Blackbird
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DC-8 "Cut-Back" Pylon Question

Wed Jul 25, 2007 4:01 am

If I recall on the DC-8 (and 707/CV-880/B-720) the wing-spar, and the wing ran through the engine pylon or something like that if I recall for strengthening purposes.

How did Douglas manage to get around that when developing the DC-8-62/-63?

Andrea K

[Edited 2007-07-24 21:02:41]

[Edited 2007-07-24 21:04:15]
 
miamiair
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RE: DC-8 "Cut-Back" Pylon Question

Wed Jul 25, 2007 5:02 am

Quoting Blackbird (Thread starter):
How did Douglas manage to get around that when developing the DC-8-62/-63

Foot Stools.

If you can find it, I think it is DC-8 SB -57-87
Molon Labe - Proud member of SMASH
 
Blackbird
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RE: DC-8 "Cut-Back" Pylon Question

Wed Jul 25, 2007 8:40 am

DC-8 SB-57-87? I don't even know what that is? I assume it's a document, but I have no idea where it's from or where to find it.

Can someone enlighten me as to what a 'foot stool" is?

Andrea Kent
 
aeroweanie
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RE: DC-8 "Cut-Back" Pylon Question

Wed Jul 25, 2007 9:01 am

The early model DC-8s had a pylon that attached to the bottom of the wing and had an "arm" that ran up above the wing to another attach point. The DC-8-62 and -63 had cut-back pylons that only attached to the bottom of the wing. This arrangement was heavier, but gave a drag reduction.
 
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RE: DC-8 "Cut-Back" Pylon Question

Wed Jul 25, 2007 9:18 am

So, it was not put on the original design because of weight? How much extra weight did it add, and how much performance was gained out of it?

Do they currently use the same basic pylon design strategy today?

Andrea K
 
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RE: DC-8 "Cut-Back" Pylon Question

Wed Jul 25, 2007 9:42 am

The "over the wing" pylons have a several % drag penalty compared to the cut-back pylons. It significant enough that ABX retrofitted their DC-8-61s with cut-back pylons removed from -62s and -63s when they became -72s and -73s.

Despite this, the 737 Classics and 737NGs have over the wing pylons, due need to mount the engines high due to ground clearance problems. The C-17 has over the wing pylons because the engines needed to be mounted close to the wing for flap blowing.
 
Blackbird
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RE: DC-8 "Cut-Back" Pylon Question

Wed Jul 25, 2007 1:20 pm

Couldn't you just make the over the pylon in such a way so that it's height (what would normally be over-wing) was the same height as the rest of the wing nearby it (thicker skin and proper support structure) which includes the wing-leading-edge portion and then the pylon below it. That way the spar could run through the structure the same way.

Wouldn't that work?

Andrea K
 
miamiair
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RE: DC-8 "Cut-Back" Pylon Question

Wed Jul 25, 2007 9:24 pm

Here's a sketch of the foot stools:

http://i30.photobucket.com/albums/c325/miamiair/Foot-Stools.jpg
Molon Labe - Proud member of SMASH
 
aeroweanie
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RE: DC-8 "Cut-Back" Pylon Question

Thu Jul 26, 2007 1:09 am

Quoting Blackbird (Reply 6):
Couldn't you just make the over the pylon in such a way so that it's height (what would normally be over-wing) was the same height as the rest of the wing nearby it (thicker skin and proper support structure) which includes the wing-leading-edge portion and then the pylon below it. That way the spar could run through the structure the same way.

Wouldn't that work?

See my explanation of how vortilons work in your parallel thread DC-8 Wing / How'd They Avoid Vortex Generators? (by Blackbird Jul 25 2007 in Tech Ops)

Pylons that extend above the attachment line increase cruise drag, as they interfere with the attachment line.
 
n8076u
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RE: DC-8 "Cut-Back" Pylon Question

Thu Jul 26, 2007 2:46 am

At the time of the original DC-8 design, Douglas thought their pylon setup was fairly efficient, and touted the design in early documentation. During later testing, it was discovered just how much extra drag the over-the-wing design was causing, and so they sought to improve that.

