Double taper spar

[Simon Chaddock]

Some time ago I built a 60" wing with a full depth balsa/Depron/balsa spar.

The wing section was a standard 11.7% Clark Y section tapering from 10.5" (267 mm) chord at the root to 6" (152 mm) chord at the tip.

The spar flanges are the same thickness as the Depron skin (3 mm) and they taper from 19 mm wide at the root to 3 mm at the tip.

It has not broken - yet!

As the flanges are fully supported by the Depron web I can make the spar just as strong by using flanges 3 times wider but only 1 mm thick.

This means the spar will be much wider so will a simple taper in width towards the tip actually maintain a constant stress in the flange?

A uniform load imposes a "squared" bending moment function which suggest a non linear taper to the spar but the load is not uniform (the wing is tapered) and the depth of the spar reduces from root to tip.

With so many variables the mathematics to actually calculate the spar dimensions is well beyond me so maybe a straight taper is as good as any!

My intention is to build an 80" higher aspect ratio version of the 60" wing.

Edited By Simon Chaddock on 14/06/2012 22:25:59

Edited By Biggles' Elder Brother - Moderator on 14/06/2012 23:26:12

[kc]

Nobody has commented so far.

I would say a simple practical test to destruction of a sample would yield some info. I recall RCMW printing an article some years ago on testing various Warren truss balsa constructions against plain square bracing. Very informative, especially the simple test method used -hanging weights on the end. Alasdair Sutherland did it if memory serves. Failure was by twisting eventually and I suppose twisting has to be taken into account for a wing as well as bending.

[riverlandgirl.]

Hi there. If it is any help, when I build solid polystrene wings, I use wide thin spars, top and bottom. This would be very simmilar to what you are doing. I make the spar paralell sided for 1/3 the span then taper the trailing edge to half the width at the tip. I have never had a failure, even on high aspect slope soarer wings. The spars on my 8'' farm girl' would be 2' wide at the root, by 1/8 thick, hard balsa.

[SDF]

RLG, having a bit of a Spinal Tap moment there? I hope you mean 8' and 2"

[Simon Chaddock]

riverlandgirl

Your wide thin spars are exactly what I had in mind.

Parallel for 1/3 and then tapered would give a reasonable approximation to the shape required by the bending moment but I think ideally it would be a sort of reverse ellipse and in an ideal world it should also taper to zero at the tip where there is no bending moment.

Tapering only the rear edge is probably quite sensible as it would in effect move the spar forward in the wing section towards the tip so encouraging a degree of stress relieving 'washout' to develop under high load.

The spar flanges on my 80" wing will be 1.75" wide at the root tapering to 1/16" at the tip but in just 1/32" hard balsa (all dimensions in inches) .

It is intended to be very light, hopefully no more than 6oz including the aileron servos.

The only problem I can see with such a thin flange is that it will have to built accurately to the wing profile as any sanding would seriously weaken it.

 

Edited By Simon Chaddock on 17/06/2012 00:48:35

[riverlandgirl.]

5DP You are right of coarse! lost me blinking glasses again!!!! Simon, I generally put in an over thick spar in a shallow rebate and then sand down to thicknes and profile. (you could also taper spar thickness this way too) Sorry if I make diction mistakes, my lap top is very small and this text I struggle to read!

[Simon Chaddock]

My original 60" wing has a Clark Y section as with 88% of its under surface flat it can be built up starting with the bottom skin.

This time I thought I would try the Eppler E195, a section with no flat surfaces. It will require the complete wing to be built up from just the spar.

This shows the root section with the broad 1mm balsa flanges on an otherwise all Depron structure.

With shear webs between the ribs at both the front and back of the flange will in effect create a hollow box spar but with the ribs running through it.

I intend to build a short parallel wing section to check that this method is really practical.

 

Edited By Simon Chaddock on 17/06/2012 22:00:48

[Simon Chaddock]

The 1mm balsa/2mm Depron lower spar flange of the 'test' wing.

The ribs glued to the lower spar flange.

A 'construction' front spar that rests on the building board is added to ensure the ribs line up exactly.

The shear webs being added.

Once the top spar fange is in place the 'box' structure will be rigid enough to complete the wing skin.

With the spar flanges being the same thinckness as the wing skin (3mm) does mean the ribs are 'simple' with no cutouts.

[Simon Chaddock]

The 4" "test" wing complete (less the nose profile section)

The spar flanges follow the Eppler E195 section.

It is remarkably rigid and weighs 0.37 oz (10.6g).

This of course represents the biggest and heaviest part of the projected 80" wing. Based on this I calculate the mean will be 0.0625 oz/in giving 5 oz (140g) in total.

The only change to the method of construction would be to make the ribs from thicker (3mm) Depron, but I would still need 42 of them!

[Simon Chaddock]

Well here goes with a proper wing although I have chickened out on two counts.

The span is 72" not 80 and the wing section will be a modified Clark Y. It will be so much easier to build with a true flat bottom over much of the lower surface.

The first pair of 1mm hard balsa spar flanges.

It will be the standard 11.7% thickness at the root but reducing to a thinner 8% at the tip.

I am using PVA wood glue which with its long drying time slows the build down a bit but does provide a really strong joint and there are alot of joins, about 1300 linear inches worth in the complete 72" span!

