Spar options for high aspect ratio balsa wings

[David Ramsden]

Hello there,
Today I had an own-design high aspect ratio glider wing fold 150' above the slope. Ouch.
The spar was spruce. The wing folded at the root end of the flap despite the fact that the top and bottom 3/32" center-section sheeting extended beyond that point. So a fully sheeted wing would probably still have folded. I need to build a new wing. 
I'd be interested to hear any tips on construction methods for high a/r balsa wings. For example, if carbon spars are better, would you go for tube or rectangular(?) hollow or solid? One or two? And what's the best glue for bonding balsa to carbon?
Thanks,
David

[Jonathan M]

Sorry to hear that, but glad the whole model wasn't a total right-off.

 

I'm no engineer, so forgive my loose terminology, however:

 

The leverage forces are greatest closest to the root, the greater the A/R the more so, so that's where the maximum strength needs to be in terms of preventing bending failure.  Sheeting the wing much beyond this area won't - as you say - have made any difference, but you'd need to ensure that you don't build any dramatic stress-risers into your new wing:  i.e. no sudden reduction in strength/stiffness at the junction of the new stronger centre-section and the structure beyond that, which would just shift the point of a potential future failure outboard; better a light 'tapering' of the spar/webbing stiffness to smooth things out.

A solid carbon spar would give you most in terms of material stiffness, but a hollow one would give a much greater strength to weight ratio and - assuming adequate wall thickness - should be more than strong enough.  A hollow spar would then allow for a solid wing-joiner of carbon or aluminium, thus increasing the stiffness at the root bays, and beyond depending on how long the joiner extends.  The robustness of the spar could also be beefed up with balsa sheer-webbing at the root area, gradually reducing as you move outboard (to prevent stress-rising as per the above).

 

A second smaller spar (and joiner) aft of the main one would help, also with resisting wing twisting (which we haven't discussed - could this have been a contributing factor to your folding crisis?).

 

But all difficult to tell/advise without knowing the size/span/chord of your own-design wing or its original structure.  Can you post a picture or a sketch?

 

CA is perfectly adequate for most carbon to balsa glueing, epoxy for very high stress areas.

[Jonathan M]

In this lightweight 2m RES thermal soarer for bungee-launch, the spar is hollow tube, the ribs and riblets making up the root and the first two bays are lite-ply.  The small transparent plastic tube forward of the spar is for the small-diameter steel joiner (lightness is key and space in the fuselage is tight, so is a better solution for this thermal model than a hoofing great cylindrical carbon joiner into the main spar).  The gap between the small joiner tube and spar is filled with epoxy for increased strength.  The large diameter clear tube behind the spar is just a ballast chamber, and the mid-rib sheeting from the third bay is just for each spoiler and its surrounding frame. But you can see how this whole assembly adds strength to the root bays while avoiding any sudden stress-risers.

 

[Jonathan M]

The above photo is of the Eli F3RES (I'm half-way through building mine) see:  Hyperflight Eli

 

Below is a photo of the wing of a 2m Quark slow aerobatic (not yet started building mine) under construction, less ailerons yet.  The bottom spar is yet to go on (wing upside down here) and there'll be sheeting added to the forward and root areas etc, but you can see the main hollow carbon tube going through the first two bays, and the secondary smaller (in his case) aluminium tube aft but just through the first bay. 

 

 

Quark short kit from https://islandmodels.ie/home/3-quark-2m.html

[Trevor]

David, were there vertical shear webs between the upper and lower spars? This is the best way of improving the bending strength on a conventionally built wing and also, by closing off the leading edge 'D' box, vastly improves torsional stiffness.

[EarlyBird]

7 hours ago, David Ramsden said:

Hello there,
Today I had an own-design high aspect ratio glider wing fold 150' above the slope. Ouch.
The spar was spruce. The wing folded at the root end of the flap despite the fact that the top and bottom 3/32" center-section sheeting extended beyond that point. So a fully sheeted wing would probably still have folded. I need to build a new wing. 
I'd be interested to hear any tips on construction methods for high a/r balsa wings. For example, if carbon spars are better, would you go for tube or rectangular(?) hollow or solid? One or two? And what's the best glue for bonding balsa to carbon?
Thanks,
David

Not your beautiful DR 420 by any chance ?

[David Ramsden]

Hello Jonathan, Trevor and Early Bird,

Thanks for all the photos Jonathan. Yes it was my DR 420 but a new wing center-section which extended the span a bit. There is (was) no wing joiner at the center, the joiners were half way out on each wing (between the flaps and the ailerons). I will post some photos later. Bit busy with family right now. What about adhesive for carbon/balsa joints? 

[David Ramsden]

[David Ramsden]

The outer wing sections are still perfect so I'll be building a new center section this winter.

 

[Martin_K]

2 hours ago, David Ramsden said:

..... What about adhesive for carbon/balsa joints? 

