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HomeScaleAircraftHelicopterScale Model RC Helicopters › Tach 3 Blade Head
10-25-2009 03:39 PM  8 years agoPost 1
Benski

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Perth, Australia

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Got a questions for those how didn't fail high school math. I've got a Hirobo 3 blade on my lama. When I use my optical skytach to check the rotor rpm, and I get the blades to stand still in the viewing window, the indicated RPM is clearly very different to the true RPM. My question is what ratio is the inticated RPM to the true RPM. From my guesstimation the indicated RPM on the tach is about 2/3 of what the head is actually spinning at?

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10-26-2009 12:11 AM  8 years agoPost 2
ruizmilton

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Carolina, Puerto Rico, US

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As far as I know, the optical tach is based on the stroboscopic effect, it does not matter how many blades you have if they are still, RPM should be correct, what is the reading on the tach? what is the rpm you estimate it to be running at?

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10-26-2009 12:27 AM  8 years agoPost 3
Peter Wales

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Orlando Fl

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You can get several different readings with multiblade heads. Someone measured my turbine Alouette and told me the headspeed was 1350 rpm. He had a tach which went from 1000-2000 rpm.

The actual headspeed was 900 rpm! The blades will appear to be stationary every 450 rpm on the tach so you need to have a basic idea of what the headspeed is. In my case, if he had a tach which could have read it, he would have seen 900, 1350,1800 etc

Peter Wales
http://scalehelicopters.org

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10-26-2009 01:04 AM  8 years agoPost 4
Super-Hornet

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Singapore

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Are u doing the reading on sunlight or artificial light? U need to do it on sunlight. Artificial light like florescent light will give u inaccurate reading due to the light itself is flickering (100times if your country A.C. is 50Hz). That flickering causes something like strobe light effect.

SH

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10-26-2009 01:43 PM  8 years agoPost 5
Benski

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Perth, Australia

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Thanks for the replies. The speed I was reading on the tach was 1000 rpm, but I was expecting 1500, however the weird thing was I couldn't get the blades stationary at 1500. Anyway it seems the "harmonics" (for lack of a better word) happen at a different ratio to that of a 2 blade head.

Peter, you confirmed the ratio I thought it was i.e.
900/1350 = 0.666666 = 2/3

I just bought your scale helicopters book Peter. Allot of good info for noobs like me. Embarking on my second scale build, a Hirobo Dauphin 2.

Cheers
Ben

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