dihedral break in middle of center section

[raymondbird]

I just want to make sure I understand... you're building a metal wing to attach to a wooden fuselage?

If it's a wooden wing, just build the center section with the dihedral break like you would at the middle of any one-piece wood spar. Attachment stays the same. When doing the design, don't forget that there is an axial component along the spar due to lift and your shear web design will need to account for this.

If you're doing a metal wing, I think BJC has the ticket--split at the middle, and spliced together. With the two-bolt attachment, shear will be zero between the bolts (assuming the wings are loaded evenly) but bending moment will be at its maximum value between the bolts. Therefore the splice at the middle will need some attention to carry the moment, but you could treat it similar to the design of an ordinary wing panel joint (heavier here because of the high moment) or you could make the caps using built up 1/8" curved "straps" (RV style) or by curving the angle as BBerson suggests.

You might also look into @Will Aldridge's SpitSairWulfStang project--it had a curved spar for a in inverted gull, but I don't remember the material or details.

Ya, that's the thought anyway, bolt a metal wing to a wood fuselage. Mainly because I need a three piece wing and that is easier in metal I believe.
Will's Corsair/KR2 wood center section looks great too but can it be up-sized for 2500 lb. gross weight . . . ?

The other problem with wood is the spar becomes really wide at required strength. The Jurca spar is 10" wide and since the wheels must fit ahead of the spar, the spar has to be very far back in the wing. Being wide means the center of strength/stiffness is even further back. Hate to bore you with all this but an expert advised me to get the spar narrower if possible for structural divergent reasons. I've had it re-designed in carbon fiber even which is great - only 1.25" wide but still one piece. I'll attach a pic of how far back original spar would be. A metal spar would be narrow too.

 

[BJC]

Will's Corsair/KR2 wood center section looks great too but can it be up-sized for 2500 lb. gross weight . . .

See The Spruce Goose


BJC

 

[Tiger Tim]

Committing to finding the space to build the per-plans one piece wing will take much less time than redesigning the wing entirely. Probably on the order of years. I’m not saying you can’t do it, you probably can, but I recommend thinking really hard about whether you want to do it.

Having said that, my first reaction is to come as close to copying the PT-19/23/26/Cornell as you can. It has enormous wood box spars with a join just outboard of the landing gear.

 

[pictsidhe]

Ya, that's the thought anyway, bolt a metal wing to a wood fuselage. Mainly because I need a three piece wing and that is easier in metal I believe.
Will's Corsair/KR2 wood center section looks great too but can it be up-sized for 2500 lb. gross weight . . . ?

The other problem with wood is the spar becomes really wide at required strength. The Jurca spar is 10" wide and since the wheels must fit ahead of the spar, the spar has to be very far back in the wing. Being wide means the center of strength/stiffness is even further back. Hate to bore you with all this but an expert advised me to get the spar narrower if possible for structural divergent reasons. I've had it re-designed in carbon fiber even which is great - only 1.25" wide but still one piece. I'll attach a pic of how far back original spar would be. View attachment 103453 A metal spar would be narrow too.

You could move the shear centre forward somewhat with some tailoring of the shear webs. Same way you'd tailor a composite spar. A D-box in front of the wheel would also help. You really should be redesigning the whole wing if you want to alter the spar.

 

[Hot Wings]

My first thought looking at the plans sheet - Why not just add some structure to the spar at the end of the wheel well bay and break it into 3 parts there?
But the spar is so far aft it is also looks like the main torsion element for the center section?

So another non intuitive idea:
Break the stock wood spar at the 'logical' point, add some structure, but don't bolt it back together like a normal wood 'I' been splice. Bolt it together with a lot of little fasteners around the perimeter of the spar, like the wing attach on a DC-3.

Another thought: Think about this from the perspective of fixing a broken wood spar - except build it 'broken' and 'repair' it on final assembly.

 

[raymondbird]

Committing to finding the space to build the per-plans one piece wing will take much less time than redesigning the wing entirely. Probably on the order of years. I’m not saying you can’t do it, you probably can, but I recommend thinking really hard about whether you want to do it.

