having an unusual hair dryer-like shape. Therefore, when you spot an Airbus A340-300, a Boeing 757, or certain A320s with such distinct engine forms, you’ll be well aware of their intended purpose.
First love never dies. I fell in love with airplanes and aviation when I was a kid. My dream was to become a pilot, but destiny led me to another path: to be an aviation digital media content creator and a small business owner. My passion for aviation inspires me to bring you quality content through my website and social accounts. Aviation is indeed in my blood and blog!
