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Of the various use cases of the Pratt & Whitney JT8D (the most-produced low-bypass turbofan in history, and the second-most-produced turbofan of any description1), those installed on the 737-100 appear to use a set of auxiliary suck-in doors, similar to those so familiar from the earlier JT3D:

JT8D on 737-100; note suck-in doors

(Image by Steve Fitzgerald at Airliners.net, via Fæ at Wikimedia Commons, modified by Fæ and Marc Lacoste at Wikimedia Commons.)

The always-useful 737 Technical Site confirms that these are, indeed, suck-in doors:

History

The original choice of powerplant was the Pratt & Whitney JT8D-1, but before the first order had been finalised the JT8D-7 was used for commonality with the current 727. The -7 was flat rated to develop the same thrust (14,000lb.st) at higher ambient temperatures than the -1 and became the standard powerplant for the -100. By the end of the -200 production the JT8D-17R was up to 17,400lb.st. thrust.

Auxiliary inlet doors were fitted to early JT8D's [sic] around the nose cowl. These were spring loaded and opened automatically whenever the pressure differential between inlet and external static pressures was high, ie slow speed, high thrust conditions (takeoff) to give additional engine air and closed again as airspeed increased causing inlet static pressure to rise.

In contrast, the JT8Ds installed on other types of aircraft, so far as I can tell, do not have suck-in doors:


DC-9

JT8D on DC-9

(Image by Cory W. Watts at flickr, via Josve05a at Wikimedia Commons.)


727

JT8D on 727

(Image by Pablo Andrés Ortega Chávez at flickr, via Wikimedia Commons.)


737-200

JT8D on 737-200

(Image by Andre Gustavo Stumpf Filho at flickr, via Wikimedia Commons.)


Caravelle 10

JT8D on Caravelle 10

(Image by duch.seb at Wikimedia Commons.)


Mercure

JT8D on Mercure

(Image by Michel Gilliand at Airliners.net, via Fæ [again] at Wikimedia Commons, modified by Fæ at Wikimedia Commons.)


Why did the JT8D need suck-in doors on the 737-100, and not on any of the many other aircraft it was attached to?


1: Behind only the General Electric/SNECMA CFM56 high-bypass turbofan, used on (among many, many other aircraft) the 737-300 through -900.

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  • $\begingroup$ Could be an after thought design change due to insufficient take off thrust found late in the project. $\endgroup$ – user3528438 Feb 27 at 0:14
  • $\begingroup$ @user3528438: But the Caravelle 10B, which uses the same type and number of engines as the 737-100, and has a significantly higher MTOW (123,500 lbs., versus the 737-100's 110,000 lbs.), would be expected to need this extra thrust more than the 737-100... and, yet, the Caravelle 10B's JT8D-7s don't require suck-in doors, whereas the 737-100's JT8D-7s do. $\endgroup$ – Sean Feb 27 at 0:58
  • $\begingroup$ Perhaps different aerodynamics due to wing mounting vs tail mounting? All the shown examples other than the 737-100 are tails (but I definitely don't know that's constant...) $\endgroup$ – ljwobker Apr 2 at 22:02
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    $\begingroup$ @ljwobker: coughcough 737-200 coughcough Mercure coughcough $\endgroup$ – Sean Apr 2 at 23:24
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    $\begingroup$ SWAG here: Was the -100 certified for high & hot or short-field operations that the others weren't certified for? This may have been enabled by the suck-in doors allowing it to generate additional thrust. (Was it rated at higher thrust in the -100 than in other installations?) OK, not so much a SWAG as just a WAG... $\endgroup$ – FreeMan May 15 at 16:27

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