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Blow In Air Intakes On Boeings  
User currently offlineSLCGuy From United States of America, joined Jul 2008, 182 posts, RR: 0
Posted (1 year 1 month 2 weeks 1 day 9 hours ago) and read 6435 times:

Question for the mechanic/tech types out there. Looking at pics of the of the blow in doors on engine intakes of some of the early Boeings, namely the JT-3D(707-320), JT-8D(737-1/200) and the JT-9D(747-100). While I realize that they open to allow more air in under high power do they open automatically from vacumn/low pressure in the intake or open mechanically by throttle setting?

Edit: To moderators, guess this should have been posted in Technical/Operations forum, sorry about that, move it if you can.

[Edited 2013-11-04 05:47:31]

20 replies: All unread, jump to last
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17172 posts, RR: 66
Reply 1, posted (1 year 1 month 2 weeks 1 day 8 hours ago) and read 6163 times:

Quoting SLCGuy (Thread starter):
While I realize that they open to allow more air in under high power do they open automatically from vacumn/low pressure in the intake or open mechanically by throttle setting?

If memory serves, they were spring loaded on the 741 and opened due to increased pressure differential. Apparently there were massive problems with jamming doors, leading to massive use of WD-40.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineLH707330 From United States of America, joined Jun 2012, 848 posts, RR: 0
Reply 2, posted (1 year 1 month 2 weeks 1 day 6 hours ago) and read 6071 times:

The ones on the 707 were likewise spring-loaded, the first ones being aft-hinged and the later ones being front-hinged. You can push the doors in with about 5 lbs/2.3 kg of force.

Quoting Starlionblue (Reply 1):
If memory serves, they were spring loaded on the 741 and opened due to increased pressure differential. Apparently there were massive problems with jamming doors, leading to massive use of WD-40.

Yes, IIRC they later designed a fatter cowl lip as a retrofit that allowed the air to turn the corner better. This got rid of the door jamming and reduced noise.


User currently offlineViscount724 From Switzerland, joined Oct 2006, 25989 posts, RR: 22
Reply 3, posted (1 year 1 month 2 weeks 23 hours ago) and read 5854 times:

Quoting LH707330 (Reply 2):
The ones on the 707 were likewise spring-loaded, the first ones being aft-hinged and the later ones being front-hinged. You can push the doors in with about 5 lbs/2.3 kg of force.

Can see the difference between the early 707 (720B in this case) doors (left) and the later ones (right).


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Click here for bigger photo!

Photo © Fenrich Family
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Photo © Wolodymir Nelowkin



User currently offlinetimz From United States of America, joined Sep 1999, 6902 posts, RR: 7
Reply 4, posted (1 year 1 month 2 weeks 23 hours ago) and read 5849 times:

Anyone know if the bigger doors mean JT3D-3 and the smaller are always JT3D-1?

User currently offlineDocLightning From United States of America, joined Nov 2005, 20336 posts, RR: 59
Reply 5, posted (1 year 1 month 2 weeks 20 hours ago) and read 5740 times:

What was changed about the intakes to make the doors obsolete?

User currently offlineStarlionblue From Greenland, joined Feb 2004, 17172 posts, RR: 66
Reply 6, posted (1 year 1 month 2 weeks 20 hours ago) and read 5717 times:

Quoting DocLightning (Reply 5):
What was changed about the intakes to make the doors obsolete?

The issue is changing airflow requirements during different regimes. I would say three things have improved.
- Better understanding of the aerodynamics involved.
- Improved variable stator vanes and inlet guide vanes enabling fine airflow control without the need for inlet doors.
- Better engine control to prevent surges and stalls in case of varying airflow. FADEC in particular is pure magic in this regard. Without digital engine control, current big fans would probably not be possible.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineLH707330 From United States of America, joined Jun 2012, 848 posts, RR: 0
Reply 7, posted (1 year 1 month 2 weeks 4 hours ago) and read 5460 times:

Quoting timz (Reply 4):
Anyone know if the bigger doors mean JT3D-3 and the smaller are always JT3D-1?

No, there are always exceptions. I think it was mostly a timing thing, the change must have been in 1964ish. There are JT3D-1 707-138Bs with both older and newer doors, as seen in these photos.

Likewise, the first LH 707 delivered came with with JT3D-3s that initially had the older style doors and long spinner cones plus the ventral fin, as seen here:
http://famgus.se/Postcards/Aviation/Airlines/LH%20Lufthansa/LH-B707-D-ABOS-1.jpg
In 1965, the same frame shows up with the new doors and without the ventral fin:
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Click here for bigger photo!

Photo © Mel Lawrence

Quoting Starlionblue (Reply 6):
The issue is changing airflow requirements during different regimes. I would say three things have improved.
- Better understanding of the aerodynamics involved.
- Improved variable stator vanes and inlet guide vanes enabling fine airflow control without the need for inlet doors.
- Better engine control to prevent surges and stalls in case of varying airflow. FADEC in particular is pure magic in this regard. Without digital engine control, current big fans would probably not be possible.