[Simon Chaddock]

This is proving a nightmare to build.

19 ribs and the shear web added to the lower skin.

The tp flange added to create a box spar.

I supose I had better steel myself and get on with the other half!

[Erfolg]

I have only just read the thread. The triangular shape is exactly what I would have anticipated, in that shear forces and bending stresses would be zero at the tip. That is during normal flying and looping.

On the other hand, my Father said to me after doing something similar. Son this is the real world. From what I have seen your bending stresses etc., only occur when you are on the line (towline/bungee). There is another part of your regime where your assumptions are clearly wrong. How is that I asked? Well, a lot of your landings are so disorderly, that your wing tip is seeing loads far greater than you are assuming. Of course he was correct, my wing tips were more often damaged than I would have liked.

There is another issue, and that is torsion. Weak tip structures may not be resistant enough to torsion issues, which can manifest themselves in flutter at speed. Then again, the Depron sheeting could be enough.

I do admire your commitment to Depron.

[Simon Chaddock]

Erfolg

When does a commitment become an obsession?

I would agree entirely about 'unplanned' tip stresses however I am a firm believer in a bit of 'give' so this wing will be held on by rubber bands - and just 2 at that.

Obviously torsional loads are a worry. however Depron has a very fine cell structure, hence not the lightest of foams, but it is reasonably rigid. Nevertheless it is still light enough to allow full sheeting in areas where normally a film covered 'open' structure would be used.

Finally with its 10x3.8 'slow fly' prop the plane has static thrust to spare but a strictly limited top speed, hopefully below any flutter regime.

No prop stall here!

[Erfolg]

Should that not be a 3.8 * 10 and 400w motor?

[Simon Chaddock]

The RH wing LE skinned and the nose profile added.

The aileron servo has to be fitted and cabled up before the rear top skin can be glued on.

The standard (11.7%) Clark Y root section.

At the tip the section is thinner at 8.7% but with a proportionally thicker & more rounded nose profile to delay the stall - I hope!

Although the under surface is flat from root to tip the change in wing section does produce a small aerodynamic washout.

Erfolg

I think I will stick with the 10x3.8 and 220W motor as it already has unlimited vertical using just 8C from the battery.

This raises a question - with just how low a C value could sustained vertical be achieved?

 

Edited By Simon Chaddock on 29/06/2012 22:35:03

[Erfolg]

What is a "C" value?

[Simon Chaddock]

A battery "C" rating gives its maximum designed output. e.g. 25C or 25 times its Ah capacity.

The above plane achieves 'vertical' drawing 24A which is the equivalent of discharging its 3000mAh battery at a rate of 8C.

I was wondering what are the design features that would allow a plane to achieve a sustained vertical performance from an even lower battery discharge rate? (without building a helicopter!)

[Erfolg]

8C Simon? I buy budget, they are rated at 20C, costing a modest circa £8-10 from HK.

[Simon Chaddock]

8C is not the maximum rate for the battery (its actually labelled at an unlikely 35C) but the rate needed to achieve vertical performance.

With really gentle flying the discharge rate drops close to 1.5C or 45 minutes power duration.

[Simon Chaddock]

Still quite a bit to do but the LH wing has almost caught up. The full 72".

At this point any differencies between the wings become very apparent!

[riverlandgirl.]

That is going to be a very strong and stiff wing. I would be very interested to know the final weight,less hard wear, to compare to one of my lighter polystyrene wings.

[Simon Chaddock]

Each 38" (0.97m) wing half (including the not yet fitted rear top skin and the aileron) weighs 2.02oz (57.3g).

As it will be a one piece wing I estimate actually joining the halves together will only add 1/4oz, say 4.3oz (122g) in total.

How does that compare?

So far the the only strength test I have done is to put a 5oz load on the tip with the root area held down firmly. Not its ultimate load but I didn't want to over stress it before it was even complete!

A 5oz tip load may not sound much but done like this it imposes a bending stress equivalent of pulling about 4g in the 22oz all up plane this wing will be fitted to.

When the final piece of wing skin is added (after the hardware has been installed) it should be a bit stiffer, particularly in torsion.

[Simon Chaddock]

I noticed that a crescent shaped tip on a small high speed prop was significantly quieter that a square one so on the basis it might be a touch more efficient I have added a scale version to this wing pushing the span close to 2 metres.

Made of 2 skins of carefully formed 2mm Depron it will also protect the aileron a bit.

The 3.7g aileron servo just fits between the wing skins and is positioned so the horn acts close to the mid point of the aileron.

Leaving just the tip of the horn sticking out underneath.

[riverlandgirl.]

Thanks Simon, will get down in the workshop tomorrow and do a bit of a weigh and measure, and let you know, your results look hard to beat and this stage though!

[riverlandgirl.]

Hi There. Here are my results. (I don't want to mess up your nice neat thread, so the photos are on my album, if you wish to take a look) The wing is off my glider. Results are for 1 half of a 2 piece wing. Span 41 1/2 ", constant cord 9", thickness 1/2 ", solid polystyrene, slight undercamber. Spars are 3/32" soft balsa, 1 1/4' wide, tapering to 3/4", full span. Weight 5 oz. This equates to 1.98 oz per square foot. Stress tested to a little over 1 lb. Two spars only, one top one bottom. No shear webs, other than at joiner box.