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HomeAircraftHelicopterHelicopter Main Discussion › Question about head speed
12-31-2010 03:13 AM  7 years agoPost 1
Mr. Miyagi

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Detroit, Michigan

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When people generally measure head speed, are they measuring from the blade root or the blade tip? Because the tips would be spinning considerably faster according to physics.

You're not a "pilot" if you fly an RC helicopter. Sorry to crush your dreams #HardTruth

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12-31-2010 03:15 AM  7 years agoPost 2
Funky Trex

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Westerville, OH - USA

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When people generally measure head speed, are they measuring from the blade root or the blade tip? Because the tips would be spinning considerably faster according to physics.
Head speed is measured by rpms (revolutions per minute). This would be the same at the root or the tip of the blade.

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12-31-2010 03:15 AM  7 years agoPost 3
Supercoolheli

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Dallas, TX

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umm...just point the tach at the heli? It should be the same if you're as far away as you should be

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12-31-2010 03:19 AM  7 years agoPost 4
dirtguy

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lake elizabeth, CA-USA

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It is generally measured in rpm which is the same at the root or tip, though you are correct if you wanted a speed measurement it would be faster at the tip.

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12-31-2010 03:21 AM  7 years agoPost 5
jsenicka

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Eagle River, WI

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Regardless of distance from the head toward tip, or distance from heli to tach holder guy, RPM is RPM.

A rotor spinning at 1900 RPM is spinning at 1900 RPM from center of main shaft to tip of blade.

The Optical tachometers we use on helis are measuring RPM by using a spinning shutter. The read RPM by you seeing the blades as stopped. They are not measuring distance traveled (like a bicycle speedometer needing to know circumference of the wheel)

Jim Senicka
Team Manager, GrandRC Flight Team

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12-31-2010 05:58 AM  7 years agoPost 6
brcg123

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Wagoner OK USA

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Is this a question posed from a "pilots" view, or are you wanting to start your "toy" up? Did they not already go over this in your Pilots class?








Trex700N, Trex600N, Raptor70, RaptorTitan, GMPCrickett, Visa

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12-31-2010 07:13 AM  7 years agoPost 7
Mr. Miyagi

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Detroit, Michigan

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brcg123
Key Veteran
Location: Wagoner OK USA

My Posts This: Topic Forum Is this a question posed from a "pilots" view, or are you wanting to start your "toy" up? Did they not already go over this in your Pilots class?
Hey brcg123,

if you want to go for a ride in the real thing let me know. I'm sure you've been dreamin about it for a while there Captain finky.

and depending on where you are pointing the optical tachometer you will get a different blade speed. If you point it closer to the hub the blade speed will be slower. If you point it at the blade tips it will be faster. So do you guys just point it in the middle? Or is the difference so small it doesn't matter? Here are two videos.. one closer to the center and one closer to the tips.

http://www.youtube.com/watch?v=s1cUqE3-djE

http://www.youtube.com/watch?v=NF7z...feature=related

You're not a "pilot" if you fly an RC helicopter. Sorry to crush your dreams #HardTruth

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12-31-2010 07:43 AM  7 years agoPost 8
RCSavager

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Topeka, KS

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I think you're relying on the results of two different kinds of meters versus what others in the hobby may use. The bottom line is as stated above by other posters. Headspeed is measured by RPM (revolutions per minute, just to be clear). A spinning prop (heli blades in this case) is the same revolutions per minute whether it's close to the main shaft or at the blade tips. The quality of the measuring tool depends on it's accuracy to properly measure this rotation, but they WILL be the same irregardless of that accuracy. The inside of the blades spin at the same rate as the tips because they are a solid unit spinning during a time of measurement.

Many use either electronic units (eagletree) that measures the pulses of the motor rotation that can be figured knowing your gear ratio and others use an optical tack that you look through and adjust until you see the blades 'stop' as described above. End results is that the quality (or lack thereof) in the measuring tool 'may' show different results but that is the fault of the tool, not the operator. What you've been advised above is still correct as well as my feeble attempt at clarification.

Ask around and find someone with another tool and have them check their results with yours and you should find an answer that you can rely on.

Best of luck to you.

Once you try steak, is hard to go back to hotdogs.

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12-31-2010 07:47 AM  7 years agoPost 9
Mr. Miyagi

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Detroit, Michigan

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Yeah.. I guess I am confused on how the different types of measuring tools work. I thought it shot out a laser beam and measured the pulses between rotations of the blade and then somehow calculates a RPM relative to the speed the blades are passing. From where the tools seem to pointing in both videos doesn't make sense to me. I don't understand how you get "RPM" from a laser pointing at the blades spinning? True RPM would be sampled from the ratio of the transmition to the main rotor and motor shaft revolutions per minute or at the mr shaft itself.