The long-duct engine pod found on the -62 and -63 reduced drag by about 5% over the -50 and -61 series pods. The new pylons on the -62/-63 achieved another 5% in drag reduction by moving the engines forward and by eliminating the over-wing portion of the pylon.

Chris
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RE: DC-8 "Cut-Back" Pylon Question

Thu Jul 26, 2007 8:27 am

Weren't the thinner, cut-back pylons one of the reasons why the DC-8-62 and DC-8-63 could only use reverse on inboard engines? (BTW: Was it only idle reverse, or up to full power)?

Andrea Kent
 
Starglider
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RE: DC-8 "Cut-Back" Pylon Question

Thu Jul 26, 2007 8:28 pm

Quoting Blackbird (Reply 10):
Weren't the thinner, cut-back pylons one of the reasons why the DC-8-62 and DC-8-63 could only use reverse on inboard engines? (BTW: Was it only idle reverse, or up to full power)?

On the DC-8-62 and -63 all 4 engines use reverse and may be used up to maximum reverse.

I was just reading an accident report regarding a DC-8-63 which ran off the side of a wet runway due to hydroplaning. One of the findings was that reversers were not used to the maximum in a timely manner to try and stop the aircraft which came to rest in a swamp. Fortunately there were no injuries and as a tribute to the DC-8 being built like a tank, it was repaired.


Starglider
 
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RE: DC-8 "Cut-Back" Pylon Question

Fri Jul 27, 2007 3:35 am

Starglider,

I meant mid-air reverse thrust... If I recall the -62 and -63 can only use inboard engine reverse (although I'm not sure what thrust level is allowed). I'm not sure if this applies to the -61, although to my knolwedge all of the early DC-8's including the -50's could use reverse on all four engines.

To my knowledge you could use full reverse on all four engines on the -10 series, inboards on full reverse, outboards on some percentage max-continuous thrust for the -20, -30, -40, and -50. Or something like that.

Andrea Kent
 
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RE: DC-8 "Cut-Back" Pylon Question

Fri Jul 27, 2007 6:57 am

I've previously compiled some information from several flight manuals regarding in-flight reverse thrust on the DC-8:

-Maximum speed for extending thrust brakes - 390 kts. or M = .88
-thrust reversers must not be used in flight below a speed of 200 knots (Kalitta flight manual states 190 knots).
-Wing flaps must be completely retracted for all inflight thrust brake operations.


---Maximum thrust usable during thrust brake operation is as follows:

***** Outboard Engines #1 and #4*****

-thrust brake stop limit for JT3C engines (DC-8-10) - idle reverse.
-thrust brake stop limit for JT4A engines (used on DC-8-20 and DC-8-30) - Approx. 31% max continuous.
-on JT3D powered airplanes (some -50, all -61 series) power settings up to idle reverse available with gear extended.
-on all other JT3D and all CFM56 powered airplanes, mechanical stop prevents inflight use (some -50 series, all -62 series, all -63 and all -70 series).

***** Inboard Engines #2 and #3*****

-maximum continuous thrust for JT3C engines (series -10)
-maximum continuous thrust for JT4A engines (series -20 and -30)
-maximum continuous thrust for JT3D engines (series -50, -60)
-maximum continuous thrust for CFM56 engines (series -70)

Sorry, no specific info for the -40 series. Not sure what the difference is between the different -50 series aircraft to cause them to have different limitations for the outboard engines.

Chris
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Blackbird
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RE: DC-8 "Cut-Back" Pylon Question

Fri Jul 27, 2007 8:41 am

I thought you could use full reverse on all four engines on the -10 variants, hmm.

Andrea Kent
 
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RE: DC-8 "Cut-Back" Pylon Question

Fri Aug 17, 2007 6:37 am

I remember something about some problems with the cutback pylons wobbling a bit or something like that on the DC-8-62 when fuel went low, requiring a redesign of the fuel control system that kept some fuel in the wing most of the time.

Also... how much weight did the cutback design add about?

