RC India

Can anyone tell me how to design a spar??
i want a round or square spar ,so i would like to know how to find the diameter or the sides for it.

@naresh9807,
This probably needs an entire semester of an engineering course to answer!

If you are talking about a spar for a model airplane, it depends on the weight, wing span and wing thickness, and expected G loads, as you probably already know.

Over the last hundred years, model airplane wing spars have evolved, more through trial and error and weight considerations than theoretical analysis.

I could offer suggestions for span 10”-60”, weight 10gms to 3kg, airspeed 2-50 m/sec. Outside that range, I do not have the experience to comment.

You may get useable inputs if you post details of the project being tackled, instead of a general query about spar design. Unless of course theory of spar design is what you are looking for  ;D

@sanjayrai55,
Sir, perhaps you could help?

BTW, I looked at your earlier posts, and found one asking “what’s the difference in voltage in USA and India”

Quite surprised!
Taken aback by such a query from someone asking about spar design  ;D

No Iyer sir, I must decline help. I'm "sparred" by this question

Sent from my Moto G (4) using Tapatalk

Aah, spared. Not speared.  ;D

@naresh9807,

My wing span is 1m and chord is 0.191m. weight of the plane is 1kg and has a speed of 10m/s.

If I were to build such a wing (assuming thickness of ~20m), I’d use one of the following:

1. I beam + D tube. Using balsa. Top and bottom spars of 3mm sq with vertical grain webbing of 1.5mm between them. Leading edge of 5mm sq on edge (diamond shape). Top and bottom 1mm sheet between spars and leading (full span)

2. Monocoque. Hotwire cut thermocol core. Using thermocol like in refrigerator packing, not what is sold in stationery shops. Covered entirely with 18gm/sqm fibreglass cloth using 20 minute finish cure epoxy. Add a second layer about 10cm each side of center. For foam core see vortex-rc.com, and Hobbyking for the fibreglass and epoxy.

3. Simple beam of ~3mm thickness of hardwood ‘baton’ (window beading) if using coro or depron skin.

4. If a round or square cross section is essential for any reason, a carbon fibre square or round tube of 6mm x 1mm wall thickness will be more than enough.

5. Full depth spar of 3mm balsa capped top and bottom with 1mm x 3mm CF strip with 3mm CF tube for leading edge.

Hope this helps.
Regards

Thankyou sir for your reply

@naresh9807,
Sorry if my (practical) suggestions did not meet your (theory) requirements.
Suggestions were based on experience of this kind of wing.
Since I have no data on compressive, tensile, bending strengths of the materials I have access to, not
possible for me to calculate the bending (and torsional) strength, of, say, an I beam spar with Warren Truss webbing.

However, if you are actually building this wing, (and not just doing a theoretical analysis), I hope you’ll find my suggestions helpful.

Regards

Its ok sir. I did want a theoretical as well as a practical answer. Your answer was helpful. Thankyou sir

@naresh9807,
I try to avoid saying things that I don’t have personal experience of.
So I didn’t really answer your ‘theoretical’ part.
Partly because I’m not sufficiently qualified.
And partly because I believe that flights of theory should be grounded in practical realities.

Let me share a sample.
Tested a 30” CF tube of 5mm OD, 4mm ID
Loaded at the centre, and rested at 1/4 length from each end, in the simplest approximation of spanwise wing loading distribution

Takes 2.5kg with no sign of breaking.
But it flexes. To avoid which, additional structure is required.

Hence I beam / D tube type structures suggested, in preference to tubular spar.
Especially since torsional stiffness issues too need to be considered...

I’ll stop my inputs here.
As I can’t afford to ‘test till failure’
Without which I cannot support/rebutt theory.
After all, there’s more to a wing than just a spar...
Thank you for listening uncritically so far.
Regards and goodbye.

Thankyou sir

I often used finely sliced bamboo as spars. The beauty of this is that it allowed the wing to flex. I seem to love a little flexing in the wing. And this also gave a beautiful arc .