Building Materials, Structures & Processes.

Phydelis
I fear you may find it rather difficult to achieve a meaningful stress calculation.

Just for fun and with the help of on line calculators for second moment of inertia and bending stress I have just about managed to calculate the surface stress and deflection for a simple caltilever "I" beam made of spruce under a given uniform load.
But
I do not have much confidence as to its accuracy as the calculators are really intended for steel beams.
And
A simple I beam is not a very good representation of a structurally efficient wing structure.

Maths was never my strogest subject and I expect a proper aeronautical stress engineer could probably do a lot better but the calculations are far from simple.

Quorneng wrote: Phydelis

I do not have much confidence as to its accuracy as the calculators are really intended for steel beams.
And
A simple I beam is not a very good representation of a structurally efficient wing structure.

Maths was never my strogest subject and I expect a proper aeronautical stress engineer could probably do a lot better but the calculations are far from simple.

This is the part that worries me & up to now, I can't see an answer.
If we were talking about a material like steel or aluminium then we may get consistent reliable results .... but if we are talking about a wooden structure then we are talking about a plant!
You'll never find two pieces the same so the calculations will be almost meaningless.
You may be able to measure that a certain structure has a minimum strength & then repeat that structure .... calculating the strength from the design alone would require the data for every piece of wood in advance. .... This is a difficult one but I'm still thinking about it.

My concern is not so much the variability of wood (which of course it has) but more the sheer difficulty of the stess calculations as even a simple built up constant chord wing is actually quite a complex structure.
If you add in a tapered wing profile and built of dissimmilar materials (i.e. balsa/hardwood) it requires (I think!) some really complex differential calculus.
Then there is the problem of determining the true disposition of the wing loading as it is not constant over the entire area, particularly if the wing incidence varies along the span, whether by design (washout) or as a result of twisting under load.

So much easier to build something, test it and then see if you can make the next one stronger and/or lighter!

Just to add insult to injury, we may then need to start calculating in resonance factors.
Many of us will have had a model that has suffered from "Flutter" on one of the control surfaces ... but this can still happen on a set of wings.
Imagine the load distribution on a wing if the tip decided to oscillate to the extent of going from "wash in" to "wash out" at umpteen times per second .... Garry does it all the time.... or was that his arse making that noise?

Phydelis.... just to give you some idea as to the properties of various types of wood & how they differ, you may find this PDF document useful.
You may be able to figure out just how complex any calculations can become from this information.
www.fpl.fs.fed.us/documnts/pdf2001/green01d.pdf

Let's imagine a built up wing as a simplified structure. Instead of using an aerofoil section, imagine it as a rectangular box measuring 24ins x 6ins x 1inch. .... a reasonable size for a model wing.
(Two of these would give a 48ins span with an aspect ratio of 8 : 1)
let's imagine that this structure (rectangular box) is built out of 1/4 inch square section wood strip.
How are you going to calculate the torsional strength?
It would be possible to anchor one end of this structure & apply a twisting load to the other. You could even do this with a simple weight acting through a specific distance ... e.g. 1 foot or 1 meter.
Let's say for example that a load of 1lb applied through a 1 foot leverage caused the end of the lever to move 1 inch.
The simple addition of small triangular gussets at each joint will drastically reduce this twisting ... this is often done by modellers between wing ribs simply because we have learned just how much this will improve wing rigidity, but we have no formula that tells us just how much of a difference it will make.
You could then repeat this test on a humid day & get completely different results because of the moisture content of the wood..

One way you could possibly devise a formula for how much a wing will bend under load is with simple algebra.
Imagine a foam veneered wing .... the foam is very flexible & can be considered negligable for this purpose.
Imagine the wing once again as a rectangular block (for simplicity)
As the wing bends under load, the top skin of veneer is under compression, the bottom skin is under tensile strain. ... the wing is tying to form a circle with an inside & outside diameter.
If you know how much the veneer will stretch or compress under load then you could calculate the diameter of this circle. .... thus the bend in the wing.
If we don't know how much the veneer will stretch or compress then use algebra to express this as formula with unknown values...... it will apply to any material. (Make the bloody tutors look up the material characteristics tables) hehehheeeeeeee

I'm still curious Phydelis, you may be going too far here. I have asked this earlier but what exactly is the project question?
You did say that you needed to know the following.
Quote Phydelis.
I want to know the following:
1.Reviewing the current practice in model aircraft construction
2.Investigate and review potential improved materials and processes
3.To suggest improvement in model aircraft construction processes
4. Case study: Improving a design using materials, structures and processes
Knowing all this will help me get a broad knowledge of what i need to know to complete my project.
Unquote.

You didn't mention anywhere here that your tutor expected extremely complicated calculations & I don't think they will do.
They may expect something of a design study where a manufacturer has, for example, changed from a built up wing to a foam wing ... I don't think they expect you to calculate the physical attributes of that wing.

If you do need to start thinking about lift/drag ratios then you might want to look at this first.
en.wikipedia.org/wiki/Lift-to-drag_ratio

Have I scared you off yet Fidelis? ....
Seriously, keep the questions coming, it's got us all thinking.
How much time do you have left?
I've got to build a simple foam veneered wing for a model & if it would help you I could video it & post it here.

Pol.

So much thanks to you guys even when pol said i shouldnt sound too kind. We had a lengthy conversation on phone so am going with the option of testing it and taking the measurement instead of calculating it. So please i need more explanation/ video of ways of measuring bending/torsional stess and ways of building a wing made with foam and balsa. You guys have made this topic so interesting to me. Once again thanks all.

Phydelis
My suggestion is to set yourself some parameters to cut down a bit on the number of variables.
First I understand the competition rules limit the plane to 2M span.
I believe you have selected to build a monoplane so next you need to pick the chord (width) of the wing. As a weight lifter you want plenty of wing area so lets say a simple plank wing with a constant chord of 1/4 span = 500mm.
Next pick the wing section. Just to show the process lets pick a simple Clark Y section.

It has a maximum depth of 11.7% of the chord. With a 500mm chord it would be 59mm deep.
So could a 2M wing like this be built to support the total estimated load at its centre and how much 'reserve' strength should there be. Doubling the normal load would be considered the minimum safety margin.
This is by no means the end of the design process but it gives an idea of some of the early stages.

Quorneng wrote: First I understand the competition rules limit the plane to 2M span.

You're thinking of a different thread.
Phydelis is doing an engineering degree & this is part of his studies.
You might be thinking of this thread, it's a school competition for indoor flight duration.
www.modelchat.org.uk/forum/building-thre...door-flight-duration