Been thinking of getting into the hobby of building ultralight helicopters, and would PVC tubing be a good choice to build the overall airframe?
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1$\begingroup$ Not only is the strength to weight ratio poor, PVC also has a rather low modulus, so the airframe will resonate at a multitude of frequencies. While tolerable in a glider, in a helicopter this is unwise. The resonance with all that is vibrating in a helicopter will shake the airframe apart before first flight. $\endgroup$ – Peter Kämpf Nov 25 '20 at 17:28
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No. The strength to weight ratio of PVC is poor compared to aluminum and you would end up with a very heavy, albeit cheap, machine, and will be even worse when compared to carbon fibre tubing, which would be the optimal choice performance-wise. Extruded aluminum tubing would likely be the best choice cost wise. Forget about building a plumb-O-copter.
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$\begingroup$ If you're talking about the sort of PVC tubing used for irrigation systems, not only is it not very strong, it's also quite flexible. Hold a 10-ft length (standard size in US hardware stores) in the middle, and it will deflect at least 6" on either end. $\endgroup$ – jamesqf Jul 10 '20 at 3:23
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$\begingroup$ I think he has in mind Sched 40 or Sched 80 rigid plumbing pipe. $\endgroup$ – John K Jul 10 '20 at 11:55
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$\begingroup$ If I'm not mistaken (I'm no expert), schedule 40 PVC is the white pipe commonly used for irrigation &c. $\endgroup$ – jamesqf Jul 11 '20 at 5:19
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$\begingroup$ Sched 40 is the standard PVC for residential house plumbing, good for a 2 or 3 hundred psi burst. Schedule 80 is for higher pressure applications and has nearly double the wall thickness (smaller ID). $\endgroup$ – John K Jul 14 '20 at 20:00
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$\begingroup$ Sure, but bursting strength is not really what's important in building airframes :-) $\endgroup$ – jamesqf Jul 15 '20 at 21:05
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In my experience using PVC to build scale models, PVC doesn't make a good construction material:
- it's too flexible. PVC pipe will bend under its own weight.
- the glue typically used to join PVC is pretty weak, impact loads will break the bond.
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PVC is no where near stiff enough for a given weight. And its strength to weight ratio is poor. The structural efficiency of air machines is absolutely critical otherwise you are building a machine that may not fly at all.
Plus, do a search for helicopter ground resonance, especially watch some youtube videos of helicopters destroying themselves.
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$\begingroup$ Ground resonance only comes into play when the helicopter has a rough landing. With PVC tubing it will resonate so much that it will be shaken to pieces even before first flight. $\endgroup$ – Peter Kämpf Nov 25 '20 at 17:34
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1$\begingroup$ I'm pretty sure I have seen videos of ground resonance without a preceding landing of any sort 🤔 $\endgroup$ – Jpe61 Nov 25 '20 at 20:12
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PVC pipes are strong and flexible. 1 1/2" diameter pipe type SDR-26 is thinner-walled and weighs only .3 pounds per foot! That's three pounds per ten-foot length which only costs eight dollars each at Home Depot. PVC cement can form a solid integrated bond when applied thickly and left on for ten seconds to melt the surface before mating.
However, PVC is breakable when bent, so the airframe must have a lot of reinforcement braces to reduce the amount of bending of longer sections.
Strength-TEST.!! Test test, the strength of every section using sandbags. Get an EAA member to advise you on this.
Regarding self-bendability: I just tested a 10' length of 1 1/2" PVC type SDR-26 plastic pipe and when suspended it at center and each end drooped down 1/2" is all. I doubt that any ultralight designer would incorporate any unbraced pipe lengths longer than 2'.
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4$\begingroup$ I have removed your "signature" twice now. Please do not add it again or we'll have to consider you a spammer. $\endgroup$ – Federico♦ Nov 25 '20 at 15:15