Andrea Kent
 
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RE: DC-8 "Cut-Back" Pylon Question

Fri Aug 17, 2007 7:58 am

Quoting Blackbird (Reply 15):
I remember something about some problems with the cutback pylons wobbling a bit or something like that on the DC-8-62 when fuel went low, requiring a redesign of the fuel control system that kept some fuel in the wing most of the time.

If what you describe affected the DC-8-62, it should also have affected the DC-8-63 but i can't recall that was ever a problem on these aircraft. Perhaps you mean the CV-990 which had an excessive wobble of its outboard pylons with fuel in the outboard wing-mounted anti-shock bodies. The CV-990 had a speed limitation (which differed, depending on the model) if it carried fuel in those outboard anti-shock bodies. Only after the outboard anti-shock bodies were empty the aircraft was allowed to accelerate to M0.89 cruise (Mmo is M.0912).

Starglider
 
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RE: DC-8 "Cut-Back" Pylon Question

Fri Aug 17, 2007 8:50 am

Starglider,

Was there any-way they could have beefed up or made the pylons more sturdy without excessive weight-gain?

Andrea
 
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RE: DC-8 "Cut-Back" Pylon Question

Sun Aug 19, 2007 9:20 am

Quoting Blackbird (Reply 17):
Was there any-way they could have beefed up or made the pylons more sturdy without excessive weight-gain?

In this case (assuming you mean the CV-990 design anomaly) it is not a simple matter of beefing up the pylon structure but a total wing/pylon redesign. Both have natural oscillating frequencies and the wing and pylons need to flex, to counter-act the wing's bending moment. With fuel in the outer anti-shock bodies, apparently the wing flexing characteristics altered in such a way that the wing flexed less than desired which resulted in the forces being transferred from the wing to the outer pylons, causing the wobble at higher speeds. Beefing up the pylon structure only would probably have resulted in higher stress levels in certain components which would expose the structure to a higher level of fatigue. Therefore, with the anti-shock bodies fueled, a speed limitation was chosen instead of a very expensive redesign.

Starglider
 
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RE: DC-8 "Cut-Back" Pylon Question

Sun Aug 19, 2007 10:22 am

I was talking more of the DC-8 cutback pylon design.

Was there anything they could have done back then, reasonably speaking to have made a cut-back pylon that would not have experienced the oscillations as experienced on the DC-8-62?


Also... as a hypothetical -- with 707-120 and DC-8-10/-20 era design, could either hypothetically have designed a cutback style pylon that would not have had the aforementioned oscillation problems (a'la DC-8-62) if either realized the problems that would have ensued with the over the wing-pylon configuration?


Andrea Kent
 
ex52tech
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RE: DC-8 "Cut-Back" Pylon Question

Sun Aug 19, 2007 5:22 pm

Quoting Starglider (Reply 18):
In this case (assuming you mean the CV-990 design anomaly) it is not a simple matter of beefing up the pylon structure but a total wing/pylon redesign.

In the case of the CV-990,and it's outboard pylons vibration problems, the outboard pylons were modified on all of the 990s. If you can find a picture of CV-990 ship one, right at roll out, you will see the outboard pylons look to have the same angle on the leading edge as the inboard pylons. The fix for the vibration problems was to decrease the angle on the leading edge of the pylon, for close to half the distance back, then increase the angle sharply to reach the leading edge of the wing.

The anti-shock bodies caused their own vibration problems when the fuel in the outboard ASB's was more than half. They found that a complete wing redesign was the only cure for that problem, as you have stated Starglider, and they were already behind in deliveries, which cost the 990 it's future.

You did a great job of explaining the cause and cure Starglider. I just wanted to make clear that these were two separate problems, and weren't related to each other.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
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RE: DC-8 "Cut-Back" Pylon Question

Tue Aug 21, 2007 6:36 am

To Ex52tech,

Thanks for the clarification. I had always understood that the pylon and ASB problems were related. I have never heard about the vibration problem of the ASBs as a separate issue. To quote Jon Proctor on the subject regarding pylon interference:

Quote
"Two problem areas that did not surface in the wind tunnel became evident during early flight tests. First, turbulence generated at the junction of the inboard engine pylons and the wing leading edge carried back to the tail and decreased elevator effectiveness. To fix it, electrically operated Krueger-type flaps had to be added in the wing leading edge between the inboard pylons and fuselage.

