Jet engine cowlings keep planes quiet, stable
Question: I noticed that jet engine cowlings are scalloped on the rear edge on some engines and not on others. What is the purpose of the scalloped edge?
– Dean, Layton, Utah
Answer: Newer engines have the saw-tooth cowlings. They help make the engine quieter, particularly at higher power.
Q: I have noticed some tiny little fins on the sides of some jet engine cowlings. For example, the 747-8 has them on the upper, inside surface (toward the fuselage) of each engine. What are they for and why on only one side?
– Rodney, Atlanta
A: These are known as strakes. They smooth the flow of air around the nacelle and reduce drag.
Q: What is the purpose of the “fin” on the exterior portion of a jet engine? Does it have something to do with aerodynamics?
– PTrebesch, Salt Lake City
A: On some cowlings, the air flowing over causes a disturbance (vortex) that would disturb airflow over the wing, so the strake is installed to keep the airflow over the wing above the engine stable.
Q: Regarding reverse thrust, why do some mechanisms slide back while others have a clamshell design that opens outwards? Which is more effective?
– Jose Silva, Chicopee, Massachusetts
A: The older engines used a targettype reverser where the aft part of the cowling moved aft and then blocked the exhaust flow. Modern high-bypass engines use blocking doors that block the fan air in the duct out the side of the cowling. My experience is they are equally effective; it is hard to tell the difference operationally.
John Cox is a retired airline captain with US Airways.