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Gull wing spar design

Posted By:
William Aldridge
3
Posts
1
#1 Posted: 10/2/2010 22:22:24

I'm designing a LSA with an inverted Gull wing.  I know it's going to be difficult and not very practical but I'm doing it anyway.  I just need to figure out how (not using wood).  For any familiar with the WAR replica corsair the spar is shaped similarly.

 

I have a couple of ideas:

1:  Aluminum I-beam with aluminum web with t shaped extrusions with notches cut in the leg of the T where the piece has to bend around the web.

2:  Make a steel tube truss type spar that would be integrated into a steel tube fuselage.  The curves in the spar would be accomplished by bending the tube.  Downside is that bending tubes can cause cracks when welded, although my preliminary design has all welds at the ends of the bends.

 

Any comments or suggestions are appreciated.



Joanne Palmer
Young Eagles Pilot or VolunteerHomebuilder or Craftsman
276
Posts
68
#2 Posted: 10/4/2010 09:30:48

William:

 

You are biting off a lot but I'll go through your ideas and perhaps make a few suggestions.  The big issue here is to react the huge bending moment as it passes through the bend in the spar.  This is a considerable force if you assume that the lifting forces are all on this outboard wing and the inner portion is providing no lift.  If you assume that the center of all this lift is at 1/2 the remaining span you can see that the moment is substantial. This moment needs a continuous metal structure to carry it.  Don't forget to add the g factors required.

I have a couple of ideas:

1:  Aluminum I-beam with aluminum web with t shaped extrusions with notches cut in the leg of the T where the piece has to bend around the web.

Rather than cut the notches and bend the remaining flange.  It would be better to cut the spar at the precise angles necessary and then add gusset plates to the joint on both sides.  These would then be riveted or better yet, bolted together with HiLocks and Hilock collars.  These gussets should not only bolt to the web of the I-beam (not a  "T" section) but also through the flanges.     Here you have two options.  You can machine the gussets out of plate stock  and make them fit on all three sides, OR make them up out of machined fittings our of bar stock and use an additional web only gusset cut and profiled.  Again, you need to make a continuous amount of metal to carry this large moment.  Once the loads are known proper sizing may dictate the selection of the exact configuration.

2:  Make a steel tube truss type spar that would be integrated into a steel tube fuselage.  The curves in the spar would be accomplished by bending the tube.  Downside is that bending tubes can cause cracks when welded, although my preliminary design has all welds at the ends of the bends.

This option is OK IF you have adequate bracing as metal that is bent already has some odd metallurgical properties that are hard to design with.  In most cases it is easier to do something similar to what is described above using sockets (internally or externally).  Also,  I've never liked tubes (round I presume) for essentially uniplanar loads anyway as the properties required sometimes get you into a larger and heavier structure than what is really needed.

If you want a quick picture of the concept for your Option 1, reply requesting one.

 

 



Craig Cantwell
43
Posts
8
#3 Posted: 10/4/2010 14:41:38

William: If you go to the following link and scroll down a couple of pictures, it will give a good idea how Vought did it. It will also show why new spars for Corsair restorations are in the megabucks price range.

http://worldwartwozone.com/forums/index.php?/topic/8079-corsair-restoration-pics/

 

Hope the photos help some,

Craig C.

 



William Aldridge
3
Posts
1
#4 Posted: 10/4/2010 20:51:56

Joanne,

 

I appreciate your input and yes I would like to see an illustration of your idea. 

 

Craig,

Thanks for the link, i have seen that before.

 

I am a huge fan of the F2G Super Corsair, this is one of the reasons I want to design a gull wing aircraft, but my plane won't be a scaled down Corsair.  I want it to be unique and definitely one of a kind.  I don't see the point in designing my own plane if there is already something out there that is pretty much the same thing. 

 

Anyway I appreciate the comments.

