D Box wing construction

[kc]

Many designs use 1/16 th balsa to sheet the D box part of the wing. This takes lots of balsa which is in short supply and likely to increase in cost. Is there a practical alternative?

I notice that certain designs don't use sheeting on most of the D box - for example the 60 inch Ugly Stik uses a few spruce stringers instead. Plenty of Ugly Stiks around so the design seems well proven.

The Bandito also uses stringers instead of a sheeted D box. Centre section is fully sheeted though. Perhaps the stringers use about a quarter of the balsa that the D box sheeting would use. So a saving of about 3 sheets of balsa per wing.

Clearly scale models may need sheeting to look scale, but for sports models what do you think?

 

Edited By kc on 02/02/2021 19:38:25

[Piers Bowlan]

The point of a D box, as you know, is to make the wing torsionally stiff, which is highly desirable. The film covering gives a bit unfortunately but the old doped nylon covering added quite a bit of torsional stiffness. Depron adds some stiffness but isn't very strong in the event of an 'arrival'.

[john stones 1 - Moderator]

You're compromising the structural integrity Cap'n

Be quiet Scotty.

Price went up, that's life.

[Matt Carlton]

What might be the cost of NOT sheeting the wing?

Not only for the sake of structural integrity, but also in terms of aerodynamics and ding resistance.

Stringers help support covering but little else, so it's very dependent on the role, speed, weight, size etc of the model. Sheeting even with 1/32" balsa is a very cost effective, light and simple way to stiffen up an open structure, especially in torsion, compared to any other viable alternative.

[Mike T]

Many designs sheet only the upper surface, so omitting the lower sheeting may be an option. Personally, I'd just suck it up...

[Allan Bennett]

In theory two opposing diagonal struts in each bay instead of solid sheeting would make the main spar into a truss which might be as strong as sheeting. But is it worth it?

Edit:  Or even a single strut in each bay, alternating in direction like / \ / \ would give you a good truss.  Thinking about it even more (maybe too much!), each strut could in theory be a tension member instead, like the flying and landing wires in airplane rigging.

Edited By Allan Bennett on 03/02/2021 02:44:50

[Nigel R]

Posted by Mike T on 03/02/2021 00:45:26:

Many designs sheet only the upper surface, so omitting the lower sheeting may be an option. Personally, I'd just suck it up...

Which gives little in strength, and only serves to make the upper leading edge surface a little more accurate.

As a design choice I can't see much to recommend sticking with upper surface sheeted and lower surface open, particularly since everyone started using film (little strength) instead of nylon (lots of strength).

Case in point - Ugly Stik, circa 1966, original covered in doped nylon.

As discussed in the other thread, it would seem the expensiveness of balsa is being overstated in relation to overall hobby cost.

[Doug Campbell]

An egg box wing especially if used with capping strips can be very strong and light as thinner wood can be used. An open structure needs to get it's strength from thicker ribs and spars. I normally go the d box route but it is up to you which method you choose

[J D 8]

The wing of an old Easy Pigeon being repaired. [ someone had done a right horrible cobble up butt gluing many broken parts.] The D box had survived the crash. I only had to replace a small bit of the top sheet. All structure aft of box was replaced .

In the 80's/90's the fixing of wings in place with nylon or metal bolts became more fashionable probably because it looks better than elastic bands. Fine for most types but not so good for trainer type models or those operating from rough ground. What likely happened to the Pigeon was it caught the wing on landing, the bolts held and the rear wing structure collapsed in compression. Wing will be held by bands when all is fixed up.

[kc]

First i should mention I posted this as an idea for discussion rather than being something I intended to do. It seemed relevant that we should discuss this now because people may not have enough balsa to complete their models. Balsa cost is a secondary consideration - a model that lasts and doesn't crash is cheaper!

What made me raise the matter was I saw the Tyro Major plan has only top sheeting on the D box and as I am using Tyro Major ribs for a diffferent model I thought it worth discussing.

The Ugly Stik article from May 1985 RCM ( the updated plan ) says it was covered in Super MonoKote - so clearly doped nylon was not needed. Weighed 6 pounds for a 60 inch model! However this designs uses riblets together with 1 spruce stringer top and 1 bottom. Seems to work as countless Ugly Stiks have flown!

Personally I like sheeted D boxes and a fully sheeted wing appeals even more when practical.

[kc]

Allen. I remember David Boddington used tape ( from a drapers shop) instead of shear web on 1 of his designs. The tape made diagonals between ribs.

[Nigel R]

Posted by kc on 03/02/2021 12:17:48:

I saw the Tyro Major plan has only top sheeting on the D box and as I am using Tyro Major ribs for a diffferent model I thought it worth discussing.

The Ugly Stik article from May 1985 RCM ( the updated plan ) says it was covered in Super MonoKote - so clearly doped nylon was not needed. Weighed 6 pounds for a 60 inch model! However this designs uses riblets together with 1 spruce stringer top and 1 bottom. Seems to work as countless Ugly Stiks have flown!

Personally I like sheeted D boxes and a fully sheeted wing appeals even more when practical.

The original 1966 Stik was nylon... Monokote like any plastic will slack off in the heat and provide no structural help. When shrunk tight it provides some strength in tension but nothing like the old school coverings.

w.r.t. Tyro does not have a D box until the underside is sheeted! Which I would do.