I have used thin CA (Deluxe Rocket Hot) for close fitting carbon tube to balsa joints. Assemble first then add glue. This on a DLG subject to vigorous, and many not too elegant, launches. No internal joints have failed. A carbon throwing peg fixed with the same glue did loosen in a ply wing tip but after re-gluing has stayed put.

Edited October 23, 2021 by Martin_K
add detail

[David Ramsden]

5 hours ago, Martin_K said:

I have used thin CA (Deluxe Rocket Hot) for close fitting carbon tube to balsa joints. Assemble first then add glue. This on a DLG subject to vigorous, and many not too elegant, launches. No internal joints have failed. A carbon throwing peg fixed with the same glue did loosen in a ply wing tip but after re-gluing has stayed put.

Hi Martin. The only time I tried dry assembly and adding thin CA to the joints, excess CA ran down from some of the joints and stuck the wing to the polythene covering my plan - and made quite a mess. Is there an easy way around that? Thanks.

[David Ramsden]

10 hours ago, Trevor said:

David, were there vertical shear webs between the upper and lower spars? This is the best way of improving the bending strength on a conventionally built wing and also, by closing off the leading edge 'D' box, vastly improves torsional stiffness.

Hi Trevor, There was only a single spruce spar but the top and bottom LE sheeting was 3/32" and the D-box was finished with vertical 3/32" at the back, well glued to the spar. The entire inner wing section seemed pretty stiff even before I'd covered it. I think part of the problem was that I'd increased the span from 2.4m to 3.15m which but a lot more leverage on the centre. She did fly well though.

[David Ramsden]

13 hours ago, Jonathan M said:

A solid carbon spar would give you most in terms of material stiffness, but a hollow one would give a much greater strength to weight ratio and - assuming adequate wall thickness - should be more than strong enough.

Hi Jonathan,

Thanks for all the advice. The carbon tube in your RES photo doesn't look anything like the smooth tubes in my local model shop. Please can you recommend somewhere to find the best carbon tube spars online? Thanks.

[Martin_K]

36 minutes ago, David Ramsden said:

Hi Martin. The only time I tried dry assembly and adding thin CA to the joints, excess CA ran down from some of the joints and stuck the wing to the polythene covering my plan - and made quite a mess. Is there an easy way around that? Thanks.

I agree that very thin CA is not easy to use. I have also inadvertently  glued my plan's cover sheet to the wing. I will also admit to some moments of panic after gluing my index finger and thumb together. (Peeling the digits apart in a basin of warm water with plenty of soap worked).

 

To minimise these hazards I apply as little CA as possible via narrow bore tube held right against the joint. The glue supplier provides applicators. I also roughen the surface of the carbon with fine abrasive to help the joint. Possibly that also makes glue flow along the tube rather then drip? Another issue is appearance. Even when applying very little thin CA I always have some white residue left around the joint.

 

So, it's difficult, messy, and can be hazardous. Here, however, we are talking about what makes a sound joint. Thin CA has delivered that for me. I also like this way of working for letting me check all the joints are correctly aligned before applying glue.

Edited October 23, 2021 by Martin_K

[Jonathan M]

David,

 

For carbon tubes I'd look first at Hyperflight:  https://www.hyperflight.co.uk/products.asp?cat=Materials&subcat=Carbon+Tube 

 

To prevent thin CA running everywhere too fast, use small bottles and fit the nozzle with those extra thin tubes, e.g. https://www.modelshopleeds.co.uk/catalog/product_info.php?products_id=14334

 

Re the wing failure, yes the increased span would have been a contributing factor, but the main culprit would have been the single spar.

 

Assuming your spruce spar was inset horizontally into the top of the ribs, and the sheer webbing below it, then you'd have a "T" section or something close to it.  Notwithstanding the D-box balsa sheeting, a "T" section beam is much weaker than an "I" section one (imagine an old-fashioned capital "I" with horizontals at both top and bottom, as per the picture below - but for our purposes ignore the radius detail).

 

If you apply an upward force to one end (wing-tip) of a "T" section beam while the other end (wing-root) is fixed, then the top horizontal (your top only spruce spar) will be in compression.  Apply the same upward force to an "I" section beam, then in addition to the top horizontal being in compression, there's a bottom horizontal (the missing bottom spruce spar) which will be in tension.  Your wing broke because there was nothing of any substance except some puny balsa D-box sheeting to hold the bottom in tension in flight.

 

 

 

 

 

[David Ramsden]

2 hours ago, Jonathan M said:

Your wing broke because there was nothing of any substance except some puny balsa D-box sheeting to hold the bottom in tension in flight

Yes. With hindsight it was a mistake. Or was it lack of foresight? All previous LE sheeting I've done was with 1/16" balsa. I mistakenly thought that upping that to 3/32" (top & bot) would give a lot more strength. It certainly seemed to give rigidity before covering but when I saw her in flight (it was a gusty 9-14mph) the entire length of the wings were bending a lot. 

