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Secondary Air Inlet Doors  
User currently offlineBigGiraffe From United States of America, joined Jan 2000, 257 posts, RR: 0
Posted (15 years 4 months 2 weeks 4 days 17 hours ago) and read 1904 times:

Some commercial jets such as the Fanjet 707s and early 737s (747s too, I think) had secondary air inlet doors on the engine nacelles to allow extra air flow with high thrust and low speed. (These doors are located around the circumference of the nacelle near the engine inlet). Later 737s and today's aircraft don't have these doors. What design changes and improvements allowed their deletion?


3 replies: All unread, jump to last
User currently offlineDl_mech From United States of America, joined Feb 2000, 2111 posts, RR: 8
Reply 1, posted (15 years 4 months 2 weeks 4 days 16 hours ago) and read 1850 times:

I've been wondering this myself, my guess would be aerodynamics. UA still had this on older 732's-- are there any UA people that know for sure?

This plane is built to withstand anything... except a bad pilot.
User currently offlineOPNLguy From , joined Dec 1969, posts, RR:
Reply 2, posted (15 years 4 months 2 weeks 4 days 14 hours ago) and read 1848 times:

That's what these were called, informally, on the 737s at least. Their purpose was to allow additional airflow into the engine at higher aircraft angles-of-attack, and they were on the 737-100s and only the -200 "basic" models. For the 737-200 "advanced" model (produced after 1974 or so), Boeing made several design changes to the forward engine cowling (including lengthening it) that all resulted in significant enough aerodynamic improvements such that the sucker doors were no longer needed.

User currently offlineBigGiraffe From United States of America, joined Jan 2000, 257 posts, RR: 0
Reply 3, posted (15 years 4 months 2 weeks 3 days 8 hours ago) and read 1814 times:

These doors are spring-loaded shut and would open when pressure was low enough on the intake side. Once the aircraft is up to speed there would be enough ram pressure to keep them closed. They are sort of a pain on USAF C-5 aircraft because you get corrosion and cracks around them. I can see why owners would want to be rid of them. But still I wonder how things were changed so they are no longer needed?

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