Tach Vs. Hobbs Time

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.