Tach Vs. Hobbs Time

In article , John Roncallo
writes:

Is there any text book way to check this. Our FBO seems to feel that
calibrating the Tach is a big deal. I'm currently thinking of just
replacing the Tach without tring to calibrate. Right now it looks to me
as if we just replaced and engine at 1700 hr thinking it had 2000 hr.


I got a free program that uses the engine sound to give you RPM. I don't know
the web site but the program says email is . Just
take a laptop to the plane and check it out.

Chuck

"PaulaJay1" wrote in message ...

I got a free program that uses the engine sound to give you RPM. I don't know
the web site but the program says email is . Just
take a laptop to the plane and check it out.

Engine sound or prop sound? How does it work on geared engines?

(just kidding).

On Sun, 21 Dec 2003 17:56:56 -0500, "Ron Natalie"
wrote:


"PaulaJay1" wrote in message ...

I got a free program that uses the engine sound to give you RPM. I don't know
the web site but the program says email is . Just
take a laptop to the plane and check it out.

Engine sound or prop sound? How does it work on geared engines?

(just kidding).

Someone suggested using a street light as a strobe to check your tach.
Do the math to figure out what speed will stop the prop and see if
your tach agrees
Drew

Drew Dalgleish wrote:

Do the math to figure out what speed will stop the prop and see if
your tach agrees

Any multiple of 60.

George Patterson
Great discoveries are not announced with "Eureka!". What's usually said is
"Hummmmm... That's interesting...."

It is 1800 for a 2 blade prop, and 1200 and 2400 for a 3 blade prop,
if my memory serves me.
I worked the math once. Light goes off 60 times per second, which is
3600 times a minute. And you have to account for the number of blades.
Just pull up with a street light to your back and try it. The prop
will stop. Should be at one of the numbers above.

It is not every multiple of 60.


"G.R. Patterson III" wrote in message ...
Drew Dalgleish wrote:

Do the math to figure out what speed will stop the prop and see if
your tach agrees

Any multiple of 60.

George Patterson
Great discoveries are not announced with "Eureka!". What's usually said is
"Hummmmm... That's interesting...."

"Doug" wrote in message om...

It is not every multiple of 60.

Yep, and if you have a geared engine there's still a bit of math. We checked my tach
once with one of those strobotach things (looking at the prop). The tach is turning faster
than the prop by a 120:77 (or something like that) ratio.

Of course, this only applies in North America and other 60Hz areas. You
Europeans, Asians, etc., are a bit slower at 50Hz; 1,500rpm for 2 and
1,000/2,000 for 3 (non geared) blades. Right???

"Doug" wrote in message
om...
It is 1800 for a 2 blade prop, and 1200 and 2400 for a 3 blade prop,
if my memory serves me.
I worked the math once. Light goes off 60 times per second, which is
3600 times a minute. And you have to account for the number of blades.
Just pull up with a street light to your back and try it. The prop
will stop. Should be at one of the numbers above.

It is not every multiple of 60.

Or 50 here in Europe.

Paul

"G.R. Patterson III" wrote in message
...
Any multiple of 60.

Drew Dalgleish wrote:

On Sun, 21 Dec 2003 17:56:56 -0500, "Ron Natalie"
wrote:


"PaulaJay1" wrote in message ...


I got a free program that uses the engine sound to give you RPM. I don't know
the web site but the program says email is . Just
take a laptop to the plane and check it out.


Engine sound or prop sound? How does it work on geared engines?

(just kidding).


Someone suggested using a street light as a strobe to check your tach.
Do the math to figure out what speed will stop the prop and see if
your tach agrees
Drew

I guess you are saying the street light flickers at 60Hz. Than a 2
bladed prop should look stopped at 1800 RPM? I might give this a try but
I think this would be better for calibrating the tach neadle rather than
the tach hour indicator.

John Roncallo

"John Roncallo" wrote in message . com...

I guess you are saying the street light flickers at 60Hz.

It flickers at 120 Hz.