The info on 'I' section is interesting but I'm thinking of moving away from spruce spars altogether. The end 40% of each wing (with the ailerons) will still be spruce-sparred because those sections were undamaged (and I want to re-use them) but I'm thinking maybe two carbon tubes for the center-wing section with everything else in balsa except the ply wing joining braces. I'll probably use 3/32" LE sheeting T & B again and might as well D-box it again too. I'll have to think carefully about the wing at the inner ends of the flaps which is an obvious "stress-riser". Maybe the TE ply brace should be longer and the center-section sheeting could start tapering off one rib further out.

Still open to ideas.

I will check out hyperflight.
Thanks Jonathan

[Jonathan M]

Sounds good David. Very nice looking glider by the way!

[Peter Jenkins]

Jonathan is quite correct about an I beam.  The point of the two flat sections is to stabilize the vertical spar from buckling as it's loaded up.  So, the problem with high aspect ratio wings is that you have to counter both the bending moment (the technical term) and the torsion caused by lift generation.  Hence typically you would have a spar that was thickest at the root and tapering as you move outwards.  Alternatively, you can have two spars and there might be a number of ways of arranging those.

 

You can use a doubling up of the spar at the root and then gradually sand or plane down the second spar thickness as you move to say half span and glue both together.  That coupled with another thinner spruce cap strip top and bottom could make up your main spar.  The D box is there to resist torsion but if you fully sheet the wing and then gradually reduce the amount of sheeting chordwise as you get to 1/4 span that will also help with resisting the bending moment.  The top sheet will be in compression with the bottom sheet in tension.  

 

Two spars, as I mentioned earlier, would also help to resist the bending moment.  Again with cap strips to make an I beam section.  You can test the strength by using small bags filled with sand to distribute the load across the wing span.  Remember that it will be quite easy to pull up to 10g by heaving back on the elevator!  That 5 lb model now weighs 50 lb so your wing needs to be strong enough to take that.  Most full size gliders, of the non aerobatic type, are cleared to +3.5 and -1 g.  Aerobatic ones go much higher in both positive and negative limits.

[David Ramsden]

1 hour ago, Jonathan M said:

Very nice looking glider by the way

Thanks Jonathan!

Just ordered 4 carbon tubes from Hyperflight. Thanks for the recommendation.
What's the model in your profile picture?

[David Ramsden]

23 minutes ago, Peter Jenkins said:

Two spars, as I mentioned earlier, would also help to resist the bending moment

Thanks for the response Peter. As you may have read, I've decided on two parallel carbon tube spars rather than spruce 'I' beams. Hopefully having the two will resist both the bending and the torsion. I'm thinking of also having the  'D' box balsa sheeting so it's 'belt and braces' this time. I liked how she flew & looked but I might reduce the span just a tad. Maybe 3m instead of the 3.15m you can see below. Plus, the entire wing would then just fit in the car!

[Peter Jenkins]

David, spars on their own won't resist torsion.  That's where the wing sheeting come in.  As a matter of fact, all aeronautical structures are tubes of varying cross sections.  A closed tube is very much stronger in resisting bending and torsion forces.

[Martin Harris - Moderator]

5 minutes ago, Peter Jenkins said:

That's where the wing sheeting come in. 

I found it interesting (and maybe slightly unnerving) that the 17m SHK I used to own a small share in had only an 8 foot spar in each wing.  It was built with a form of geodetic construction and resisted all attempts by the mainly low hours syndicate members to break it!  Wing structure and sheeting certainly did the trick!

[Jonathan M]

6 hours ago, David Ramsden said:

Thanks Jonathan!

Just ordered 4 carbon tubes from Hyperflight. Thanks for the recommendation.
What's the model in your profile picture?

 

No problem David - I've also been gradually learning the technical principles as I've gone along!

 

The pink beastie in my profile picture is from the maiden flight of my Chris Foss Middle Phase which I built a while ago, but with the semi-symmetrical section aileron wing for added enjoyment rather than the basic trainer version which has a flat bottom section and dihedral.

[Martin_K]

David,

 

You may find some inspiration in looking at the plans for Mark Drela designs, e.g. the Allegro and Bubble Dancer. These are the progenitor of many kits found on sites like Hyperflight. The Drela approach is highly technical and detailed in terms of wing loadings, materials, airfoils. I used it more for ideas than a working method.

[David Ramsden]

4 hours ago, Martin_K said:

The Drela approach is highly technical and detailed in terms of wing loadings, materials, airfoils. I used it more for ideas than a working method.

Hi Martin. I looked at the Bubble Dancer wing plan. Looks like a balsa spar with carbon caps. Interesting stuff. Maybe one day!