Here you can exchange your experience and datas about your home build boards
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tweoistom
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Postby tweoistom » Fri Feb 05, 2021 4:23 pm
Hi everyone,
I'm hoping for some guidance on what to aim for in regards to the stiffness of a mast I am building. It's 3d printed core with glass reinforcing - approx 900g/m^2 each side. Naturally it has some flex but what I am trying to work out what should I be aiming for. I understand some flex is useful for making things more comfortable
I've never had experience of commercial foils and masts so have nothing to compare it to.
I'm basically looking for a way to quantify and then some figures to compare to. The textbook example to figure this out would be If I were to bolt the mast to a work bench and then hang a weight and measure deflection.
Maybe a direct comparison with an aluminium roof rack bar (closest thing to an aluminium mast in my possession)?
Thanks
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direnc
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Postby direnc » Fri Feb 05, 2021 7:02 pm
Hi,
It needs to be super stiff. IMHO, you're better off by just buying an aluminum mast and plate from gong (90eu mast + 30eu plate + shippping). I and many DIY foil builders did so:)
Still not impossible to build one yourself. But really needs to be super stiff. So stiff you can stand on it.
Check out these forum threads:
http://kitefoil.forumactif.org/t1580-me ... ion-du-mat
http://kitefoil.forumactif.org/t3831-ma ... s-question
There used to be deflection and twist data for a given weight for commercially available foils. Some may be mentioned on the threads above.
As for building your own, you may consider building something like the project cedrus mast. You may print a simple 3d printed mold to build one, and get quite stiff a mast...
The patent is available online should you want to go that way...
https://projectcedrus.com/
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tweoistom
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Postby tweoistom » Fri Feb 05, 2021 7:58 pm
Thanks this is exactly what I am after. My original design had a piece of aluminium tube square tube 20x20 running the length with an aerofoil section added front and back with glass reinforcing the lot but wanted to get something a little slimmer. It is still tempting given the cedrus mast though.
Based on the test on those links I am at around 30mm
Which isn't too bad but about twice what some are seeing given the mast length 750mm. Excuse the mess please!
Maybe more reinforcing or a redesign.
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direnc
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Postby direnc » Fri Feb 05, 2021 8:37 pm
Not bad for a DIY mast indeed!
BTW, mast stiffness makes quite a difference while riding. I can even feel some aluminum masts wobble and it's no good. Also some carbon ones are unfortunately not as stiff as some aluminum ones
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tegirinenashi
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Postby tegirinenashi » Sat Feb 06, 2021 1:34 am
I did 5kg test for two masts:
Alum LF second gen Rocket/Happy:
flex = 13 mm
torsion = 3-4 mm
Carbon mast from aliexpress (98 cm tall, 14 mm thick):
flex = 12 mm
torsion = 22 mm
Since then I have layered 400-500 gsm 45 degree twill, and it is noticeably less twisty, but the thickness went up to about 15 mm.
For carbon mast it is essentially a compromise: 100% unidirectional would be very stiff in flex but twisty, while 100% 45 degree twill would make it rigid in torsion, but flexier than comparable alum.
With, fiberglass there is no way making a stiff mast, other than raising thickness substantially. I'd suggest that to have it comparable to carbon you need to have 20 mm thickness, minimum. (The stiffness is proportional to the cube of thickness).
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plummet
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Postby plummet » Sat Feb 06, 2021 2:29 am
The above test is the datum for testing mast flex.
I agree with tegirinenashi re glass flex. I will be too flexible. Get in there with some carbon Uni on the vertical and also corner to corner to give torsional stiffness. 900G glass matt will not cut it.....
Remember you want the fibers going in the direction of where you want the stiffness.
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tweoistom
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Postby tweoistom » Sat Feb 06, 2021 4:56 pm
Cheers, I'll see what materials I have left over after making a new wing as keen not too buy much more resin if I don't need to. Fairly keen to get this mast working just from a point of satisfaction of making the whole lot and learning the process along the way.
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tweoistom
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Postby tweoistom » Mon Feb 15, 2021 2:27 pm
Thought it worth an update as I was surprised how much difference a small amount of carbon uni had - 100mm wide tape on each side of strut @ 250g/m^2 has dropped deflection down to 15mm. Which is about the width of the mast. Well worth it. It'll have another layer of glass over the top to keep everything tidy so will likely improve a little after this.
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BayAreaKite
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Postby BayAreaKite » Wed Feb 17, 2021 5:12 pm
Hello everyone, it's great to see others building their own masts. I wanted to offer a little insight regarding proper testing of mast stiffness, so you don't waste time and material in the wrong places:) Fixturing the mast at the base and pushing on the end does NOT test mast stiffness, it simply tests the stiffness of the joint to the board. If you look up shear and moment diagrams for cantilever beam, you will see max moment at the fixed end. If your max moment is at the fixed end, you will be primarily testing this region and not the actual section of the mast itself. A proper method to test mast stiffness in bending is to apply an eccentric axial load, which imparts a constant moment throughout the length of the mast. This is equivalent to getting on the rail of your board, and making a hard lean. You can see some animated contour plots of this applied load condition here:
https://projectcedrus.com/cedrus-develo ... -analysis/
A lot of big foil brands are testing this was as well, and it's not correct. Again, as the maximum stress is occurring at the base of the mast, you are not capturing the true characteristics of the mast section (Ixx). Don't get me wrong, it's important to have a good stiff connection to the board, but it's just as important to have a stiff section down where it connects to the fuselage and testing this way does not measure stiffness at the fuselage end of the mast.
Happy to help offer any additional insight.
Kyle @Project Cedrus
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