If you want to reduce wood use, cut the ribs from foamboard and use the balsa to sheet the D box instead

[Piers Bowlan]

And what do you find works best to glue the foam board to the balsa sheet Nigel? Foam board ribs make a lot of sense. Foam ribs are used in some full sized designs too.

[kc]

The Tyro Major section is well proven I think so I will use it as desgned. Actually I realise it's the same construction as the RM Trainer - identical except size - and I built that back in 1984 and it's still OK and not warped. I seem to remember Nigel that you may have built that more recently - if so did you make a full Dbox!

Basically if it's good enough for Boddo and the Ugly Stik good enough for Phil Kraft then it's good enough! Plenty of film covered Ugly Stiks to prove that film is OK I reckon.

I was expecting that someone would suggest Laminating Film as D box covering? Or even cardboard.....

[Phil McCavity]

With all the comparison to the old ugly stik and the Tyro why not take the lesson from them and go back to nylon and dope to compensate for the balsa you remove?

[Nigel R]

Piers

Copydex would be my first choice for the sheeting - I like contact adhesive for wing sheeting - jig up the wing and then just roll on the sheet. You can do all the surfaces in one session. I did try evo stik on my last build - but you have to be ridiculously careful to avoid getting any liquid stuff on the foam itself. I ended up running just the thinnest of beads onto the edge of the paper with my fingertip which was far too damn fiddly... never again!

Or, plain old PVA - weight the sheet down with magazines overnight. Go fairly light on the glue or the paper skin on the foamboard will go soggy and deform, don't ask me how I know that.

In most other places (ribs to spars, spar webs to ribs, etc) I found medium or thick cyano was perfectly ok.

Super thin cyano was too hot when it cured - the foam goes a bit melty.

Foam tac, I have not tried.

kc

I did build the RM Trainer... but... I used a "proper" wing section printed out from DevWing, and sheeted the entire wing with 1/16".

With the "stressed skin" approach, the internal structure can be almost entirely dispensed with. Next time I make something full sheeted in that four to five foot region, I am thinking of making the ribs from foamboard, using a full depth slotted foamboard "spar" just to hold the ribs in place and provide a bit of resistance to handling/crushing. 1/4" balsa LE and TE complete the frame, stick on the sheet, shape the LE, and job done. Halfway house to a full foam wing I suppose. I guess you could also go with the foamboard eggbox and just sheet the D Box for a lighter model, much the same as the Panic wings used to be bead foam core with leading edge and pretend ribs / capstrips... Joining would need to be glass like a normal foam wing.

Nylon provides excellent strength in tension. On an open wing this translates to excellent torsional rigidity. Coupled with a decent webbed spar for bending loads that would be a strong wing overall. Film provides bugger all strength in tension. YMMV

[Bob Cotsford]

I don't think the Ugly Stick gives the whole story. If it's wing flexes it's not the end of the world with it's thick section and low aspect ratio. Aerodynamic it ain't. Turbulence generated by the poor leading edge, maybe the spars act as turbulators and actually improve it's agility?

Now raise the aspect ratio and airspeed and you start to get the benefits of a sheeted D box maintaining an accurate section over the front of the wing where most of the work is done. Stiffening the wing helps to keep the AOA where you want it, preventing flutter.

Then there's the question of aesthetics, a nice smooth wing or the starved horse look with the structure poking up?

I like the idea of foamboard ribs with balsa (or thin depron) sheet covering though I think if the spar is made from foam it should be quite chunky to support the skin and stop it buckling under stress. Depron with brown paper covering should prove quite dingproof, didn't Colin Leighfield amongst others try something like that?

[Nigel R]

I think up to around .20 size it would be a no brainer, people have made sparless wings before and even a slightly squashy foamboard spar should be enough to stop buckling.

I could see .40 size needing more than one eggbox spar. Perhaps just double up, put one spar at the 1/3 and another at the 2/3 chord point.

.60 size, perhaps moving to a "proper" spar might be prudent?

Need a brave soul to do a bit of destruction testing!

[Bob Cotsford]

Posted by Nigel R on 03/02/2021 17:06:11:

....

Need a brave soul to do a bit of destruction testing!

Nope, only us chickens in here

[Matt Carlton]

I would imagine that it should be possible to use thin corrugated cardboard as an alternative to balsa. I believe the Regal Eagle did this?

1/16" thick corrugated cardboard is reasonable in price (about £1.50 for a 1200 x 1200 sheet) might not be as smooth as sanded balsa but for a trainer or knockabout model, adequate.

There was a range of kits (craftsman?) which were more or less entirely cardboard many moons ago.

[Martin Harris - Moderator]

I don't want to spread alarm and despondency but the latest shortage is...

Cardboard!

Apparently, so much shopping is being done on line that packaging is getting scarce and, of course, more expensive.

[Phil McCavity]

......and Marmite!

Breweries are making less beer because of lockdown so the yeast extract produced as a by-product is more scarce. I love marmite!

[Dave Cunnington]

Yuk, each to his own, but I can well live without that stuff

[Matt Carlton]

I'd rather chew cardboard than marmite. It is truly awful.

[Christopher Wolfe]

Try Vegemite - It's yummy on toast!

skippy