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HomeScaleAircraftScale HeliScale Helicopter Main Discussion › Question about taching a multibladed head
01-10-2018 08:41 PM  8 months agoPost 1
Copter Doctor

rrProfessor

Enterprise/ft.rucker ,al- home of army aviation

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Is there a formula used for getting an accurate reading using an optical sky tach? I used a sky tach to get the head speed of my 4 bladed machine but it read about 2100 rpm which I know is in correct as I am pretty sure it is much lower than that.

drive a rotary, fly a rotorcraft

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01-10-2018 11:02 PM  8 months agoPost 2
payne1967

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uk

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if the tach you are using is for a 2 blade head then half the reading with a 4 blade head
so your 2100 would be
2100/2 = 1050

www.alcesterhelicopterclub.bmfa.org

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01-10-2018 11:20 PM  8 months agoPost 3
chopper37

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NJ and Long Island

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In the past I’ve made a white disc and put a black stripe on it then screwed it to head. Basically a large head button, then tach work same as two blade head. Now just setup in gov mode.

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01-10-2018 11:30 PM  8 months agoPost 4
Phoenix NOTAR

rrApprentice

Tallahassee, Florida USA

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Emile,

This was posted in RunRyder back in 2010 by Jeff (ozghost1952):

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)

I think on a 4 blade head the blades would show as "stopped" at the actual rpm (maybe 1050 in your case). the difficult is with odd number of blades.

What I do is just place a thin strip of bright colored tape across a head button, or in your case just across one opposed set of blade grips. Then any head becomes a two blade when you look at the tape.

Or you can just buy the very nice "Chinook" brand tach.

http://www.western-robotics.com/CHNK2E

Sandy

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01-11-2018 01:39 AM  8 months agoPost 5
Flyin for Jesus

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Troy, IL. 62294

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The skytach is an optical tach that you look through the shutter and when the blades stop spinning, that's the RPM.

There is no dividing needed. 1 blade, 2 blades, 3 or 4.

The trouble is this. With a 4 blade head spinning 750 RPM and the tach reading 1000, the blades will look stationary because the head has only traveled 3/4 of a revolution but all the blades line up. A 2 blade head would look like it has 4 blades and a 3 blade head would look like it has 6 blades. So you know 1000 RPM isn't what the HS is. Same with a HS of 1000 and the tach at 750, the head would do 1 1/4 revolutions and still look like it is standing still.
With a 2 blade head, you need to be off by 50% to make it look stationary , a 3 blade head off by 33% get it stationary again. 4 blade 25% and a 5 blade head 20%.
So the more blades, the closer your starting point has to be or else you can end up way off as you are.
You'll have to calculate the battery voltage, gear ratio, kv of the motor and run the head up to say 50% throttle and by calculating that, you can start the tach pretty close to the correct HS.
Otherwise you'll need 3 black blades and one white blade and as long as the white blade is always in the right spot, you're good unless your tach is reading double the HS, then the white blade would be the same spot just doing 2 revolutions per shutter opening.

Hope that helps.

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01-11-2018 01:41 AM  8 months agoPost 6
Copter Doctor

rrProfessor

Enterprise/ft.rucker ,al- home of army aviation

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Thank you gentlemen, I was thinking dividing it by 2 inn the case of a 4 bladed head. the idea of the striped head button is genious! I wouldn't have thought about that.
I was just curious about the head speed i was running since I am constanly asked but can never answer. At 1050 it sounds and feels great and doesn't scare the crap outta me

drive a rotary, fly a rotorcraft

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01-14-2018 09:53 PM  8 months agoPost 7
Mojave

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Palos Verdes, Ca. USA

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As previously stated, a piece of tape works just fine.
Barry

All helis and planes have an expiration date stamped on them...you only find it after you crash!!

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01-15-2018 12:13 PM  8 months agoPost 8
CoachE

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Cincinnati, ohio -usa

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Question??? Are you all doing all of this because you are using Esc's that have no data logging capability?

