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Old Nov 18, 2012, 01:04 PM
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United States, CA, Sacramento
Joined May 2011
89 Posts
You are correct, I have practically no experience with very large, unstabilized RC helicopters. My experience comes from 3D helis and small scale (450) sized projects.

I will concede the pure pitching up tendency, I've "learned" this before (read in book and manuals), it just always seems to me that the helo would want to roll as well, but I do concede to your experience in this matter.

With multi-blade setup, obviously RPM will decrease. I had a 5 blade head on a 450 scale project that was unstabilized, so I am just a little familiar with their operation. I don't remember what RPM I ran it at, but it was far, far less than a typical 2 blade 3D RPM speed. I could fly it just fine, but the particular MD500 fuselage that I wanted has been sold out practically all year so I abandoned that project and I'm now doing a 450 size Bell 212 build (flybared).

I guess the only thing I can't agree with your post is the blade tips approaching the speed of sound. I fail to see how 1500 rpm with a 70" rotor diameter can produce tip speeds close to the speed of sound.

(rotor radius [in])*(2)*(pi)*(RPM)*(1[ft]/12[in])*(60[min]/1[hr])*(1[mi]/5280[ft])= 35*2*pi*1500/12*60/5280=312mph... not even half the speed of sound. Even with the RPMs a 3D Trex 800 will produce, the tip speed is still under 400mph.

How close to the speed of sound do the rotor tips have to be going to to cause trouble? (I'm not trying to sound snarky, I'm genuinely asking to being schooled on this)
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