LH707330
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Why The Gap In The 367-80B Core Duct?

Wed Nov 06, 2013 9:49 pm

I noticed the other day that the 367-80B has a gap in the core duct that is not in the production 707s, does anybody know why this is? I would think the aerodynamics of the former would be unfavorable.

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Viscount724
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RE: Why The Gap In The 367-80B Core Duct?

Thu Nov 07, 2013 1:18 am

Isn't that just the rear part of the thrust reverser in the deployed position?


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jetmech
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RE: Why The Gap In The 367-80B Core Duct?

Thu Nov 07, 2013 3:31 am

Quoting LH707330 (Thread starter):

I suspect it may have something to do with attempts at reducing noise. When a jet of fluid is issued into another, quiescent (still) body of fluid, the turbulence and shearing produces much vibration of the fluid body, which results in the generation of much noise. Increasing the relative speed of the jet, results in more emitted noise.

The gaps in the core allow the hot exhaust gasses exiting from the last turbine stage to induce cold, slower moving ambient fluid into the exhaust jet. The two streams will mix to some extent inside the exhaust cone, and thus, a cooler, slower jet of fluid will be issued from the exhaust cone outlet. The end result is less shearing and turbulence, and thus, less emitted noise.

The multitube / cookie-cutter exhaust of RR Conway powered 707's reduced noise by using the numerous tubes / convolutions to increse the shear area of the exhaust jet, which again, promoted more rapid mixing with the freestream. In this case, most of the mixing took place outside the engine.

http://www.adastron.com/707/gallery/vh-ebg-2.jpg

http://www.adastron.com/707/gallery/vh-ebg-2.jpg


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The cookie-cutter nozzle is still used on some modern engines such as the RB211-524G2.


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Regards, JetMech

[Edited 2013-11-06 20:14:02]
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
boeingfixer
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RE: Why The Gap In The 367-80B Core Duct?

Thu Nov 07, 2013 4:08 pm

Quoting LH707330 (Thread starter):
I noticed the other day that the 367-80B has a gap in the core duct that is not in the production 707s, does anybody know why this is? I would think the aerodynamics of the former would be unfavorable.
Quoting Viscount724 (Reply 1):
Isn't that just the rear part of the thrust reverser in the deployed position?

It is indeed the core thrust reverser cascades you see there. The T/R's on the JT3D have gone through many different configurations and this was just a style without the translating cowl at the aft of the engine. Another thing to consider is the fact the Dash 80 was a development airframe and had of lot of non-standard items on it compared to production 707's and 720's.

Cheers,

John
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LH707330
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RE: Why The Gap In The 367-80B Core Duct?

Thu Nov 07, 2013 6:30 pm

Quoting Viscount724 (Reply 1):
Isn't that just the rear part of the thrust reverser in the deployed position?
Quoting boeingfixer (Reply 3):
It is indeed the core thrust reverser cascades you see there.

I found a few more where the plane is either flying or taxiing, in this one at left it would look like a highly unsafe maneuver if it is indeed a deployed T/R:

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Quoting jetmech (Reply 2):
I suspect it may have something to do with attempts at reducing noise. When a jet of fluid is issued into another, quiescent (still) body of fluid, the turbulence and shearing produces much vibration of the fluid body, which results in the generation of much noise. Increasing the relative speed of the jet, results in more emitted noise.

If this is the case, it must have been some experimental design. IIRC, N70700 was retired around 1980, before the 707 hush-kit party got going, and I doubt that the test team was concerned about noise before that, unless this was some experiment to meet upcoming noise regs. The three 707 hush kit designs I'm aware of are 1) an extended bypass duct, 2) an integrated mixer design, or a 3) lengthened duct and intake, but I've never seen the gap in the core duct:

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boeingfixer
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RE: Why The Gap In The 367-80B Core Duct?

