Rodrigo C

Hello,

I am designing a Tiger Moth with 100" wing span, and don't know what Wood Thickness to use on the Ribs.

I got 2 options, 3/32" balsa or 1/8" with lightening holes (I think using 1/8" without any lightening holes will make the plane too heavy), but I don't really know which thickness will give me best resistance x weight ratio.

So was hoping the experts out there could give me some tips .

Thanks.

David Cutler

Whichever you choose, its worth considering using thinner ribs, but with cap strips.

It's amazing how much strength cap strips give, as the extra strength is exactly where it needs to be, i.e. on the skin of the wing, and you could probably get away with thinner ribs, and possibly even lighter structure in total.

-David C.

Rodrigo C

The only problem I see in using cap strips is that the width can't be too large otherwise the scale characteristics will be lost, but the idea sounds interesting.

So, use 3/32 Ribs (with or without any holes?) with 1/16 Cap strips with 4mm width? 1/4" I think it's too much.

I know I shouldn't worry that much about the weight, but I have no idea on how much it will height and I'm kinda freaking out about that, so I'm trying to bring the weight down the maximum without afecting the structure too much.

I plan to use a 50cc engine on it.

Also how many servos should I use? let's say 1 per aileron, 1 per stabilizer, 1 for rudder?

David Cutler

If you do use cap strips, even fairly thin ones, you could almost certainly remove more material than the strips themselves from the middle of each rib, as long as you keep enough meat there to keep the caps apart and maintain the shape.

If you really are paranoid about weight, (and that's not a bad way to be!) always consider the possibility of moving weight to where it's needed. That is, rely a lot on shape, and not bulk to get strength.

For example, in another field, hardly any of the strength in a truck's ladder frame comes from the web, which is only really there to give the flanges (which is where the strength really lies) structural stability, and keep them apart.

David C.

Rodrigo C

Sounds right David, think I'll do that.

Got another question. I have never built Giant scale airplanes, but I have built several smaller ones.

Since the Tiger Moth uses no sheeting (only sheeting is in the leading edge and will have about 50mm witdth, the chord is about 370mm, And I will use 2 8mm spars at the max thick of the airfoil and another 8mm botton spar at about 55% of the airfoil.
The wings will be split, cause I can't fit a 100" (aprox. 2500mm) wing into my car, and I will use aluminium tube to join them. One of the problems is that since the lower wing has 4 degrees of Dihedral I can only extend the tube for about 240mm inside botton wing. And about 320mm inside the top wing that has 3 Degrees of dihedral.

The big question is, will the wings hold it together or will the break up?

Sorry for my ignorance

P.S. and sorry for using metric and royal system, but I'm used with metric system since I'm from Brazil, and I know americans use inches.

BMatthews

I would suggest that the best way to join the wings is with shorter stub type rods and then use the scale rigging to supply the actual structure for the flight loads.

If you do go for self supporting wings rather than using functional scale rigging then look into how the glider models couple the rods and tubes into the spars. As long as the joiners are reasonably long (your's are long enough) it's much more important that the joiners be coupled into the spars properly.

Rodrigo C

Hi matheus, sorry, I don't know if I'm really tired hehe, but I didn't get the meaning of what you said.

Rod is the tube?

What is scale Rigging (don't know what Rigging means)

Do you mean where I place the joiner? I was thinking to place it where the maximum thickness of the wing is located between the 2 main spars.

Forgot to ask something, should I place webbing between all the Ribs? since the wing doesn't have sheeting, or it doesn't strenghten the structure?

CoosBayLumber

My Feedback: (1)

I did the parts for a Dennis Bryant Tiger Moth of 78 inch span about six years ago. It used 1/8 inch wide ribs at two inch spacing. It also had a plywood center section plate for the top wing in which allowed for removals. The model uses four spars of 1/4 inch by 1/8 inch and some shear webs near center of 3/32 inch thickness.

If you use contest quality wood, should be no need for lightening holes as mentioned. This eliminates the decision between 3/32 inch and 1/8 inch thick ribs.

You know that this A/C has already been designed, flown, and drafted up a few times now by others. This may be of interest in your larger design.



Wm.

BWooster

Judging from my 51 1/2" Tiger Moth, I guess your wing chord is going to about 40 cm. Wouldn't it be possible to build-up ribs out of thicker wood, much like the original (including the half ribs)?

I noticed at the local air museum they are rebuilding a Norseman, and there really isn't much to the ribs. I think most of the strength is in the fabric -- the rib just holds the airfoil shape.

Also, I think some of the Canadian Moths had a sheeted leading edge. That may let you build a stronger wing, and still be scale.

Sounds like a great project. Good luck with it.

BMatthews

Rod is tube but for most smaller (as in thinner wings like the Tiger Moth) we usually use music wire rods instead of aluminium tubes. The aluminium needs to be a large diameter for strength and the TM wing is not thick enough for that.

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What is scale Rigging (don't know what Rigging means)
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Scale rigging is the scale flying and landing wires between the wings. For a model this size I would use stainless steel control line cable and small turnbuckles for line tension just like the full size airplane. For a model the size you want this can be a fine way to make the wings strong enough but light as well. And since you want to put the rigging wires in place for the scale look you may as well make them work for you. Using the scale rigging also lets you make the rest of the wing structure much lighter and more scale without becoming too weak. The two wings PLUS the wires work together to give you an overall structure sort of like a girder bridge for cars.


This is NOT the place to put in the joiner rods. The joiner rods should be at the spar locations and the wires need to go between the upper and lower spars or alongside the single spars and be joined very strongly to the spars. If you put the joiner between the spars then the 2 or 3 ribs that run between the spars where the joiner is located must take all the wing loading. This would be a very poor engineering solution.

This is the part where you should look at plans for thermal gliders that use the winch to launch them. Copy the way the joiners are made for these types and read the following paragraph to go with this.

Here again for a large model of this sort it's best to copy the scale spar locations and position a joiner at each spar. Using the scale spar locations also gives you the easy solution for mounting the struts and aileron hinges. Just use the scale locations The joiner rods (probably 1/4 inch music wire for this size) will go into aluminium tubing sockets in the wings. The sockets are glued to fit between the spars at the wing roots using epoxy and microballoon putty mix to fill in the spaces and glue the parts together. 1/32 ply webs for these rib bays should be used on front and back for strength and to hold in the epoxy putty/glue mixture. Spruce spars is a must on this size model and 1/32 ply webbing is not out of place either for the first half of the span and change to 1/16 hard balsa webbing for the outer half of the wing

If you're using an upper and lower spar then yes you should use webbing. I gather the Tiger Moth uses leading edge sheeting back to the front spar. But for the rear spar it may be better to make the top spar cutout in the ribs a little deeper and then fill in the gap in the ribs afterwards. Sand the little fill strips down to match the ribs and you'll have a lovely looking covering job. The part where the airlerons go can just be filled in with a long balsa cap for the covering to stick to. Here again it would help it look very scale when finished.

If you have not figured it out yet from the other comments a 100 inch Tiger Moth is a very large model. This does not mean you should forget about keeping the structure light but it does mean you can use the heavy woods like spruce and plywood or even balsa in scale or slightly larger cross sections where required. I would have no problem with using 1/8 for the ribs without lightening holes for this size model but if I remember right the Tiger Moth has LOTS of ribs so 3/32 is probably fine as it will give lots of support to the leading and trailing edges. Contest light grade wood for this size model should not be needed. Just pick the lighter 6 to 8 lb stock available readily from the hobby shops.

Good luck with your model.