 

 




Spencer Gould
Homebuilder or Craftsman
32
Posts
1
#5 Posted: 10/8/2010 01:13:31

 

A composite spar with a C, I or box cross section could easily take on that shape. You would really need to know your material allowables and composite design. Although I used pultruded pre made carbon “graphlite” on my own design these kinds of “rod” materials would tend to fight taking on the curve with out a serious vacuum or clamp setup. However older uni directional carbon or S-glass layup methods could do the trick. As for the mold you could use a flat table with foam formers to take on the shape of the caps covered in duct tape its about as cheap as your going to get on tooling.

 

Composite skins in the compound curve area is another thing to think about.

 

Good hard points on the spar(s) will allow you to mechanically transition to a fuselage built out of any other kind of material out there.  

 

All you have to do is get it back to the non curved section like the F4U pics showed then the rest of the outboard section could be more conventional methods

 

Spencer G

 

EAA tech counselor #5426    



Sonja Englert
Homebuilder or Craftsman
18
Posts
1
#6 Posted: 10/9/2010 12:08:40

This is right, a composite spar would be easier and cheaper to build than a metal structure of this shape. If you need engineering assistance, let me know. I am an aeronautical engineer (www.caro-engineering.com).

 

Sonja



William Aldridge
3
Posts
1
#7 Posted: 10/9/2010 13:53:40

Thanks Spencer I had at one time seriously contemplated making the plane out of composites, but someone told me that metal would be far cheaper and easier to work with.  While I was considering the composite idea I had had a similar idea.  Someone said that Rutan laid fiberglass into a piece of steel C-channel removed it and repeated the process once more then joined the 2 pieces back to back to get an I-beam then used carbon fiber strips for the spar caps.  I had thought to use a similar technique that seams to be exactly what you are describing.

 

I have given up the I-beam idea completely since someone informed me that I-beams don't handle Torsion very well and that a box beam would be a lot better.  I have since come up with the idea of making a kind of sandwich/box beam.

 

In essence I would use a thin piece of corrugated aluminum (6-8 inches between corrugations) between 2 slightly thicker face sheets of aluminum.  I don't exactly know what I would use for spar caps yet but i had the idea of using multiple layers of thin aluminum formed laminated together to make the spar caps.  I would make a form the shape of the wing spar and then bend the strips to make C-channels out of them kind of like a rib forming.  The sides would then be riveted into the face of the spar and when it was to to attach the wing skin the rivets for the skin would also hold all the laminated strips together. 

 

Sonja,

 

I would love any help you are willing to offer.  In essence I have the whole design roughed out and what I need to do now is start calculating loads on everything.  I even have some of the load paths thought out.  Trouble is I don't know where to start.  I have been buying and reading all the books I can on the subject but I still don't know everything i need to do the job.  The project has been going in fits and starts.  As I learn something new I can progress but there are long periods when nothing gets done because I lack the knowledge of how to proceed. 



Tony Pileggi
Homebuilder or CraftsmanAirVenture Volunteer
54
Posts
24
#8 Posted: 10/10/2010 08:40:05

I think you should go with a composite spar. Since it has been done, you won't have to re-invent it. Take a look at my web page for more info.

www.Corsair82.com

 

Tony Pileggi



All composite 82% F4U-1A Corsair replica www.Corsair82.com
Sonja Englert
Homebuilder or Craftsman
18
Posts
1
#9 Posted: 10/10/2010 09:30:23

William,

 

Please contact me directly so we can discuss it, sonja.englert@juno.com  541-389-3284.



Spencer Gould
Homebuilder or Craftsman
32
Posts
1
#10 Posted: 10/14/2010 22:30:28

 

Thought some visuals may help from my design. I used the foam & duct tape technique to form the lower corners of my fuselage, viewed from the top and side the mid section is curved where it transitions from a constant cross section to a tapered one going towards the tail. All on a bent sky jump shaped mould table covered in clear packing tape on the flat section. I use duct tape on the blue foam as it adheres to it better then the clear tape. This corner also features a 1” radii. The compound curve section spans about 30” the foam formers / moulds where hot wired out. Relief cuts helped them to go around the corner. The final product came out great. Not sure you will need to go as fancy but this shows you the bang for the buck you get using these methods.

 

Hope you can use something similar.

 

Spencer



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