BD Team Pilot / Get !T - Got !T - Perfect !T

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01-15-2018 03:47 PM  8 months agoPost 9
Copter Doctor

rrProfessor

Enterprise/ft.rucker ,al- home of army aviation

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no

Not at all, I am using the castle esc. the sky-tachs are clearly designed for use with two bladed systems. the straight line across the top of the head button works perfectly without having to do any calculations.

drive a rotary, fly a rotorcraft

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01-15-2018 04:03 PM  8 months agoPost 10
Flyin for Jesus

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Troy, IL. 62294

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Copter Doctor
the sky-tachs are clearly designed for use with two bladed systems.
Incorrect. All optical tachometers work the same with any number of blades.

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01-15-2018 07:23 PM  8 months agoPost 11
Copter Doctor

rrProfessor

Enterprise/ft.rucker ,al- home of army aviation

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Incorrect. All optical tachometers work the same with any number of blades
ok maybe thats partially true, since with anything more than two, you have to calculate the math to get the accurate reading.
with my tach I use for my fixed wing birds, the tach has a selector swith so you can use it for 2, 3 and 4 bladed props.
an observation I made while using the stripe across the head button was when the blades appeared frozen, the stripe was still spinning. when the stripe appeared to be frozen as one stripe, the blades were still spinning.
the frozen stripe gave me the same reading directly as when I did the math after using the blades for reference. Maybe I should say the LCD readout on the tach is designed for two blades.

drive a rotary, fly a rotorcraft

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01-15-2018 08:45 PM  8 months agoPost 12
Flyin for Jesus

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Troy, IL. 62294

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Just to keep things friendly here, I'm only doing this for anyone in the future to know its okay to use optical tachs with a viewfinder on a head with any number of blades and is just as accurate for any configuration. I'm not trying to win anything, just clearing up mis-information.

Just to make sure we're all on the same page here... this is what you are using? http://www.modelavionics.com/tach.asp

It has a window you look through at the spinning head of your heli and then there is a shutter inside that when the shutter speed matches the helis HS, the blades "appear" to not be moving.

The tach you use for an airplane prop. I assume you use something like this. https://www.towerhobbies.com/cgi-bi...r0aAtdHEALw_wcB

It sees light and then sees shade. All it does is count how many times the lens is shaded per minute and divide by the number of blades to give the RPM. This method is absolute and with proper lighting is always correct.

Your skytach is not the same. I mentioned in one of my prior posts how you can get error. Example 1000 on the tach and 750 actual HS you will still see the blades stationary on a 4 blade head. Because the head rotates 3/4 of a revolution then the shutter opens again and you see the blades in the same spot. But you're not reading the true HS. There is no calculation to correct for this. It is simply an incorrect reading.

Quick chart
2 blade head @ 1000RPM
stationary with a skytach
500___2 rotations per shutter opening
667___1 1/2 rotations per shutter opening
1000__1 rotation per shutter opening
2000__1/2 rotation per shutter opening

3 blade head @ 1000 RPM
stationary with a skytach
500___2 rotations per shutter opening
600___1 2/3 rotations per shutter opening
750___1 1/3 rotations per shutter opening
1000__1 rotation per shutter opening
1500__2/3 rotation per shutter opening
3000__1/3 rotation per shutter opening

4 blade head @ 1000 RPM
stationary with a skytach
500___2 rotations per shutter opening
571___1 3/4 rotations per shutter opening
667___1 1/2 rotations per shutter opening
800___1 1/4 rotation per shutter opening
1000__1 rotation per shutter opening
1333__3/4 rotation per shutter opening
2000__1/2 rotation per shutter opening
4000__1/4 rotation per shutter opening

There are fewer possibilities to get an incorrect reading with a 2 blade head. But in your case that you were up at double the HS, you would still have been seeing stationary blades even if you were using a 2 blade head.

