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HelicopterScale Model RC Helicopters › reading multi blade headspead
05-31-2010 01:38 PM  7 years agoPost 1
cgflyer

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Boca Raton, FL

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Happy memorial Day!!
I have been playing with head speed on my latest scale and although I usually go by sound and performance, I thought I would check the final head speed. Problem is that the sky tack seems to read high on multi blade heads. (for obvious reasons) I know there are some propeler tachs that allow you to select the amount of blades before checking, but is there a formula to convert the speed measured with the Sky Tach when used on multi blade rotor heads.

Rich

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05-31-2010 03:58 PM  7 years agoPost 2
Rodan

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Prescott Valley, AZ

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This was giving me fits until I tried a trick suggested by a member on SRCH...

Put a short piece of fine line tape across the head button and use your tach, with the heli on the ground. Use the tape as your reference instead of the blades. Works great!

I would have thought a shutter type tach would have worked regardless of the number of blades, but apparently that is not the case!

BTW, GREAT looking heli! Is that a modified Century fuse, or custom?

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05-31-2010 03:59 PM  7 years agoPost 3
The Captain

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Fort Irwin,CA

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If it's the same sky tach I have, where you look through the window and adjust the shutter speed until the blades seem to stop, then it does not matter how many blades you have.

I have an older airplane tach that you have to set the number of blades for it to work right, but you also have to hold it within 3" of the spinning prop, which isn't very practical for heli use.

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05-31-2010 04:00 PM  7 years agoPost 4
HS10

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Elk River, MN

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I am about ready to do the same thing...

Help me understand the issue of the optical tachs and multi rotor blade?

Thanks

Logo 14/24 (Agusta 109A-MKII) (MH-6 Little Bird) (OH-58A) Navy - Retired

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05-31-2010 04:24 PM  7 years agoPost 5
cgflyer

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Boca Raton, FL

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Thanks for the compliments and replies. I too thought the shudder would work however though it does stop the blades the same, it was showing 2200rpm and I was certainly no where near that. Great idea Roden and I will give it a try.

Roden, this was originaly the Seaking fuselage from Century but since no one makes the Sikorsky S62, I had to make my own. Here is a link to the build and how it started out if interested.

https://rc.runryder.com/helicopter/t548243p1/

Rich

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05-31-2010 04:47 PM  7 years agoPost 6
Rodan

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Prescott Valley, AZ

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If it's the same sky tach I have, where you look through the window and adjust the shutter speed until the blades seem to stop, then it does not matter how many blades you have.
You would think this is the case, but it doesn't work...

By using the tape across the head button, I was able to use my tach to confirm what all my calculations predicted for the motor/gearing/batteries/throttle setting I was using. This was on a 3 blade head, and six blades were visible at that speed. When I got the tach to show 3 stopped blades, the numbers were completely out of whack.

I don't know why it doesn't work with multis, but it doesn't....

The only drawback of the tape method, is you can't tach it in the air, but it will get you in the ballpark, and you can make small adjustments from there....

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05-31-2010 05:59 PM  7 years agoPost 7
The Captain

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Fort Irwin,CA

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After thinking about it for a bit, I take back my earlier statement. I should have thought about this earlier.

A Sky Tach is calibrated for use with a traditional 2 blade head. How it does this is the shutter flashes at half period intervals (period is defined as one full rotation). The blades rotate 180 between each flash of the shutter, basically the shutter is flashing twice the speed of the rotor RPM but the tach knows this and spits out the proper RPM.

When you try to tach a multiblade head, I'll use a 3 blade head as an example, when the blades "stop" in the air the shutter is actually flashing at 1/3 period, the blades are rotating only 120 between each shutter flash. shorter distance = shorter time = faster displayed RPM. The sky Tach doesn’t know to account for a shorter period so it makes sense that the RPM reading is high.

To use a sky tach on a multiblade head try this. I haven’t done it yet, only as a thought experiment but I bet it would work.

make one of your blades identifiable, i.e. color the tip with a permanent marker(you can use rubbing alcohol to take the marker off afterwards) Take-off and have your buddy tach your heli, only this time find the RPM setting that makes the one blade with the colored tip stop in the same place. The shutter will now be flashing at 1 period, blades rotate 360 between each shutter flash. Double whatever RPM the tach is giving you and that should be the accurate RPM.

The only problem with this is that I believe the Sky Tach can only read down to 1000 RPM, so if your actual RPM is anything less than 2000 it wouldn’t work.

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05-31-2010 06:20 PM  7 years agoPost 8
cgflyer

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Boca Raton, FL

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Interesting! I still lke Rodan's idea but after reading how the sky tach works, I think two marks should be made on the head button simulating two blades. The head can then be measured on the ground as if it were a two bladed head by stopping the marks and not the blades. Great discussion guys.

Rich

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05-31-2010 08:01 PM  7 years agoPost 9
Rodan

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Prescott Valley, AZ

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Excellent observation, Captain! I had always thought of the shutter rotating at a 1:1 relationship, but on my tach, the 'shutter' is a slot through a rotating drum, so your theory makes sense, and explains why I would see 6 blades at the correct RPM with my three blade head. The 'snapshots' are 180 degrees out on the rotation of the head...

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06-01-2010 07:26 PM  7 years agoPost 10
seattle_helo

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Seattle, WA USA

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Yeah, I had serious issues with this until I just put tape on the rotor head cap in desperation. I almost threw out a $5000 set of mechs I was getting so pissed!

Much of the saga is solved here:

http://scalerchelis.com/phpBB2/view...hp?f=24&t=10863

nick

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06-02-2010 08:53 AM  7 years agoPost 11
ozghost1952

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Melbourne Australia

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Using the Skytach - try this ...

Reading of 2200 with blades stationary
3 blades x .66 = 1452 (2/3)
4 blades x .50 = 1100 (2/4)

Seems to correlate with my onboard tach readings

Jeff

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06-02-2010 05:58 PM  7 years agoPost 12
cgflyer

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Boca Raton, FL

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Thanks Jeff, another good idea. Using your caclulations the head sounds closer to the what the tach was reading.

Rich

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06-03-2010 01:22 PM  7 years agoPost 13
Jason Lewis

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Whitby, Ontario, Canada

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I went with an Eagletree system, records RPM (with the optional sernsor) and a lot more stuff, really lets you know is going on with your power system.

Jason

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