How do I make a two piece wing?

[Jackson T]

I've just finished covering the wing, and the two piece wing mech works smoothly, except I have to make sure the posicle sticks come out both sides of the sheeted part. It feels very strong and sturdy! I don't think this method would be quite strong enough for my next balsa build though, which is hopefully going to be a 2.5m glider

 

[Merv]

Looks great

 

[Craftydan]

Moving to a longer wing, you do want to switch to a stronger material for the spar inserts, but for the pockets, it looks like you've got the right idea.

As the wing gets longer, you can also consider switching to a 3-piece wing instead of a two piece -- The center wingbox is permanent, but you have two removable joints at convenient points farther out. You still end up with reasonable sized pieces, but the joints don't have to be as strong. Easy peasy with a simple dihedral (since that's set into in the center panel), or makes polyhedral more manageable . . . if you're into that sort of thing

 

[Jackson T]

Moving to a longer wing, you do want to switch to a stronger material for the spar inserts, but for the pockets, it looks like you've got the right idea.

As the wing gets longer, you can also consider switching to a 3-piece wing instead of a two piece -- The center wingbox is permanent, but you have two removable joints at convenient points farther out. You still end up with reasonable sized pieces, but the joints don't have to be as strong. Easy peasy with a simple dihedral (since that's set into in the center panel), or makes polyhedral more manageable . . . if you're into that sort of thing

I have thought about that, but I think a joiner in the middle will be easier because it will be polyhedral. That way I won't have to worry about any angled wing joining spars. You gave me a good idea if I make a big normal dihedral plane though!

 

[Jackson T]

So, for my next build which I'm planning now, the plan is as follows. The main spar will be an I-beam spar, and I thought I could maybe put some 1/16 walls on the sides and slide a strip of aluminium through for the wing joining mech. Any thoughts? The slot for the wing joining spar will be around 7mm high. How long and thick should the aluminium spar be? Also, should I use two spars (one for the front section and one for the back), or should I use just one?

 

[TooJung2Die]

That'll be a strong spar but you may have over-engineered it. Normally a spar is a box shaped cross section. There are top and bottom spars and shear webbing front and rear. The wing joiner fills the empty box and extends into each wing for about three ribs.

Here is a box spar with the plywood wing joiner dry fit into the spar opening.

 

[Jackson T]

That'll be a strong spar but you may have over-engineered it.

Do you mean it will be stronger than it needs to be, or I'm just making things complicated for myself?

 

[Hai-Lee]

Do you mean it will be stronger than it needs to be, or I'm just making things complicated for myself?

It may be far stronger then you need but importantly the big disadvantage of an over-engineered structure is a significant weight penalty.

As for the webbing in between top and bottom spar members should be installed so that the distance between the top and bottom spar remains constant under all loads. Ideally the Grain of the webbing should run from the top spar member to the bottom spar member rather than spanwise.

Another thing to consider is that the wing extremities do not require as much strength as the centre sections and so you can reduce the strength required in the spar as you progress towards the wing tips. Normally where using webbing the thickness of the material can be reduced as you approach the wing tips Or the box spar build can revert to the "I" beam with a single web piece.

Just a few thoughts on wing structure!

Have fun!

 

[Jackson T]

It may be far stronger then you need but importantly the big disadvantage of an over-engineered structure is a significant weight penalty.

As for the webbing in between top and bottom spar members should be installed so that the distance between the top and bottom spar remains constant under all loads. Ideally the Grain of the webbing should run from the top spar member to the bottom spar member rather than spanwise.

Another thing to consider is that the wing extremities do not require as much strength as the centre sections and so you can reduce the strength required in the spar as you progress towards the wing tips. Normally where using webbing the thickness of the material can be reduced as you approach the wing tips Or the box spar build can revert to the "I" beam with a single web piece.

Just a few thoughts on wing structure!

Have fun!

I'm good with far stronger at the cost of some weight. It's going to be so light that it just might be to slow and fly below the sub-critical Reynolds Number anyway, which would mean I would have to put lead on to make it fly faster. My estimated AUW with battery is 800-900g, which is quite light for a 2.5m wingspan as far as I know. The main spar for the first 20cm out from the centre will be the wing joining mech, the next 42.5cm will be an "I" beam, and the last 62.5cm will be a simple strip of balsa, like on my current build. Does that sound strong enough?

By the way, here is a top view of the plane. Ignore the finer details like the ailerons because this is one of the first drawings. I've decided to draw it all up on CAD from now on.