Having said that, my first reaction is to come as close to copying the PT-19/23/26/Cornell as you can. It has enormous wood box spars with a join just outboard of the landing gear.

Hey, sounds great thanks. Will see what I can find about that

 

[raymondbird]

My first thought looking at the plans sheet - Why not just add some structure to the spar at the end of the wheel well bay and break it into 3 parts there?
But the spar is so far aft it is also looks like the main torsion element for the center section?

So another non intuitive idea:
Break the stock wood spar at the 'logical' point, add some structure, but don't bolt it back together like a normal wood 'I' been splice. Bolt it together with a lot of little fasteners around the perimeter of the spar, like the wing attach on a DC-3.

Another thought: Think about this from the perspective of fixing a broken wood spar - except build it 'broken' and 'repair' it on final assembly.

Very clever and will have to think about that and talk to an engineer. You are correct too about the original 10" wide spar being the main torsion element. Seems to me after reading all Bilski's posts though, among others, is that this wing would still twist the wrong way when loaded. Spar is too far back.

 

[Hot Wings]

, is that this wing would still twist the wrong way when loaded. Spar is too far back.

Yes, aeroelastic distortion is going to be a significant concern with the spar that far behind the aerodynamic center - just like a forward swept wing.
Your structural engineer will have to consider this and may find that it is the driving/limiting factor.

If you are paying an engineer to do all of these calculations the others on this post may have a pretty good case with regard to spending that money on a larger work area. It might be cheaper overall?

 

[mcrae0104]

Since it does appear that the spar is carrying the torsion, you'll need to ensure that it can still do so if you switch to metal. You really do need to re-engineer the whole wing in that case.

If you stck with wood, I'd break it outside the wheel well as suggested, leaving one beefed-up rib (truss rib w/ plywood both sides) at the outboard end of the center section. This would allow a third connector near the leading edge so that the torsion can be more effectively transferred from the outer panel to inner. Definitely need to sharpen your pencil on that.

 

[Riggerrob]

How much of a D-spar can you install forward of the wheel-well?

 

[raymondbird]

How much of a D-spar can you install forward of the wheel-well?

Wheel is about 4" from leading edge of wing when retracted and the depth there is about 6". Main spar is 9" deep at the root

 

[Riggerrob]

Try looking at sailplane wing attachment fittings for inspiration.
You will find that the farther outboard spar attachment fittings are ... the lighter they can be. The greatest bending stresses are at the centre-line, ergo require the strongest and heaviest fittings. Some competition sailplanes have 3-piece wings with joints at half-span.
Consider that any joint will weigh 4 times as much as a straight spar. Consider the two pieces of spar butted against each other, then doubler plates, then bolts, then add a safety factor.
By far the greatest bending stresses are at wing roots. That is why many World War 2 assault gliders (Hoadrian, Messerschmitt Gigant, etc.) had vestigial struts to beef-up wing roots.

Another glider idea is to overlap spars inside the fuselage. On many gliders, wing spars just rest in a "saddle" at the fuselage wall, then extend all the way to the far fuselage wall. The end of the spar has a simple, metal pin that plugs into the far wing. This transmits spanwise loads with the lightest structure. It helps that spars overlap where bending loads are greatest. If you want to maintain Messerchmitt 109 external loft lines, keep the top of the spar halves flat and only taper the bottom edge of the spar web. Then the challenge is to cut bottom spar caps to the precise curve to make it look Messerschmitt-like.

Consider the US Navy's changing attitudes towards wing fold configurations. Their first folding wing monoplane was the Grumman Wildcat which folded wings just outboard of the fuselage. This allowed for the narrowest folded configuration to allow stowing the maximum number of airplanes below decks.
By the end of World War 2, Grumman Bearcat only folded outer wing panels, and they barely folded narrow enough to fit in elevators. Grumman saved hundreds of pounds by moving the wing-fold hinge outboard.

To that end, I recommend that you build your wings in three parts (ala. Thatcher CX-5). Make the centre section barely narrow enough to conform to local highway trailer rules (usually 8 feet wide). You could also learn from Thatcher's construction methods where he starts building the cockpit and wing centre-section together. He completes the wing build, then un-bolts the outer wing panels and stores them (hanging from ceiling). Thatcher wing centre-sections are permanently riveted to the fuselage. Then Thatcher turns the fuselage 90 degrees and completes the aft fuselage. Building the wing in one large piece helps keep alignment precise.

