Tube Spars and Foam Cored Wings

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ToddK

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I think you are on to something. It seems to me like a foam wing, with round aluminum tube spars, a thin layer of fiberglass, with a length of wood (false spar) on the back to connect flaperon hinges to might be the easiest and fastest way to build wing.
 

ToddK

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This is what I have been knocking around. A thin, solid foam core made by bonding together sheets of 2" Dow Foam, that comprise the leading edge of the wing, and the necessary internal structure along with aluminum tube spars, all glassed. This structure could be thinned in the middle more then what I have drawn. Only the leading edge portion would need detailed sanding and smoothing prior to glassing, as that is the only part exposed to airflow. Hard points for mounting flight controls (probably made of wood), would also need to be installed as well as paths for cables prior to glassing. Foam ribs are then hot wire cut, and glued to the now fully fiber glassed core. A single layer of class capstrip is applied to top and bottom of each rib and going back to the trailing edge. The wing would then be fabric covered.
This method would use a minimum amount of foam and glass, and, would require a minimum amount of detailed shaping and smoothing.

The first inboard section/bay of ribs would be likely be a solid section, as well as the sections on either side of the ailerons for strength and to prevent warping when the fabric is shrunk. Other options might be to angle the ribs into each other for strength at the trailing edge.

I believe a wing such as this would be pretty light, and very fast to build, especially if full length ailerons were used.

This construction method could also be adapted to numerous legacy designs. Simplifying wing construction, while keeping the wood/tube and fabric aesthetic, and utilizing easier to procure materials. The foam can be procured at the local big box store, and the aluminum tube appropriate for the design loads, from the many local and on line metal dealers.
 

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ToddK

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This probably deserves its own thread for best consideration of the idea.
How do the loads get to the spar? Passing concentrated loads through the foam is usually best avoided, unless the loads are small and the foam can take them.
Single spar, so the twisting moments also need to get into the spar. I'm not clear on how that happens. Bonding AL to foam?
Is the rear side of the D-cell glassed in, the ribs glued to the back?
That already-built D-cell is a great place for some pultrusion spar caps and a web layed right on the back of the cell. Or, just lots of uni layers instead of the pultrusions (heavier, but still lighter than an AL tube, and easier to ship. Also avoids the AL bonding challenges).
For those who want fabric, this is an interesting idea.
You might be right about its own thread. My thought was why mess with taking out a bunch of foam, when you can just make the necessary portion smaller.

As drawn, I was thinking a front and rear spar, the front spar being a larger diameter then the rear. The entire inner core, the grey shaded area is constructed first, and completely glassed. The ribs are then added.

In terms of passing loads, my initial thought is that the foam can probably handle it just fine. If a kitfox with 9 plywood ribs can do it, then 16 feet of continuous foam should be able to. If that is not the case, then it would be pretty easy to include some plywood to help out with that.
 
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ToddK

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Looks interesting. I don't see a need for the aluminum tube spars.
My brain is still thinking in terms of more traditional aircraft construction. I kind of figured the inner glassed foam section would be pretty strong on its own. Without the tube spars, I am not sure how the wing would be attached to the fuselage and to the struts.
 

Riggerrob

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You still need a tubular main spar root to mate with tubular fuselage fittings.
But you also have the option of inserting a vertical spar web part way into the fuselage, then building whatever outer wing panels you want.

A new BD-4 wing would benefit from a cambered airfoil to better climb and cruise.
I still like Jim Bede's telescoping (short-span) wing panels. Can you still use original concept wing panels on both sides with only a single end rib to close off the wing tip or wing root?

As for a new BD-5 wing ... there is little point to tapering it with Reynolds numbers that low. At those low Reynolds numbers, smoothness is a big issue driving you towards precise female molds. I have seen at least one BD-5 fitted with after-market wing cuffs that substantially increase camber towards the leading edge. These cuffs are glued to the metal top skin, forward of the main spar. BD-5 The thicker leading edge tames stall characteristics ... closer to what new private pilots can handle. If the thicker leading edge is incorporated - from the start - in a new BD-5 wing, it can increase fuel volume just forward of the centre-of-gravity.
 

wsimpso1

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You still need a tubular main spar root to mate with tubular fuselage fittings.
Wel, only if you have tube fuselage fittings. I have looked at a lot of homebuilts, and only a few designs (mostly Jim Bede designs) use round tube spars and fittings. Most use some sort of channel, I beam, box spar, etc, with the bending strength tailored along the spar to the expected bending moment. And they use bolts to make the connections.
 

wsimpso1

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You have this massive solid core wing. Now what? How do you attach it to the airplane?
ToddK,

Is that a serious question? Monocoque and Semi-monocoque have been around airplanes since the 1930's. First it was aluminum, later fiberglass and then carbon fiber, but there is nothing magic about it in metal and rivets, and it works fine in composites.