 

[Hai-Lee]

I'm good with far stronger at the cost of some weight. It's going to be so light that it just might be to slow and fly below the sub-critical Reynolds Number anyway, which would mean I would have to put lead on it to make it fly faster. My estimated AUW with battery is 800-900g, which is quite light for a 2.5m wingspan. The main spar for the first 20cm out from the centre will be the wing joining mech, the next 42.5cm will be an "I" beam, and the last 62.5cm to the wingtip will be a simple strip of balsa, like my current build. Do you think that will be strong enough? Also, I'm really keen on aerodynamics, and learnt about a wingtip device called a whitcomb winglet from a book I borrowed from the library. I am planning on having a removable winglet system on the plane so I can experiment with whitcomb winglets and other wingtip devices.

By the way, here is a top view of the plane:

You have not specified the thickness and grade of the balsa used for the outer spar to the wing tip. If you find that it is too flexible you can give it a light coating of glue for added strength. The next stronger method is to laminate thin thin balsa to the solid spar BUT do so as if it was a web, (grain running from spar top to bottom and not spanwise. I prefer the use of 1mm Balsa for this as it stops the compression of a solid balsa spar and therefore wing flex and profile distortions for a very little weight penalty.

You do not mention the LE and TE structure but for the tip sections they will be taking on a greater deal of the torsional and the like forces so what you may save on the spar weight and structure should be added in part at least to the LE and TE for a more rigid and resilient structure.

Just a few thoughts!

 

[TooJung2Die]

Do you mean it will be stronger than it needs to be, or I'm just making things complicated for myself?

Yes and Yes. You drew a combination of I-beam spar and a box spar. Use one or the other like Hai-Lee suggests. Box is better for the center because a wing joiner will slip inside and strengthen the joint. I-beam is lighter and better towards the wing tips. I would pick one type of spar and use it for the whole wing letting it taper towards the tips.

 

[Jackson T]

You have not specified the thickness and grade of the balsa used for the outer spar to the wing tip. If you find that it is too flexible you can give it a light coating of glue for added strength. The next stronger method is to laminate thin thin balsa to the solid spar BUT do so as if it was a web, (grain running from spar top to bottom and not spanwise. I prefer the use of 1mm Balsa for this as it stops the compression of a solid balsa spar and therefore wing flex and profile distortions for a very little weight penalty.

You do not mention the LE and TE structure but for the tip sections they will be taking on a greater deal of the torsional and the like forces so what you may save on the spar weight and structure should be added in part at least to the LE and TE for a more rigid and resilient structure.

Just a few thoughts!

The outer panel (where the taper starts on the plan), will have a spar 9-10mm tall by 3mm thick, tapering to 5 or 6mm tall by 3mm thick. The wing has a middle chord of 167mm and a tip chord of 100mm. I'm using a NACA 2408 aerofoil (8% thick), so my aerofoil is ~13mm tall at the middle section tapering down to ~8mm at the tip. I don't have a clue what grade my balsa is, because I buy it all from Bunnings (Australia's home depot). I think the wing spar for the outer panel will be strong enough, but I will definitely keep those techniques in mind. The leading edge for the outer panel will be something like 6.5x6.5mm at the middle side tapering to 6.5x5mm at the tip. The trailing edge on the outer panel has the aileron, so minus the aileron is only 10mm by 6.5mm tapering down to 10mm by ~5.5mm.

 

[Hai-Lee]

The grades of Balsa available from Bunnings are all over the place. The weight is an indicator but most don't carry scales with them.

Apart from that it sounds like you have a plan and know what you are doing! Will watch this as the build progresses!

have fun!

 

[Jackson T]

So, major change of plans. I've decided to make a 3m scale glider instead of the previously mentioned 2.5m glider. I purposely want to make it on the heavy side (~1600grams AUW) so it carries more energy and flies more like the full scale ones. To help make the long and very thin wings strong, I'm going to sheet the whole upper surface of the wing and probably to the main spar on the underside. The wing will be normal dihedral, not polyhedral. I can't decide whether to make the wing 2 piece or 3 piece. If I make it 3 piece, there will be more weight outboard from the centre, causing slower roll response, and I would have to get creative with servo connections as well, but the joints won't have to be as strong. If I make it a 2 piece wing, I only have to make one joining mech, the weight stays central, and the servo connections will be easier, but it will have to include the dihedral, and it will have to be a fair bit stronger (50%).