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[fuselage/skin] Doublers  
User currently offlineDC10student From United Kingdom, joined May 2008, 9 posts, RR: 0
Posted (6 years 5 months 3 weeks 11 hours ago) and read 5263 times:

I've come across two descriptions for types of doublers. One being "finger-style" and the other "scalloped" and for the life of me i cannot imagine what they look like.

Anyone like to hazard a guess?

11 replies: All unread, jump to last
 
User currently offlineATLAMT From United States of America, joined Sep 2001, 240 posts, RR: 3
Reply 1, posted (6 years 5 months 2 weeks 6 days 21 hours ago) and read 5217 times:

Here is a link to some images.

Airplane Design Book



Fwd to MCO and Placard
User currently offlineDC10student From United Kingdom, joined May 2008, 9 posts, RR: 0
Reply 2, posted (6 years 5 months 2 weeks 6 days 11 hours ago) and read 5171 times:

Thanks for that, seems quite a good book!
The preview didn't let me see those pages which was a shame and its on long loan out of the library too.

But definetly useful to know about


User currently offlineATLAMT From United States of America, joined Sep 2001, 240 posts, RR: 3
Reply 3, posted (6 years 5 months 2 weeks 6 days 10 hours ago) and read 5161 times:

You couldn't see page 136?

How about this.



Not your typical finger doubler but you can see the idea behind it.



Fwd to MCO and Placard
User currently offlineDC10student From United Kingdom, joined May 2008, 9 posts, RR: 0
Reply 4, posted (6 years 5 months 2 weeks 6 days 10 hours ago) and read 5156 times:

Thanks.

Odd thing happened, i did another search and got the same book to preview again. But this time let me different pages with the pictures you suggested.
(plus there is a few more bits on the DC-10 in there)

Thanks for pointing me to this book!!!


User currently offlineDALMD88 From United States of America, joined Jul 2000, 2571 posts, RR: 14
Reply 5, posted (6 years 5 months 2 weeks 6 days ago) and read 5105 times:



Quoting ATLAMT (Reply 3):
Not your typical finger doubler but you can see the idea behind it

True, most of the ones I've used the top part, with the two rows is soild across. The finger doubler is also very thin compared to the rest of the repair bulid up. I've only really seem them used on Douglas aircraft. I've never really had a good explaniation of their pupose. Time for one of our engineering types to chime in.


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 6, posted (6 years 5 months 2 weeks 6 days ago) and read 5097 times:

What's a scalloped-doubler look like?

Andrea Kent


User currently offlineAbqwildcat From United States of America, joined Jan 2008, 6 posts, RR: 0
Reply 7, posted (6 years 5 months 2 weeks 5 days 23 hours ago) and read 5094 times:



Quoting Blackbird (Reply 6):
True, most of the ones I've used the top part, with the two rows is soild across. The finger doubler is also very thin compared to the rest of the repair bulid up. I've only really seem them used on Douglas aircraft. I've never really had a good explaniation of their pupose. Time for one of our engineering types to chime in.

The best explanation I've ever gotten on these finger doublers is that if one of the "fingers" cracks in service, the total strength of the doubler isn't reduced by much and the crack won't propagate across the entire doubler "unzipping" from hole to hole. I've never seen or designed a doubler with fingers for planes in production, but it's certainly possible there are cases when they'd be appropriate. I also can't remember seeing any finger doublers in the SRM for 747, 767, or 777.


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 8, posted (6 years 5 months 2 weeks 5 days 19 hours ago) and read 5068 times:

Uh, DALMD88 said what you quoted, not me.

Andrea Kent


User currently offlineAbqwildcat From United States of America, joined Jan 2008, 6 posts, RR: 0
Reply 9, posted (6 years 5 months 2 weeks 5 days 3 hours ago) and read 5015 times:

Sorry there Blackbird. I selected the text and hit "quote selected text" but failed to check to ensure it wasn't quoting the wrong person. How odd.

User currently offlineTZTriStar500 From United States of America, joined Jan 2004, 1453 posts, RR: 9
Reply 10, posted (6 years 5 months 2 weeks 4 days 20 hours ago) and read 4973 times:



Quoting Abqwildcat (Reply 7):
The best explanation I've ever gotten on these finger doublers is that if one of the "fingers" cracks in service, the total strength of the doubler isn't reduced by much and the crack won't propagate across the entire doubler "unzipping" from hole to hole. I've never seen or designed a doubler with fingers for planes in production, but it's certainly possible there are cases when they'd be appropriate. I also can't remember seeing any finger doublers in the SRM for 747, 767, or 777.

This is partly true. The Douglas concept of finger doublers do two things. They are thin fatigue doublers that allow for a better transition of load across a joint by introducing a lower initial load than the primary doubler(s) and spreads the load across the primary doubler(s) more evenly. They also act as a crack indicator and thus a higher life without supplemental inspections or access from the back side to inspect. For example if a crack starts, it would be first detected in the finger since it is not covered externally. Also, if I am not mistaken, they are not considered part of the overall static strength of a repair, but an extra feature.

Finger doublers are actually quite a novel idea and one of the the best joint designs for long fatigue life with few, if any, supplemental inspections. Unfortunately, they are difficult to install because of this. An airline has to stock this pre-made material or make it which is time-consuming. The DC-10 SRM does allow for not using one in some circumstances, but it does significantly reduce its fatigue life.

One of my theories as to why McDonnell-Douglas ultimately failed was this design. While Douglas airframes are sturdy and utilized some great structural engineering, they were complicated structurally, more expensive to build, and perhaps built a bit too well. What I mean by that is they last too long and thus did not have enough obsolescence built-in like a Boeing aircraft which used and still uses simpler lap splices while not as optimal structurally, do very well for their intention and are easy to assemble.

Actually Lockheed used a fatigue doubler in many of their L-1011 fuselage repairs which actually did some of what Douglas finger doublers did. They aided the structural life of the repair but where not considered integral to its static strength.



35 years of American Trans Air/ATA Airlines, 1973-2008. A great little airline that will not be soon forgotten.
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 11, posted (6 years 5 months 2 weeks 2 days 19 hours ago) and read 4909 times:



Quoting Blackbird (Reply 6):
What's a scalloped-doubler look like?

The edge is wavy, rather than straight like a normal doubler. Every fastened joint has a defined edge margin (distance from fastener to the edge of the material). This forms an imaginary circle around each fastener that must lie within the metal. If you make a big row of these, like a typical skin joint, you can see that there will be some material outside the edge margin of two adjacent fasteners. You can cut this excess material out and save weight without weakening the joint at all, which gives you a panel that has a wavy edge. The trade off is that you have extra manufacturing steps, and hence higher cost.

Tom.


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