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Sep 22, 2012, 01:21 PM
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Top/Bottom or I-Beam spars in rubber powered plane wings???

I built a Guillows "Arrow", modified into a 2Ch RC glider, expanded the wingspan from 28" to 37" and put an I-beam(top and bottom spars with vertical grain shear webbing inbetween) spar into the wings and have not regretted it at all and am very glad I did, but, it's getting launched pretty hard and fast with a hi-start and carrying 2 mini-servos, a dinky Berg reciever and dinky dog collar battery. So, there's a lot more of a load on the wings considering what I did, BUT it didn't seem to add much weight with the result of a very durable wing.

Now, I'm building a 21"wingspan rubber powered Sig "Tiger", and my RC glider sensibilities tell me to put at least a top and bottom spar of 1/16" thick, 1/4" wide strip balsa spars in for better load bearing, I think shear webbing would be overkill.

I'd rather spend a little more time building and a lot more time flying and much less time fixing later.

****So, How many of you ended up wishing you had built up your main load bearing capablilties of your wings on your rubber powered planes, and/or ended up doing it later, vs how many did this and wish they had not afterward?

Thanks - Paul
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Sep 22, 2012, 03:27 PM
B for Bruce
BMatthews's Avatar
A 21 inch span rubber model simply will not generate enough load to worry about this. Keep in mind that with FF you don't have any sort of elevator control to suddenly input a big G loading. So even in a full on death dive the trim of the model will produce very little G force on the wing during the pullout.

Any beefing up you do would depend on the current spar and leading/trailing edge options. Some of us, me included, have not seen the wing plan for the Tiger so if you could post up a view of the rib showing the spar and leading/trailing edge wood sizes that would help a lot.

Having said that when I built my Sig Cabinaire many years ago I went with the stock spar options and have not had any regrets. And on a model of this sort if I were designing it myself I'd likely use something like 1/8sq on the leading edge, two upper spars of 1/8x1/16 spaced at around 20% and 60% of the chord and a 3/32x3/8 trailing edge. The leading edge and spars would be selected from straight grained rock hard balsa. The trailing edge from medium to light C grain. It doesn't sound like much to an RC sort of guy but it's more than enough for sporty free flight.
Sep 22, 2012, 04:26 PM
Registered User

The Tiger, I believe, has a single spar on the underside of the wing, looks to be something around 1/8" square, but no top spar it appears;

(not my model)

I'm not sure I like that. Not sure how good my ideas are, but I'd be more happy with a 1/16" x 1/16" spar top and bottom at the max thickness point and another on the lower surface only at about 60%. I seem to remember a few of the old KK jobs were like that, possibly the Achilles?
Sep 22, 2012, 06:02 PM
Registered User

Sig Tiger Wing Spars

I'll be gone for a week after 5 pm west coast time, so I won't be able to post a picture until then(at least).

The best description I can make is this:

It has fairly dense(but not really,really dense) spar of 1/16" x 1/4" (by my visual memory of it) so that the spar is oriented vertically and runs along the bottom of the wing(so the 1/16" bottom edge touches the tissue paper when covered, but not extending to the top of the wing). This spar is about 60% from the leading edge and 40% from the trailing edge. It seems OK for flight strain loads, It just seems a wee bit underbuilt for a crash.

So, all I would be doing is changing the orientation of the supplied spar to a flat horizontal orientation on the bottom, and adding exactly the same size of material and putting it on the top of the wing(in a notch, of course) so they are facing each other. Or, I would move both top and bottom spars forward to the highpoint of the airfoil or a little forward to act as a turbulator as well.

I think even shifting the single spar to the top of the wing alone to take a compression load and let the leading and trailing edges take the tension load would possibly be better.

The leading edge is 2 pieces, a flat 1/16" thick by 1/8" wide strip with a 1/16" square strip on top of that and the forward end of the rib is notched for that "L" shape to fit into. Once built, the leading edge is then sanded round. The trailing edge is a single 1/16" x 1/18" flat strip(with some curved tip pieces).

