HermansCVR580 From United States of America, joined Jul 1999, 494 posts, RR: 1 Posted (5 months 9 hours ago) and read 3921 times:
If you look at the front outer areas on the engines on 707 and older 737-200's they have what appear to be openings on the out side of the engines. What are they for? Can they be adjusted during flight?
AF1624 From France, joined Jul 2006, 573 posts, RR: 0 Reply 1, posted (5 months 7 hours ago) and read 3870 times:
This is adjusted in flight simply my moving the throttle levers to a higher power setting. These doors open so that, basically, more air gets sucked in.
I'm sure someone will come up with a detailed explanation but that's the basics.
They're called blow-in doors. They allow more air into the inlet at low airspeed and high power. Old inlets didn't work well at low speed (low ram pressure) so they had auxiliary doors to let more air in when inlet pressure drops below ambient.
Not directly. They're sprung doors and just respond to pressure changes across the inlet wall. They close up on their own when inlet pressure gets high enough (appropriate relationship of airspeed and engine thrust).
HermansCVR580 From United States of America, joined Jul 1999, 494 posts, RR: 1 Reply 3, posted (5 months 3 hours ago) and read 3699 times:
Very interesting, see I learn something new everyday.
So for example on the 737's some of the same planes that had these blow in doors in earlier pictures, then years later the exact same place no longer has them on the engine. Was the aircraft re-engined even though its still the older style straight pipe Pratts?
The right decision at the wrong time, is still a wrong decision. "Hal Carr"
timz From United States of America, joined Sep 1999, 6468 posts, RR: 8 Reply 4, posted (5 months 3 hours ago) and read 3689 times:
Some early 747s had them too-- for some reason they could meet noise rules better without the blow-in doors and later 747s didn't have them. Maybe 737s were quieter without them too?
(Don't call 737 JT8Ds "straight-pipe"-- bypass ratio was around 1.)
Spacepope From Vatican City, joined Dec 1999, 2739 posts, RR: 1 Reply 5, posted (5 months 3 hours ago) and read 3683 times:
Quoting HermansCVR580 (Thread starter): f you look at the front outer areas on the engines on 707 and older 737-200's they have what appear to be openings on the out side of the engines. What are they for? Can they be adjusted during flight?
dlednicer From United States of America, joined May 2005, 506 posts, RR: 6 Reply 6, posted (5 months 3 hours ago) and read 3644 times:
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Blow-in (or more properly, suck-in) doors were also used on Spey-engined Phantoms (side of aft fuselage), Northrop F-5Es (side of aft fuselage), Dassault Mirages (side of inlets), IAI Kfirs (side of inlets), Sukhoi Su-22s (side of inlet), Su-27s (bottom of inlets), later model MiG-21s (side of fuselage ahead of wing), Mikoyan-Gurevich MiG-29s (top of inlets), Lockheed F-117s (top of inlets), General Dynamics F-111s (side of inlets) and many other military aircraft.
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amccann From United States of America, joined Mar 2008, 171 posts, RR: 0 Reply 9, posted (4 months 4 weeks 1 day 19 hours ago) and read 3415 times:
Quoting dlednicer (Reply 6): Mikoyan-Gurevich MiG-29s (top of inlets)
I'm not positive, but I believe the inlets on the leading edge upper surface of the Mig 29 are auxiliary inlets used when the main inlets are closed to prevent the "injection" of FOD from the main inlets.
Auxiliary inlet doors were fitted to early JT8D's 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.
HermansCVR580 From United States of America, joined Jul 1999, 494 posts, RR: 1 Reply 12, posted (4 months 4 weeks 1 day 9 hours ago) and read 3244 times:
Wow to me this is really amazing. I also never knew that military aircraft had these also, but after learning about them a bit they are a really simple device on a high tech engine. And to think now days there really is no need for them with the high bypass engines. We sure have come a long way.
The right decision at the wrong time, is still a wrong decision. "Hal Carr"
KC135Hydraulics From United States of America, joined Nov 2012, 165 posts, RR: 0 Reply 15, posted (4 months 4 weeks 22 hours ago) and read 3006 times:
The E-8s here in the Deid also have these doors on them. An E-8 is basically a 707, and we've got the old engines on them. I've watched them a few times during engine run-ups/maintenance, and the doors definitely do not open and close symetrically! Some lag behind the others though they do all eventually open and close.
TupolevTu154 From UK - England, joined Aug 2004, 2119 posts, RR: 31 Reply 17, posted (4 months 3 weeks 6 days 3 hours ago) and read 2700 times:
I never really knew how they worked either but then I sat next to them for an hour or so back in September and it was certainly a noise I won't soon forget! Here's a short video I took on my phone showing how they flap around on high power settings. I was fascinated watching them;
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 19, posted (4 months 3 weeks 5 days 19 hours ago) and read 2577 times:
Quoting JoeCanuck (Reply 18): I find the suction vents on the Hawker Harrier very prominent.
Harrier was a corner case because it had enormous thrust for its size and had to run at full thrust with zero (or even negative) forward airspeed. Even that giant inlet wasn't enough to move the requisite mass of air without blow-in doors.
dlednicer From United States of America, joined May 2005, 506 posts, RR: 6 Reply 20, posted (4 months 3 weeks 5 days 16 hours ago) and read 2538 times:
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Quoting tdscanuck (Reply 19): Harrier was a corner case because it had enormous thrust for its size and had to run at full thrust with zero (or even negative) forward airspeed. Even that giant inlet wasn't enough to move the requisite mass of air without blow-in doors.