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01-15-2018 09:28 PM  8 months agoPost 13
Copter Doctor

rrProfessor

Enterprise/ft.rucker ,al- home of army aviation

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Yes, i am using the same and have usex the one from miniature aircraft as well. m not saying it cant be used, things just get a little more involved with calculating the results when theres more than 2 blades based on the readout. With the tape across the headbutton i know instantly what the readout tells me is accurate

drive a rotary, fly a rotorcraft

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01-15-2018 10:43 PM  8 months agoPost 14
ozghost1952

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Melbourne Australia

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Anyone used the HeliTach app? It uses frequency to measure head speed
For electric Heli’s it translates the “note” of the blades and for IC engines the “note” from the engine plus gear ratios
Jeff

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01-15-2018 10:56 PM  8 months agoPost 15
Flyin for Jesus

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Troy, IL. 62294

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Copter Doctor
things just get a little more involved with calculating the results when theres more than 2 blades based on the readout.
That's my point. there is no calculating.

You found a solution to reading the wrong HS, with that mark. You can use any mark. Be it 3 black blades and one white one, or even just a piece of tape at the root of one blade. If you see that tape in different places even though the blades appear stopped, then you are reading on of the possibilities I wrote out in the chart.

@ozghost1952
A friend at the field tried one of the phone tach apps ( don't know which one he used ) just to see if it matched my telemetry numbers. It was right on the money. Guess it just gets the RPM by the sound frequency.

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01-16-2018 12:24 AM  8 months agoPost 16
Copter Doctor

rrProfessor

Enterprise/ft.rucker ,al- home of army aviation

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That's my point. there is no calculating.
You found a solution to reading the wrong HS, with that mark. You can use any mark.
This is where I think I may have lost you, see, when I first used the blades, the read out was waaay higher than what I knew the hs should be. that's why I made the original post of which I got your answer as well as others. of all the answers I got, the tape idea appealed to me most because it was the simplest and what the readout says is exactly what the hs is, no dividing or calculating, plus with the odd number of blades, the tape reacts no differently.

drive a rotary, fly a rotorcraft

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01-16-2018 01:07 AM  8 months agoPost 17
Flyin for Jesus

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Troy, IL. 62294

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I'm with you and I now see we're both on the same page. The tape piece is a great idea and I have personally used it when my GOV was wayyyyy off from where it should have been. Since it was soo far off, using any marker ( tape being easiest IMO ) gets rid of all the possibilities I wrote in my chart that have a fraction in them. Only 500 or 1000 on the tach will show the tape in the same spot at 1000HS.

I'm glad you got it sorted ( like a week ago ) I juat wanted to make sure people know its prefectly fine to us an optical viewfinder type of tach on heads of any configuration.

Happy flying.

Dan

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01-16-2018 01:15 AM  8 months agoPost 18
Copter Doctor

rrProfessor

Enterprise/ft.rucker ,al- home of army aviation

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yes for sure. I have never really been concerned about what hs I was running on my ships, i am a little old school and tune by feel and sound. i have been asked so many times what my hs was and i always replied that i have no idea

drive a rotary, fly a rotorcraft

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01-16-2018 04:57 AM  8 months agoPost 19
Flyin for Jesus

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Troy, IL. 62294

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I have a scale heli I need to finish up. Its a nitro converting to electric but too old to buy a conversion kit. I made the motor to clutch piece, just need to 3d print a motor mount for test fit before making it out of aluminum.
I want to fly with the HS low as possible for a more scale look. That will be guess and check but hopefully with no crash in the middle of the process.

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01-16-2018 06:01 AM  8 months agoPost 20
Copter Doctor

rrProfessor

Enterprise/ft.rucker ,al- home of army aviation

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what machine is it? all my machines are once nitro that i converted to electric making/modifying parts myself. maybe i can offer some help.
i too am all about the lowest head speed i can possible get away with for the scale appearance and feel.

drive a rotary, fly a rotorcraft

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HomeScaleAircraftScale HeliScale Helicopter Main Discussion › Question about taching a multibladed head
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