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DC-8 "Cut-Back" Pylon Question  
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Posted (6 years 9 months 4 days 22 hours ago) and read 4713 times:

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]

33 replies: All unread, showing first 25:
 
User currently offlineMiamiair From , joined Dec 1969, posts, RR:
Reply 1, posted (6 years 9 months 4 days 21 hours ago) and read 4706 times:

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


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 2, posted (6 years 9 months 4 days 17 hours ago) and read 4665 times:

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


User currently offlineAeroWeanie From United States of America, joined Dec 2004, 1606 posts, RR: 52
Reply 3, posted (6 years 9 months 4 days 17 hours ago) and read 4661 times:
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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.

User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 4, posted (6 years 9 months 4 days 17 hours ago) and read 4654 times:

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


User currently offlineAeroWeanie From United States of America, joined Dec 2004, 1606 posts, RR: 52
Reply 5, posted (6 years 9 months 4 days 16 hours ago) and read 4646 times:
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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.


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 6, posted (6 years 9 months 4 days 13 hours ago) and read 4625 times:

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


User currently offlineMiamiair From , joined Dec 1969, posts, RR:
Reply 7, posted (6 years 9 months 4 days 5 hours ago) and read 4608 times:

Here's a sketch of the foot stools:

http://i30.photobucket.com/albums/c325/miamiair/Foot-Stools.jpg


User currently offlineAeroWeanie From United States of America, joined Dec 2004, 1606 posts, RR: 52
Reply 8, posted (6 years 9 months 4 days 1 hour ago) and read 4578 times:
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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.


User currently offlineN8076U From United States of America, joined Jun 2006, 425 posts, RR: 9
Reply 9, posted (6 years 9 months 3 days 23 hours ago) and read 4553 times:

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



Don't blame me, I don't work here...
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 10, posted (6 years 9 months 3 days 17 hours ago) and read 4523 times:

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


User currently offlineStarglider From Netherlands, joined Sep 2006, 668 posts, RR: 44
Reply 11, posted (6 years 9 months 3 days 5 hours ago) and read 4486 times:

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


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 12, posted (6 years 9 months 2 days 22 hours ago) and read 4456 times:

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


User currently offlineN8076U From United States of America, joined Jun 2006, 425 posts, RR: 9
Reply 13, posted (6 years 9 months 2 days 19 hours ago) and read 4438 times:

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



Don't blame me, I don't work here...
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 14, posted (6 years 9 months 2 days 17 hours ago) and read 4429 times:

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

Andrea Kent


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 15, posted (6 years 8 months 1 week 4 days 19 hours ago) and read 4272 times:

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


User currently offlineStarglider From Netherlands, joined Sep 2006, 668 posts, RR: 44
Reply 16, posted (6 years 8 months 1 week 4 days 18 hours ago) and read 4259 times:

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


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 17, posted (6 years 8 months 1 week 4 days 17 hours ago) and read 4246 times:

Starglider,

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

Andrea


User currently offlineStarglider From Netherlands, joined Sep 2006, 668 posts, RR: 44
Reply 18, posted (6 years 8 months 1 week 2 days 17 hours ago) and read 4154 times:

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


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 19, posted (6 years 8 months 1 week 2 days 16 hours ago) and read 4152 times:

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


User currently offlineEx52tech From United States of America, joined Dec 2006, 559 posts, RR: 1
Reply 20, posted (6 years 8 months 1 week 2 days 9 hours ago) and read 4137 times:

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"
User currently offlineStarglider From Netherlands, joined Sep 2006, 668 posts, RR: 44
Reply 21, posted (6 years 8 months 1 week 19 hours ago) and read 4074 times:

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


User currently offlineEx52tech From United States of America, joined Dec 2006, 559 posts, RR: 1
Reply 22, posted (6 years 8 months 1 week 16 hours ago) and read 4061 times:

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 



"Saddest thing I ever witnessed....an airplane being scrapped"
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 23, posted (6 years 8 months 1 week 6 hours ago) and read 4039 times:

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


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 24, posted (6 years 8 months 1 week 5 hours ago) and read 4036 times:

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.


25 Blackbird : 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
26 Tdscanuck : Redesign is always an option, just usually an expensive one. I'm sure I meant C-17. I would assume it's pretty complex on the C-5 and C-141 as well.
27 Starglider : 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
28 Blackbird : Starglider, I'm not really sure. It seemed to be the result of the thinner pylons... so it may have started there... Andrea K
29 Blackbird : Oh... and Starglider... Back then older speedgauges weren't properly designed to take into account some compressibility effects. Especially at maximum
30 Blackbird : 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 JT3
31 Post contains links and images Viscount724 : 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 19
32 Blackbird : 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 persue
33 Blackbird : 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 anybod
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