I'll probably do something different since I haven't made a plane completley per-plans yet and they all seem to fly reasonably well(my almost completely altered/modified Great Planes Spirit 2M flies REALLY well, better than stock from what others have told me who have built/owned them).

I do remember hearing a freeflight joke somewhere that went like this:
"Every time your plane crashes, examine it carefully because every piece that isn't broken is overbuilt and needs to be reduced by half".

Thanks - Paul
Sep 22, 2012, 07:14 PM
Balsa Flies Better!

I'd suggest getting in a somewhat different mindset. You're trying to beef stuff up, but most of the airplanes I build, I try to lighten up. Dave Aronstein pointed out that "Zero mass, needs zero strength." The airplanes that withstand running into stuff better than anything are the really light indoor duration models. When you try to beef stuff up- you make the likelihood of a crash greater- and the force of the impact.

The LE sounds nicely designed to take some impact- the TE sounds typical. I often like to use a top and bottom spar at the high point. I'd probably just use 1/16" by 1/16", but if I had light wood, I might go to something larger. On the other hand, if I didn't feel like cutting a bunch of new ribs, the existing spar system will work fine.

Sep 23, 2012, 12:09 AM
B for Bruce
BMatthews's Avatar
The spar as shown on the plan will work fine. But having said that I'm not a big fan of the spars for wings of this sort being on the bottom. It allows the top surface covering shrinkage to curve up the wing too much for my tastes.

If I were going to bother to make new ribs for a wing of this sort I'd likely switch that bottom spar for two top spars of 1/16 x 1/8 hard balsa. I'd put one about 15 to 20% back from the leading edge and the other at around 50 to 60% depending on how the spars visually split the surface up and with consideration given to trying to best support the tissue upper covering evenly. But as I said there's really nothing big to be gained other than a flatter wing after the tissue shrinks.
Sep 23, 2012, 12:07 PM
Registered User
When I built a Comet Phantom Fury recently, I ran into similar things. The wing has two bottom spars 1/16" X 1/8" as designed, and the ribs are wide enough apart to get a fair amount of sagging in the tissue. I added a 1/16" square top spar at the peak of the ribs, and used basswood. The rest of the stripwood was hard balsa, so that would have been OK too. The tissue and thinned Sig Lite Coat dope added a lot of strength. See my post on this.

I've flown the Fury twice since it was built, and the wing has taken inverted landings with no problem. At first, I had enough warp in the wings to induce a roll a few seconds after launch. The next time I flew I'd fixed that by strapping each wing to a shelf board, propping the T.E. at the tips enough for a little washout, the same at each tip this time, and spraying with 70% rubbing alcohol. After it dried, the wing did fine, at least under power. Got good rise off table takeoffs and no roll.

Don't need much for these wings to give them enough strength. The Fury wing is a lot longer than the Tiger wing. Biggest problem is keeping them straight. Check for warps every time. Does the Tiger wing come off? It's certainly easier to straighten if it does.
Sep 23, 2012, 01:12 PM
yes, its a flying lamb :)
draganbt's Avatar
It seems to me that adding diagonals will give you the extra strength that you seek
Sep 25, 2012, 06:05 PM
Registered User
I agree with just about all of the above but prefer bass for 1/16" stringers and never
seem to have hard enough 1/16" sheet to cut them. Not for crash loads, but
to stop ' helpers ' breaking the spars when they pick the model up.
Like to use good stiff balsa for the trailing edge to prevent trailing edge bending
after shrinking covering, almost impossible to get out.
Sep 26, 2012, 04:28 PM
B for Bruce
BMatthews's Avatar
Those darn "well intentioned" helpers have been responsible for more than a few busted models over the years. But instead of 1/16 basswood or similar I prefer to just go with 1/16x3/32 or 1/16x1/8 to "harden" my models against their paws. It reduces the need to maintain a mixed material bank. It's not any lighter or less strong, just simpler to use balsa that I already have instead of shopping for something else.