Thu Nov 07, 2013 7:36 pm

Quoting LH707330 (Reply 4):
I found a few more where the plane is either flying or taxiing, in this one at left it would look like a highly unsafe maneuver if it is indeed a deployed T/R:

The T/R isn't deployed. It's only the cascades you see which are always in view with that configuration of nacelle. The blocker doors(Clamshells) are in the core exhaust gas path and can be seen stowed in the following pic of the same engine.

http://www.flickr.com/photos/chrisst...3075730507/sizes/l/in/photostream/

Quoting LH707330 (Reply 4):
but I've never seen the gap in the core duct:

That's not a gap but just the exposed cascade vanes. Looks to me like the translating portion of the aft nacelle has been disabled and the section covering the cascades was modified to expose the cascades.

Exposed cascade vanes are nothing new. The 727 had them as well as early 737-100's. Other aircraft that also had this feature were the Vickers VC10 and the Il-62.

Cheers,

John
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LH707330
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RE: Why The Gap In The 367-80B Core Duct?

Thu Nov 07, 2013 8:08 pm

Quoting boeingfixer (Reply 5):
The T/R isn't deployed. It's only the cascades you see which are always in view with that configuration of nacelle. The blocker doors(Clamshells) are in the core exhaust gas path and can be seen stowed in the following pic of the same engine.

This explanation makes sense, thanks.

Quoting boeingfixer (Reply 5):
Looks to me like the translating portion of the aft nacelle has been disabled and the section covering the cascades was modified to expose the cascades.

Maybe they modded this to test the layout for the 727s. I found a few old gray photos from the early/mid 1960s with this layout, so it's nothing new.
 
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kanban
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RE: Why The Gap In The 367-80B Core Duct?

Fri Nov 08, 2013 2:23 am

Then again, it may just be a museum display setting to show more of the engine.
 
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jetmech
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RE: Why The Gap In The 367-80B Core Duct?

Fri Nov 08, 2013 6:23 am

Quoting LH707330 (Reply 4):
If this is the case, it must have been some experimental design.

Apparently, there was a fair bit of design work carried out on the J57 exhaust which I believe was in relation to the 367-80 and DC-8. The photos below show a variety of jet pipe modifications that were tested in the wind tunnel.

http://grcimagenet.grc.nasa.gov/shar...ons&maxcnumber=44496&maxcyear=1957

http://grcimagenet.grc.nasa.gov/shar...ons&maxcnumber=44496&maxcyear=1957

http://grcimagenet.grc.nasa.gov/shar...ons&maxcnumber=44496&maxcyear=1957

http://grcimagenet.grc.nasa.gov/shar...ons&maxcnumber=44496&maxcyear=1957

Quoting LH707330 (Reply 4):
I've never seen the gap in the core duct

The combined cookie-cutter / gapped (ejector) nozzle design seems to have been an approach used on the DC-8 and some of the retro-fitted hush kits seen today.


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Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
boeingfixer
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RE: Why The Gap In The 367-80B Core Duct?

Fri Nov 08, 2013 8:50 pm

Quoting kanban (Reply 7):
Then again, it may just be a museum display setting to show more of the engine.

Pretty positive it's not. As seen in the following photo the reverser cascades are normally exposed.


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This really looks like a one-off installation which was very common on the Dash-80.

Quoting jetmech (Reply 8):
Apparently, there was a fair bit of design work carried out on the J57 exhaust which I believe was in relation to the 367-80 and DC-8. The photos below show a variety of jet pipe modifications that were tested in the wind tunnel.

I would surmise that these tests are for the C-135 and not for commercial 707's or DC-8's as the engines are J57's and not JT3C's. I know that they are essentially the same engine but there are subtle differences due to civil certification of the JT3C.

Quoting jetmech (Reply 8):
The combined cookie-cutter / gapped (ejector) nozzle design seems to have been an approach used on the DC-8 and some of the retro-fitted hush kits seen today.

The 737-200 hush kit from Nordam also had a slotted ejector. It was a tremendously heavy hush kit and not too many of them were sold compared to AvAero's installation.


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Cheers,

John
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