I think DocLightning was asking specifically about the 741, which never got FADEC. There's an old tech/ops thread about the redesigned nose cowl that discusses it.


User currently offlineDocLightning From United States of America, joined Nov 2005, 20336 posts, RR: 59
Reply 8, posted (1 year 1 month 1 week 6 days 22 hours ago) and read 5317 times:

Quoting Starlionblue (Reply 6):
The issue is changing airflow requirements during different regimes. I would say three things have improved.
- Better understanding of the aerodynamics involved.
- Improved variable stator vanes and inlet guide vanes enabling fine airflow control without the need for inlet doors.
- Better engine control to prevent surges and stalls in case of varying airflow. FADEC in particular is pure magic in this regard. Without digital engine control, current big fans would probably not be possible.

Most modern engines don't have inlet guide vanes. I guess my question is specifically, what was the issue that the doors solved and how is that now solved without the doors?


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17172 posts, RR: 66
Reply 9, posted (1 year 1 month 1 week 6 days 21 hours ago) and read 5286 times:

Quoting DocLightning (Reply 8):
I guess my question is specifically, what was the issue that the doors solved and how is that now solved without the doors?

Ah sorry. I'll try and the experts can clarify/correct.

The intakes are shaped for cruise thrust at speed. If you make them too large high speed performance suffers.

At speed, air is rammed into the inlets by forward movement and you get the requisite airflow. The problem comes when standing still or moving slowly while at high thrust. The inlets don't let enough air in, and the doors open to increase air intake.

Taking in air through the doors also slows airflow in the main inlet, preventing excessive intake speeds.

Modern engines have much higher bypass ratios, meaning the inlet is much larger anyway. If nothing else, modern noise requirements mean the doors are a non-starter. There is may be some performance penalty at high speed from not having the doors but this trade-off is presumably accepted.


You can read a lot about it here, starting around page 50: http://books.google.com.hk/books?id=...et%20doors%20jet%20engines&f=false



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineferpe From France, joined Nov 2010, 2805 posts, RR: 59
Reply 10, posted (1 year 1 month 1 week 6 days 15 hours ago) and read 5208 times:

Quoting Starlionblue (Reply 9):
Modern engines have much higher bypass ratios, meaning the inlet is much larger anyway. If nothing else, modern noise requirements mean the doors are a non-starter. There is may be some performance penalty at high speed from not having the doors but this trade-off is presumably accepted.

A lot of the problem is solved with better shapes of the inlet lips, initially these where simple radius round lips and you would get flow separation on the inside cowl directly after the lips at TO and on the outside (overspill) at high speed. This could then lead to fan disturbances and higher cruise drag. By research one found that elliptical shapes would make it easier for air to be sucked in when the inlet would be on the small side ie at take-off and could also make the outer lip let the overspill air follow the nacelle at speed. Thus one could avoid the blow in doors and still have a good inlet.



Non French in France
User currently offlineTristarsteve From Sweden, joined Nov 2005, 4068 posts, RR: 33
Reply 11, posted (1 year 1 month 1 week 6 days 12 hours ago) and read 5134 times:

Quoting Starlionblue (Reply 9):
The problem comes when standing still or moving slowly while at high thrust. The inlets don't let enough air in,

The Tristar (never had doors) was designed with a 42000 lb thrust engine, which was changed on the -200 to a 52000lb thrust engine. 25pc more thrust with the same intake area! The S duct to nbr 2 engine was exactly the same. This caused big problems on the ground. When we were running the engine for maintenance we could not achieve take off power. In the AMM were tables that gave limits for engine running, about 90pc N1 was the max.
I was working at GF at the time, and we had an engine shop that replaced modules, but no test bed. So when we installed the engine in the aircraft, we had to test it properly, tests 1 to 17. But to fit an engine to Nbr 2 position meant it could not be tested, so we had to always fit a ready tested engine in Nbr 2. This meant some lengthy engine changes when then was no ready tested engine available.


User currently offlineCALTECH From Poland, joined May 2007, 2319 posts, RR: 26
Reply 12, posted (1 year 1 month 1 week 1 day 1 hour ago) and read 4622 times:

Quoting DocLightning (Reply 8):
Most modern engines don't have inlet guide vanes. I guess my question is specifically, what was the issue that the doors solved and how is that now solved without the doors?

GEs and Pratts still have Inlet Guide Vanes. If you mean the IGVs that were out in front of the 1st stage compressor or fan, yes, those are gone. Might lose a hand or foot in the old days, now one just turns into red mush.



UNITED We Stand
User currently offlineferpe From France, joined Nov 2010, 2805 posts, RR: 59
Reply 13, posted (1 year 1 month 1 week 15 hours ago) and read 4461 times:

Quoting CALTECH (Reply 12):
GEs and Pratts still have Inlet Guide Vanes.

