slcguy
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Blow In Air Intakes On Boeings

Mon Nov 04, 2013 1:14 pm

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]
 
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Starlionblue
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RE: Blow In Air Intakes On Boeings

Mon Nov 04, 2013 2:26 pm

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." - John Ringo
 
LH707330
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RE: Blow In Air Intakes On Boeings

Mon Nov 04, 2013 4:15 pm

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.
 
Viscount724
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RE: Blow In Air Intakes On Boeings

Mon Nov 04, 2013 11:13 pm

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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timz
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RE: Blow In Air Intakes On Boeings

Mon Nov 04, 2013 11:25 pm

Anyone know if the bigger doors mean JT3D-3 and the smaller are always JT3D-1?
 
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DocLightning
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RE: Blow In Air Intakes On Boeings

Tue Nov 05, 2013 2:17 am

What was changed about the intakes to make the doors obsolete?
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Starlionblue
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RE: Blow In Air Intakes On Boeings

Tue Nov 05, 2013 2:41 am

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.
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LH707330
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RE: Blow In Air Intakes On Boeings

Tue Nov 05, 2013 6:46 pm

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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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.
 
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DocLightning
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RE: Blow In Air Intakes On Boeings

Wed Nov 06, 2013 12:19 am

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?
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Starlionblue
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RE: Blow In Air Intakes On Boeings

Wed Nov 06, 2013 1:36 am

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." - John Ringo
 
ferpe
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RE: Blow In Air Intakes On Boeings

Wed Nov 06, 2013 7:04 am

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.
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Tristarsteve
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RE: Blow In Air Intakes On Boeings

Wed Nov 06, 2013 10:16 am

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.
 
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CALTECH
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RE: Blow In Air Intakes On Boeings

Mon Nov 11, 2013 9:12 pm

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.
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ferpe
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RE: Blow In Air Intakes On Boeings

Tue Nov 12, 2013 7:29 am

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.
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HAWK21M
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RE: Blow In Air Intakes On Boeings

Thu Nov 14, 2013 3:05 pm

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.
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LH707330
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RE: Blow In Air Intakes On Boeings

Thu Nov 14, 2013 6:37 pm

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.
 
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CALTECH
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RE: Blow In Air Intakes On Boeings

Sat Nov 16, 2013 2:43 pm

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.
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BravoOne
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RE: Blow In Air Intakes On Boeings

Sat Nov 16, 2013 3:13 pm

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.
 
LH707330
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RE: Blow In Air Intakes On Boeings

Sat Nov 16, 2013 10:05 pm

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.
 
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HAWK21M
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RE: Blow In Air Intakes On Boeings

Mon Nov 25, 2013 11:58 am

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.
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sovietjet
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RE: Blow In Air Intakes On Boeings

Mon Nov 25, 2013 4:33 pm

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


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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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