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Old Mar 28, 2005, 07:41 PM
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How to measure head speed (on my own)

Want to measure the head speed on my Trex.

I have a head speed gauge - point it at the blades and it shows 10s of RPM.

Can I do this on my own or do I need an accomplice to hold the heli

i.e. could I just set a 0 pitch curve, spool up on the ground, and check the speed at different throttle settings? What should I be looking for at 50% throttle - or should I be interested in head speed at a different throttle setting (60%?).

If pitch is required then I guess I need to hold the heli down somehow.... any suggestions?
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Old Mar 28, 2005, 08:32 PM
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San Carlos, California, United States
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I have a "testing 2x4" that I put through the skids of helicopters when I tach them.

You'd need something smaller for a T-rex, though.

Be sure to weight both ends with something heavy, like a cinderblock.

Toshi
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Old Mar 29, 2005, 12:37 PM
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Upstate NY
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If I didn't have an "accomplice" to hold the heli, I would weight the heli down as Toshi said. I use a piece of 1 x 4 pine through the skids and a couple of bricks (out beyond the radius of the rotor). I also tape the tach to something and place it under the rotor so I have both hands free. Then I can stand at a safe distance from the heli with both hands on the Tx (ready to cut power if needed).

It would really stinks to lay on your belly holding the tach and then knock over the TX and have the throttle stick get jammed up!!!

BTW: I have an analog tach that doesn't have the greatest meter resolution, so I read under the blades and paddles and then divide the results by 2.
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Old Mar 29, 2005, 06:25 PM
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Thanks, I tried this briefly today and was able to measure the head speed (I think).. not too happy with what I saw (only 1200 RPM if I believe the tacho!). Cannot help but think something was not right as it flies OK so must be getting more like 2000 rpm I guess.

Weather permitting I'll try again tomorrow.
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Old Mar 29, 2005, 10:12 PM
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You can test your tach by pointing it at a flourescent light. You should read 3600.
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Old Mar 29, 2005, 11:06 PM
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It sounds like it may be setup for 3 bladed prop mode, that would put it at 1800RPM (1.5x1200RPM). dbcaster has a very good suggestion for checking.

-Kai
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Old Mar 30, 2005, 05:26 AM
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I think it was set for 2 blades - it toggles 2 - 3 - 4 - off using the one button on it.

Flurescent light showed 3000 BUT I was maybe 8 or 10 feet from it (only flourescents are in the room of my garage) and I'm not sure if the 3600 thing changes if you're on 240 volts (UK)
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Old Mar 30, 2005, 09:18 AM
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You may find it is 3000 in the UK as we're on 50hz mains as apposed to 60hz in the US which would give 3600 (or about a 17% difference).

Quote:
Originally Posted by ukgroucho
I think it was set for 2 blades - it toggles 2 - 3 - 4 - off using the one button on it.

Flurescent light showed 3000 BUT I was maybe 8 or 10 feet from it (only flourescents are in the room of my garage) and I'm not sure if the 3600 thing changes if you're on 240 volts (UK)
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Old Mar 30, 2005, 10:11 AM
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I just hold my trex firmly by the frame and hold it above head level (to avoid injury should something let loose) and then spool it up to my desired throttle point - I then check the RPM with the optical tach. This needs to be done outside (for the optical tach to read properly and just in case you accidently let go!).

I don't think that the stock Trex will lift off at much less than ~1600 RPM - if your tach is reading much less, then it's either the tach or your technique. Don't forget about the paddles - you want your tach on 2 blades to be measuring the blades, not the blades and paddles.
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Old Mar 30, 2005, 04:55 PM
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please ignore
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Old Mar 30, 2005, 05:01 PM
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Quote:
Originally Posted by Paul G
You may find it is 3000 in the UK as we're on 50hz mains as apposed to 60hz in the US which would give 3600 (or about a 17% difference).
Good thinking.. It was late when I posted, I worked out that different power might make a difference (110 V / 240v) but just had not thought about the frequency difference. Add up just right.
So... good news is the tacho works OK. Had to be technique 'cos the Trex definitely flies. Maybe if the weather is better tomorrow I can get outside and try again.
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