arluna From United States of America, joined Jun 2006, 81 posts, RR: 1 Reply 3, posted (2 years 2 months 1 week 3 days 4 hours ago) and read 3598 times:
Those little doors on the sides of the intakes are spring loaded and are designed to open when the engines are operating at high power settings and low airspeed. They allow more air to enter the intake to compensate for the lack of ram air at the intake mouth. As airspeed increases and the amount of ram air avilable at the intake increases the little doors will slowly close.
Northwest727 From United States of America, joined Jul 2005, 491 posts, RR: 1 Reply 5, posted (2 years 2 months 1 week 3 days 4 hours ago) and read 3576 times:
arluna is correct, those are the auxiliary intake doors, which are spring loaded and open when the engines are at high power settings and low airspeeds. I believe that due to "sharper" lips around the air intake, this robbed the early engines of ram air when they were operated at high power and at slow airspeeds. The early 737s and early 747s had them as well; in the mid 1970s when noise regulations were starting to come into play, engineers discovered that these doors allowed high amounts of noise to escape. A redesign of the nacelles created a "blunter" lip which eliminated the airflow problems, thus eliminated the need for these doors. That is why you don't see them today.
Here is one of such study, showing the 1960's era JT9D intake vs. the "quiet design" of the 1970s (that later became standard, and retrofits were offered for earlier 747s):