Tach Vs. Hobbs Time

"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

Ben Jackson wrote:
In article ,
C J Campbell wrote:

The tach should almost always run slower than the Hobbs. The Hobbs is a
clock, but the tach is connected directly to the engine and should only
click off one tach hour when the engine has turned 60 x max rpm.


But it's not "max rpm". In my Comanche (260hp IO-540, redline 2700RPM)
it's 1:1 at 2300RPM. I think the "definition" of tach time is 1:1 at
cruise.


I guess this is the question I'm asking. Where does 1:1 occur. On the
particular trip I was running 65% which at that altitude was 2425 RPM.

John Roncallo

Ben Jackson wrote:

In article ,
C J Campbell wrote:

The tach should almost always run slower than the Hobbs. The Hobbs is a
clock, but the tach is connected directly to the engine and should only
click off one tach hour when the engine has turned 60 x max rpm.


But it's not "max rpm". In my Comanche (260hp IO-540, redline 2700RPM)
it's 1:1 at 2300RPM. I think the "definition" of tach time is 1:1 at
cruise.


This sounds like it would be appropriate for a CS prop because you can
always load the prop up at these low RPMs and still get 65% even at 8000
ft. For example the C-172RG also with an O-360 180HP Lycoming will put
out 68% power at 8000 ft 2300 RPM and 22MP.

I would think that the Archer fixed pitch prop would be calibrated
differently. To get 65% on a standard day at 8000 ft requires ~2480 RPM.

John Roncallo

In article ,
John Roncallo wrote:

I would think that the Archer fixed pitch prop would be calibrated
differently. To get 65% on a standard day at 8000 ft requires ~2480 RPM.

Why? Do the Archer's engine components wear less at high RPM than an
engine on an Arrow?

--
Ben Jackson

http://www.ben.com/

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.

Ben Jackson wrote:

In article ,
John Roncallo wrote:

I would think that the Archer fixed pitch prop would be calibrated
differently. To get 65% on a standard day at 8000 ft requires ~2480 RPM.


Why? Do the Archer's engine components wear less at high RPM than an
engine on an Arrow?


Yes they probably do wear less. They are under less load for the given
circumstance. Cylinder pressures are much higher in an engine at 65%
power 2300 RPM than they are at 65% 2480 RPM.

John Roncallo