Tach Vs. Hobbs Time

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/

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

"C J Campbell" wrote in message
...
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.

The ratio of turns to "tach time" isn't necessarily set on max rpm.

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...."

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