A more serious problem lay in the outboard engine oscillations; the power plants would swing side-to-side at cruise speeds when the outboard anti-shock body fuel tanks were full. This more serious problem required shortening of the out board pylons by 28 inches, accomplished by moving the engines rearward 29 inches." unquote.

Apart from shortening and decreasing the angle of the outboard pylon as you mentioned, there were also changes to the nacelle/pylon fairing on the production airplanes by giving them area rule properties. This configuration resulted in a significant increase in the drag-rise Mach number for the aircraft, from about 0.80 for the basic configuration to about 0.89 for the modified aircraft. See illustration below:

Big version: Width: 233 Height: 169 File size: 12kb
CV-990 Inboard and outboard pylon mods for production aircraft.


There were more improvements reducing drag on the CV-990 but that is beyond the scope of this topic.

Quoting Blackbird (Reply 19):
I was talking more of the DC-8 cutback pylon design.

Was there anything they could have done back then, reasonably speaking to have made a cut-back pylon that would not have experienced the oscillations as experienced on the DC-8-62?

Do you have any reference about the oscillations as experienced on the DC-8-62? I have never found anything mentioning or addressing the problem of oscillations. I do know that, like Convair had drag problems with the cv-990 nacelle/pylons, Douglas also had similar drag problems with the nacelle/pylons in the prototype phase nacelles for the -62 and -63 series.. See sketch and explanation/solution below:

Big version: Width: 230 Height: 166 File size: 8kb


During the prototype flight investigation of a new long duct nacelle for the DC-8-62, flight results obtained with a proposed new nacelle afterbody resulted in a much greater interference drag than had been indicated by wind-tunnel tests. In fact, the penalty measured in flight was double the wind-tunnel value for representative cruise conditions. Examination of pressure distributions on the nacelle in the channel between the wing and nacelle indicated that the shock in the channel was significantly stronger and farther aft in flight than in the wind tunnel; this caused very high levels of drag. The difference between the tunnel and flight results was attributed to the differences in boundary-layer growth because of corresponding differences in Reynolds number. Applications of Whitcomb's local area-rule methodology resulted in fairing candidates that eliminated the problem. The successful application of the area-rule process and the elimination of what would have been a major performance penalty for the long duct nacelle configuration provided Douglas with the confidence and enabling technology to proceed with the new versions of the DC-8, the highly successful "Super Sixties" (DC-8-62 and DC-8-63).

Regards,
Starglider
 
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RE: DC-8 "Cut-Back" Pylon Question

Tue Aug 21, 2007 9:57 am

To Starglider

When I read your remarks about what John Proctor wrote, you jogged my somewhat dusty memory banks on the subject.
When you mentioned the pylon fairings and moving the outboards back 28", the idiot light came on above my head, and I started to remember what I had read.
I spent the last few hours digging up my material on the 990. After reading it, and it has been a while, I realized that you were 100% correct, that the two problems were related, and I took the liberty of relying on memory for my statements.

I stand humbly corrected, while I might joke around a little in here, I take this Tech/Ops area very seriously, and try to be as factually correct as possible.

Thank you.

I will now go and bang my head on the wall for a while.

 banghead 
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RE: DC-8 "Cut-Back" Pylon Question

Tue Aug 21, 2007 8:16 pm

Dear Starglider,

There was a book written about the DC-8 by Terry Waddington, it said in regards to the DC-8-62 design the plane experienced rather violent flutter as it's speed was increased to 418 kts: "the cause was traced to the new slimmer pylons which were more flexible than the earlier version and the cantilivered location of the engine pod. The problem was solved by re-design of the fuel-management system, which allowed retention of reserve fuel in the outer portion of the wing, thereby increasing it's stiffness" (pg 69-70)

This was allegedly AFTER the pylon was extended to bring the exhaust-cown forward 40-inches almost directly under the wing leading edge, which reduced drag five percent.