Of course both our options fail if we're building decidely small models. I've gotten tired of losing those small models eventually in thermals despite care being taken to fly only when the air feels "dead". I've simply found out the hard way one too many times that I was wrong and had to watch some very cute and satisfying models to fly go bye-bye overhead. Now it's all about the 19 inch or bigger models that can more easily carry at least a viscous button timer and the other stuff needed for a DT with reasonable ease.
Sep 27, 2012, 11:56 AM
Registered User
Can I ask a question Bruce? Its kind of related, and you seem to be the chap who might know.

On lightweight models, such as the sort of rubber powered model we are discussing here, does a shear web between top and bottom spars have any benefit? I was imagining vertical grain 1/32". I was thinking it would add a little "crush" resistance for young fingers (my son recently cracked the spars in the OD "dolphinesque" I made for him)
Sep 27, 2012, 01:19 PM
B for Bruce
BMatthews's Avatar
There's no doubt at all that it would greatly aid with crush resistance from the "delicate handling" done by a young'un. It does leave open the question of the longevity of the tissue covering though... Perhaps it would be wiser to focus on some all sheet designs for the next few years?

Although it's got little to do with your question or even the topic of this thread in terms of such spars on SMALL models it's interesting to look at some of the open class rubber model designs that are/were popular over in Britain. Some of these were built with silly things like 1/8x1/16 up to 3/16 x 1/16 spars top and bottom joined by 1/32 or even sanded down thinner webbing all in an effort to achieve enough strength and stiffness with the absolute minimum of weight. Such models typically used full "Union Jack" style geodetic ribs of 1/32 balsa along with these minimalist spars and supremely delicate leading and trailing edge wood sizes that placed light weight above all else. Yet we're talking about models with 40 to 48 inch wingspans. Truly this was the age of OCD model designing....
Sep 27, 2012, 02:04 PM
Registered User
Perhaps it would be wiser to focus on some all sheet designs for the next few years?
Thats possibly true, he had a lot of fun with a few catapult gliders I made for him. The other one is a little tired and patched by now I must admit!
Yet we're talking about models with 40 to 48 inch wingspans. Truly this was the age of OCD model designing
Oh definitely, and they do look stunning when done well, but one does have to wonder, I remember looking at a few such things at Old Warden meetings a few times. Some of the "wakefield" models seemed to push building complexity to a fairly ridiculous level, judging by what I've seen in old plan catalogues etc.

It seems to me, from my experience anyway, that lightness comes as much from proper wood selection (where did all the light balsa go?) and accurate building, I was amazed when I started looking into how much glue weighs.
Sep 27, 2012, 02:44 PM
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P_J_Glor's Avatar

Fly Aways

Originally Posted by BMatthews View Post

Of course both our options fail if we're building decidely small models. I've gotten tired of losing those small models eventually in thermals despite care being taken to fly only when the air feels "dead". I've simply found out the hard way one too many times that I was wrong and had to watch some very cute and satisfying models to fly go bye-bye overhead.
I have to agree with you there. A thermal can certainly result in a lost model, even if a little on the heavy side. I had a PeeWee .020 30" Goldberg ranger (all sheet balsa with colored dope) fly away from me years ago, and also recall seeing Walt Mooney's all sheet balsa "Old Ironsides" disappear over the US/Mexican border at a 4th of July fly in on its maiden flight. It was still climbing as we lost sight of it.

Sep 29, 2012, 02:31 PM
Registered User

Thanks for the info.......

Hey, I'm back from our 6 day backpacking trip and it was really a good trip. Hopefully I'll be getting back to the "Tiger" soon.

I think I'm going to do a little building up of the wing but in a way that won't involve cutting "new" ribs, just a little modification of the ones in the box. I'll look at what I have on hand since I seem to have a bunch of wood stuff piled up that needs to get used up.

As far as added weight goes, yes, I agree from personal experience that too much added weigth adds momentum to what ever part(s) impact something in a "crash" and I want to keep this as "floaty" as possible.

Hopefully, tomorrow the Willamette Modelers Club will be having another outdoor freeflight event, I need to check on that first. The last 2 were pretty interesting.

Thanks - Paul

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