4 Metre Glider Scratch Build

[Jackson T]

I read fairly quickly, but are you going to taper the leading edge of the control surfaces? The image is from CA hinge instructions, but it’s a great illustration of the taper. You can minimize the gap with a taper. See the attached picture. The build looks great, nice work!

The hinge center axis part is not recessed into the balsa, so there is naturally a small (~2mm) gap. The gap is big enough to give me plenty of throw, so I don't need to bevel anything. Sorry to sound really dumb, but what's a CA hinge?

 

[TooJung2Die]

Sorry to sound really dumb, but what's a CA hinge?

Sorry but if you're not part of the inner circle of people in the know with all these acronyms you get left out.

CA is Cyanoacrylate Adhesive better known as Super Glue. A CA hinge is flexible sheet material that wicks up CA glue quickly. Push them into a slot, apply a drop or two of CA and you have a hinge. You can buy them but I make mine from Tyvek. There are lots of other flexible materials that work very well.

 

[Jackson T]

Sorry but if you're not part of the inner circle of people in the know with all these acronyms you get left out.

CA is Cyanoacrylate Adhesive better known as Super Glue. A CA hinge is flexible sheet material that wicks up CA glue quickly. Push them into a slot, apply a drop or two of CA and you have a hinge. You can buy them but I make mine from Tyvek. There are lots of other flexible materials that work very well.

Yeah, I learnt what CA meant when I was investigating the active ingredients of super glue. So CA hinges are a piece of flexible material that's glued into either side of the hinge, right? Do you put CA on the flexible material before you install it so it can push into the slot? Wouldn't the CA bind up the hinge?

 

[Jackson T]

I'm out of CA glue, and my local bunnings (Australia's version of lowes) doesn't sell the thin CA glue I like anymore. They do sell medium/thick CA, but I don't know if it will dry quick enough. Does anyone use medium or thick CA on their balsa builds? How long does it take to dry?

 

[TooJung2Die]

You put thin CA on the hinge after it is inserted into the slot. Use thin CA so it get sucked into the slot. Try to avoid getting CA on the flex part of the hinge but getting some is unavoidable. It doesn't affect the flexibility. There are how-to videos on Youtube. Medium and thick CA are useful where there are gaps in the glue joint. I have not found them necessary for my balsa builds. I use thin CA because it wicks into tight joints and the wood grain.

 

[Jackson T]

Did most of the fuselage bottom sheeting this arvo. I ran out of CA glue almost a week ago, but I couldn't wait till we go to town next or for my next Hobbyking order, So I used PVA glue and rubber bands. I almost forgot to put some fuselage side thingys in so that the rubber bands don't just squeeze the fuselage together! I'm glad I remembered.

Then, I realised that the top side was getting squeezed together, so I cut some strips and put them between the sides to support the fuselage while the rubber bands held the sheeting down nice and tight.

I'll hopefully finish the sheeting tonight, than it will be time to make a start on the wing!

 

[rockyboy]

Rubber bands are handy things - but dangerous to watermelons and balsa if too many are used It looks like your bracing method is working well though.

I've found some joints are good to hold together with masking tape while the glue dries too - it doesn't put as much squeezing pressure as rubber bands which can be both a good and a bad thing depending on the joint.

 

[Jackson T]

Rubber bands are handy things - but dangerous to watermelons and balsa if too many are used It looks like your bracing method is working well though.

I've found some joints are good to hold together with masking tape while the glue dries too - it doesn't put as much squeezing pressure as rubber bands which can be both a good and a bad thing depending on the joint.

Thats a good idea! You just might see a bit of masking tape later in the build

 

[Jackson T]

Finished the fuselage sheeting!

 

[Jackson T]

On to the wing! This is what the aerofoil cross section spar plan is.

I'm thinking 10mm leading edge, 35mm trailing edge 3 x 25mm spars, and 1.5mm shear webbing. The spars are long and thin like that so I can easily fit an aluminium/balsa sandwich in for the wing joining spar. This is my first plane over 1.4m, so any advice is very much appreciated. Do you think I will need to sheet the leading edge back to the spar?

 

[rockyboy]

On to the wing! This is what the aerofoil cross section spar plan is. View attachment 128421
I'm thinking 10mm leading edge, 35mm trailing edge 3 x 25mm spars, and 1.5mm shear webbing. The spars are long and thin like that so I can easily fit an aluminium/balsa sandwich in for the wing joining spar. This is my first plane over 1.4m, so any advice is very much appreciated. Do you think I will need to sheet the leading edge back to the spar?

I think sheeting back to the spar would be a good idea at this size, and if you can swing it run a carbon fiber (CF) strip down the whole length. Doesn't even need to be a solid stick of CF - running a length of CF tow material across the top and bottom of the spar does amazing things for strength. At the scale you're doing I'd suggest making a spar for some strength testing, but I'm pretty sure a fully sheeted "D box" with dual carbon tow strips will do the trick.

 

[Piotrsko]

I'd rotate the spars so the long edge is vertical like the webbing or double the thickness on the webbing. The carbon fiber will transfer loads to the spar better, but I think the break point will be the webbing fails near a dihederal joint. 4 meter wings need to be really tough, more so than combat or acro.

Classic structural engineering suggests a wing is a beam freestanding with one end fixed. The longer the span the higher the moment forces become on the fixed point.

 

[Jackson T]

I think sheeting back to the spar would be a good idea at this size, and if you can swing it run a carbon fiber (CF) strip down the whole length. Doesn't even need to be a solid stick of CF - running a length of CF tow material across the top and bottom of the spar does amazing things for strength. At the scale you're doing I'd suggest making a spar for some strength testing, but I'm pretty sure a fully sheeted "D box" with dual carbon tow strips will do the trick.