In the Long-EZ and all of its derivatives, you have a structural skin composite fuselage, a beefy rectangular or channel shaped spar or two bonded in, and then extra plies of composite tapes reinforcing the fuselage near the spar and distributing loads between the fuselage and the spar. The center section spar typically extends a couple feet out from the fuselage wall and has wing skins around it. The outer wing panels are built with the spar going inboard past where the flying surfaces end, and all of the loads funnel into the rectangular or channel shaped spar or spars. The outer panel main spar usually overlaps the center section, and is pinned to the center section spar with a couple bolts (axis fore-aft) a couple feet apart. On some designs there is only one set of spars at the thickest part of the wing, and the spars are wide things intended to carry all loads including pitching moments. Other designs run some form of a drag spar in the outer panel and connect to a comparitively light center section drag spar with a bolt or two. The drag spar is also bonded to the fuselage with local reinforcements.

Three part wing was also done on hollow wing Lancairs and others. My bird is being done that way. Billski's Fiberglass Bird | Page 2 | HomeBuiltAirplanes.com , with posts 33 and 35 showing the combination. You could have a ball looking over my log... Not much tube in my bird. Lessee, rudder/brake pedals, parts of the rudder system, stick system, flap actuation, some control bits, nose gear, and the engine mount.

Then there are two piece wings. Sailplanes have been doing this a long time and they take them apart and put them in the trailer after every day of flying. Instead of a center section, there is a beefy box of some sort built into the fuselage, the wings have spars that protrude inboard with the left and right spars overlapping each other through the fuselage, and attached with two big pins one near each fuselage wall. A drag spar may be present and be pinned or just be contained by a pin and socket.

Then there are one piece wings. Yep, one long spar and wing. The Glasair I, II, and III all had a one piece wing that was continuous, even the skins carried across the whole span and through the cockpit, just like some of Pazmany's birds did. I imagine the fuselage sat on a contoured pad, but that there were a few tabs with bolts to hardpoints in fuselage. This approach is also how the canard is typically built and installed on Long-EZ's and derivatives. My horizontal tail is built this way and will be bonded to a saddle in the bottom of my fuselage, then the vertical tail will be built around and bonded to it, then the top half of the fuselage bonded on and to both tail planes. That works too, where I only have nine feet of span and five feet of height to get down the road. Rutan style canards have too much canard span for that move, so they use lift tabs and a cover.

I have personally viewed a Panzl (nee Staudacher) unlimited aerobatic mount, and it has a one piece wood and carbon fiber wing with the aero surfaces removed through the fuselage with two bolts forward (through the main spar) and two aft (through the drag spar) that all run fore-aft through hardpoints in the steel tube fuselage. Oh, and there is this neat take-apart bridge that Greg fashions that closes the bottom of the fuselage and connects the front and back of the bottom longerons together too.

I hope that this does not blow your mind, there are lots of ways to do things. While the Tailwind and various Pitts are way cool, there is a whole 'nother world out there making wings connect to fuselages in other ways.

Billski
 
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ElectricFlyer

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My guess is not much. I sure would not use it as an example of sturdy construction.
To be fair - he said he was not doing anything silly in them like barrel rolls - and he did some reasonable load testing on the last one, over 3G for something that was just for floating over the fields. However I liked some of his previous wings that he covered in FG instead of vinyl like on latest.
Cheers
 

Geraldc

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I wonder how oratex would work instead of vinyl? Would need foam that could stand the heat of ironing.
 

wsimpso1

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I wonder how oratex would work instead of vinyl? Would need foam that could stand the heat of ironing.
You shrink Oratex with the iron set at 300-350 F. Various polyester fabrics used for aircraft covering also need temperatures in that range.

The prominent plastic foams used in homebuilt airplanes for cores are extruded polystyrene, polyurethane, and polyvinylchloride. Let's talk about each:
  • Polystyrene - The only foam we know about that can be hotwire sawn safely, is not self extinguishing, max temperature around 180F, low strength and low crush resistance;
  • Polyurethane - Easily mechanically worked and shaped, some have been known to delaminate easily, modest strength, is not usually self extinguishing, max temperatures can be obtained over 300F;
  • Polyvinylchloride - High toughness, self-extinguishing, max temperature around 200 F, highest strength of the three.
So, one will not hotwire foam and then be able to shrink Oratex onto it.

One might specify a high temp polyurethane foam and use a CNC bandsaw to sculpt cores, bond, etc, then shrink Oratex or other polyester fabric. All of the high temp foams I found were high density tooling foams...

Billski
 

proppastie

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I might need more input here but my fabric manuals seem to say to glue 2" over lap at leading edge .....do not glue to the ribs, and shrink the panels between the ribs tight. So perhaps one could have foam ribs but as long as you kept the iron away from the ribs when shrinking you might still use a heat shrink Dacron?
 

TFF

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When you shrink, the fabric is best to slip across the ribs so there is no pent up or uneven pull. Under-cambered has its issues as well as people gluing to ribs instead of rib stitching. So shrinking next to ribs is done. It can have pull marks if that might bother you. I believe the vinyl Peter used was Oratex trim vinyl. The different temps some systems use has more to do with the paints. Polyfiber Chemicals donot shrink. Dope shrinks even if it’s non shrinking. Polyfiber instructions have you tighten the fabric to max. Dope has to leave some extra shrink allowances for the dope, so it does not get shrunk as tight. It can pretzel structure In time.
 
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