Was there any way they could have designed the pylon back then, that they wouldn't have had to use elaborate fuel-juggling to prevent flutter? Couldn't they have stiffened the strut in some areas?


Andrea Kent
 
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RE: DC-8 "Cut-Back" Pylon Question

Tue Aug 21, 2007 8:40 pm

Quoting Blackbird (Reply 23):
Was there any way they could have designed the pylon back then, that they wouldn't have had to use elaborate fuel-juggling to prevent flutter? Couldn't they have stiffened the strut in some areas?

It would certainly be possible to redesign the pylon, but expensive and time consuming. Changing fuel management is just a procedure change...it's basically free. And, if it's lighter, may actually be a better solution.

Don't forget that the 707's and DC-8's had an extremely sophisticated full-time fuel management system (the flight engineer). Fuel management on even the 747 is a bit of a Byzantine procedure. The 707 and DC-8 fuel procedures are child's play compared to a B-52 or C-17.

Tom.
 
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RE: DC-8 "Cut-Back" Pylon Question

Tue Aug 21, 2007 10:18 pm

But they could have redesigned the pylon had they wished?

BTW: C-17? Aren't those fly by wire? You sure you didn't mean C-5 or C-141?

Andrea Kent
 
tdscanuck
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RE: DC-8 "Cut-Back" Pylon Question

Tue Aug 21, 2007 10:42 pm

Quoting Blackbird (Reply 25):
But they could have redesigned the pylon had they wished?

Redesign is always an option, just usually an expensive one.

Quoting Blackbird (Reply 25):
BTW: C-17? Aren't those fly by wire? You sure you didn't mean C-5 or C-141?

I'm sure I meant C-17. I would assume it's pretty complex on the C-5 and C-141 as well.

Fly-by-wire refers, typically, to flight controls. It's a whole separate system from fuel management.

In the case of the C-17, the fuel management is automated so the flight crew doesn't have to pay much attention to it but the automated system is implementing a very complicated procedure. My point was more about complexity of fuel management than automation vs. human workload. Changing a procedure, if it's an option, is usually cheaper and easier fix for something than redesigning structure provided you have enough resources (crew and automation) to implement the changed procedure.

In the 707/DC-8 era you probably couldn't have automated the fuel management very easily but, since they had a dedicated flight engineer, they could still have complicated fuel management without driving the pilot/copilot workloads through the ceiling.

On original B-52's, fuel management was practically a full-time job for the copilot. They have a B-52 control panel in the Museum of Flight in Seattle...it's incredible.

Tom.
 
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RE: DC-8 "Cut-Back" Pylon Question

Wed Aug 22, 2007 6:51 am

To Ex52tech,

Nobody is perfect and this is not a perfect world, it would be a dull place if it were. You can stop beating your head now, i see it has been going on for 20 hours. Thanks for the effort and taking the time of digging into the material.

Quoting Blackbird (Reply 23):
There was a book written about the DC-8 by Terry Waddington, it said in regards to the DC-8-62 design the plane experienced rather violent flutter as it's speed was increased to 418 kts: "the cause was traced to the new slimmer pylons which were more flexible than the earlier version and the cantilivered location of the engine pod. The problem was solved by re-design of the fuel-management system, which allowed retention of reserve fuel in the outer portion of the wing, thereby increasing it's stiffness" (pg 69-70)

I'm still looking for a copy of that book, i think it is out of print. The violent flutter, was that flutter to the wing or flight controls or the pylons themselves? In other words, was the flexing of the pylon due to flutter of the wing or did the pylons flexing initiate the flutter to the wing? Furthermore, the flutter occurred at a speed of 418kts (i presume IAS), but the Type Certificate (rev. date May 1, 2006) mentions a VMo of 338kts at SL increasing to 351kts (MMo 0.847) at 23k ft reducing to 255kt (MMo 0.88) at 42k ft for the DC-8-62, nowhere near 418kt (unless measured in TAS which is not logical because aerodynamic performance is usually measured in IAS). If it was 418kt IAS then the flutter appeared well beyond the operating limits of the aircraft.