GE, Pratt and RR all have inlet guides vanes on the first compressor stages, in fact you have to search long to find any turbofan that does not use inlet guide vanes to regulate the amount of air entering the compressors as the RPM drops, all to avoid surges. They all also combine that with air bleed into the fan channel for the same reason.



Non French in France
User currently offlineHAWK21M From India, joined Jan 2001, 31702 posts, RR: 56
Reply 14, posted (1 year 1 month 5 days 7 hours ago) and read 4059 times:

If not mistaked the earlier JT8D-7/7A/9/9A had these Blow in doors on the Inlet fan cowl, these were spring loaded to closed position & opened up during Low thrust settings & were closed when Inlet Airflow requirements were met.


Think of the brighter side!
User currently offlineLH707330 From United States of America, joined Jun 2012, 848 posts, RR: 0
Reply 15, posted (1 year 1 month 5 days 4 hours ago) and read 4006 times:

Quoting LH707330 (Reply 7):
Quoting timz (Reply 4):
Anyone know if the bigger doors mean JT3D-3 and the smaller are always JT3D-1?

No, there are always exceptions. I think it was mostly a timing thing, the change must have been in 1964ish. There are JT3D-1 707-138Bs with both older and newer doors, as seen in these photos.

Update/correction on this point, the a/c I referenced (707-138B ln 29) had JT3D-3B fans retrofitted, at which point the blow-in doors were changed. This proves that JT3D-3 types had both doors (the LH bird D-ABOS has JT3D-3s), perhaps the JT3D-3 to JT3D-3B change is the change to the wider doors and short/round spinner cones.

Quoting HAWK21M (Reply 14):
these were spring loaded to closed position & opened up during Low thrust settings & were closed when Inlet Airflow requirements were met.

I think you've got it backwards, at lowe speed/high thrust they were open, otherwise closed.


User currently offlineCALTECH From Poland, joined May 2007, 2319 posts, RR: 26
Reply 16, posted (1 year 1 month 3 days 8 hours ago) and read 3748 times:

Quoting ferpe (Reply 13):
GE, Pratt and RR all have inlet guides vanes on the first compressor stages, in fact you have to search long to find any turbofan that does not use inlet guide vanes to regulate the amount of air entering the compressors as the RPM drops, all to avoid surges. They all also combine that with air bleed into the fan channel for the same reason.

Yes, GE and Pratt still call them IGVs, Rolls-Royce calls them on the RB-211s, IP Compressor Case Vanes.



UNITED We Stand
User currently offlineBravoOne From United States of America, joined Apr 2013, 328 posts, RR: 0
Reply 17, posted (1 year 1 month 3 days 7 hours ago) and read 3737 times:
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I don't believe that any 707-138B, with the one exception of the Travolta airplane operated with the larger cowl doors. The fan conversion done on these aircraft converted the old engines to the JT3D-1, not a 3B engine as earlier stated. These engines were bastardized -3B's and not eligable for the USAF AMARC program thus the 138B was a very lonesome step child late in life. At one time you could have bought the Travolta airplane for less than 200K.

Interesting to note that the TWA 707-331B/C's never used the larger doors on their aircraft.


User currently offlineLH707330 From United States of America, joined Jun 2012, 848 posts, RR: 0
Reply 18, posted (1 year 1 month 3 days ago) and read 3673 times:

Quoting BravoOne (Reply 17):
I don't believe that any 707-138B, with the one exception of the Travolta airplane operated with the larger cowl doors. The fan conversion done on these aircraft converted the old engines to the JT3D-1, not a 3B engine as earlier stated.

All the 138Bs were delivered or retrofitted with the JT3D-1 and narrow doors in the early 1960s, but at least two of them had the new cowl doors later in life, namely the Travolta plane and the first 138B, which were photographed together here. The first Qantas bird (VH-EBA/XBA) was first converted from JT3C-6 to JT3D-1s in 1961, then to JT3D-3B in 1983. I had initially thought they just changed the doors, but it appears they changed the engines too. Whether the same thing happened on the Travolta bird, I'm not sure.


User currently offlineHAWK21M From India, joined Jan 2001, 31702 posts, RR: 56
Reply 19, posted (1 year 3 weeks 3 days 11 hours ago) and read 3252 times:

Quoting LH707330 (Reply 15):

I think you've got it backwards, at lowe speed/high thrust they were open, otherwise closed.

Thats correct....thinking in one language and writing in another.....thanks.



Think of the brighter side!
User currently offlinesovietjet From Bulgaria, joined Mar 2003, 2648 posts, RR: 17
Reply 20, posted (1 year 3 weeks 3 days 6 hours ago) and read 3232 times:

These doors were also on the Tu-154 (except the Tu-154M)


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Photo © Alexander Tarasenkov - St.Petersburg Spotters



However I find it interesting that they were not on the Il-62 (uses the same NK-8 engines) or the Il-86 (uses the NK-86 which is a development of the NK-8). Does anybody know why only the Tu-154 used them?


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