Do you have any pics to illustrate? I'm having a hard time visualising all of that. I was really hoping to not need anything but balsa, but if you think it's necessary, I would rather my plane fly a bit heavier than not at all. Where could I get carbon fibre tow from? Would glass fibre work? How would I glue it to the spars? Thanks for the advice, BTW!

 

[rockyboy]

Here's the stuff I'm talking about, but it's possible to find in smaller "hobby sized" quantities as well. Tower Hobbies used to sell it in much smaller quantities - like 25 feet, but I wasn't able to find it on their site. But it's not too pricey on Amazon, and there are other online sellers as well.

https://www.amazon.com/dp/B015NLZ31Q/?tag=lstir-20

One idea would be to put down your bottom balsa spar piece, then put in all the ribs, then glue on your top spar and leading edge pieces to keep things in place. Depends on how you're designing your ribs I suppose. But anyway, once it's all assembled to that point but prior to sheeting the leading edge, epoxy on a strip of tow that starts at the root on the bottom of the wing, goes all the way out to the tip, wraps around, and comes all the way back to the root on the top. As a single piece it's incredibly strong at resisting tension forces. Then put in all your sheer webbing front and back of the spar - and doubling up on the inner 1/3 or so of the wing like @Piotrsko suggested (great idea btw!). Then do the sheeting over the leading edge of the wing, top and bottom.

In a lot of ways this is sort of a poor mans approach to a high performance glider wing design - it might also benefit from a 3/4' layer of fiberglass cloth over the sheeting, but that could just be unnecessary weight.

Maybe some of the glider guys like @Craftydan or @FAI-F1D can help chime in with their experience too.

 

[Jackson T]

I'd rotate the spars so the long edge is vertical like the webbing or double the thickness on the webbing. The carbon fiber will transfer loads to the spar better, but I think the break point will be the webbing fails near a dihederal joint. 4 meter wings need to be really tough, more so than combat or acro.

Classic structural engineering suggests a wing is a beam freestanding with one end fixed. The longer the span the higher the moment forces become on the fixed point.

I had the main spars oriented that way because in this layout the top spar is in compression and the bottom spar is in tension when under load, instead of just bearing the weight like a normal beam. Also, because I can fit a balsa/aluminium spar in easily for the wing joiners. Because they are in compression and tension instead of just being bearers, a wider cross section should still have plenty of compression and tension strength (hopefully), but my dad had the same thoughts. The webbing's grain is oriented vertically to withstand twisting of the main spars, so I didn't think 3mm was necessary. Just a note for everyone, this plane is not going to be high started or looped, I would be happy with a 2G rating if that's all I could get. If it turns out really strong though, I might have a go at tow line with it (running with a string) as it's hopefully going to be very light and slow.

 

[Jackson T]

Here's the stuff I'm talking about, but it's possible to find in smaller "hobby sized" quantities as well. Tower Hobbies used to sell it in much smaller quantities - like 25 feet, but I wasn't able to find it on their site. But it's not too pricey on Amazon, and there are other online sellers as well.

https://www.amazon.com/dp/B015NLZ31Q/?tag=lstir-20

One idea would be to put down your bottom balsa spar piece, then put in all the ribs, then glue on your top spar and leading edge pieces to keep things in place. Depends on how you're designing your ribs I suppose. But anyway, once it's all assembled to that point but prior to sheeting the leading edge, epoxy on a strip of tow that starts at the root on the bottom of the wing, goes all the way out to the tip, wraps around, and comes all the way back to the root on the top. As a single piece it's incredibly strong at resisting tension forces. Then put in all your sheer webbing front and back of the spar - and doubling up on the inner 1/3 or so of the wing like @Piotrsko suggested (great idea btw!). Then do the sheeting over the leading edge of the wing, top and bottom.

In a lot of ways this is sort of a poor mans approach to a high performance glider wing design - it might also benefit from a 3/4' layer of fiberglass cloth over the sheeting, but that could just be unnecessary weight.

Maybe some of the glider guys like @Craftydan or @FAI-F1D can help chime in with their experience too.

I just viewed the carbon fibre, and it doesn't ship to Australia . Dodgy Amazon! Any other sellers that might have it relatively cheap? How thick is the stuff? Do you mean using 3mm shear webbing for the leading edge side full span, or for the middle third of the wing on both sides?

 

[Jackson T]

How about this stuff? https://www.ebay.com.au/itm/TORAY-T...m3aa6be80ef:g:Q6kAAOSwBahVID16&frcectupt=true

 

[Wildthing]

I just viewed the carbon fibre, and it doesn't ship to Australia . Dodgy Amazon! Any other sellers that might have it relatively cheap? How thick is the stuff? Do you mean using 3mm shear webbing for the leading edge side full span, or for the middle third of the wing on both sides?

I checked also, don't ship to Canada but I did check their own website and it was the same price. I never keyed in my info but they wanted $14.95 extra for shipping.
https://www.google.ca/url?sa=t&rct=...on_fiber_all&usg=AOvVaw3OWdwAAXR6uHSdXzoDwh1k

 

[Jackson T]

I checked also, don't ship to Canada but I did check their own website and it was the same price. I never keyed in my info but they wanted $14.95 extra for shipping.
https://www.google.ca/url?sa=t&rct=...on_fiber_all&usg=AOvVaw3OWdwAAXR6uHSdXzoDwh1k

Thanks! I had a look, and they want $15 shipping! What do you think about the ebay one I linked above?

 

[Wildthing]

Thanks! I had a look, and they want $15 shipping! What do you think about the ebay one I linked above?

Price per/ft is the same $0.10 , the shipping appears to be cheaper