The original P.1127 prototype even had inflatable inlet lips, to provide a better shape for high thrust/no speed operation:
The early prototype YAV-8Bs had two rows of blow-in doors to cope with these problems:
musang From United Kingdom, joined Apr 2001, 755 posts, RR: 7 Reply 22, posted (4 months 3 weeks 5 days 10 hours ago) and read 2483 times:
Quoting tdscanuck (Reply 2): They're sprung doors and just respond to pressure changes across the inlet wall
To give an idea of how "sprung" they are, you can push them in manually.
(And while on the tangential subject of things you can do manually, you can lift the thrust reverser shutters on a DC-8-50!)
Quoting dlednicer (Reply 6): used on Spey-engined Phantoms (side of aft fuselage), Northrop F-5Es (side of aft fuselage),
Interesting. If they're so far back, presumably they're not allowing air into the intake, but augmenting (adding) airflow into the exhaust duct?? Does the exhaust section demand more if after burning (reheating)?
Quoting Viscount724 (Reply 14): JT3Ds on 707s and 720Bs had two types of blow-in doors.
The narrower type of door looks hinged at the rear, almost as if its a dump vent, i.e. letting air out. It would appear that this type forces the air to make an "s" turn into the intake.
Different spring pressure, hinge lubrication might contribute, but I'd contend that uneven vent opening would be likely due to the uneven pressure inside the intake, which I would further guess might be caused by airflow interference due to the proximity of the ground, and any crosswind. Most of that was conjecture!
dlednicer From United States of America, joined May 2005, 506 posts, RR: 6 Reply 23, posted (4 months 3 weeks 5 days 2 hours ago) and read 2379 times:
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Quoting musang (Reply 22): Quoting dlednicer (Reply 6):
used on Spey-engined Phantoms (side of aft fuselage), Northrop F-5Es (side of aft fuselage),
Interesting. If they're so far back, presumably they're not allowing air into the intake, but augmenting (adding) airflow into the exhaust duct?? Does the exhaust section demand more if after burning (reheating)?
The engines on the F-5 are mounted very far aft, so the doors are at the inlet:
musang From United Kingdom, joined Apr 2001, 755 posts, RR: 7 Reply 24, posted (4 months 3 weeks 5 days 1 hour ago) and read 2371 times:
Thanks for that, it sorts out the F-5. I had no idea they were so far aft.
However the Phantom doors are way aft of the engine face. Could they be to do with ventilation of the engine bay due to overheating of the RR Spey engines in the Royal Navy variant?
ferpe From France, joined Nov 2010, 1785 posts, RR: 57 Reply 26, posted (4 months 3 weeks 4 days 15 hours ago) and read 2281 times:
The blow in doors are there to help with the conflicting sizing requirements of the inlet at no or low speed and high speed. The problem is very well shown in the Harrier flow pictures, at low speed you need very broad and round lips as you are sucking air from a larger area then your inlet, even from behind the inlet lips at start. If the air does not follow the lip around to the inside you get inlet air separation which gives an airflow meeting the fan or compressor face which is uneven, can lead to surge.
At high speed it is the inverse, there is so much air available due to your forward speed stuffing air in the inlets that you can't consume all of it, ie you have air spilling over the sides of the inlet and if it does not attach around the outside lip you get drag. This problem is bad for an airliner (Inlet Mach range 0 to 0.85) but even worse for a fighter (Inlet should work over a Mach range 0 to 1.5).
For airliners advanced inlet lip shapes has given a good enough compromise so that you can do without the doors nowerdays, for fighters the Mach range is most of the time to large, especially if you have an inlet which has an optimized high Mach design like those with variable shock lips or bodies, these have almost invariably suck-in doors. Notable is that the JSF35 does not have doors (would cause radar reflections) or variable inlets (once again difficult to do with a low radar signature), therefore it has problems flying faster then M 1.5, today it better to be slower and not be seen then the opposite.
HermansCVR580 From United States of America, joined Jul 1999, 494 posts, RR: 1 Reply 27, posted (4 months 3 weeks 4 days 10 hours ago) and read 2217 times:
Do these blow in doors make the engines any more louder or do they have no effect on engine noise?
The right decision at the wrong time, is still a wrong decision. "Hal Carr"
Gr8Circle From Canada, joined Dec 2005, 2988 posts, RR: 4 Reply 29, posted (4 months 3 weeks 3 days 18 hours ago) and read 2080 times:
I remember doing a pre-flight walk around of an AI B707-337, and as he walked past the engine, he pushed in one of the blow-in doors on the P&W engine and shone his torch inside for a quick inspection........it just moved in easily, and swung shut on being released, as though it was held back in place by a spring.......
HAWK21M From India, joined Jan 2001, 31201 posts, RR: 58 Reply 30, posted (4 months 3 weeks 2 days 13 hours ago) and read 1882 times:
Quoting Viscount724 (Reply 28): If not mistaken, one of the reasons they were eliminated on later engine versions was to reduce noise.
Franking speaking the noise reduction vide Noise supressors or Vortex generators may not be humanly noticeable due to the small variation in db levels.But exists when measured using test equipment.
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