You're not a "pilot" if you fly an RC helicopter. Sorry to crush your dreams #HardTruth

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12-31-2010 07:51 AM  7 years agoPost 10
RCSavager

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Topeka, KS

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In my personal experience I have had great luck with this item (http://www.modelavionics.com/tach.asp) and have no need to try anything else. It's very accurate and seems to be what most heli guys/gals use. I can only speak for the Phoenix area but I'm sure it's the same everywhere.

Once you try steak, is hard to go back to hotdogs.

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12-31-2010 10:44 AM  7 years agoPost 11
brcg123

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Wagoner OK USA

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Yeah.. I guess I am confused on how the different types of measuring tools work. I thought it shot out a laser beam and measured the pulses between rotations of the blade and then somehow calculates a RPM relative to the speed the blades are passing. From where the tools seem to pointing in both videos doesn't make sense to me. I don't understand how you get "RPM" from a laser pointing at the blades spinning? True RPM would be sampled from the ratio of the transmition to the main rotor and motor shaft revolutions per minute or at the mr shaft itself.
If your using a tach made for heli's, it works on the principle of looking through a site window on the tach, and a wheel spinning the same direction of rotation as the blades on the heli, and applying more rpm of the tach wheel until the tach blades catch up with the blades on the heli, and reading the rpm when they are turning the same to where the blades appear to stop in motion, its a visual thing, no beam of laser being used.
If your using a tach made for an airplane, then you set the number of blades, put it in front of the running blades on the airplane and the tach reads the amount of times the beam breaks in a set given time frame, and that gives you the rpm. this cant be used on a heli, because of the positioning of the heli to check the rpm would be kinda dangerous








Trex700N, Trex600N, Raptor70, RaptorTitan, GMPCrickett, Visa

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12-31-2010 11:16 AM  7 years agoPost 12
jcoats

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central VA

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Nobody here are (real pilots),whyvare you still asking us questions? ...

Did nobody see this creeps rant on Christmas day..

Tool

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12-31-2010 11:51 AM  7 years agoPost 13
JeffKollin

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Jenison, MI

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Aaaahhhhh is this the tach that real pilots use....

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12-31-2010 11:58 AM  7 years agoPost 14
jcoats

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central VA

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Lol.. Jeff

He should go ask his real pilot friends about these toys head speeds...mmmm. the rotors don't tilt...dousxhe

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12-31-2010 05:43 PM  7 years agoPost 15
Mr. Miyagi

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Detroit, Michigan

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lol... I guess I am never going to live that one down.

the guy in this video says he is using an optical tach, but I think this is a prop tach, right? From what I have gathered the term "optical tach" is used interchangably with "prop tach"? hmmm

http://www.youtube.com/watch?v=NF7zGrGTqfA

anyways, I downloaded the manual for Model Aviationics "optical tach" it says that the rpm sensed is accurate +/- 2.5rpm which would mean that optical tachs can't offer a true reading of rotor RPM.

A brushless motor sensor seems to be the most accurate from what I can tell because it measures the pulses from the ESC. Does anyone disagree?

You're not a "pilot" if you fly an RC helicopter. Sorry to crush your dreams #HardTruth

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12-31-2010 07:45 PM  7 years agoPost 16
RCSavager

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Topeka, KS

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All comes down to how important 2.5rpm accuracy is to you I suppose.

Anything within 100rpm would work for me at least.

Once you try steak, is hard to go back to hotdogs.

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12-31-2010 09:08 PM  7 years agoPost 17
AirWolfRC

rrProfessor

42½ N, 83½ W

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anyways, I downloaded the manual for Model Aviationics "optical tach" it says that the rpm sensed is accurate +/- 2.5rpm which would mean that optical tachs can't offer a true reading of rotor RPM.

A brushless motor sensor seems to be the most accurate from what I can tell because it measures the pulses from the ESC. Does anyone disagree?
Accuracy, from either device is a matter of the accuracy of the internal electronic time base reference. One is not necessarily better than the other.

Speed . . . of a rotating object . . . is measured in RPM.

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01-01-2011 05:07 PM  7 years agoPost 18
Mr. Miyagi

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Detroit, Michigan

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I understand they both have error, but the brushless motor sensor reads the pulses of each revo of the motor. I think that would be a little less error right?

You're not a "pilot" if you fly an RC helicopter. Sorry to crush your dreams #HardTruth

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01-01-2011 05:39 PM  7 years agoPost 19
AirWolfRC

rrProfessor

42½ N, 83½ W

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You're missing the point . . . how accurate is your electronic clock ? ?

You can sense pulses off a burshless motor and you can sense pulses off a photo detector . . . but how accurate is the clock you're measuring those pulses with ?

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01-02-2011 01:38 AM  7 years agoPost 20
flyen_3d

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BIRMINGHAM, ALABAMA

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What The ----
Last I heard head speed is measured in RPM, revolutions per min. It does not matter if its at the root or 100' out

RPM is RPM

you have too much time on your hands to think of crazy stuff like that

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