Optical Tach?

[basic]

Ok, so i've been looking for a tachometer for heli/air use and have some questions.

A lot of the ones i see that are designed for use on RC stuff are really expensive when compared to the traditional reflective optical tachs you can get. Is there any real difference? i mean i can understand on the RC ones you can set up number of blades etc, but it's just as easy to divide by 2 on a heli to get your actual RPM. So would one of the reflective ones like this http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=41727 work or does it need to be something made for RC? that's just one example of what i found, you can find really cheap ones (28 bucks) on ebay like that. i mean the tolerances don't have to be ultra tight, i think ± 5% would be good one those. i mean at 2400 RPM you'll be within 120 rpm, and 5% is a HUGE window. I guess if you were taching a motor spinning at 30000 RPM. the margin of error would be a little larger, 1500 rpm.

I guess my biggests questions are these - what do you all use, do the normal ones like the one linked above work and if not where can i get a cheap one that will?

[estarter]

I have this one, too big and can not set the # of blades.

[basic]

ij'm not really worried about having to set the number of blades, simple math will tell me after i get a reading, i just want to make sure that a plain one will still work, although i don't know why they wouldn't. GUess i just wanted to make sure.

[estarter]

The one I have use LED sensor, works indoor with fluorescent lights. Since all my helis are with 2 blades, simple math for me.

[swatson144]

I use 2 methods cheapest is the 20-30$ tach from Hangar9 and a videao camera.

(http://www.youtube.com/watch?v=o_-CFpqFMsQ)http://www.youtube.com/watch?v=o_-CFpqFMsQ (http://www.youtube.com/watch?v=o_-CFpqFMsQ)

You can see 2500 at lift off before the paddles get in and cause it to increase.

next comes the Eagle tree data logger whith brushless rpm addon. It takes pulses from the motor and does the math and charts headspeed for a flight ~ 100$. It also operates real time on the bench.

http://www.rchelicoptertown.com/forum/attachment.php?attachmentid=641&d=1175311093

I'd say the datalogger is about a no brainer now if you are gonna spend any money to mention. It's such a versatile tool.

Steve

[basic]

i've been lookgin at those eagle tree data loggers, i think they also have an optical tach addon so that i could use it with the gasser.

[BobinNJ]

How do you use an optical tach safely?

[Firadiz]

The instructions state that while one person controls the Heli, another person uses the Optical Tach to visualize the RPM's. This prevents a mishap to where the operator is not in control of his/her bird at all times. That is the safest way. I'm sure there are other safety ways, but with an optical tach, someone has to be looking through the slit, so IMO there needs to be 2 people to perform the RPM's.

[estarter]

I usually do it by myself, the tach I use can read RPM from up to 36 inches. I flip my Tx at idle up, take the reading at 0 pitch and at -9 pitch. It's a little scarely to take the reading and I usually stand about couple feet away from the bird, extend my arm to take the reading.

Good thing is the tach stores the value until it's power down.