Hello all,
this is my silly question:
for a carbon fiber 1 1/2" wing tube with thick wall (0.078")

moment of inertia Jx is 0.088 inch^4 (3.68 cm^4)
stregth modulus Wx is 0.118 inh^3 (1.93 cm^3)

let's assume the following:

a 118" TOC plane weights 40 lb (roughly 20 kg)
at the end of a 10g dive it has a stalled wing
the plane now weights 400 lbs (200 kg) over a single wing.

The wing loading is triangularly distributed over half the span.
The center of loading is roughly at 1/3 half the span that is 20" (50 cm).

The bending moment on the wing tube thus results 8.000 lbs*inch (1.000 kgcm)
Under such a load the material stress results in 67.919 lbs/inch^2 (518 kg/cm^2).

This stress is less than 1/10 of the maximum stress for that material.

Now, why should we under-employ the material strength or, as counterpart, oversize the structural elements?.

Is there anything wrong in the calculations?
Are there other reasons I don't know?

I hope I have performed correctly the units conversions (I'm Italian, I know, nobody's perfect), and I apologize for my poor english.

Many thanks to all.
Enrico

It is over engineered because, in all likelyhood, it was never engineered to begin with!

What Jim says.
Model designers tend to find what the limits are empiricly. If something breaks, make the next one bigger.
For a TOC plane which is a major investment, make it way too big.
You could investigate what a smaller acceptable tube would be, and compare it's weight to the first one. It probably won't be that much different.
Bigger is better when it comes to holding wings on expensive airplanes.

Thanks fellows,
I posted this thread before realizing, in another one, that acceleration was measured on TOC models up to 30g.
In this case what appeared to me as a waste of strength now can be called security factor and is about 3.
I was thinking of structural buildings calculation (it's my job), where this factor is a a bit more than 2 (2.400 kg/cm2 working stress, 5.000 kg/cm2 breaking stress) and mostly for linearity of stress to strain ratio.
In aerodyinamics, where wheight is an issue more then in static buildings, it appeared all natural applying some engineering.
OK, next week I'll call my brain surgeon.
Greetings.
Enrico

You considered a 400 lb load on one wing due to a stalled wing during a 10 g maneuver. One way of looking at it is, is a 400lb load possible? For a wing with area of 1600in^2 (800 per side) it would take a speed of more than 350 miles per hour at a lift coefficient of 1.5 to produce 400 lbs of lift from one side. If the other side produces no lift, the roll rate would be awesome.

I think your English is excellent.

I've read that the load on winch launched model gliders is the highest loading encountered by model airplanes. I guess, because of the long wings spans, and thin airfoils.

If you search for info about design of glider wings, I think you'll find some sound analysis of the forces and structures, in that case.

I've stumbled across such info on the www, but can't put my finger on it at this moment.

Securtiy factor is a good name for it.

When I worked in design (a few years before going into aero full time) I was amazed that the smallest fitting was analyzed and if weight could be saved, even the smallest amount, it would be shaved off. Enough ounces saved here and there over several thousand parts starts to be a lot of pounds and airplane performance gained.

With models however no one wants to pay for that kind of analysis to be done. It is cheaper to hook an airplane to a winch and see if the wing joiner breaks. Then increase sizes of materials until it doesn't. If it didn't break to start with, flies OK and you already have the plans drawn why make it smaller - it costs too much to redraw the plans.

In the case of the aerobatic ship the savings in weight is just a few ounces so the cost in money to do the analysis is not a good trade off either. Just get a bigger motor.

It's good to keep in mind that the "overkill" approach doesn't work so well on airplanes. When you oversize one part, you are in effect making all the other parts weaker, since the total weight and hence all the loads increase.

For example, if you double all the material gauges on an airplane, the weight and the loads would also roughly double, and the strength margin would not change very much. So the real path to the strongest possible airplane is structural balance -- making everything just strong enough or stiff enough for the job, and no more. Overkill on selected parts will never get you there. In practice, structural balance is compromised for a number of reasons:

* Time and cost
* Limited gauge availability
* Manuacturing constraints
* Varying consequence of failure among parts

.............. When you oversize one part, you are in effect making all the other parts weaker, since the total weight and hence all the loads increase.............

If you have a case where in the process of oversizing one part you decide that the total airplane can now carry more loads then what you said is true. If you oversize everything then what you said is true.

I was thinking of this case in which all the other parts are adequately sized for the loads. Then just oversizing one item such as a wing joiner is usually not a big thing since the total weight does not increase that much (of course it depends on what part it is and what the increase in weight is).

Least someone misunderstands oversizing one item in itself does not make the other parts weaker, of course they can still carry the same loads they did before.

Thanks Jim,
if you find that URL once again I'll be pleased to learn something more about airplane structures calculations.

I'm on Mark side.
just to follow up Ben exemple (and it's truely my actual problem), if I oversize wing connector, I have to cut out a bigger hole in the root rib, thus weakening it.
In case of a overload break, I don't care much if the collapse happened in the tube or in the actual wing.
Effect is in all cases a crash.
Greetings.
Enrico

.

Enrico, the rib isn't the structure that takes the bending load of the tube. The spars do that. The hole in the rib can be any size, just so long as the tube itself is supported in the wing.

Yes, you don't want to tube that is too small in a foam wing. THat would leave a lot of foam between the skin and the tube structure. Bigger is better. Being that the bending loads need to be transferred from the skin to the tube, this is important.

In some cases, the foam is strong enough to transfer the loads but only if the bearing strenth isn't too high. A large diameter helps reduce bearing strength.

In most cases, especially for aerobatics, the loads are transfered by a ribs at the outboard and root ends of the tube. THe hole size thru the rib is not critical. THere isn't a lot of chordi=wise bending load in a rib that would be a problem, and it will be surrounded by skin that will keep it bound in place.

What I don't understand is why I don't see more wing tubes installed with shear webs between the tube socket and the wing skin. Seems to me it would be a more efficient way to transfer the loads to the tube than the point load of an outboard rib. I had a set of wing tubes come loose on me, and I installed new ones with shear webs out to the skin. Seemed bulletproof to me.

Thanks all.
Actually was chordwise bending moment my problem and I'm happy to realize isn't that much.
John, I agree that transeferring loads by point is not an efficient way. Excuse my poor understanding, what do you intend with shear web?
Greetings.

Ok, I drew up a sketch. It is looking from the front, at a cutaway view of the wing along the spar line.

The shear webs are vertical pieces of balsa between the tube and the wing skin. It would transfer bending loads gradually and along the direction they are occuring. The web could even extend out into the wing beyond the tube to try to reduce stress concentration at the end of the tube.

Thanks John,
now everithing's clear.
Greetings.
Enrico