 

[Will Aldridge]

Also take a look at the KRSuper2 website is KR-Super2 it's a 3 piece wing bolted to the fuselage made from graphlite cf rod for the spar caps and fiberglass cloth for the shear web. The cf rods are very flexible individually and can easily make your dihedral.

 

[raymondbird]

Also take a look at the KRSuper2 website is KR-Super2 it's a 3 piece wing bolted to the fuselage made from graphlite cf rod for the spar caps and fiberglass cloth for the shear web. The cf rods are very flexible individually and can easily make your dihedral.

Wow, you guys are the absolute best! Such an amazing wealth of knowledge in this group.

Hey Will, I was indeed inspired already by the KRSuper2, and even went so far as to get Jim Marske to design a carbon rod replacement for my massive Jurca wood spar. I have it built already too. Only problem is I've lost my big workshop years ago. I'm ready to continue work now but can find no way to get room enough to build a one piece 24' span wing. I like the 8' center section idea as well because, like you said, it would still be trailer-able and with the landing gear and under wing radiators all permanently built in as a bonus.

Maybe I could get a re-design and overlap carbon fiber rod spars? (If they wouldn't get in the way of all the gear retraction bits) Didn't think of that actually. I was thinking I needed metal spars for the super high stress butt connector wing attach strap method. Maybe that could be done with a carbon rod spar as well but my understanding is you can't drill through them for bolts . . . ?

Thanks again!

 

[Hot Wings]

went so far as to get Jim Marske to design a carbon rod replacement for my massive Jurca wood spar. I have it built already too. O

Maybe I could get a re-design and overlap carbon fiber rod spars?

I again offer the idea to 'repair' the existing piece.

Many years ago I asked Marske about the repair process for the Graphlite rod. The essence of his reply was that it needed to be overlapped by 50X the diameter. Of course in your situation you will need to add extra structure to mount the fittings. These might be added in the same way that they are at the root - per his book?

Marske may still have his calculations for your spar and would be the best person to ask if this idea is possible or practical.

 

[Victor Bravo]

Look at the Monnett Moni for an example of a center-join metal wing with dihedral in the middle. May be worthwhile for your use, I am not sure. But a good data point.

 

[raymondbird]

I again offer the idea to 'repair' the existing piece.

Many years ago I asked Marske about the repair process for the Graphlite rod. The essence of his reply was that it needed to be overlapped by 50X the diameter. Of course in your situation you will need to add extra structure to mount the fittings. These might be added in the same way that they are at the root - per his book?

Marske may still have his calculations for your spar and would be the best person to ask if this idea is possible or practical.

I am going to do just that thanks, and will let you know what he says. BTW, I think I may have found a solution and all wood - the Siers Barracuda, and plans are still available I hope. It's wing spar is 3 piece and surprisingly narrow for the same gross weight that I need. No dihedral in the center section but how hard could that be to change in wood.
Salute!

 

[BJC]

BTW, I think I may have found a solution and all wood - the Siers Barracuda, and plans are still available I hope. It's wing spar is 3 piece and surprisingly narrow for the same gross weight that I need. No dihedral in the center section but how hard could that be to change in wood.

Ensure that you retain the required torsional rigidity.


BJC

 

[dominique]

Hello
I'm trying to follow this interesting post
I take this opportunity to ask you a question
I take the example of the KR2 and its 3-part spar
I'm assuming that the spar caps thicknesses need to be increased at the connecting fittings compared to a one-piece spar to account for the spar being drilled.
Do you know how we can calculate or estimate the necessary increase in thickness?
thank you very much
Dominique .... sorry "google" translation !

 

[mcrae0104]

Do you know how we can calculate or estimate the necessary increase in thickness?

You will need to make sure that the section maintains its required moment if inertia after the material is removed for the bolt holes. Check out Jeff Hanson's videos for more on moment of inertia. Of course you will need to check the bolt connection and that may also require adding more material.