Starglider
 
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RE: DC-8 "Cut-Back" Pylon Question

Wed Aug 22, 2007 7:22 am

Starglider,

I'm not really sure. It seemed to be the result of the thinner pylons... so it may have started there...

Andrea K
 
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RE: DC-8 "Cut-Back" Pylon Question

Wed Aug 29, 2007 9:08 am

Oh... and Starglider...

Back then older speedgauges weren't properly designed to take into account some compressibility effects. Especially at maximum speed the number would just creep up and up and up.

Andrea Kent
 
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RE: DC-8 "Cut-Back" Pylon Question

Thu Sep 06, 2007 8:18 am

When was the first time Boeing considered making the JT3D turbofan, and when was the first time Douglas or Boeing etc were first made aware of the JT3D?

Regarding the interference effects with the new cutback pylons with the long-duct fan-pod... would the JT4A or RR Conway pod have caused a similar disruption as they were around the same diameter?


Andrea Kent
 
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RE: DC-8 "Cut-Back" Pylon Question

Thu Sep 06, 2007 2:56 pm

Quoting Blackbird (Reply 30):
When was the first time Boeing considered making the JT3D turbofan, and when was the first time Douglas or Boeing etc were first made aware of the JT3D?

Assume you mean when Pratt & Whitney (not Boeing) considered making the JT3D. According to the P&W website, the JT3D made its first test flight in 1958 even before the first 707 went into service, so JT3D development must have started at least a couple of years earlier. The 707 and DC-8 were the only aircraft types that used the commercial version of the JT3D so I'm sure Boeing (and Douglas once the DC-8 program was launched) were very closely involved in its development since it was intended for their aircraft.

I believe the first aircraft delivered with JT3Ds was this 720B on February 3, 1961 (when it was AA's N7538A):


View Large View Medium
Click here for bigger photo!

Photo © Kjell Nilsson



However the first aircraft actually built with JT3Ds appears to be the AA 707-120B below. It made its first flight June 22, 1960 but wasn't delivered until almost a year later, May 25, 1961. Assume Boeing kept it for test purposes.


View Large View Medium
Click here for bigger photo!

Photo © Bob Garrard
View Large View Medium
Click here for bigger photo!

Photo © Frank C. Duarte Jr.



The first DC-8 delivered with JT3Ds was this KLM DC-8-53 on April 3, 1961:


View Large View Medium
Click here for bigger photo!

Photo © Mel Lawrence



The first 707s and DC-8s (and the world's first commercial aircraft) with turbofans were of course not any of the above, but the aircraft below, the first DC-8-40 and 707-420 with Rolls-Royce Conways delivered to AC (then TCA) February 7, 1960 and to Air India 10 days later on February 18, 1960.


View Large View Medium
Click here for bigger photo!

Photo © John F. Ciesla
View Large View Medium
Click here for bigger photo!

Photo © John F. Ciesla



<

[Edited 2007-09-06 08:02:12]
 
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RE: DC-8 "Cut-Back" Pylon Question

Fri Sep 07, 2007 8:57 am

While slightly off topic --I remember seeing a drawing of a shorter-ranged DC-8 concept which was like a 720 competitor. Douglas never actually persued it much further than the drawing board, but it featured an inboard glove like the 720 and 707-120B did. The glove didn't carry all the way to the inboard engines. I'm wondering if the DC-8 had a glove that extended just around to where the inboard pylons are, would they have needed to modify the inboard pylon at all to compensate for the airflow difference or would it make little difference even if a cutback was used?

While not entirely related to the original


Viscount724,

Yes I meant Pratt & Whitney. From what I read in a book just today it would seem around 1956 to 1958 although I'm not sure of an exact date of when they came up with the idea.


BTW: Does anybody have the answer to the lower half of Reply 30 (did the wider RR Conways and JT4A's have interference problems like the JT3D long-duct pods did)


Andrea Kent
 
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RE: DC-8 "Cut-Back" Pylon Question

Sat Sep 15, 2007 4:18 am

Did the RR Conways and JT4A's have interference effect problems with the wings leading-edge like the DC-8-62's JT3D Long-Duct Fan Pod did?

Does anybody have an answer?

Blackbird

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